BEFORE PROCEEDING to the third article in the “From the
Ground Up” engine series, I am going to review the first installment,
published in the April issue. MA has received numerous comments on
the information presented in that article. Many were favorable, but
several pointed out that the theoretical information was incomplete,
poorly explained, or just plain wrong. And in some instances they
have valid points.
So I’ll review some of “Engines 101”’s basics, keeping in mind
the comments that were offered. In all cases they were offered in
good faith and in the hopes of improving the series and modelers’
understanding of engine basics.
Probably the most incomplete section was the beginning of the
article. I did not take the space, which is precious in any article, to
properly explain the intent of the engine theory I was writing about or
the manner in which it was to be presented. I’ll do it now as it should
have been done in “Engines 101.”
The engine theory I am presenting is intended solely to provide
beginning RC pilots with enough knowledge of a model-engine’s
workings to understand why choices about proper mixture settings,
fuel, propellers, and other items to be discussed later will be made.
There is no intent to fully detail an engine’s intricate machinery.
The new RC pilot is not going to be designing or disassembling (I
hope) his or her first few engines, but this person will be setting highand
low-speed mixture settings.
All the theory I present will be from a strictly operations
viewpoint. Proper engine operation is the only goal in this discussion.
Where true technical names for parts may be confusing, I will use
descriptive terms instead. Since most new modelers have no
knowledge of two-stroke engines, but do have at least a passing
familiarity with their cars’ engines, I’ll try to reference parts with
confusing names in more recognizable terms.
As with all things mechanical, a model engine’s true operation is
complicated. Operations that are explained separately and appear to
be independent actually overlap, and sometimes interfere with, other
operations. To simplify the theoretical presentation, I will explain
each action as if it were the only one happening at that time.
A great deal of confusion would have been avoided if I had started
“Engines 101” with the preceding. Because of the decision to
simplify and avoid confusion, the rotary disk induction valve became
the crankshaft intake slot.
The true engineer’s name is correct but leads one to look for a
moving valve such as those found in a car or a rotating valve. There
isn’t one, and the “valve” is a slot. The name also sounds as if some
sort of “pumping” action is happening when it is not. The same
48 MODEL AVIATION
Engines 101
Rear ball bearing is partially visible above and to either side of
counterweight in Webra Speed .61 (L). K&B .65 (R) uses bronze
bushings to support crankshaft, so it has only case metal in this
area.
Photo clearly shows SuperTigre’s boost port on left side. K&B
.61 (R) has no such boost port and therefore a much narrower
cylinder case.
Follow-up to April issue’s “Engines 101” clears the air
by Frank Granelli
Revisited
Photos by the author
08sig2.QXD 5/24/04 8:47 am Page 48
rationale was used when
discussing the “intake” and
“boost intake” ports, actually
known as transfer (also called
bypass) and boost transfer
(bypass) ports.
Why “transfer”? Because
these ports allow the fuel/air mix
in the crankcase to transfer from
the crankcase to the combustion
chamber, but their function is
that of fuel/air intake ports.
When traveling, you do not need
to know a street’s name—just
where it goes. In this case it goes
into the combustion chamber.
I also simplified the partial
vacuums found in our engines’
operations—actually lowpressure
areas since there is
nothing even approaching a
physicist’s definition of a partial hard
vacuum in our engines (there is just too
much gas density everywhere inside)—as
just “vacuums,” as most automotive books
do.
I completely ignored the function of an
engine’s timing advance, all references to
Top Dead Center (TDC) operations, and
especially all references to interference and
benefits one operation may have compared
with another.
None of this theory would help newer RC
pilots operate their engines better.
Explaining these operationally irrelevant, but
theoretically important, functions would
have taken almost a full article themselves.
Similarly, I called the methanol in our
fuel a “heat exchanger” and pointed out that
methanol helps cool the engine; that is why a
richer mixture is important. I felt that “heat
exchanger” was a simple, generally
understood term that would not require
definition but get the point across.
However, in technical terms, methanol
cools our engines because it has a high heat
of evaporation. During carburetor air intake,
methanol in the fuel is transformed into a gas
requiring a great deal of heat. The process—
called refrigeration—therefore removes heat
from the surrounding lower engine sections
to have the energy to transform the
methanol.
The same process cools the food in your
household refrigerator, but without the
combustion part. Your refrigerator
substitutes an electrically driven pump and
evaporator for crankcase pumping and
venturi action at the carburetor. But this
process is not technically one of heat
exchange. A heat exchanger does not
transform into another state of matter, but
absorbs heat from one source and transfers it
to another.
There is a true heat exchanger in our
fuel that removes heat from the engine and
transfers it to the earth’s atmosphere via
the exhaust. We call it the “fuel’s oil.” As
everyone who has cleaned his or her model
aircraft knows, some oil is not burned
during combustion and escapes through the
exhaust. While escaping, it also transfers
some of the engine’s heat.
The main point remains, however.
Whether through heat exchange or
refrigeration, the fuel lubricates and cools
our engines. RC engines are not cooled
solely through contact with the air; therefore,
the proper high-speed fuel mixture settings
are critical and must not be ignored.
In retrospect, I probably should have used
the technical terms and part names for the
preceding and for other items in “Engines
101.” It is usually better to explain the
technical, rather than “street,” names at the
start, even if doing so requires much
additional explanation. In this way, regular
RC pilots and engine technicians will
eventually have the same reference names.
So sit down, tighten your engine cap on
your head, and let’s explore the true, fully
detailed, roller-coaster operation of the
thermodynamically controlled contraption
we call the two-stroke, internal-combustion
engine. Along the way I will point out areas
where “Engines 101” was unclear, poorly
explained, or technically incorrect.
“Engines 101” started with the engine
before the first combustion and then
followed the operation cycle. It was assumed
that there was a fresh air/fuel charge in the
crankcase. That caused confusion about fuel
transfer (intake) timing versus
exhaust timing.
Since no combustion had yet
occurred, I ignored the fact that
the exhaust port opens slightly
sooner than the transfer ports do.
The main point was that the
exhaust port became fully open at
the same time the transfer ports
were opened completely.
But that might lead one to
wonder why the fuel/air mixture
just doesn’t go right across the
cylinder and out the exhaust.
Rather than explain that yes, it
does do that (somewhat), I
ignored it.
Actually, some fresh fuel/air
mixture does exit the exhaust,
especially during start-up. This is
one of the two-stroke engine’s
inefficiencies that engine designers strive to
minimize. This is also one reason why you
may notice some fuel condensing in the
muffler during operation—especially during
“rich” operation. Only the fact that the
engine’s parts are moving quickly helps
reduce this unwanted fuel/air loss.
This time I’ll start with the engine’s
piston at Bottom Dead Center (BDC),
meaning that it is as far down in its
movement (called stroke) as it can get. The
engine has not started, and there is no fuel
anywhere inside the engine. There is no fuel
anywhere except in the fuel tank.
(Operationally, it is important to have fuel in
the tank before trying to run the engine.)
Starting an engine from this position is
difficult until fuel flows from the tank,
through the fuel lines, and into the
carburetor. Therefore, we need to draw the
fuel from the tank, into the carburetor. We
will use the “suction” effect that permits the
engine to run in performing this task. Where
does the suction come from? While at BDC,
the rotary disk induction valve—the intake
slot in the crankshaft—is fully closed.
As the engine is hand-rotated
counterclockwise, the piston begins to move
upward. It first closes all the transfer (intake)
ports. At this point the rotary valve (for
short) begins to open, but the exhaust is also
still slightly open. However, there is no
connection between the exhaust port and the
engine’s lower crankcase at this point, so
that is irrelevant now.
As the piston continues to move upward,
the crankcase volume (not area, as was
written in “Engines 101”; that was a
misstatement) begins to increase. As this
volume increases with continued upward
movement of the piston (not cylinder—
another misstatement in the original article),
a low-pressure area is created in the
crankcase.
This happens because the now-sealed
crankcase volume is bigger than it was, but it
still contains only the original amount of air.
The air expands to fill the increased volume
and therefore has a lower pressure.
But do you remember that rotary
induction valve that was opening just as the
August 2004 49
Larger brass “tube” on right is fuel jet. Smaller brass fitting on
left is low-speed needle valve that regulates amount of fuel
flowing through fuel jet at reduced throttle levels.
K&B .65 (L) has larger upper cylinder case
to accommodate boost transfer ports.
Non-boost-ported K&B .61 (R) has almost
straight-walled cylinder case.
08sig2.QXD 5/24/04 8:47 am Page 49
transfer ports were closing? The valve opens
more as the piston travels upward. It is now
fully open, and that means the crankcase
section is no longer sealed.
The rotary valve is located just under the
carburetor. If the carburetor throttle barrel is
open, air rushes through the carburetor,
through the rotary valve (crankshaft), and
into the crankcase. Remember this process; it
will be repeated shortly, once fuel is added to
the mix.
Now we have plenty of air rushing into
and through the engine as we hand-rotate the
propeller. What happens if we put an
obstruction, such as a thumb, over the
carburetor’s air inlet?
Low pressure returns to the lower
crankcase since it is again sealed, even when
the rotary valve is open. But the piston is still
moving and re-creating the low-pressure
condition with each revolution. You can
actually feel the suction with your thumb.
This suction effect draws fuel and air into
the carburetor.
This low-pressure condition seeks relief
from wherever it can. Since the only
possible pressure relief is the small brass
fuel inlet—the fuel jet—fuel is drawn from
the fuel tank and into the fuel jet.
The photo showing the venturi process
was not sent in with the original article. That
caused confusion about the venturi process
because the picture’s caption explained it in
detail. I’ve included the photo here to better
illustrate the venturi process. The low
pressure—we call it suction—continues
through the small fuel inlet, through the
lines, and into the fuel tank.
Now remove the obstruction. As the
rotary valve opens, the crankcase’s lower
pressure draws fuel through the small brass
tube in the picture (the fuel jet) and air from
the atmosphere into the rotary induction
valve.
As the air is pulled through the
carburetor, it speeds up to go through the
narrow carburetor intake passage. The added
velocity means that the intake air gains
kinetic energy and, in order to maintain
balance, the potential energy (temperature
and pressure) drops. When the engine is
running or hand-cranked, this lowered
pressure is seen at the fuel jet, and the
difference between this low-pressure area
and the outside air pressure (seen at the fuel
tank vent) “sucks” fuel into the carburetor as
if your thumb were still there!
When the piston reaches as far upward as
it can—TDC—the fully open rotary valve
begins to close but draws fresh air and fuel
into the crankcase for another 70°-90° of
crankshaft rotation. The valve closes
completely before the exhaust port begins to
open. The crankcase and combustion
chamber are again sealed. But the piston still
has a ways to go before reaching BDC. It
continues downward, compressing the
fuel/air mixture inside the engine’s
crankcase.
That results in the crankcase’s now being
a high-pressure region. I left the following
part out of “Engines 101” because it is not
the major reason why the fuel/air mixture
flows into the combustion chamber. But still,
this high-pressure condition does exist, and
for now, when there has been no
combustion, it is the only transfer
mechanism in operation. The piston
continues compressing the crankcase mixture
and increasing the pressure.
But before reaching BDC, the piston
uncovers the transfer and boost transfer ports
(bypass ports). The high crankcase pressure
now has an exit. The fuel/air mixture under
50 MODEL AVIATION
Chart illustrates what ports and valves are open during one crankshaft rotation from viewpoint of looking directly at propeller end of
crankshaft. Each valve or port is open for length of its labeled area in curve. Drawing courtesy Dean Pappas.
08sig2.QXD 5/24/04 8:47 am Page 50
pressure rushes up through the transfer ports
and into the volume just above the piston.
Since the exhaust port is also fully open at
BDC, some of this precious mixture is lost
out the exhaust port. But some remains
above the piston.
(One advantage of the Schnuerle boost
transfer port system is that less incoming
fuel/air mix flowing from these sidemounted
ports is lost out the exhaust. The
Schnuerle ports are not aimed straight out
the exhaust port, as is the main transfer port.)
As BDC is passed, the piston travels
upward, pushing more of the fuel/air mixture
upward and into the already filled
combustion chamber. Yet some still goes out
the exhaust port—another inefficiency. Once
the exhaust port closes, the piston begins to
compress the fuel/air mix as it continues
upward. If the glow plug is lit, and the
fuel/air mixture is in the proper proportions,
a prolonged, controlled explosion called
“combustion” occurs.
The model two-stroke is part of the class
of engines known as “combustion ignition,”
which includes diesels. But there is a
subclass known as “catalytic enhanced
combustion ignition” engines. Our engines
fit into that category, as do many automobile
diesels with “glow plugs” that are constantly
receiving electric current (still not a true
chemical catalyst effect) and are therefore
always “lit.”
It seemed easier to just call our engines
“diesels” in the original article to
differentiate them from model gas ignition
engines rather than go through the true
technical explanation, as I just did.
Consider all of the preceding and add the
fuel/air mixture to the now-running engine.
How does the process differ? “Engines 101”
basically assumed that it didn’t, and for
operational understanding it doesn’t vary.
Proper operation and equipment selections,
except for tuned pipes that few sport pilots
use anymore, do not depend on any of the
following information. Still, this knowledge
could be important for a full understanding
of our engines’ operating theory.
Do you recall the intake process I
described in the preceding? Consider the
same process but with the engine running at
full speed. The piston is at BDC with most of
the exhaust gases gone, receiving a fresh
charge of fuel/air from the crankcase into the
now-vacant volume above the piston, right?
Well, not really. The exhaust port opens
only slightly before the transfer ports, called
the “exhaust lead” or “blowdown.” The
exhaust gases have not fully exited the
cylinder when the transfer ports begin to
open. The relationship between these
openings is part of the engine’s timing. The
accompanying illustration summarizes many
sport engines’ timing in this regard.
In practice, this timing means that fresh
fuel/air mixture is flowing into the cylinder
even as exhaust gases are exiting. Why
would an engine designer do this?
The hot, still expanding exhaust gases are
exiting at a high velocity. This forms a lowpressure
area just above the piston, “behind”
the exiting exhaust gases. The fresh fuel/air
mixture is “pulled” through the transfer
ports, into the low-pressure area in the
cylinder at the same time the descending
piston is compressing the mixture in the
crankcase and pushing it into the bypasses.
We say the exhaust gases “scavenge” the
fuel/air mixture into this section. The
scavenging effect increases the velocity, and
hence the amount of the fresh fuel/air
mixture that is drawn into the engine.
Just as the scavenge action is finishing
(the exhaust gases’ momentum is exhausted)
and the pulling of intake from the crankcase
through the bypasses is ending, the rotary
valve opens. This helps start the flow of
fresh fuel/air mixture into the crankcase for
the next power stroke.
At exceptionally low speeds, such as idle,
the scavenging action goes to completion,
and you are back to having pressure in the
crankcase at the moment the rotary valve
closes because of the descending piston. You
can sometimes tell that this is happening as
the engine spits fuel from the carburetor at
slow speeds.
Therefore, the scavenge effect is the
major force our engines use to put fuel and
air into the combustion chamber, but
crankcase pressure does play an important
part in the initial charge’s transferring into
the cylinder. Together, these alternating,
thermodynamically produced high- and lowpressure
conditions, neither a true or even
partial vacuum, allow our engines to run.
Several exhaust systems are available that
August 2004 51
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08sig2.QXD 5/24/04 8:48 am Page 51
advanced that detonation occurs, meaning
that the fuel/air mixture ignites before it
should. This condition may sometimes be
identified by a loud “frying egg” sound
(crackling) as the engine is run at full speed.
When you hear this sound, your engine may
be in for problems from overheating and
detonation. Land and readjust the high-speed
mixture.
I am going to stop discussing the process
at this point. The preceding is a far more
complete and technically correct explanation
of two-stroke engines’ operation than I wrote
in “Engines 101.” In deference to that article,
this installment has required nearly 2,000
words to cover the same topic as did its
roughly 600 words, without adding new
operational information that less-experienced
RC pilots could use to run their engines
better.
The long explanation would have left
little space for all the other topics I discussed
in “Engines 101,” but the shortcuts caused
confusion that would have been avoided with
the longer version. Yet even this explanation
covers only the basics of our easy-to-use but
complicated machines.
If you want to learn more, Dave Gierke
has written the excellent engine book Two-
Stroke Glow Engines, Volume 1, available
directly from him at 1276 Ransom Rd.,
Lancaster NY 14086. It is $18.95 including
shipping.
In “Engines 101,” I erred in writing that the
piston in an aluminum-brass-chrome (ABC)
engine is larger in diameter than its
respective cylinder. I took the liberty of
exaggeration to make the thermal expansion
point.
Actually, the piston is the same diameter
as the cylinder, which still expands more than
the piston to allow space for the piston to
move efficiently. Sometimes the piston is
larger in new engines, but by no more than
one to two ten-thousandths of an inch. This
quickly wears to the same diameter. I took
poetic license to make the point in few
words, but it was technically incorrect.
The main operational point was that ABC
engines are more tolerant of lean fuel/air
mixtures than ringed engines are. This is
because of the thermal expansion differences
inherent in this design. But ringed engines
usually outlast the ABC type if the fuel/air
mixture is always correct and the engines are
always properly maintained. And it is true
that ringed engines have a bit more torque
than corresponding ABC engines.
Regarding torque, I mentioned that fourstroke
engines have more than corresponding
two-strokes. I meant more usable torque but
wanted to avoid using extra space to explain
what that meant. In fact, two- and fourstrokes
have roughly the same amount. But
the four-stroke produces its maximum torque
at sufficiently low rpm so that most sport
fliers can “prop” their engines to reach this
speed.
Many two-strokes (not all, since older, socalled
long-stroke engines did not) have their
“torque band” or “curve” (the rpm range at
will increase the scavenging effect. I will
discuss them later, but now you understand
how and why they could increase an
engine’s power by increasing the
scavenging effect.
During the charge cycle, some fresh
fuel/air mixture is drawn out the exhaust
along with the escaping gases. This is lost
power and poor fuel economy that engine
designers strive to recover as much as
possible.
An additional complication is that the
combustion occurs before the piston reaches
TDC. It continues even when the engine
reaches TDC and ends at or after TDC.
The amount of advance is shown in
the drawing.
It may seem strange to put combustion
pressure against the piston’s upward
movement, but combustion takes time, and
our fuel doesn’t explode all at once.
Therefore, the prolonged explosion used to
burn as much of the fuel/air charge as
possible is made achievable by the
“advanced timing.” The relationship
between the piston’s movements and
ignition is a delicate balance. Too much
advance, and the piston is damaged; too
little means that insufficient combustion
occurs.
However, running an engine too lean
produces extra heat that can change this
delicate balance. Hot engines can
experience timing that becomes so
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08sig2.QXD 5/24/04 8:48 am Page 52
August 2004 53
which maximum torque is produced and
remains nearly constant) at relatively high
rpm. Many times the torque band is higher
than 13,000 rpm and may require impractical
propeller selection to reach—especially for
sport pilots using low-nitromethane-content
fuel.
In “Engines 101” I wrote that in theory, a
two-stroke engine should have twice the
power of an equivalent four-stroke. This
assumed that the two-stroke was 100%
efficient. As I have pointed out, it is far from
that efficiency level.
I also wrote that modern four-stroke
engines have roughly 70%-80% of the
“power” of an equivalent two-stroke. That is
true, but I should have added that I was
considering only sport 40-60 engines since
the article was addressing only
noncompetition pilots.
Some supercharged, fuel-injected
competition four-strokes can reach power
parity with the two-stroke, but at a much
higher cost. I didn’t mention these engines
because they are not usually relevant to new
RC pilots, but they are fine engines that are
worth more than their cost in the long run. I
know because I use them in RC Aerobatics
(Pattern) competition.
“Engines 101” contained two major
bloopers in addition to the two errors I
already mentioned, one of which was that
ball bearings provide “more” crankshaft
support than bronze bushings. I meant that
they provide “better” support since ball
bearings reduce crankshaft friction loads.
The bushing actually provides “more”
support since more area is in contact with the
crankshaft.
The point I made was valid, but the image
described was incorrect. If you are interested,
you can see in that article’s photo of the
K&B .65’s insides that it does indeed have
bushed crankshaft bearings.
An embarrassing mistake was that I
identified the oil-retention groove as a score
mark. When looking at the photo, I noticed
the thin line. Since that engine had not been
moved in nearly 15 years, I thought the
camera had captured oil buildup from the
cylinder onto the long-stationary piston. My
eyes are no longer good enough to see that
thin line without the camera’s magnification,
so I never noticed it before.
I learned something new here. The oil
groove serves to retain some lubrication in
the piston/cylinder contact area. I am happy
to correct this mistake and thank all those
who wrote in to point it out.
The last error involves the K&B .65
shown in “Engines 101.” I assumed that the
bushed engine followed the older, non-
Schnuerle-ported K&B design, but it does
not. The non-Schnuerle-ported engine should
have been the K&B .61 shown here in the
comparison photo that I originally thought
was too dirty to print.
Not all Schnuerle engines show the boost
transfer port on the outside. Some enlarge the
entire upper cylinder case to fit the boost
port, as shown in the photo comparing the
two K&B “60s.” These engines are more
difficult to spot.
Revisiting “Engines 101” has taken time
and space that could have been used for the
third article in this segment of the “From the
Ground Up” series, which will be published
next month. I thank all those who sent in
their suggestions and comments. Although
the important operation steps I presented in
“Engines 101” are valid, those comments
helped identify those areas that needed more
explanation. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
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08sig2.QXD 5/24/04 8:48 am Page 53
Edition: Model Aviation - 2004/08
Page Numbers: 48,49,50,51,52,53
Edition: Model Aviation - 2004/08
Page Numbers: 48,49,50,51,52,53
BEFORE PROCEEDING to the third article in the “From the
Ground Up” engine series, I am going to review the first installment,
published in the April issue. MA has received numerous comments on
the information presented in that article. Many were favorable, but
several pointed out that the theoretical information was incomplete,
poorly explained, or just plain wrong. And in some instances they
have valid points.
So I’ll review some of “Engines 101”’s basics, keeping in mind
the comments that were offered. In all cases they were offered in
good faith and in the hopes of improving the series and modelers’
understanding of engine basics.
Probably the most incomplete section was the beginning of the
article. I did not take the space, which is precious in any article, to
properly explain the intent of the engine theory I was writing about or
the manner in which it was to be presented. I’ll do it now as it should
have been done in “Engines 101.”
The engine theory I am presenting is intended solely to provide
beginning RC pilots with enough knowledge of a model-engine’s
workings to understand why choices about proper mixture settings,
fuel, propellers, and other items to be discussed later will be made.
There is no intent to fully detail an engine’s intricate machinery.
The new RC pilot is not going to be designing or disassembling (I
hope) his or her first few engines, but this person will be setting highand
low-speed mixture settings.
All the theory I present will be from a strictly operations
viewpoint. Proper engine operation is the only goal in this discussion.
Where true technical names for parts may be confusing, I will use
descriptive terms instead. Since most new modelers have no
knowledge of two-stroke engines, but do have at least a passing
familiarity with their cars’ engines, I’ll try to reference parts with
confusing names in more recognizable terms.
As with all things mechanical, a model engine’s true operation is
complicated. Operations that are explained separately and appear to
be independent actually overlap, and sometimes interfere with, other
operations. To simplify the theoretical presentation, I will explain
each action as if it were the only one happening at that time.
A great deal of confusion would have been avoided if I had started
“Engines 101” with the preceding. Because of the decision to
simplify and avoid confusion, the rotary disk induction valve became
the crankshaft intake slot.
The true engineer’s name is correct but leads one to look for a
moving valve such as those found in a car or a rotating valve. There
isn’t one, and the “valve” is a slot. The name also sounds as if some
sort of “pumping” action is happening when it is not. The same
48 MODEL AVIATION
Engines 101
Rear ball bearing is partially visible above and to either side of
counterweight in Webra Speed .61 (L). K&B .65 (R) uses bronze
bushings to support crankshaft, so it has only case metal in this
area.
Photo clearly shows SuperTigre’s boost port on left side. K&B
.61 (R) has no such boost port and therefore a much narrower
cylinder case.
Follow-up to April issue’s “Engines 101” clears the air
by Frank Granelli
Revisited
Photos by the author
08sig2.QXD 5/24/04 8:47 am Page 48
rationale was used when
discussing the “intake” and
“boost intake” ports, actually
known as transfer (also called
bypass) and boost transfer
(bypass) ports.
Why “transfer”? Because
these ports allow the fuel/air mix
in the crankcase to transfer from
the crankcase to the combustion
chamber, but their function is
that of fuel/air intake ports.
When traveling, you do not need
to know a street’s name—just
where it goes. In this case it goes
into the combustion chamber.
I also simplified the partial
vacuums found in our engines’
operations—actually lowpressure
areas since there is
nothing even approaching a
physicist’s definition of a partial hard
vacuum in our engines (there is just too
much gas density everywhere inside)—as
just “vacuums,” as most automotive books
do.
I completely ignored the function of an
engine’s timing advance, all references to
Top Dead Center (TDC) operations, and
especially all references to interference and
benefits one operation may have compared
with another.
None of this theory would help newer RC
pilots operate their engines better.
Explaining these operationally irrelevant, but
theoretically important, functions would
have taken almost a full article themselves.
Similarly, I called the methanol in our
fuel a “heat exchanger” and pointed out that
methanol helps cool the engine; that is why a
richer mixture is important. I felt that “heat
exchanger” was a simple, generally
understood term that would not require
definition but get the point across.
However, in technical terms, methanol
cools our engines because it has a high heat
of evaporation. During carburetor air intake,
methanol in the fuel is transformed into a gas
requiring a great deal of heat. The process—
called refrigeration—therefore removes heat
from the surrounding lower engine sections
to have the energy to transform the
methanol.
The same process cools the food in your
household refrigerator, but without the
combustion part. Your refrigerator
substitutes an electrically driven pump and
evaporator for crankcase pumping and
venturi action at the carburetor. But this
process is not technically one of heat
exchange. A heat exchanger does not
transform into another state of matter, but
absorbs heat from one source and transfers it
to another.
There is a true heat exchanger in our
fuel that removes heat from the engine and
transfers it to the earth’s atmosphere via
the exhaust. We call it the “fuel’s oil.” As
everyone who has cleaned his or her model
aircraft knows, some oil is not burned
during combustion and escapes through the
exhaust. While escaping, it also transfers
some of the engine’s heat.
The main point remains, however.
Whether through heat exchange or
refrigeration, the fuel lubricates and cools
our engines. RC engines are not cooled
solely through contact with the air; therefore,
the proper high-speed fuel mixture settings
are critical and must not be ignored.
In retrospect, I probably should have used
the technical terms and part names for the
preceding and for other items in “Engines
101.” It is usually better to explain the
technical, rather than “street,” names at the
start, even if doing so requires much
additional explanation. In this way, regular
RC pilots and engine technicians will
eventually have the same reference names.
So sit down, tighten your engine cap on
your head, and let’s explore the true, fully
detailed, roller-coaster operation of the
thermodynamically controlled contraption
we call the two-stroke, internal-combustion
engine. Along the way I will point out areas
where “Engines 101” was unclear, poorly
explained, or technically incorrect.
“Engines 101” started with the engine
before the first combustion and then
followed the operation cycle. It was assumed
that there was a fresh air/fuel charge in the
crankcase. That caused confusion about fuel
transfer (intake) timing versus
exhaust timing.
Since no combustion had yet
occurred, I ignored the fact that
the exhaust port opens slightly
sooner than the transfer ports do.
The main point was that the
exhaust port became fully open at
the same time the transfer ports
were opened completely.
But that might lead one to
wonder why the fuel/air mixture
just doesn’t go right across the
cylinder and out the exhaust.
Rather than explain that yes, it
does do that (somewhat), I
ignored it.
Actually, some fresh fuel/air
mixture does exit the exhaust,
especially during start-up. This is
one of the two-stroke engine’s
inefficiencies that engine designers strive to
minimize. This is also one reason why you
may notice some fuel condensing in the
muffler during operation—especially during
“rich” operation. Only the fact that the
engine’s parts are moving quickly helps
reduce this unwanted fuel/air loss.
This time I’ll start with the engine’s
piston at Bottom Dead Center (BDC),
meaning that it is as far down in its
movement (called stroke) as it can get. The
engine has not started, and there is no fuel
anywhere inside the engine. There is no fuel
anywhere except in the fuel tank.
(Operationally, it is important to have fuel in
the tank before trying to run the engine.)
Starting an engine from this position is
difficult until fuel flows from the tank,
through the fuel lines, and into the
carburetor. Therefore, we need to draw the
fuel from the tank, into the carburetor. We
will use the “suction” effect that permits the
engine to run in performing this task. Where
does the suction come from? While at BDC,
the rotary disk induction valve—the intake
slot in the crankshaft—is fully closed.
As the engine is hand-rotated
counterclockwise, the piston begins to move
upward. It first closes all the transfer (intake)
ports. At this point the rotary valve (for
short) begins to open, but the exhaust is also
still slightly open. However, there is no
connection between the exhaust port and the
engine’s lower crankcase at this point, so
that is irrelevant now.
As the piston continues to move upward,
the crankcase volume (not area, as was
written in “Engines 101”; that was a
misstatement) begins to increase. As this
volume increases with continued upward
movement of the piston (not cylinder—
another misstatement in the original article),
a low-pressure area is created in the
crankcase.
This happens because the now-sealed
crankcase volume is bigger than it was, but it
still contains only the original amount of air.
The air expands to fill the increased volume
and therefore has a lower pressure.
But do you remember that rotary
induction valve that was opening just as the
August 2004 49
Larger brass “tube” on right is fuel jet. Smaller brass fitting on
left is low-speed needle valve that regulates amount of fuel
flowing through fuel jet at reduced throttle levels.
K&B .65 (L) has larger upper cylinder case
to accommodate boost transfer ports.
Non-boost-ported K&B .61 (R) has almost
straight-walled cylinder case.
08sig2.QXD 5/24/04 8:47 am Page 49
transfer ports were closing? The valve opens
more as the piston travels upward. It is now
fully open, and that means the crankcase
section is no longer sealed.
The rotary valve is located just under the
carburetor. If the carburetor throttle barrel is
open, air rushes through the carburetor,
through the rotary valve (crankshaft), and
into the crankcase. Remember this process; it
will be repeated shortly, once fuel is added to
the mix.
Now we have plenty of air rushing into
and through the engine as we hand-rotate the
propeller. What happens if we put an
obstruction, such as a thumb, over the
carburetor’s air inlet?
Low pressure returns to the lower
crankcase since it is again sealed, even when
the rotary valve is open. But the piston is still
moving and re-creating the low-pressure
condition with each revolution. You can
actually feel the suction with your thumb.
This suction effect draws fuel and air into
the carburetor.
This low-pressure condition seeks relief
from wherever it can. Since the only
possible pressure relief is the small brass
fuel inlet—the fuel jet—fuel is drawn from
the fuel tank and into the fuel jet.
The photo showing the venturi process
was not sent in with the original article. That
caused confusion about the venturi process
because the picture’s caption explained it in
detail. I’ve included the photo here to better
illustrate the venturi process. The low
pressure—we call it suction—continues
through the small fuel inlet, through the
lines, and into the fuel tank.
Now remove the obstruction. As the
rotary valve opens, the crankcase’s lower
pressure draws fuel through the small brass
tube in the picture (the fuel jet) and air from
the atmosphere into the rotary induction
valve.
As the air is pulled through the
carburetor, it speeds up to go through the
narrow carburetor intake passage. The added
velocity means that the intake air gains
kinetic energy and, in order to maintain
balance, the potential energy (temperature
and pressure) drops. When the engine is
running or hand-cranked, this lowered
pressure is seen at the fuel jet, and the
difference between this low-pressure area
and the outside air pressure (seen at the fuel
tank vent) “sucks” fuel into the carburetor as
if your thumb were still there!
When the piston reaches as far upward as
it can—TDC—the fully open rotary valve
begins to close but draws fresh air and fuel
into the crankcase for another 70°-90° of
crankshaft rotation. The valve closes
completely before the exhaust port begins to
open. The crankcase and combustion
chamber are again sealed. But the piston still
has a ways to go before reaching BDC. It
continues downward, compressing the
fuel/air mixture inside the engine’s
crankcase.
That results in the crankcase’s now being
a high-pressure region. I left the following
part out of “Engines 101” because it is not
the major reason why the fuel/air mixture
flows into the combustion chamber. But still,
this high-pressure condition does exist, and
for now, when there has been no
combustion, it is the only transfer
mechanism in operation. The piston
continues compressing the crankcase mixture
and increasing the pressure.
But before reaching BDC, the piston
uncovers the transfer and boost transfer ports
(bypass ports). The high crankcase pressure
now has an exit. The fuel/air mixture under
50 MODEL AVIATION
Chart illustrates what ports and valves are open during one crankshaft rotation from viewpoint of looking directly at propeller end of
crankshaft. Each valve or port is open for length of its labeled area in curve. Drawing courtesy Dean Pappas.
08sig2.QXD 5/24/04 8:47 am Page 50
pressure rushes up through the transfer ports
and into the volume just above the piston.
Since the exhaust port is also fully open at
BDC, some of this precious mixture is lost
out the exhaust port. But some remains
above the piston.
(One advantage of the Schnuerle boost
transfer port system is that less incoming
fuel/air mix flowing from these sidemounted
ports is lost out the exhaust. The
Schnuerle ports are not aimed straight out
the exhaust port, as is the main transfer port.)
As BDC is passed, the piston travels
upward, pushing more of the fuel/air mixture
upward and into the already filled
combustion chamber. Yet some still goes out
the exhaust port—another inefficiency. Once
the exhaust port closes, the piston begins to
compress the fuel/air mix as it continues
upward. If the glow plug is lit, and the
fuel/air mixture is in the proper proportions,
a prolonged, controlled explosion called
“combustion” occurs.
The model two-stroke is part of the class
of engines known as “combustion ignition,”
which includes diesels. But there is a
subclass known as “catalytic enhanced
combustion ignition” engines. Our engines
fit into that category, as do many automobile
diesels with “glow plugs” that are constantly
receiving electric current (still not a true
chemical catalyst effect) and are therefore
always “lit.”
It seemed easier to just call our engines
“diesels” in the original article to
differentiate them from model gas ignition
engines rather than go through the true
technical explanation, as I just did.
Consider all of the preceding and add the
fuel/air mixture to the now-running engine.
How does the process differ? “Engines 101”
basically assumed that it didn’t, and for
operational understanding it doesn’t vary.
Proper operation and equipment selections,
except for tuned pipes that few sport pilots
use anymore, do not depend on any of the
following information. Still, this knowledge
could be important for a full understanding
of our engines’ operating theory.
Do you recall the intake process I
described in the preceding? Consider the
same process but with the engine running at
full speed. The piston is at BDC with most of
the exhaust gases gone, receiving a fresh
charge of fuel/air from the crankcase into the
now-vacant volume above the piston, right?
Well, not really. The exhaust port opens
only slightly before the transfer ports, called
the “exhaust lead” or “blowdown.” The
exhaust gases have not fully exited the
cylinder when the transfer ports begin to
open. The relationship between these
openings is part of the engine’s timing. The
accompanying illustration summarizes many
sport engines’ timing in this regard.
In practice, this timing means that fresh
fuel/air mixture is flowing into the cylinder
even as exhaust gases are exiting. Why
would an engine designer do this?
The hot, still expanding exhaust gases are
exiting at a high velocity. This forms a lowpressure
area just above the piston, “behind”
the exiting exhaust gases. The fresh fuel/air
mixture is “pulled” through the transfer
ports, into the low-pressure area in the
cylinder at the same time the descending
piston is compressing the mixture in the
crankcase and pushing it into the bypasses.
We say the exhaust gases “scavenge” the
fuel/air mixture into this section. The
scavenging effect increases the velocity, and
hence the amount of the fresh fuel/air
mixture that is drawn into the engine.
Just as the scavenge action is finishing
(the exhaust gases’ momentum is exhausted)
and the pulling of intake from the crankcase
through the bypasses is ending, the rotary
valve opens. This helps start the flow of
fresh fuel/air mixture into the crankcase for
the next power stroke.
At exceptionally low speeds, such as idle,
the scavenging action goes to completion,
and you are back to having pressure in the
crankcase at the moment the rotary valve
closes because of the descending piston. You
can sometimes tell that this is happening as
the engine spits fuel from the carburetor at
slow speeds.
Therefore, the scavenge effect is the
major force our engines use to put fuel and
air into the combustion chamber, but
crankcase pressure does play an important
part in the initial charge’s transferring into
the cylinder. Together, these alternating,
thermodynamically produced high- and lowpressure
conditions, neither a true or even
partial vacuum, allow our engines to run.
Several exhaust systems are available that
August 2004 51
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08sig2.QXD 5/24/04 8:48 am Page 51
advanced that detonation occurs, meaning
that the fuel/air mixture ignites before it
should. This condition may sometimes be
identified by a loud “frying egg” sound
(crackling) as the engine is run at full speed.
When you hear this sound, your engine may
be in for problems from overheating and
detonation. Land and readjust the high-speed
mixture.
I am going to stop discussing the process
at this point. The preceding is a far more
complete and technically correct explanation
of two-stroke engines’ operation than I wrote
in “Engines 101.” In deference to that article,
this installment has required nearly 2,000
words to cover the same topic as did its
roughly 600 words, without adding new
operational information that less-experienced
RC pilots could use to run their engines
better.
The long explanation would have left
little space for all the other topics I discussed
in “Engines 101,” but the shortcuts caused
confusion that would have been avoided with
the longer version. Yet even this explanation
covers only the basics of our easy-to-use but
complicated machines.
If you want to learn more, Dave Gierke
has written the excellent engine book Two-
Stroke Glow Engines, Volume 1, available
directly from him at 1276 Ransom Rd.,
Lancaster NY 14086. It is $18.95 including
shipping.
In “Engines 101,” I erred in writing that the
piston in an aluminum-brass-chrome (ABC)
engine is larger in diameter than its
respective cylinder. I took the liberty of
exaggeration to make the thermal expansion
point.
Actually, the piston is the same diameter
as the cylinder, which still expands more than
the piston to allow space for the piston to
move efficiently. Sometimes the piston is
larger in new engines, but by no more than
one to two ten-thousandths of an inch. This
quickly wears to the same diameter. I took
poetic license to make the point in few
words, but it was technically incorrect.
The main operational point was that ABC
engines are more tolerant of lean fuel/air
mixtures than ringed engines are. This is
because of the thermal expansion differences
inherent in this design. But ringed engines
usually outlast the ABC type if the fuel/air
mixture is always correct and the engines are
always properly maintained. And it is true
that ringed engines have a bit more torque
than corresponding ABC engines.
Regarding torque, I mentioned that fourstroke
engines have more than corresponding
two-strokes. I meant more usable torque but
wanted to avoid using extra space to explain
what that meant. In fact, two- and fourstrokes
have roughly the same amount. But
the four-stroke produces its maximum torque
at sufficiently low rpm so that most sport
fliers can “prop” their engines to reach this
speed.
Many two-strokes (not all, since older, socalled
long-stroke engines did not) have their
“torque band” or “curve” (the rpm range at
will increase the scavenging effect. I will
discuss them later, but now you understand
how and why they could increase an
engine’s power by increasing the
scavenging effect.
During the charge cycle, some fresh
fuel/air mixture is drawn out the exhaust
along with the escaping gases. This is lost
power and poor fuel economy that engine
designers strive to recover as much as
possible.
An additional complication is that the
combustion occurs before the piston reaches
TDC. It continues even when the engine
reaches TDC and ends at or after TDC.
The amount of advance is shown in
the drawing.
It may seem strange to put combustion
pressure against the piston’s upward
movement, but combustion takes time, and
our fuel doesn’t explode all at once.
Therefore, the prolonged explosion used to
burn as much of the fuel/air charge as
possible is made achievable by the
“advanced timing.” The relationship
between the piston’s movements and
ignition is a delicate balance. Too much
advance, and the piston is damaged; too
little means that insufficient combustion
occurs.
However, running an engine too lean
produces extra heat that can change this
delicate balance. Hot engines can
experience timing that becomes so
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08sig2.QXD 5/24/04 8:48 am Page 52
August 2004 53
which maximum torque is produced and
remains nearly constant) at relatively high
rpm. Many times the torque band is higher
than 13,000 rpm and may require impractical
propeller selection to reach—especially for
sport pilots using low-nitromethane-content
fuel.
In “Engines 101” I wrote that in theory, a
two-stroke engine should have twice the
power of an equivalent four-stroke. This
assumed that the two-stroke was 100%
efficient. As I have pointed out, it is far from
that efficiency level.
I also wrote that modern four-stroke
engines have roughly 70%-80% of the
“power” of an equivalent two-stroke. That is
true, but I should have added that I was
considering only sport 40-60 engines since
the article was addressing only
noncompetition pilots.
Some supercharged, fuel-injected
competition four-strokes can reach power
parity with the two-stroke, but at a much
higher cost. I didn’t mention these engines
because they are not usually relevant to new
RC pilots, but they are fine engines that are
worth more than their cost in the long run. I
know because I use them in RC Aerobatics
(Pattern) competition.
“Engines 101” contained two major
bloopers in addition to the two errors I
already mentioned, one of which was that
ball bearings provide “more” crankshaft
support than bronze bushings. I meant that
they provide “better” support since ball
bearings reduce crankshaft friction loads.
The bushing actually provides “more”
support since more area is in contact with the
crankshaft.
The point I made was valid, but the image
described was incorrect. If you are interested,
you can see in that article’s photo of the
K&B .65’s insides that it does indeed have
bushed crankshaft bearings.
An embarrassing mistake was that I
identified the oil-retention groove as a score
mark. When looking at the photo, I noticed
the thin line. Since that engine had not been
moved in nearly 15 years, I thought the
camera had captured oil buildup from the
cylinder onto the long-stationary piston. My
eyes are no longer good enough to see that
thin line without the camera’s magnification,
so I never noticed it before.
I learned something new here. The oil
groove serves to retain some lubrication in
the piston/cylinder contact area. I am happy
to correct this mistake and thank all those
who wrote in to point it out.
The last error involves the K&B .65
shown in “Engines 101.” I assumed that the
bushed engine followed the older, non-
Schnuerle-ported K&B design, but it does
not. The non-Schnuerle-ported engine should
have been the K&B .61 shown here in the
comparison photo that I originally thought
was too dirty to print.
Not all Schnuerle engines show the boost
transfer port on the outside. Some enlarge the
entire upper cylinder case to fit the boost
port, as shown in the photo comparing the
two K&B “60s.” These engines are more
difficult to spot.
Revisiting “Engines 101” has taken time
and space that could have been used for the
third article in this segment of the “From the
Ground Up” series, which will be published
next month. I thank all those who sent in
their suggestions and comments. Although
the important operation steps I presented in
“Engines 101” are valid, those comments
helped identify those areas that needed more
explanation. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
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08sig2.QXD 5/24/04 8:48 am Page 53
Edition: Model Aviation - 2004/08
Page Numbers: 48,49,50,51,52,53
BEFORE PROCEEDING to the third article in the “From the
Ground Up” engine series, I am going to review the first installment,
published in the April issue. MA has received numerous comments on
the information presented in that article. Many were favorable, but
several pointed out that the theoretical information was incomplete,
poorly explained, or just plain wrong. And in some instances they
have valid points.
So I’ll review some of “Engines 101”’s basics, keeping in mind
the comments that were offered. In all cases they were offered in
good faith and in the hopes of improving the series and modelers’
understanding of engine basics.
Probably the most incomplete section was the beginning of the
article. I did not take the space, which is precious in any article, to
properly explain the intent of the engine theory I was writing about or
the manner in which it was to be presented. I’ll do it now as it should
have been done in “Engines 101.”
The engine theory I am presenting is intended solely to provide
beginning RC pilots with enough knowledge of a model-engine’s
workings to understand why choices about proper mixture settings,
fuel, propellers, and other items to be discussed later will be made.
There is no intent to fully detail an engine’s intricate machinery.
The new RC pilot is not going to be designing or disassembling (I
hope) his or her first few engines, but this person will be setting highand
low-speed mixture settings.
All the theory I present will be from a strictly operations
viewpoint. Proper engine operation is the only goal in this discussion.
Where true technical names for parts may be confusing, I will use
descriptive terms instead. Since most new modelers have no
knowledge of two-stroke engines, but do have at least a passing
familiarity with their cars’ engines, I’ll try to reference parts with
confusing names in more recognizable terms.
As with all things mechanical, a model engine’s true operation is
complicated. Operations that are explained separately and appear to
be independent actually overlap, and sometimes interfere with, other
operations. To simplify the theoretical presentation, I will explain
each action as if it were the only one happening at that time.
A great deal of confusion would have been avoided if I had started
“Engines 101” with the preceding. Because of the decision to
simplify and avoid confusion, the rotary disk induction valve became
the crankshaft intake slot.
The true engineer’s name is correct but leads one to look for a
moving valve such as those found in a car or a rotating valve. There
isn’t one, and the “valve” is a slot. The name also sounds as if some
sort of “pumping” action is happening when it is not. The same
48 MODEL AVIATION
Engines 101
Rear ball bearing is partially visible above and to either side of
counterweight in Webra Speed .61 (L). K&B .65 (R) uses bronze
bushings to support crankshaft, so it has only case metal in this
area.
Photo clearly shows SuperTigre’s boost port on left side. K&B
.61 (R) has no such boost port and therefore a much narrower
cylinder case.
Follow-up to April issue’s “Engines 101” clears the air
by Frank Granelli
Revisited
Photos by the author
08sig2.QXD 5/24/04 8:47 am Page 48
rationale was used when
discussing the “intake” and
“boost intake” ports, actually
known as transfer (also called
bypass) and boost transfer
(bypass) ports.
Why “transfer”? Because
these ports allow the fuel/air mix
in the crankcase to transfer from
the crankcase to the combustion
chamber, but their function is
that of fuel/air intake ports.
When traveling, you do not need
to know a street’s name—just
where it goes. In this case it goes
into the combustion chamber.
I also simplified the partial
vacuums found in our engines’
operations—actually lowpressure
areas since there is
nothing even approaching a
physicist’s definition of a partial hard
vacuum in our engines (there is just too
much gas density everywhere inside)—as
just “vacuums,” as most automotive books
do.
I completely ignored the function of an
engine’s timing advance, all references to
Top Dead Center (TDC) operations, and
especially all references to interference and
benefits one operation may have compared
with another.
None of this theory would help newer RC
pilots operate their engines better.
Explaining these operationally irrelevant, but
theoretically important, functions would
have taken almost a full article themselves.
Similarly, I called the methanol in our
fuel a “heat exchanger” and pointed out that
methanol helps cool the engine; that is why a
richer mixture is important. I felt that “heat
exchanger” was a simple, generally
understood term that would not require
definition but get the point across.
However, in technical terms, methanol
cools our engines because it has a high heat
of evaporation. During carburetor air intake,
methanol in the fuel is transformed into a gas
requiring a great deal of heat. The process—
called refrigeration—therefore removes heat
from the surrounding lower engine sections
to have the energy to transform the
methanol.
The same process cools the food in your
household refrigerator, but without the
combustion part. Your refrigerator
substitutes an electrically driven pump and
evaporator for crankcase pumping and
venturi action at the carburetor. But this
process is not technically one of heat
exchange. A heat exchanger does not
transform into another state of matter, but
absorbs heat from one source and transfers it
to another.
There is a true heat exchanger in our
fuel that removes heat from the engine and
transfers it to the earth’s atmosphere via
the exhaust. We call it the “fuel’s oil.” As
everyone who has cleaned his or her model
aircraft knows, some oil is not burned
during combustion and escapes through the
exhaust. While escaping, it also transfers
some of the engine’s heat.
The main point remains, however.
Whether through heat exchange or
refrigeration, the fuel lubricates and cools
our engines. RC engines are not cooled
solely through contact with the air; therefore,
the proper high-speed fuel mixture settings
are critical and must not be ignored.
In retrospect, I probably should have used
the technical terms and part names for the
preceding and for other items in “Engines
101.” It is usually better to explain the
technical, rather than “street,” names at the
start, even if doing so requires much
additional explanation. In this way, regular
RC pilots and engine technicians will
eventually have the same reference names.
So sit down, tighten your engine cap on
your head, and let’s explore the true, fully
detailed, roller-coaster operation of the
thermodynamically controlled contraption
we call the two-stroke, internal-combustion
engine. Along the way I will point out areas
where “Engines 101” was unclear, poorly
explained, or technically incorrect.
“Engines 101” started with the engine
before the first combustion and then
followed the operation cycle. It was assumed
that there was a fresh air/fuel charge in the
crankcase. That caused confusion about fuel
transfer (intake) timing versus
exhaust timing.
Since no combustion had yet
occurred, I ignored the fact that
the exhaust port opens slightly
sooner than the transfer ports do.
The main point was that the
exhaust port became fully open at
the same time the transfer ports
were opened completely.
But that might lead one to
wonder why the fuel/air mixture
just doesn’t go right across the
cylinder and out the exhaust.
Rather than explain that yes, it
does do that (somewhat), I
ignored it.
Actually, some fresh fuel/air
mixture does exit the exhaust,
especially during start-up. This is
one of the two-stroke engine’s
inefficiencies that engine designers strive to
minimize. This is also one reason why you
may notice some fuel condensing in the
muffler during operation—especially during
“rich” operation. Only the fact that the
engine’s parts are moving quickly helps
reduce this unwanted fuel/air loss.
This time I’ll start with the engine’s
piston at Bottom Dead Center (BDC),
meaning that it is as far down in its
movement (called stroke) as it can get. The
engine has not started, and there is no fuel
anywhere inside the engine. There is no fuel
anywhere except in the fuel tank.
(Operationally, it is important to have fuel in
the tank before trying to run the engine.)
Starting an engine from this position is
difficult until fuel flows from the tank,
through the fuel lines, and into the
carburetor. Therefore, we need to draw the
fuel from the tank, into the carburetor. We
will use the “suction” effect that permits the
engine to run in performing this task. Where
does the suction come from? While at BDC,
the rotary disk induction valve—the intake
slot in the crankshaft—is fully closed.
As the engine is hand-rotated
counterclockwise, the piston begins to move
upward. It first closes all the transfer (intake)
ports. At this point the rotary valve (for
short) begins to open, but the exhaust is also
still slightly open. However, there is no
connection between the exhaust port and the
engine’s lower crankcase at this point, so
that is irrelevant now.
As the piston continues to move upward,
the crankcase volume (not area, as was
written in “Engines 101”; that was a
misstatement) begins to increase. As this
volume increases with continued upward
movement of the piston (not cylinder—
another misstatement in the original article),
a low-pressure area is created in the
crankcase.
This happens because the now-sealed
crankcase volume is bigger than it was, but it
still contains only the original amount of air.
The air expands to fill the increased volume
and therefore has a lower pressure.
But do you remember that rotary
induction valve that was opening just as the
August 2004 49
Larger brass “tube” on right is fuel jet. Smaller brass fitting on
left is low-speed needle valve that regulates amount of fuel
flowing through fuel jet at reduced throttle levels.
K&B .65 (L) has larger upper cylinder case
to accommodate boost transfer ports.
Non-boost-ported K&B .61 (R) has almost
straight-walled cylinder case.
08sig2.QXD 5/24/04 8:47 am Page 49
transfer ports were closing? The valve opens
more as the piston travels upward. It is now
fully open, and that means the crankcase
section is no longer sealed.
The rotary valve is located just under the
carburetor. If the carburetor throttle barrel is
open, air rushes through the carburetor,
through the rotary valve (crankshaft), and
into the crankcase. Remember this process; it
will be repeated shortly, once fuel is added to
the mix.
Now we have plenty of air rushing into
and through the engine as we hand-rotate the
propeller. What happens if we put an
obstruction, such as a thumb, over the
carburetor’s air inlet?
Low pressure returns to the lower
crankcase since it is again sealed, even when
the rotary valve is open. But the piston is still
moving and re-creating the low-pressure
condition with each revolution. You can
actually feel the suction with your thumb.
This suction effect draws fuel and air into
the carburetor.
This low-pressure condition seeks relief
from wherever it can. Since the only
possible pressure relief is the small brass
fuel inlet—the fuel jet—fuel is drawn from
the fuel tank and into the fuel jet.
The photo showing the venturi process
was not sent in with the original article. That
caused confusion about the venturi process
because the picture’s caption explained it in
detail. I’ve included the photo here to better
illustrate the venturi process. The low
pressure—we call it suction—continues
through the small fuel inlet, through the
lines, and into the fuel tank.
Now remove the obstruction. As the
rotary valve opens, the crankcase’s lower
pressure draws fuel through the small brass
tube in the picture (the fuel jet) and air from
the atmosphere into the rotary induction
valve.
As the air is pulled through the
carburetor, it speeds up to go through the
narrow carburetor intake passage. The added
velocity means that the intake air gains
kinetic energy and, in order to maintain
balance, the potential energy (temperature
and pressure) drops. When the engine is
running or hand-cranked, this lowered
pressure is seen at the fuel jet, and the
difference between this low-pressure area
and the outside air pressure (seen at the fuel
tank vent) “sucks” fuel into the carburetor as
if your thumb were still there!
When the piston reaches as far upward as
it can—TDC—the fully open rotary valve
begins to close but draws fresh air and fuel
into the crankcase for another 70°-90° of
crankshaft rotation. The valve closes
completely before the exhaust port begins to
open. The crankcase and combustion
chamber are again sealed. But the piston still
has a ways to go before reaching BDC. It
continues downward, compressing the
fuel/air mixture inside the engine’s
crankcase.
That results in the crankcase’s now being
a high-pressure region. I left the following
part out of “Engines 101” because it is not
the major reason why the fuel/air mixture
flows into the combustion chamber. But still,
this high-pressure condition does exist, and
for now, when there has been no
combustion, it is the only transfer
mechanism in operation. The piston
continues compressing the crankcase mixture
and increasing the pressure.
But before reaching BDC, the piston
uncovers the transfer and boost transfer ports
(bypass ports). The high crankcase pressure
now has an exit. The fuel/air mixture under
50 MODEL AVIATION
Chart illustrates what ports and valves are open during one crankshaft rotation from viewpoint of looking directly at propeller end of
crankshaft. Each valve or port is open for length of its labeled area in curve. Drawing courtesy Dean Pappas.
08sig2.QXD 5/24/04 8:47 am Page 50
pressure rushes up through the transfer ports
and into the volume just above the piston.
Since the exhaust port is also fully open at
BDC, some of this precious mixture is lost
out the exhaust port. But some remains
above the piston.
(One advantage of the Schnuerle boost
transfer port system is that less incoming
fuel/air mix flowing from these sidemounted
ports is lost out the exhaust. The
Schnuerle ports are not aimed straight out
the exhaust port, as is the main transfer port.)
As BDC is passed, the piston travels
upward, pushing more of the fuel/air mixture
upward and into the already filled
combustion chamber. Yet some still goes out
the exhaust port—another inefficiency. Once
the exhaust port closes, the piston begins to
compress the fuel/air mix as it continues
upward. If the glow plug is lit, and the
fuel/air mixture is in the proper proportions,
a prolonged, controlled explosion called
“combustion” occurs.
The model two-stroke is part of the class
of engines known as “combustion ignition,”
which includes diesels. But there is a
subclass known as “catalytic enhanced
combustion ignition” engines. Our engines
fit into that category, as do many automobile
diesels with “glow plugs” that are constantly
receiving electric current (still not a true
chemical catalyst effect) and are therefore
always “lit.”
It seemed easier to just call our engines
“diesels” in the original article to
differentiate them from model gas ignition
engines rather than go through the true
technical explanation, as I just did.
Consider all of the preceding and add the
fuel/air mixture to the now-running engine.
How does the process differ? “Engines 101”
basically assumed that it didn’t, and for
operational understanding it doesn’t vary.
Proper operation and equipment selections,
except for tuned pipes that few sport pilots
use anymore, do not depend on any of the
following information. Still, this knowledge
could be important for a full understanding
of our engines’ operating theory.
Do you recall the intake process I
described in the preceding? Consider the
same process but with the engine running at
full speed. The piston is at BDC with most of
the exhaust gases gone, receiving a fresh
charge of fuel/air from the crankcase into the
now-vacant volume above the piston, right?
Well, not really. The exhaust port opens
only slightly before the transfer ports, called
the “exhaust lead” or “blowdown.” The
exhaust gases have not fully exited the
cylinder when the transfer ports begin to
open. The relationship between these
openings is part of the engine’s timing. The
accompanying illustration summarizes many
sport engines’ timing in this regard.
In practice, this timing means that fresh
fuel/air mixture is flowing into the cylinder
even as exhaust gases are exiting. Why
would an engine designer do this?
The hot, still expanding exhaust gases are
exiting at a high velocity. This forms a lowpressure
area just above the piston, “behind”
the exiting exhaust gases. The fresh fuel/air
mixture is “pulled” through the transfer
ports, into the low-pressure area in the
cylinder at the same time the descending
piston is compressing the mixture in the
crankcase and pushing it into the bypasses.
We say the exhaust gases “scavenge” the
fuel/air mixture into this section. The
scavenging effect increases the velocity, and
hence the amount of the fresh fuel/air
mixture that is drawn into the engine.
Just as the scavenge action is finishing
(the exhaust gases’ momentum is exhausted)
and the pulling of intake from the crankcase
through the bypasses is ending, the rotary
valve opens. This helps start the flow of
fresh fuel/air mixture into the crankcase for
the next power stroke.
At exceptionally low speeds, such as idle,
the scavenging action goes to completion,
and you are back to having pressure in the
crankcase at the moment the rotary valve
closes because of the descending piston. You
can sometimes tell that this is happening as
the engine spits fuel from the carburetor at
slow speeds.
Therefore, the scavenge effect is the
major force our engines use to put fuel and
air into the combustion chamber, but
crankcase pressure does play an important
part in the initial charge’s transferring into
the cylinder. Together, these alternating,
thermodynamically produced high- and lowpressure
conditions, neither a true or even
partial vacuum, allow our engines to run.
Several exhaust systems are available that
August 2004 51
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08sig2.QXD 5/24/04 8:48 am Page 51
advanced that detonation occurs, meaning
that the fuel/air mixture ignites before it
should. This condition may sometimes be
identified by a loud “frying egg” sound
(crackling) as the engine is run at full speed.
When you hear this sound, your engine may
be in for problems from overheating and
detonation. Land and readjust the high-speed
mixture.
I am going to stop discussing the process
at this point. The preceding is a far more
complete and technically correct explanation
of two-stroke engines’ operation than I wrote
in “Engines 101.” In deference to that article,
this installment has required nearly 2,000
words to cover the same topic as did its
roughly 600 words, without adding new
operational information that less-experienced
RC pilots could use to run their engines
better.
The long explanation would have left
little space for all the other topics I discussed
in “Engines 101,” but the shortcuts caused
confusion that would have been avoided with
the longer version. Yet even this explanation
covers only the basics of our easy-to-use but
complicated machines.
If you want to learn more, Dave Gierke
has written the excellent engine book Two-
Stroke Glow Engines, Volume 1, available
directly from him at 1276 Ransom Rd.,
Lancaster NY 14086. It is $18.95 including
shipping.
In “Engines 101,” I erred in writing that the
piston in an aluminum-brass-chrome (ABC)
engine is larger in diameter than its
respective cylinder. I took the liberty of
exaggeration to make the thermal expansion
point.
Actually, the piston is the same diameter
as the cylinder, which still expands more than
the piston to allow space for the piston to
move efficiently. Sometimes the piston is
larger in new engines, but by no more than
one to two ten-thousandths of an inch. This
quickly wears to the same diameter. I took
poetic license to make the point in few
words, but it was technically incorrect.
The main operational point was that ABC
engines are more tolerant of lean fuel/air
mixtures than ringed engines are. This is
because of the thermal expansion differences
inherent in this design. But ringed engines
usually outlast the ABC type if the fuel/air
mixture is always correct and the engines are
always properly maintained. And it is true
that ringed engines have a bit more torque
than corresponding ABC engines.
Regarding torque, I mentioned that fourstroke
engines have more than corresponding
two-strokes. I meant more usable torque but
wanted to avoid using extra space to explain
what that meant. In fact, two- and fourstrokes
have roughly the same amount. But
the four-stroke produces its maximum torque
at sufficiently low rpm so that most sport
fliers can “prop” their engines to reach this
speed.
Many two-strokes (not all, since older, socalled
long-stroke engines did not) have their
“torque band” or “curve” (the rpm range at
will increase the scavenging effect. I will
discuss them later, but now you understand
how and why they could increase an
engine’s power by increasing the
scavenging effect.
During the charge cycle, some fresh
fuel/air mixture is drawn out the exhaust
along with the escaping gases. This is lost
power and poor fuel economy that engine
designers strive to recover as much as
possible.
An additional complication is that the
combustion occurs before the piston reaches
TDC. It continues even when the engine
reaches TDC and ends at or after TDC.
The amount of advance is shown in
the drawing.
It may seem strange to put combustion
pressure against the piston’s upward
movement, but combustion takes time, and
our fuel doesn’t explode all at once.
Therefore, the prolonged explosion used to
burn as much of the fuel/air charge as
possible is made achievable by the
“advanced timing.” The relationship
between the piston’s movements and
ignition is a delicate balance. Too much
advance, and the piston is damaged; too
little means that insufficient combustion
occurs.
However, running an engine too lean
produces extra heat that can change this
delicate balance. Hot engines can
experience timing that becomes so
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08sig2.QXD 5/24/04 8:48 am Page 52
August 2004 53
which maximum torque is produced and
remains nearly constant) at relatively high
rpm. Many times the torque band is higher
than 13,000 rpm and may require impractical
propeller selection to reach—especially for
sport pilots using low-nitromethane-content
fuel.
In “Engines 101” I wrote that in theory, a
two-stroke engine should have twice the
power of an equivalent four-stroke. This
assumed that the two-stroke was 100%
efficient. As I have pointed out, it is far from
that efficiency level.
I also wrote that modern four-stroke
engines have roughly 70%-80% of the
“power” of an equivalent two-stroke. That is
true, but I should have added that I was
considering only sport 40-60 engines since
the article was addressing only
noncompetition pilots.
Some supercharged, fuel-injected
competition four-strokes can reach power
parity with the two-stroke, but at a much
higher cost. I didn’t mention these engines
because they are not usually relevant to new
RC pilots, but they are fine engines that are
worth more than their cost in the long run. I
know because I use them in RC Aerobatics
(Pattern) competition.
“Engines 101” contained two major
bloopers in addition to the two errors I
already mentioned, one of which was that
ball bearings provide “more” crankshaft
support than bronze bushings. I meant that
they provide “better” support since ball
bearings reduce crankshaft friction loads.
The bushing actually provides “more”
support since more area is in contact with the
crankshaft.
The point I made was valid, but the image
described was incorrect. If you are interested,
you can see in that article’s photo of the
K&B .65’s insides that it does indeed have
bushed crankshaft bearings.
An embarrassing mistake was that I
identified the oil-retention groove as a score
mark. When looking at the photo, I noticed
the thin line. Since that engine had not been
moved in nearly 15 years, I thought the
camera had captured oil buildup from the
cylinder onto the long-stationary piston. My
eyes are no longer good enough to see that
thin line without the camera’s magnification,
so I never noticed it before.
I learned something new here. The oil
groove serves to retain some lubrication in
the piston/cylinder contact area. I am happy
to correct this mistake and thank all those
who wrote in to point it out.
The last error involves the K&B .65
shown in “Engines 101.” I assumed that the
bushed engine followed the older, non-
Schnuerle-ported K&B design, but it does
not. The non-Schnuerle-ported engine should
have been the K&B .61 shown here in the
comparison photo that I originally thought
was too dirty to print.
Not all Schnuerle engines show the boost
transfer port on the outside. Some enlarge the
entire upper cylinder case to fit the boost
port, as shown in the photo comparing the
two K&B “60s.” These engines are more
difficult to spot.
Revisiting “Engines 101” has taken time
and space that could have been used for the
third article in this segment of the “From the
Ground Up” series, which will be published
next month. I thank all those who sent in
their suggestions and comments. Although
the important operation steps I presented in
“Engines 101” are valid, those comments
helped identify those areas that needed more
explanation. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
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08sig2.QXD 5/24/04 8:48 am Page 53
Edition: Model Aviation - 2004/08
Page Numbers: 48,49,50,51,52,53
BEFORE PROCEEDING to the third article in the “From the
Ground Up” engine series, I am going to review the first installment,
published in the April issue. MA has received numerous comments on
the information presented in that article. Many were favorable, but
several pointed out that the theoretical information was incomplete,
poorly explained, or just plain wrong. And in some instances they
have valid points.
So I’ll review some of “Engines 101”’s basics, keeping in mind
the comments that were offered. In all cases they were offered in
good faith and in the hopes of improving the series and modelers’
understanding of engine basics.
Probably the most incomplete section was the beginning of the
article. I did not take the space, which is precious in any article, to
properly explain the intent of the engine theory I was writing about or
the manner in which it was to be presented. I’ll do it now as it should
have been done in “Engines 101.”
The engine theory I am presenting is intended solely to provide
beginning RC pilots with enough knowledge of a model-engine’s
workings to understand why choices about proper mixture settings,
fuel, propellers, and other items to be discussed later will be made.
There is no intent to fully detail an engine’s intricate machinery.
The new RC pilot is not going to be designing or disassembling (I
hope) his or her first few engines, but this person will be setting highand
low-speed mixture settings.
All the theory I present will be from a strictly operations
viewpoint. Proper engine operation is the only goal in this discussion.
Where true technical names for parts may be confusing, I will use
descriptive terms instead. Since most new modelers have no
knowledge of two-stroke engines, but do have at least a passing
familiarity with their cars’ engines, I’ll try to reference parts with
confusing names in more recognizable terms.
As with all things mechanical, a model engine’s true operation is
complicated. Operations that are explained separately and appear to
be independent actually overlap, and sometimes interfere with, other
operations. To simplify the theoretical presentation, I will explain
each action as if it were the only one happening at that time.
A great deal of confusion would have been avoided if I had started
“Engines 101” with the preceding. Because of the decision to
simplify and avoid confusion, the rotary disk induction valve became
the crankshaft intake slot.
The true engineer’s name is correct but leads one to look for a
moving valve such as those found in a car or a rotating valve. There
isn’t one, and the “valve” is a slot. The name also sounds as if some
sort of “pumping” action is happening when it is not. The same
48 MODEL AVIATION
Engines 101
Rear ball bearing is partially visible above and to either side of
counterweight in Webra Speed .61 (L). K&B .65 (R) uses bronze
bushings to support crankshaft, so it has only case metal in this
area.
Photo clearly shows SuperTigre’s boost port on left side. K&B
.61 (R) has no such boost port and therefore a much narrower
cylinder case.
Follow-up to April issue’s “Engines 101” clears the air
by Frank Granelli
Revisited
Photos by the author
08sig2.QXD 5/24/04 8:47 am Page 48
rationale was used when
discussing the “intake” and
“boost intake” ports, actually
known as transfer (also called
bypass) and boost transfer
(bypass) ports.
Why “transfer”? Because
these ports allow the fuel/air mix
in the crankcase to transfer from
the crankcase to the combustion
chamber, but their function is
that of fuel/air intake ports.
When traveling, you do not need
to know a street’s name—just
where it goes. In this case it goes
into the combustion chamber.
I also simplified the partial
vacuums found in our engines’
operations—actually lowpressure
areas since there is
nothing even approaching a
physicist’s definition of a partial hard
vacuum in our engines (there is just too
much gas density everywhere inside)—as
just “vacuums,” as most automotive books
do.
I completely ignored the function of an
engine’s timing advance, all references to
Top Dead Center (TDC) operations, and
especially all references to interference and
benefits one operation may have compared
with another.
None of this theory would help newer RC
pilots operate their engines better.
Explaining these operationally irrelevant, but
theoretically important, functions would
have taken almost a full article themselves.
Similarly, I called the methanol in our
fuel a “heat exchanger” and pointed out that
methanol helps cool the engine; that is why a
richer mixture is important. I felt that “heat
exchanger” was a simple, generally
understood term that would not require
definition but get the point across.
However, in technical terms, methanol
cools our engines because it has a high heat
of evaporation. During carburetor air intake,
methanol in the fuel is transformed into a gas
requiring a great deal of heat. The process—
called refrigeration—therefore removes heat
from the surrounding lower engine sections
to have the energy to transform the
methanol.
The same process cools the food in your
household refrigerator, but without the
combustion part. Your refrigerator
substitutes an electrically driven pump and
evaporator for crankcase pumping and
venturi action at the carburetor. But this
process is not technically one of heat
exchange. A heat exchanger does not
transform into another state of matter, but
absorbs heat from one source and transfers it
to another.
There is a true heat exchanger in our
fuel that removes heat from the engine and
transfers it to the earth’s atmosphere via
the exhaust. We call it the “fuel’s oil.” As
everyone who has cleaned his or her model
aircraft knows, some oil is not burned
during combustion and escapes through the
exhaust. While escaping, it also transfers
some of the engine’s heat.
The main point remains, however.
Whether through heat exchange or
refrigeration, the fuel lubricates and cools
our engines. RC engines are not cooled
solely through contact with the air; therefore,
the proper high-speed fuel mixture settings
are critical and must not be ignored.
In retrospect, I probably should have used
the technical terms and part names for the
preceding and for other items in “Engines
101.” It is usually better to explain the
technical, rather than “street,” names at the
start, even if doing so requires much
additional explanation. In this way, regular
RC pilots and engine technicians will
eventually have the same reference names.
So sit down, tighten your engine cap on
your head, and let’s explore the true, fully
detailed, roller-coaster operation of the
thermodynamically controlled contraption
we call the two-stroke, internal-combustion
engine. Along the way I will point out areas
where “Engines 101” was unclear, poorly
explained, or technically incorrect.
“Engines 101” started with the engine
before the first combustion and then
followed the operation cycle. It was assumed
that there was a fresh air/fuel charge in the
crankcase. That caused confusion about fuel
transfer (intake) timing versus
exhaust timing.
Since no combustion had yet
occurred, I ignored the fact that
the exhaust port opens slightly
sooner than the transfer ports do.
The main point was that the
exhaust port became fully open at
the same time the transfer ports
were opened completely.
But that might lead one to
wonder why the fuel/air mixture
just doesn’t go right across the
cylinder and out the exhaust.
Rather than explain that yes, it
does do that (somewhat), I
ignored it.
Actually, some fresh fuel/air
mixture does exit the exhaust,
especially during start-up. This is
one of the two-stroke engine’s
inefficiencies that engine designers strive to
minimize. This is also one reason why you
may notice some fuel condensing in the
muffler during operation—especially during
“rich” operation. Only the fact that the
engine’s parts are moving quickly helps
reduce this unwanted fuel/air loss.
This time I’ll start with the engine’s
piston at Bottom Dead Center (BDC),
meaning that it is as far down in its
movement (called stroke) as it can get. The
engine has not started, and there is no fuel
anywhere inside the engine. There is no fuel
anywhere except in the fuel tank.
(Operationally, it is important to have fuel in
the tank before trying to run the engine.)
Starting an engine from this position is
difficult until fuel flows from the tank,
through the fuel lines, and into the
carburetor. Therefore, we need to draw the
fuel from the tank, into the carburetor. We
will use the “suction” effect that permits the
engine to run in performing this task. Where
does the suction come from? While at BDC,
the rotary disk induction valve—the intake
slot in the crankshaft—is fully closed.
As the engine is hand-rotated
counterclockwise, the piston begins to move
upward. It first closes all the transfer (intake)
ports. At this point the rotary valve (for
short) begins to open, but the exhaust is also
still slightly open. However, there is no
connection between the exhaust port and the
engine’s lower crankcase at this point, so
that is irrelevant now.
As the piston continues to move upward,
the crankcase volume (not area, as was
written in “Engines 101”; that was a
misstatement) begins to increase. As this
volume increases with continued upward
movement of the piston (not cylinder—
another misstatement in the original article),
a low-pressure area is created in the
crankcase.
This happens because the now-sealed
crankcase volume is bigger than it was, but it
still contains only the original amount of air.
The air expands to fill the increased volume
and therefore has a lower pressure.
But do you remember that rotary
induction valve that was opening just as the
August 2004 49
Larger brass “tube” on right is fuel jet. Smaller brass fitting on
left is low-speed needle valve that regulates amount of fuel
flowing through fuel jet at reduced throttle levels.
K&B .65 (L) has larger upper cylinder case
to accommodate boost transfer ports.
Non-boost-ported K&B .61 (R) has almost
straight-walled cylinder case.
08sig2.QXD 5/24/04 8:47 am Page 49
transfer ports were closing? The valve opens
more as the piston travels upward. It is now
fully open, and that means the crankcase
section is no longer sealed.
The rotary valve is located just under the
carburetor. If the carburetor throttle barrel is
open, air rushes through the carburetor,
through the rotary valve (crankshaft), and
into the crankcase. Remember this process; it
will be repeated shortly, once fuel is added to
the mix.
Now we have plenty of air rushing into
and through the engine as we hand-rotate the
propeller. What happens if we put an
obstruction, such as a thumb, over the
carburetor’s air inlet?
Low pressure returns to the lower
crankcase since it is again sealed, even when
the rotary valve is open. But the piston is still
moving and re-creating the low-pressure
condition with each revolution. You can
actually feel the suction with your thumb.
This suction effect draws fuel and air into
the carburetor.
This low-pressure condition seeks relief
from wherever it can. Since the only
possible pressure relief is the small brass
fuel inlet—the fuel jet—fuel is drawn from
the fuel tank and into the fuel jet.
The photo showing the venturi process
was not sent in with the original article. That
caused confusion about the venturi process
because the picture’s caption explained it in
detail. I’ve included the photo here to better
illustrate the venturi process. The low
pressure—we call it suction—continues
through the small fuel inlet, through the
lines, and into the fuel tank.
Now remove the obstruction. As the
rotary valve opens, the crankcase’s lower
pressure draws fuel through the small brass
tube in the picture (the fuel jet) and air from
the atmosphere into the rotary induction
valve.
As the air is pulled through the
carburetor, it speeds up to go through the
narrow carburetor intake passage. The added
velocity means that the intake air gains
kinetic energy and, in order to maintain
balance, the potential energy (temperature
and pressure) drops. When the engine is
running or hand-cranked, this lowered
pressure is seen at the fuel jet, and the
difference between this low-pressure area
and the outside air pressure (seen at the fuel
tank vent) “sucks” fuel into the carburetor as
if your thumb were still there!
When the piston reaches as far upward as
it can—TDC—the fully open rotary valve
begins to close but draws fresh air and fuel
into the crankcase for another 70°-90° of
crankshaft rotation. The valve closes
completely before the exhaust port begins to
open. The crankcase and combustion
chamber are again sealed. But the piston still
has a ways to go before reaching BDC. It
continues downward, compressing the
fuel/air mixture inside the engine’s
crankcase.
That results in the crankcase’s now being
a high-pressure region. I left the following
part out of “Engines 101” because it is not
the major reason why the fuel/air mixture
flows into the combustion chamber. But still,
this high-pressure condition does exist, and
for now, when there has been no
combustion, it is the only transfer
mechanism in operation. The piston
continues compressing the crankcase mixture
and increasing the pressure.
But before reaching BDC, the piston
uncovers the transfer and boost transfer ports
(bypass ports). The high crankcase pressure
now has an exit. The fuel/air mixture under
50 MODEL AVIATION
Chart illustrates what ports and valves are open during one crankshaft rotation from viewpoint of looking directly at propeller end of
crankshaft. Each valve or port is open for length of its labeled area in curve. Drawing courtesy Dean Pappas.
08sig2.QXD 5/24/04 8:47 am Page 50
pressure rushes up through the transfer ports
and into the volume just above the piston.
Since the exhaust port is also fully open at
BDC, some of this precious mixture is lost
out the exhaust port. But some remains
above the piston.
(One advantage of the Schnuerle boost
transfer port system is that less incoming
fuel/air mix flowing from these sidemounted
ports is lost out the exhaust. The
Schnuerle ports are not aimed straight out
the exhaust port, as is the main transfer port.)
As BDC is passed, the piston travels
upward, pushing more of the fuel/air mixture
upward and into the already filled
combustion chamber. Yet some still goes out
the exhaust port—another inefficiency. Once
the exhaust port closes, the piston begins to
compress the fuel/air mix as it continues
upward. If the glow plug is lit, and the
fuel/air mixture is in the proper proportions,
a prolonged, controlled explosion called
“combustion” occurs.
The model two-stroke is part of the class
of engines known as “combustion ignition,”
which includes diesels. But there is a
subclass known as “catalytic enhanced
combustion ignition” engines. Our engines
fit into that category, as do many automobile
diesels with “glow plugs” that are constantly
receiving electric current (still not a true
chemical catalyst effect) and are therefore
always “lit.”
It seemed easier to just call our engines
“diesels” in the original article to
differentiate them from model gas ignition
engines rather than go through the true
technical explanation, as I just did.
Consider all of the preceding and add the
fuel/air mixture to the now-running engine.
How does the process differ? “Engines 101”
basically assumed that it didn’t, and for
operational understanding it doesn’t vary.
Proper operation and equipment selections,
except for tuned pipes that few sport pilots
use anymore, do not depend on any of the
following information. Still, this knowledge
could be important for a full understanding
of our engines’ operating theory.
Do you recall the intake process I
described in the preceding? Consider the
same process but with the engine running at
full speed. The piston is at BDC with most of
the exhaust gases gone, receiving a fresh
charge of fuel/air from the crankcase into the
now-vacant volume above the piston, right?
Well, not really. The exhaust port opens
only slightly before the transfer ports, called
the “exhaust lead” or “blowdown.” The
exhaust gases have not fully exited the
cylinder when the transfer ports begin to
open. The relationship between these
openings is part of the engine’s timing. The
accompanying illustration summarizes many
sport engines’ timing in this regard.
In practice, this timing means that fresh
fuel/air mixture is flowing into the cylinder
even as exhaust gases are exiting. Why
would an engine designer do this?
The hot, still expanding exhaust gases are
exiting at a high velocity. This forms a lowpressure
area just above the piston, “behind”
the exiting exhaust gases. The fresh fuel/air
mixture is “pulled” through the transfer
ports, into the low-pressure area in the
cylinder at the same time the descending
piston is compressing the mixture in the
crankcase and pushing it into the bypasses.
We say the exhaust gases “scavenge” the
fuel/air mixture into this section. The
scavenging effect increases the velocity, and
hence the amount of the fresh fuel/air
mixture that is drawn into the engine.
Just as the scavenge action is finishing
(the exhaust gases’ momentum is exhausted)
and the pulling of intake from the crankcase
through the bypasses is ending, the rotary
valve opens. This helps start the flow of
fresh fuel/air mixture into the crankcase for
the next power stroke.
At exceptionally low speeds, such as idle,
the scavenging action goes to completion,
and you are back to having pressure in the
crankcase at the moment the rotary valve
closes because of the descending piston. You
can sometimes tell that this is happening as
the engine spits fuel from the carburetor at
slow speeds.
Therefore, the scavenge effect is the
major force our engines use to put fuel and
air into the combustion chamber, but
crankcase pressure does play an important
part in the initial charge’s transferring into
the cylinder. Together, these alternating,
thermodynamically produced high- and lowpressure
conditions, neither a true or even
partial vacuum, allow our engines to run.
Several exhaust systems are available that
August 2004 51
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08sig2.QXD 5/24/04 8:48 am Page 51
advanced that detonation occurs, meaning
that the fuel/air mixture ignites before it
should. This condition may sometimes be
identified by a loud “frying egg” sound
(crackling) as the engine is run at full speed.
When you hear this sound, your engine may
be in for problems from overheating and
detonation. Land and readjust the high-speed
mixture.
I am going to stop discussing the process
at this point. The preceding is a far more
complete and technically correct explanation
of two-stroke engines’ operation than I wrote
in “Engines 101.” In deference to that article,
this installment has required nearly 2,000
words to cover the same topic as did its
roughly 600 words, without adding new
operational information that less-experienced
RC pilots could use to run their engines
better.
The long explanation would have left
little space for all the other topics I discussed
in “Engines 101,” but the shortcuts caused
confusion that would have been avoided with
the longer version. Yet even this explanation
covers only the basics of our easy-to-use but
complicated machines.
If you want to learn more, Dave Gierke
has written the excellent engine book Two-
Stroke Glow Engines, Volume 1, available
directly from him at 1276 Ransom Rd.,
Lancaster NY 14086. It is $18.95 including
shipping.
In “Engines 101,” I erred in writing that the
piston in an aluminum-brass-chrome (ABC)
engine is larger in diameter than its
respective cylinder. I took the liberty of
exaggeration to make the thermal expansion
point.
Actually, the piston is the same diameter
as the cylinder, which still expands more than
the piston to allow space for the piston to
move efficiently. Sometimes the piston is
larger in new engines, but by no more than
one to two ten-thousandths of an inch. This
quickly wears to the same diameter. I took
poetic license to make the point in few
words, but it was technically incorrect.
The main operational point was that ABC
engines are more tolerant of lean fuel/air
mixtures than ringed engines are. This is
because of the thermal expansion differences
inherent in this design. But ringed engines
usually outlast the ABC type if the fuel/air
mixture is always correct and the engines are
always properly maintained. And it is true
that ringed engines have a bit more torque
than corresponding ABC engines.
Regarding torque, I mentioned that fourstroke
engines have more than corresponding
two-strokes. I meant more usable torque but
wanted to avoid using extra space to explain
what that meant. In fact, two- and fourstrokes
have roughly the same amount. But
the four-stroke produces its maximum torque
at sufficiently low rpm so that most sport
fliers can “prop” their engines to reach this
speed.
Many two-strokes (not all, since older, socalled
long-stroke engines did not) have their
“torque band” or “curve” (the rpm range at
will increase the scavenging effect. I will
discuss them later, but now you understand
how and why they could increase an
engine’s power by increasing the
scavenging effect.
During the charge cycle, some fresh
fuel/air mixture is drawn out the exhaust
along with the escaping gases. This is lost
power and poor fuel economy that engine
designers strive to recover as much as
possible.
An additional complication is that the
combustion occurs before the piston reaches
TDC. It continues even when the engine
reaches TDC and ends at or after TDC.
The amount of advance is shown in
the drawing.
It may seem strange to put combustion
pressure against the piston’s upward
movement, but combustion takes time, and
our fuel doesn’t explode all at once.
Therefore, the prolonged explosion used to
burn as much of the fuel/air charge as
possible is made achievable by the
“advanced timing.” The relationship
between the piston’s movements and
ignition is a delicate balance. Too much
advance, and the piston is damaged; too
little means that insufficient combustion
occurs.
However, running an engine too lean
produces extra heat that can change this
delicate balance. Hot engines can
experience timing that becomes so
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08sig2.QXD 5/24/04 8:48 am Page 52
August 2004 53
which maximum torque is produced and
remains nearly constant) at relatively high
rpm. Many times the torque band is higher
than 13,000 rpm and may require impractical
propeller selection to reach—especially for
sport pilots using low-nitromethane-content
fuel.
In “Engines 101” I wrote that in theory, a
two-stroke engine should have twice the
power of an equivalent four-stroke. This
assumed that the two-stroke was 100%
efficient. As I have pointed out, it is far from
that efficiency level.
I also wrote that modern four-stroke
engines have roughly 70%-80% of the
“power” of an equivalent two-stroke. That is
true, but I should have added that I was
considering only sport 40-60 engines since
the article was addressing only
noncompetition pilots.
Some supercharged, fuel-injected
competition four-strokes can reach power
parity with the two-stroke, but at a much
higher cost. I didn’t mention these engines
because they are not usually relevant to new
RC pilots, but they are fine engines that are
worth more than their cost in the long run. I
know because I use them in RC Aerobatics
(Pattern) competition.
“Engines 101” contained two major
bloopers in addition to the two errors I
already mentioned, one of which was that
ball bearings provide “more” crankshaft
support than bronze bushings. I meant that
they provide “better” support since ball
bearings reduce crankshaft friction loads.
The bushing actually provides “more”
support since more area is in contact with the
crankshaft.
The point I made was valid, but the image
described was incorrect. If you are interested,
you can see in that article’s photo of the
K&B .65’s insides that it does indeed have
bushed crankshaft bearings.
An embarrassing mistake was that I
identified the oil-retention groove as a score
mark. When looking at the photo, I noticed
the thin line. Since that engine had not been
moved in nearly 15 years, I thought the
camera had captured oil buildup from the
cylinder onto the long-stationary piston. My
eyes are no longer good enough to see that
thin line without the camera’s magnification,
so I never noticed it before.
I learned something new here. The oil
groove serves to retain some lubrication in
the piston/cylinder contact area. I am happy
to correct this mistake and thank all those
who wrote in to point it out.
The last error involves the K&B .65
shown in “Engines 101.” I assumed that the
bushed engine followed the older, non-
Schnuerle-ported K&B design, but it does
not. The non-Schnuerle-ported engine should
have been the K&B .61 shown here in the
comparison photo that I originally thought
was too dirty to print.
Not all Schnuerle engines show the boost
transfer port on the outside. Some enlarge the
entire upper cylinder case to fit the boost
port, as shown in the photo comparing the
two K&B “60s.” These engines are more
difficult to spot.
Revisiting “Engines 101” has taken time
and space that could have been used for the
third article in this segment of the “From the
Ground Up” series, which will be published
next month. I thank all those who sent in
their suggestions and comments. Although
the important operation steps I presented in
“Engines 101” are valid, those comments
helped identify those areas that needed more
explanation. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
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08sig2.QXD 5/24/04 8:48 am Page 53
Edition: Model Aviation - 2004/08
Page Numbers: 48,49,50,51,52,53
BEFORE PROCEEDING to the third article in the “From the
Ground Up” engine series, I am going to review the first installment,
published in the April issue. MA has received numerous comments on
the information presented in that article. Many were favorable, but
several pointed out that the theoretical information was incomplete,
poorly explained, or just plain wrong. And in some instances they
have valid points.
So I’ll review some of “Engines 101”’s basics, keeping in mind
the comments that were offered. In all cases they were offered in
good faith and in the hopes of improving the series and modelers’
understanding of engine basics.
Probably the most incomplete section was the beginning of the
article. I did not take the space, which is precious in any article, to
properly explain the intent of the engine theory I was writing about or
the manner in which it was to be presented. I’ll do it now as it should
have been done in “Engines 101.”
The engine theory I am presenting is intended solely to provide
beginning RC pilots with enough knowledge of a model-engine’s
workings to understand why choices about proper mixture settings,
fuel, propellers, and other items to be discussed later will be made.
There is no intent to fully detail an engine’s intricate machinery.
The new RC pilot is not going to be designing or disassembling (I
hope) his or her first few engines, but this person will be setting highand
low-speed mixture settings.
All the theory I present will be from a strictly operations
viewpoint. Proper engine operation is the only goal in this discussion.
Where true technical names for parts may be confusing, I will use
descriptive terms instead. Since most new modelers have no
knowledge of two-stroke engines, but do have at least a passing
familiarity with their cars’ engines, I’ll try to reference parts with
confusing names in more recognizable terms.
As with all things mechanical, a model engine’s true operation is
complicated. Operations that are explained separately and appear to
be independent actually overlap, and sometimes interfere with, other
operations. To simplify the theoretical presentation, I will explain
each action as if it were the only one happening at that time.
A great deal of confusion would have been avoided if I had started
“Engines 101” with the preceding. Because of the decision to
simplify and avoid confusion, the rotary disk induction valve became
the crankshaft intake slot.
The true engineer’s name is correct but leads one to look for a
moving valve such as those found in a car or a rotating valve. There
isn’t one, and the “valve” is a slot. The name also sounds as if some
sort of “pumping” action is happening when it is not. The same
48 MODEL AVIATION
Engines 101
Rear ball bearing is partially visible above and to either side of
counterweight in Webra Speed .61 (L). K&B .65 (R) uses bronze
bushings to support crankshaft, so it has only case metal in this
area.
Photo clearly shows SuperTigre’s boost port on left side. K&B
.61 (R) has no such boost port and therefore a much narrower
cylinder case.
Follow-up to April issue’s “Engines 101” clears the air
by Frank Granelli
Revisited
Photos by the author
08sig2.QXD 5/24/04 8:47 am Page 48
rationale was used when
discussing the “intake” and
“boost intake” ports, actually
known as transfer (also called
bypass) and boost transfer
(bypass) ports.
Why “transfer”? Because
these ports allow the fuel/air mix
in the crankcase to transfer from
the crankcase to the combustion
chamber, but their function is
that of fuel/air intake ports.
When traveling, you do not need
to know a street’s name—just
where it goes. In this case it goes
into the combustion chamber.
I also simplified the partial
vacuums found in our engines’
operations—actually lowpressure
areas since there is
nothing even approaching a
physicist’s definition of a partial hard
vacuum in our engines (there is just too
much gas density everywhere inside)—as
just “vacuums,” as most automotive books
do.
I completely ignored the function of an
engine’s timing advance, all references to
Top Dead Center (TDC) operations, and
especially all references to interference and
benefits one operation may have compared
with another.
None of this theory would help newer RC
pilots operate their engines better.
Explaining these operationally irrelevant, but
theoretically important, functions would
have taken almost a full article themselves.
Similarly, I called the methanol in our
fuel a “heat exchanger” and pointed out that
methanol helps cool the engine; that is why a
richer mixture is important. I felt that “heat
exchanger” was a simple, generally
understood term that would not require
definition but get the point across.
However, in technical terms, methanol
cools our engines because it has a high heat
of evaporation. During carburetor air intake,
methanol in the fuel is transformed into a gas
requiring a great deal of heat. The process—
called refrigeration—therefore removes heat
from the surrounding lower engine sections
to have the energy to transform the
methanol.
The same process cools the food in your
household refrigerator, but without the
combustion part. Your refrigerator
substitutes an electrically driven pump and
evaporator for crankcase pumping and
venturi action at the carburetor. But this
process is not technically one of heat
exchange. A heat exchanger does not
transform into another state of matter, but
absorbs heat from one source and transfers it
to another.
There is a true heat exchanger in our
fuel that removes heat from the engine and
transfers it to the earth’s atmosphere via
the exhaust. We call it the “fuel’s oil.” As
everyone who has cleaned his or her model
aircraft knows, some oil is not burned
during combustion and escapes through the
exhaust. While escaping, it also transfers
some of the engine’s heat.
The main point remains, however.
Whether through heat exchange or
refrigeration, the fuel lubricates and cools
our engines. RC engines are not cooled
solely through contact with the air; therefore,
the proper high-speed fuel mixture settings
are critical and must not be ignored.
In retrospect, I probably should have used
the technical terms and part names for the
preceding and for other items in “Engines
101.” It is usually better to explain the
technical, rather than “street,” names at the
start, even if doing so requires much
additional explanation. In this way, regular
RC pilots and engine technicians will
eventually have the same reference names.
So sit down, tighten your engine cap on
your head, and let’s explore the true, fully
detailed, roller-coaster operation of the
thermodynamically controlled contraption
we call the two-stroke, internal-combustion
engine. Along the way I will point out areas
where “Engines 101” was unclear, poorly
explained, or technically incorrect.
“Engines 101” started with the engine
before the first combustion and then
followed the operation cycle. It was assumed
that there was a fresh air/fuel charge in the
crankcase. That caused confusion about fuel
transfer (intake) timing versus
exhaust timing.
Since no combustion had yet
occurred, I ignored the fact that
the exhaust port opens slightly
sooner than the transfer ports do.
The main point was that the
exhaust port became fully open at
the same time the transfer ports
were opened completely.
But that might lead one to
wonder why the fuel/air mixture
just doesn’t go right across the
cylinder and out the exhaust.
Rather than explain that yes, it
does do that (somewhat), I
ignored it.
Actually, some fresh fuel/air
mixture does exit the exhaust,
especially during start-up. This is
one of the two-stroke engine’s
inefficiencies that engine designers strive to
minimize. This is also one reason why you
may notice some fuel condensing in the
muffler during operation—especially during
“rich” operation. Only the fact that the
engine’s parts are moving quickly helps
reduce this unwanted fuel/air loss.
This time I’ll start with the engine’s
piston at Bottom Dead Center (BDC),
meaning that it is as far down in its
movement (called stroke) as it can get. The
engine has not started, and there is no fuel
anywhere inside the engine. There is no fuel
anywhere except in the fuel tank.
(Operationally, it is important to have fuel in
the tank before trying to run the engine.)
Starting an engine from this position is
difficult until fuel flows from the tank,
through the fuel lines, and into the
carburetor. Therefore, we need to draw the
fuel from the tank, into the carburetor. We
will use the “suction” effect that permits the
engine to run in performing this task. Where
does the suction come from? While at BDC,
the rotary disk induction valve—the intake
slot in the crankshaft—is fully closed.
As the engine is hand-rotated
counterclockwise, the piston begins to move
upward. It first closes all the transfer (intake)
ports. At this point the rotary valve (for
short) begins to open, but the exhaust is also
still slightly open. However, there is no
connection between the exhaust port and the
engine’s lower crankcase at this point, so
that is irrelevant now.
As the piston continues to move upward,
the crankcase volume (not area, as was
written in “Engines 101”; that was a
misstatement) begins to increase. As this
volume increases with continued upward
movement of the piston (not cylinder—
another misstatement in the original article),
a low-pressure area is created in the
crankcase.
This happens because the now-sealed
crankcase volume is bigger than it was, but it
still contains only the original amount of air.
The air expands to fill the increased volume
and therefore has a lower pressure.
But do you remember that rotary
induction valve that was opening just as the
August 2004 49
Larger brass “tube” on right is fuel jet. Smaller brass fitting on
left is low-speed needle valve that regulates amount of fuel
flowing through fuel jet at reduced throttle levels.
K&B .65 (L) has larger upper cylinder case
to accommodate boost transfer ports.
Non-boost-ported K&B .61 (R) has almost
straight-walled cylinder case.
08sig2.QXD 5/24/04 8:47 am Page 49
transfer ports were closing? The valve opens
more as the piston travels upward. It is now
fully open, and that means the crankcase
section is no longer sealed.
The rotary valve is located just under the
carburetor. If the carburetor throttle barrel is
open, air rushes through the carburetor,
through the rotary valve (crankshaft), and
into the crankcase. Remember this process; it
will be repeated shortly, once fuel is added to
the mix.
Now we have plenty of air rushing into
and through the engine as we hand-rotate the
propeller. What happens if we put an
obstruction, such as a thumb, over the
carburetor’s air inlet?
Low pressure returns to the lower
crankcase since it is again sealed, even when
the rotary valve is open. But the piston is still
moving and re-creating the low-pressure
condition with each revolution. You can
actually feel the suction with your thumb.
This suction effect draws fuel and air into
the carburetor.
This low-pressure condition seeks relief
from wherever it can. Since the only
possible pressure relief is the small brass
fuel inlet—the fuel jet—fuel is drawn from
the fuel tank and into the fuel jet.
The photo showing the venturi process
was not sent in with the original article. That
caused confusion about the venturi process
because the picture’s caption explained it in
detail. I’ve included the photo here to better
illustrate the venturi process. The low
pressure—we call it suction—continues
through the small fuel inlet, through the
lines, and into the fuel tank.
Now remove the obstruction. As the
rotary valve opens, the crankcase’s lower
pressure draws fuel through the small brass
tube in the picture (the fuel jet) and air from
the atmosphere into the rotary induction
valve.
As the air is pulled through the
carburetor, it speeds up to go through the
narrow carburetor intake passage. The added
velocity means that the intake air gains
kinetic energy and, in order to maintain
balance, the potential energy (temperature
and pressure) drops. When the engine is
running or hand-cranked, this lowered
pressure is seen at the fuel jet, and the
difference between this low-pressure area
and the outside air pressure (seen at the fuel
tank vent) “sucks” fuel into the carburetor as
if your thumb were still there!
When the piston reaches as far upward as
it can—TDC—the fully open rotary valve
begins to close but draws fresh air and fuel
into the crankcase for another 70°-90° of
crankshaft rotation. The valve closes
completely before the exhaust port begins to
open. The crankcase and combustion
chamber are again sealed. But the piston still
has a ways to go before reaching BDC. It
continues downward, compressing the
fuel/air mixture inside the engine’s
crankcase.
That results in the crankcase’s now being
a high-pressure region. I left the following
part out of “Engines 101” because it is not
the major reason why the fuel/air mixture
flows into the combustion chamber. But still,
this high-pressure condition does exist, and
for now, when there has been no
combustion, it is the only transfer
mechanism in operation. The piston
continues compressing the crankcase mixture
and increasing the pressure.
But before reaching BDC, the piston
uncovers the transfer and boost transfer ports
(bypass ports). The high crankcase pressure
now has an exit. The fuel/air mixture under
50 MODEL AVIATION
Chart illustrates what ports and valves are open during one crankshaft rotation from viewpoint of looking directly at propeller end of
crankshaft. Each valve or port is open for length of its labeled area in curve. Drawing courtesy Dean Pappas.
08sig2.QXD 5/24/04 8:47 am Page 50
pressure rushes up through the transfer ports
and into the volume just above the piston.
Since the exhaust port is also fully open at
BDC, some of this precious mixture is lost
out the exhaust port. But some remains
above the piston.
(One advantage of the Schnuerle boost
transfer port system is that less incoming
fuel/air mix flowing from these sidemounted
ports is lost out the exhaust. The
Schnuerle ports are not aimed straight out
the exhaust port, as is the main transfer port.)
As BDC is passed, the piston travels
upward, pushing more of the fuel/air mixture
upward and into the already filled
combustion chamber. Yet some still goes out
the exhaust port—another inefficiency. Once
the exhaust port closes, the piston begins to
compress the fuel/air mix as it continues
upward. If the glow plug is lit, and the
fuel/air mixture is in the proper proportions,
a prolonged, controlled explosion called
“combustion” occurs.
The model two-stroke is part of the class
of engines known as “combustion ignition,”
which includes diesels. But there is a
subclass known as “catalytic enhanced
combustion ignition” engines. Our engines
fit into that category, as do many automobile
diesels with “glow plugs” that are constantly
receiving electric current (still not a true
chemical catalyst effect) and are therefore
always “lit.”
It seemed easier to just call our engines
“diesels” in the original article to
differentiate them from model gas ignition
engines rather than go through the true
technical explanation, as I just did.
Consider all of the preceding and add the
fuel/air mixture to the now-running engine.
How does the process differ? “Engines 101”
basically assumed that it didn’t, and for
operational understanding it doesn’t vary.
Proper operation and equipment selections,
except for tuned pipes that few sport pilots
use anymore, do not depend on any of the
following information. Still, this knowledge
could be important for a full understanding
of our engines’ operating theory.
Do you recall the intake process I
described in the preceding? Consider the
same process but with the engine running at
full speed. The piston is at BDC with most of
the exhaust gases gone, receiving a fresh
charge of fuel/air from the crankcase into the
now-vacant volume above the piston, right?
Well, not really. The exhaust port opens
only slightly before the transfer ports, called
the “exhaust lead” or “blowdown.” The
exhaust gases have not fully exited the
cylinder when the transfer ports begin to
open. The relationship between these
openings is part of the engine’s timing. The
accompanying illustration summarizes many
sport engines’ timing in this regard.
In practice, this timing means that fresh
fuel/air mixture is flowing into the cylinder
even as exhaust gases are exiting. Why
would an engine designer do this?
The hot, still expanding exhaust gases are
exiting at a high velocity. This forms a lowpressure
area just above the piston, “behind”
the exiting exhaust gases. The fresh fuel/air
mixture is “pulled” through the transfer
ports, into the low-pressure area in the
cylinder at the same time the descending
piston is compressing the mixture in the
crankcase and pushing it into the bypasses.
We say the exhaust gases “scavenge” the
fuel/air mixture into this section. The
scavenging effect increases the velocity, and
hence the amount of the fresh fuel/air
mixture that is drawn into the engine.
Just as the scavenge action is finishing
(the exhaust gases’ momentum is exhausted)
and the pulling of intake from the crankcase
through the bypasses is ending, the rotary
valve opens. This helps start the flow of
fresh fuel/air mixture into the crankcase for
the next power stroke.
At exceptionally low speeds, such as idle,
the scavenging action goes to completion,
and you are back to having pressure in the
crankcase at the moment the rotary valve
closes because of the descending piston. You
can sometimes tell that this is happening as
the engine spits fuel from the carburetor at
slow speeds.
Therefore, the scavenge effect is the
major force our engines use to put fuel and
air into the combustion chamber, but
crankcase pressure does play an important
part in the initial charge’s transferring into
the cylinder. Together, these alternating,
thermodynamically produced high- and lowpressure
conditions, neither a true or even
partial vacuum, allow our engines to run.
Several exhaust systems are available that
August 2004 51
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08sig2.QXD 5/24/04 8:48 am Page 51
advanced that detonation occurs, meaning
that the fuel/air mixture ignites before it
should. This condition may sometimes be
identified by a loud “frying egg” sound
(crackling) as the engine is run at full speed.
When you hear this sound, your engine may
be in for problems from overheating and
detonation. Land and readjust the high-speed
mixture.
I am going to stop discussing the process
at this point. The preceding is a far more
complete and technically correct explanation
of two-stroke engines’ operation than I wrote
in “Engines 101.” In deference to that article,
this installment has required nearly 2,000
words to cover the same topic as did its
roughly 600 words, without adding new
operational information that less-experienced
RC pilots could use to run their engines
better.
The long explanation would have left
little space for all the other topics I discussed
in “Engines 101,” but the shortcuts caused
confusion that would have been avoided with
the longer version. Yet even this explanation
covers only the basics of our easy-to-use but
complicated machines.
If you want to learn more, Dave Gierke
has written the excellent engine book Two-
Stroke Glow Engines, Volume 1, available
directly from him at 1276 Ransom Rd.,
Lancaster NY 14086. It is $18.95 including
shipping.
In “Engines 101,” I erred in writing that the
piston in an aluminum-brass-chrome (ABC)
engine is larger in diameter than its
respective cylinder. I took the liberty of
exaggeration to make the thermal expansion
point.
Actually, the piston is the same diameter
as the cylinder, which still expands more than
the piston to allow space for the piston to
move efficiently. Sometimes the piston is
larger in new engines, but by no more than
one to two ten-thousandths of an inch. This
quickly wears to the same diameter. I took
poetic license to make the point in few
words, but it was technically incorrect.
The main operational point was that ABC
engines are more tolerant of lean fuel/air
mixtures than ringed engines are. This is
because of the thermal expansion differences
inherent in this design. But ringed engines
usually outlast the ABC type if the fuel/air
mixture is always correct and the engines are
always properly maintained. And it is true
that ringed engines have a bit more torque
than corresponding ABC engines.
Regarding torque, I mentioned that fourstroke
engines have more than corresponding
two-strokes. I meant more usable torque but
wanted to avoid using extra space to explain
what that meant. In fact, two- and fourstrokes
have roughly the same amount. But
the four-stroke produces its maximum torque
at sufficiently low rpm so that most sport
fliers can “prop” their engines to reach this
speed.
Many two-strokes (not all, since older, socalled
long-stroke engines did not) have their
“torque band” or “curve” (the rpm range at
will increase the scavenging effect. I will
discuss them later, but now you understand
how and why they could increase an
engine’s power by increasing the
scavenging effect.
During the charge cycle, some fresh
fuel/air mixture is drawn out the exhaust
along with the escaping gases. This is lost
power and poor fuel economy that engine
designers strive to recover as much as
possible.
An additional complication is that the
combustion occurs before the piston reaches
TDC. It continues even when the engine
reaches TDC and ends at or after TDC.
The amount of advance is shown in
the drawing.
It may seem strange to put combustion
pressure against the piston’s upward
movement, but combustion takes time, and
our fuel doesn’t explode all at once.
Therefore, the prolonged explosion used to
burn as much of the fuel/air charge as
possible is made achievable by the
“advanced timing.” The relationship
between the piston’s movements and
ignition is a delicate balance. Too much
advance, and the piston is damaged; too
little means that insufficient combustion
occurs.
However, running an engine too lean
produces extra heat that can change this
delicate balance. Hot engines can
experience timing that becomes so
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08sig2.QXD 5/24/04 8:48 am Page 52
August 2004 53
which maximum torque is produced and
remains nearly constant) at relatively high
rpm. Many times the torque band is higher
than 13,000 rpm and may require impractical
propeller selection to reach—especially for
sport pilots using low-nitromethane-content
fuel.
In “Engines 101” I wrote that in theory, a
two-stroke engine should have twice the
power of an equivalent four-stroke. This
assumed that the two-stroke was 100%
efficient. As I have pointed out, it is far from
that efficiency level.
I also wrote that modern four-stroke
engines have roughly 70%-80% of the
“power” of an equivalent two-stroke. That is
true, but I should have added that I was
considering only sport 40-60 engines since
the article was addressing only
noncompetition pilots.
Some supercharged, fuel-injected
competition four-strokes can reach power
parity with the two-stroke, but at a much
higher cost. I didn’t mention these engines
because they are not usually relevant to new
RC pilots, but they are fine engines that are
worth more than their cost in the long run. I
know because I use them in RC Aerobatics
(Pattern) competition.
“Engines 101” contained two major
bloopers in addition to the two errors I
already mentioned, one of which was that
ball bearings provide “more” crankshaft
support than bronze bushings. I meant that
they provide “better” support since ball
bearings reduce crankshaft friction loads.
The bushing actually provides “more”
support since more area is in contact with the
crankshaft.
The point I made was valid, but the image
described was incorrect. If you are interested,
you can see in that article’s photo of the
K&B .65’s insides that it does indeed have
bushed crankshaft bearings.
An embarrassing mistake was that I
identified the oil-retention groove as a score
mark. When looking at the photo, I noticed
the thin line. Since that engine had not been
moved in nearly 15 years, I thought the
camera had captured oil buildup from the
cylinder onto the long-stationary piston. My
eyes are no longer good enough to see that
thin line without the camera’s magnification,
so I never noticed it before.
I learned something new here. The oil
groove serves to retain some lubrication in
the piston/cylinder contact area. I am happy
to correct this mistake and thank all those
who wrote in to point it out.
The last error involves the K&B .65
shown in “Engines 101.” I assumed that the
bushed engine followed the older, non-
Schnuerle-ported K&B design, but it does
not. The non-Schnuerle-ported engine should
have been the K&B .61 shown here in the
comparison photo that I originally thought
was too dirty to print.
Not all Schnuerle engines show the boost
transfer port on the outside. Some enlarge the
entire upper cylinder case to fit the boost
port, as shown in the photo comparing the
two K&B “60s.” These engines are more
difficult to spot.
Revisiting “Engines 101” has taken time
and space that could have been used for the
third article in this segment of the “From the
Ground Up” series, which will be published
next month. I thank all those who sent in
their suggestions and comments. Although
the important operation steps I presented in
“Engines 101” are valid, those comments
helped identify those areas that needed more
explanation. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
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08sig2.QXD 5/24/04 8:48 am Page 53
Edition: Model Aviation - 2004/08
Page Numbers: 48,49,50,51,52,53
BEFORE PROCEEDING to the third article in the “From the
Ground Up” engine series, I am going to review the first installment,
published in the April issue. MA has received numerous comments on
the information presented in that article. Many were favorable, but
several pointed out that the theoretical information was incomplete,
poorly explained, or just plain wrong. And in some instances they
have valid points.
So I’ll review some of “Engines 101”’s basics, keeping in mind
the comments that were offered. In all cases they were offered in
good faith and in the hopes of improving the series and modelers’
understanding of engine basics.
Probably the most incomplete section was the beginning of the
article. I did not take the space, which is precious in any article, to
properly explain the intent of the engine theory I was writing about or
the manner in which it was to be presented. I’ll do it now as it should
have been done in “Engines 101.”
The engine theory I am presenting is intended solely to provide
beginning RC pilots with enough knowledge of a model-engine’s
workings to understand why choices about proper mixture settings,
fuel, propellers, and other items to be discussed later will be made.
There is no intent to fully detail an engine’s intricate machinery.
The new RC pilot is not going to be designing or disassembling (I
hope) his or her first few engines, but this person will be setting highand
low-speed mixture settings.
All the theory I present will be from a strictly operations
viewpoint. Proper engine operation is the only goal in this discussion.
Where true technical names for parts may be confusing, I will use
descriptive terms instead. Since most new modelers have no
knowledge of two-stroke engines, but do have at least a passing
familiarity with their cars’ engines, I’ll try to reference parts with
confusing names in more recognizable terms.
As with all things mechanical, a model engine’s true operation is
complicated. Operations that are explained separately and appear to
be independent actually overlap, and sometimes interfere with, other
operations. To simplify the theoretical presentation, I will explain
each action as if it were the only one happening at that time.
A great deal of confusion would have been avoided if I had started
“Engines 101” with the preceding. Because of the decision to
simplify and avoid confusion, the rotary disk induction valve became
the crankshaft intake slot.
The true engineer’s name is correct but leads one to look for a
moving valve such as those found in a car or a rotating valve. There
isn’t one, and the “valve” is a slot. The name also sounds as if some
sort of “pumping” action is happening when it is not. The same
48 MODEL AVIATION
Engines 101
Rear ball bearing is partially visible above and to either side of
counterweight in Webra Speed .61 (L). K&B .65 (R) uses bronze
bushings to support crankshaft, so it has only case metal in this
area.
Photo clearly shows SuperTigre’s boost port on left side. K&B
.61 (R) has no such boost port and therefore a much narrower
cylinder case.
Follow-up to April issue’s “Engines 101” clears the air
by Frank Granelli
Revisited
Photos by the author
08sig2.QXD 5/24/04 8:47 am Page 48
rationale was used when
discussing the “intake” and
“boost intake” ports, actually
known as transfer (also called
bypass) and boost transfer
(bypass) ports.
Why “transfer”? Because
these ports allow the fuel/air mix
in the crankcase to transfer from
the crankcase to the combustion
chamber, but their function is
that of fuel/air intake ports.
When traveling, you do not need
to know a street’s name—just
where it goes. In this case it goes
into the combustion chamber.
I also simplified the partial
vacuums found in our engines’
operations—actually lowpressure
areas since there is
nothing even approaching a
physicist’s definition of a partial hard
vacuum in our engines (there is just too
much gas density everywhere inside)—as
just “vacuums,” as most automotive books
do.
I completely ignored the function of an
engine’s timing advance, all references to
Top Dead Center (TDC) operations, and
especially all references to interference and
benefits one operation may have compared
with another.
None of this theory would help newer RC
pilots operate their engines better.
Explaining these operationally irrelevant, but
theoretically important, functions would
have taken almost a full article themselves.
Similarly, I called the methanol in our
fuel a “heat exchanger” and pointed out that
methanol helps cool the engine; that is why a
richer mixture is important. I felt that “heat
exchanger” was a simple, generally
understood term that would not require
definition but get the point across.
However, in technical terms, methanol
cools our engines because it has a high heat
of evaporation. During carburetor air intake,
methanol in the fuel is transformed into a gas
requiring a great deal of heat. The process—
called refrigeration—therefore removes heat
from the surrounding lower engine sections
to have the energy to transform the
methanol.
The same process cools the food in your
household refrigerator, but without the
combustion part. Your refrigerator
substitutes an electrically driven pump and
evaporator for crankcase pumping and
venturi action at the carburetor. But this
process is not technically one of heat
exchange. A heat exchanger does not
transform into another state of matter, but
absorbs heat from one source and transfers it
to another.
There is a true heat exchanger in our
fuel that removes heat from the engine and
transfers it to the earth’s atmosphere via
the exhaust. We call it the “fuel’s oil.” As
everyone who has cleaned his or her model
aircraft knows, some oil is not burned
during combustion and escapes through the
exhaust. While escaping, it also transfers
some of the engine’s heat.
The main point remains, however.
Whether through heat exchange or
refrigeration, the fuel lubricates and cools
our engines. RC engines are not cooled
solely through contact with the air; therefore,
the proper high-speed fuel mixture settings
are critical and must not be ignored.
In retrospect, I probably should have used
the technical terms and part names for the
preceding and for other items in “Engines
101.” It is usually better to explain the
technical, rather than “street,” names at the
start, even if doing so requires much
additional explanation. In this way, regular
RC pilots and engine technicians will
eventually have the same reference names.
So sit down, tighten your engine cap on
your head, and let’s explore the true, fully
detailed, roller-coaster operation of the
thermodynamically controlled contraption
we call the two-stroke, internal-combustion
engine. Along the way I will point out areas
where “Engines 101” was unclear, poorly
explained, or technically incorrect.
“Engines 101” started with the engine
before the first combustion and then
followed the operation cycle. It was assumed
that there was a fresh air/fuel charge in the
crankcase. That caused confusion about fuel
transfer (intake) timing versus
exhaust timing.
Since no combustion had yet
occurred, I ignored the fact that
the exhaust port opens slightly
sooner than the transfer ports do.
The main point was that the
exhaust port became fully open at
the same time the transfer ports
were opened completely.
But that might lead one to
wonder why the fuel/air mixture
just doesn’t go right across the
cylinder and out the exhaust.
Rather than explain that yes, it
does do that (somewhat), I
ignored it.
Actually, some fresh fuel/air
mixture does exit the exhaust,
especially during start-up. This is
one of the two-stroke engine’s
inefficiencies that engine designers strive to
minimize. This is also one reason why you
may notice some fuel condensing in the
muffler during operation—especially during
“rich” operation. Only the fact that the
engine’s parts are moving quickly helps
reduce this unwanted fuel/air loss.
This time I’ll start with the engine’s
piston at Bottom Dead Center (BDC),
meaning that it is as far down in its
movement (called stroke) as it can get. The
engine has not started, and there is no fuel
anywhere inside the engine. There is no fuel
anywhere except in the fuel tank.
(Operationally, it is important to have fuel in
the tank before trying to run the engine.)
Starting an engine from this position is
difficult until fuel flows from the tank,
through the fuel lines, and into the
carburetor. Therefore, we need to draw the
fuel from the tank, into the carburetor. We
will use the “suction” effect that permits the
engine to run in performing this task. Where
does the suction come from? While at BDC,
the rotary disk induction valve—the intake
slot in the crankshaft—is fully closed.
As the engine is hand-rotated
counterclockwise, the piston begins to move
upward. It first closes all the transfer (intake)
ports. At this point the rotary valve (for
short) begins to open, but the exhaust is also
still slightly open. However, there is no
connection between the exhaust port and the
engine’s lower crankcase at this point, so
that is irrelevant now.
As the piston continues to move upward,
the crankcase volume (not area, as was
written in “Engines 101”; that was a
misstatement) begins to increase. As this
volume increases with continued upward
movement of the piston (not cylinder—
another misstatement in the original article),
a low-pressure area is created in the
crankcase.
This happens because the now-sealed
crankcase volume is bigger than it was, but it
still contains only the original amount of air.
The air expands to fill the increased volume
and therefore has a lower pressure.
But do you remember that rotary
induction valve that was opening just as the
August 2004 49
Larger brass “tube” on right is fuel jet. Smaller brass fitting on
left is low-speed needle valve that regulates amount of fuel
flowing through fuel jet at reduced throttle levels.
K&B .65 (L) has larger upper cylinder case
to accommodate boost transfer ports.
Non-boost-ported K&B .61 (R) has almost
straight-walled cylinder case.
08sig2.QXD 5/24/04 8:47 am Page 49
transfer ports were closing? The valve opens
more as the piston travels upward. It is now
fully open, and that means the crankcase
section is no longer sealed.
The rotary valve is located just under the
carburetor. If the carburetor throttle barrel is
open, air rushes through the carburetor,
through the rotary valve (crankshaft), and
into the crankcase. Remember this process; it
will be repeated shortly, once fuel is added to
the mix.
Now we have plenty of air rushing into
and through the engine as we hand-rotate the
propeller. What happens if we put an
obstruction, such as a thumb, over the
carburetor’s air inlet?
Low pressure returns to the lower
crankcase since it is again sealed, even when
the rotary valve is open. But the piston is still
moving and re-creating the low-pressure
condition with each revolution. You can
actually feel the suction with your thumb.
This suction effect draws fuel and air into
the carburetor.
This low-pressure condition seeks relief
from wherever it can. Since the only
possible pressure relief is the small brass
fuel inlet—the fuel jet—fuel is drawn from
the fuel tank and into the fuel jet.
The photo showing the venturi process
was not sent in with the original article. That
caused confusion about the venturi process
because the picture’s caption explained it in
detail. I’ve included the photo here to better
illustrate the venturi process. The low
pressure—we call it suction—continues
through the small fuel inlet, through the
lines, and into the fuel tank.
Now remove the obstruction. As the
rotary valve opens, the crankcase’s lower
pressure draws fuel through the small brass
tube in the picture (the fuel jet) and air from
the atmosphere into the rotary induction
valve.
As the air is pulled through the
carburetor, it speeds up to go through the
narrow carburetor intake passage. The added
velocity means that the intake air gains
kinetic energy and, in order to maintain
balance, the potential energy (temperature
and pressure) drops. When the engine is
running or hand-cranked, this lowered
pressure is seen at the fuel jet, and the
difference between this low-pressure area
and the outside air pressure (seen at the fuel
tank vent) “sucks” fuel into the carburetor as
if your thumb were still there!
When the piston reaches as far upward as
it can—TDC—the fully open rotary valve
begins to close but draws fresh air and fuel
into the crankcase for another 70°-90° of
crankshaft rotation. The valve closes
completely before the exhaust port begins to
open. The crankcase and combustion
chamber are again sealed. But the piston still
has a ways to go before reaching BDC. It
continues downward, compressing the
fuel/air mixture inside the engine’s
crankcase.
That results in the crankcase’s now being
a high-pressure region. I left the following
part out of “Engines 101” because it is not
the major reason why the fuel/air mixture
flows into the combustion chamber. But still,
this high-pressure condition does exist, and
for now, when there has been no
combustion, it is the only transfer
mechanism in operation. The piston
continues compressing the crankcase mixture
and increasing the pressure.
But before reaching BDC, the piston
uncovers the transfer and boost transfer ports
(bypass ports). The high crankcase pressure
now has an exit. The fuel/air mixture under
50 MODEL AVIATION
Chart illustrates what ports and valves are open during one crankshaft rotation from viewpoint of looking directly at propeller end of
crankshaft. Each valve or port is open for length of its labeled area in curve. Drawing courtesy Dean Pappas.
08sig2.QXD 5/24/04 8:47 am Page 50
pressure rushes up through the transfer ports
and into the volume just above the piston.
Since the exhaust port is also fully open at
BDC, some of this precious mixture is lost
out the exhaust port. But some remains
above the piston.
(One advantage of the Schnuerle boost
transfer port system is that less incoming
fuel/air mix flowing from these sidemounted
ports is lost out the exhaust. The
Schnuerle ports are not aimed straight out
the exhaust port, as is the main transfer port.)
As BDC is passed, the piston travels
upward, pushing more of the fuel/air mixture
upward and into the already filled
combustion chamber. Yet some still goes out
the exhaust port—another inefficiency. Once
the exhaust port closes, the piston begins to
compress the fuel/air mix as it continues
upward. If the glow plug is lit, and the
fuel/air mixture is in the proper proportions,
a prolonged, controlled explosion called
“combustion” occurs.
The model two-stroke is part of the class
of engines known as “combustion ignition,”
which includes diesels. But there is a
subclass known as “catalytic enhanced
combustion ignition” engines. Our engines
fit into that category, as do many automobile
diesels with “glow plugs” that are constantly
receiving electric current (still not a true
chemical catalyst effect) and are therefore
always “lit.”
It seemed easier to just call our engines
“diesels” in the original article to
differentiate them from model gas ignition
engines rather than go through the true
technical explanation, as I just did.
Consider all of the preceding and add the
fuel/air mixture to the now-running engine.
How does the process differ? “Engines 101”
basically assumed that it didn’t, and for
operational understanding it doesn’t vary.
Proper operation and equipment selections,
except for tuned pipes that few sport pilots
use anymore, do not depend on any of the
following information. Still, this knowledge
could be important for a full understanding
of our engines’ operating theory.
Do you recall the intake process I
described in the preceding? Consider the
same process but with the engine running at
full speed. The piston is at BDC with most of
the exhaust gases gone, receiving a fresh
charge of fuel/air from the crankcase into the
now-vacant volume above the piston, right?
Well, not really. The exhaust port opens
only slightly before the transfer ports, called
the “exhaust lead” or “blowdown.” The
exhaust gases have not fully exited the
cylinder when the transfer ports begin to
open. The relationship between these
openings is part of the engine’s timing. The
accompanying illustration summarizes many
sport engines’ timing in this regard.
In practice, this timing means that fresh
fuel/air mixture is flowing into the cylinder
even as exhaust gases are exiting. Why
would an engine designer do this?
The hot, still expanding exhaust gases are
exiting at a high velocity. This forms a lowpressure
area just above the piston, “behind”
the exiting exhaust gases. The fresh fuel/air
mixture is “pulled” through the transfer
ports, into the low-pressure area in the
cylinder at the same time the descending
piston is compressing the mixture in the
crankcase and pushing it into the bypasses.
We say the exhaust gases “scavenge” the
fuel/air mixture into this section. The
scavenging effect increases the velocity, and
hence the amount of the fresh fuel/air
mixture that is drawn into the engine.
Just as the scavenge action is finishing
(the exhaust gases’ momentum is exhausted)
and the pulling of intake from the crankcase
through the bypasses is ending, the rotary
valve opens. This helps start the flow of
fresh fuel/air mixture into the crankcase for
the next power stroke.
At exceptionally low speeds, such as idle,
the scavenging action goes to completion,
and you are back to having pressure in the
crankcase at the moment the rotary valve
closes because of the descending piston. You
can sometimes tell that this is happening as
the engine spits fuel from the carburetor at
slow speeds.
Therefore, the scavenge effect is the
major force our engines use to put fuel and
air into the combustion chamber, but
crankcase pressure does play an important
part in the initial charge’s transferring into
the cylinder. Together, these alternating,
thermodynamically produced high- and lowpressure
conditions, neither a true or even
partial vacuum, allow our engines to run.
Several exhaust systems are available that
August 2004 51
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08sig2.QXD 5/24/04 8:48 am Page 51
advanced that detonation occurs, meaning
that the fuel/air mixture ignites before it
should. This condition may sometimes be
identified by a loud “frying egg” sound
(crackling) as the engine is run at full speed.
When you hear this sound, your engine may
be in for problems from overheating and
detonation. Land and readjust the high-speed
mixture.
I am going to stop discussing the process
at this point. The preceding is a far more
complete and technically correct explanation
of two-stroke engines’ operation than I wrote
in “Engines 101.” In deference to that article,
this installment has required nearly 2,000
words to cover the same topic as did its
roughly 600 words, without adding new
operational information that less-experienced
RC pilots could use to run their engines
better.
The long explanation would have left
little space for all the other topics I discussed
in “Engines 101,” but the shortcuts caused
confusion that would have been avoided with
the longer version. Yet even this explanation
covers only the basics of our easy-to-use but
complicated machines.
If you want to learn more, Dave Gierke
has written the excellent engine book Two-
Stroke Glow Engines, Volume 1, available
directly from him at 1276 Ransom Rd.,
Lancaster NY 14086. It is $18.95 including
shipping.
In “Engines 101,” I erred in writing that the
piston in an aluminum-brass-chrome (ABC)
engine is larger in diameter than its
respective cylinder. I took the liberty of
exaggeration to make the thermal expansion
point.
Actually, the piston is the same diameter
as the cylinder, which still expands more than
the piston to allow space for the piston to
move efficiently. Sometimes the piston is
larger in new engines, but by no more than
one to two ten-thousandths of an inch. This
quickly wears to the same diameter. I took
poetic license to make the point in few
words, but it was technically incorrect.
The main operational point was that ABC
engines are more tolerant of lean fuel/air
mixtures than ringed engines are. This is
because of the thermal expansion differences
inherent in this design. But ringed engines
usually outlast the ABC type if the fuel/air
mixture is always correct and the engines are
always properly maintained. And it is true
that ringed engines have a bit more torque
than corresponding ABC engines.
Regarding torque, I mentioned that fourstroke
engines have more than corresponding
two-strokes. I meant more usable torque but
wanted to avoid using extra space to explain
what that meant. In fact, two- and fourstrokes
have roughly the same amount. But
the four-stroke produces its maximum torque
at sufficiently low rpm so that most sport
fliers can “prop” their engines to reach this
speed.
Many two-strokes (not all, since older, socalled
long-stroke engines did not) have their
“torque band” or “curve” (the rpm range at
will increase the scavenging effect. I will
discuss them later, but now you understand
how and why they could increase an
engine’s power by increasing the
scavenging effect.
During the charge cycle, some fresh
fuel/air mixture is drawn out the exhaust
along with the escaping gases. This is lost
power and poor fuel economy that engine
designers strive to recover as much as
possible.
An additional complication is that the
combustion occurs before the piston reaches
TDC. It continues even when the engine
reaches TDC and ends at or after TDC.
The amount of advance is shown in
the drawing.
It may seem strange to put combustion
pressure against the piston’s upward
movement, but combustion takes time, and
our fuel doesn’t explode all at once.
Therefore, the prolonged explosion used to
burn as much of the fuel/air charge as
possible is made achievable by the
“advanced timing.” The relationship
between the piston’s movements and
ignition is a delicate balance. Too much
advance, and the piston is damaged; too
little means that insufficient combustion
occurs.
However, running an engine too lean
produces extra heat that can change this
delicate balance. Hot engines can
experience timing that becomes so
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08sig2.QXD 5/24/04 8:48 am Page 52
August 2004 53
which maximum torque is produced and
remains nearly constant) at relatively high
rpm. Many times the torque band is higher
than 13,000 rpm and may require impractical
propeller selection to reach—especially for
sport pilots using low-nitromethane-content
fuel.
In “Engines 101” I wrote that in theory, a
two-stroke engine should have twice the
power of an equivalent four-stroke. This
assumed that the two-stroke was 100%
efficient. As I have pointed out, it is far from
that efficiency level.
I also wrote that modern four-stroke
engines have roughly 70%-80% of the
“power” of an equivalent two-stroke. That is
true, but I should have added that I was
considering only sport 40-60 engines since
the article was addressing only
noncompetition pilots.
Some supercharged, fuel-injected
competition four-strokes can reach power
parity with the two-stroke, but at a much
higher cost. I didn’t mention these engines
because they are not usually relevant to new
RC pilots, but they are fine engines that are
worth more than their cost in the long run. I
know because I use them in RC Aerobatics
(Pattern) competition.
“Engines 101” contained two major
bloopers in addition to the two errors I
already mentioned, one of which was that
ball bearings provide “more” crankshaft
support than bronze bushings. I meant that
they provide “better” support since ball
bearings reduce crankshaft friction loads.
The bushing actually provides “more”
support since more area is in contact with the
crankshaft.
The point I made was valid, but the image
described was incorrect. If you are interested,
you can see in that article’s photo of the
K&B .65’s insides that it does indeed have
bushed crankshaft bearings.
An embarrassing mistake was that I
identified the oil-retention groove as a score
mark. When looking at the photo, I noticed
the thin line. Since that engine had not been
moved in nearly 15 years, I thought the
camera had captured oil buildup from the
cylinder onto the long-stationary piston. My
eyes are no longer good enough to see that
thin line without the camera’s magnification,
so I never noticed it before.
I learned something new here. The oil
groove serves to retain some lubrication in
the piston/cylinder contact area. I am happy
to correct this mistake and thank all those
who wrote in to point it out.
The last error involves the K&B .65
shown in “Engines 101.” I assumed that the
bushed engine followed the older, non-
Schnuerle-ported K&B design, but it does
not. The non-Schnuerle-ported engine should
have been the K&B .61 shown here in the
comparison photo that I originally thought
was too dirty to print.
Not all Schnuerle engines show the boost
transfer port on the outside. Some enlarge the
entire upper cylinder case to fit the boost
port, as shown in the photo comparing the
two K&B “60s.” These engines are more
difficult to spot.
Revisiting “Engines 101” has taken time
and space that could have been used for the
third article in this segment of the “From the
Ground Up” series, which will be published
next month. I thank all those who sent in
their suggestions and comments. Although
the important operation steps I presented in
“Engines 101” are valid, those comments
helped identify those areas that needed more
explanation. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
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