IT LOOKS DIFFERENT
from the .40 cu. in. engine
that came with your RTF
trainer. There is a “bump”
on top, and the glow plug
points from the head at an
angle. The carburetor
appears to be upside down,
and the throttle arm is on the wrong side.
The sound is also different from your
engine; it’s lower in pitch with a “crack” to
it. The owner calls it a “four-stroke” and
says he wouldn’t fly with anything else.
Despite its different appearance and
sound, the model four-stroke engine is
identical to its two-stroke cousin except for
the manner in which the fuel/air mixture
enters the combustion chamber and the
way in which the burnt gases escape the
chamber after combustion.
The four-stroke is fuel and air cooled, is
fuel lubricated, runs on alcohol-based fuel,
uses glow catalytic ignition, usually has
carburetor induction, and relies on fixed,
mechanical timing for operation—just like
a two-stroke engine.
Operationally, there is no difference in
user technique or equipment between a
two- and four-stroke, with the possible
exception of fuel and glow plugs. This
commonality makes it easy for the newer
model pilot to enjoy both types of power
plants without learning new techniques or
buying additional field equipment.
So then, why the different name and
appearance?
The induction/exhaust characteristics
that differentiate a four-stroke from a twostroke
do have some effect after all.
Although they do not change the way the
engine is used, they do change almost
everything else. The label “four-stroke” is
derived from these differences.
Unlike an engine that produces power
on every up and every down piston
stroke—two strokes—the manner in which
the gases enter and leave the combustion
chamber in a four-stroke requires that it
produce power only on every other up and
down piston stroke, which is four strokes.
How Those Parts Work Together: To
understand why this happens, let’s look
closer at four-stroke operation. As we do,
keep in mind that the engines being
discussed are normally aspirated sport
engines intended for sport, high-drag
models.
Similar to the engine in your
automobile, except for rotary-powered
cars, the model four-stroke uses intake and
exhaust valves driven by a camshaft. Most
four-strokes also use pushrods from the
camshaft to move the valves, but a few use
belt-driven overhead camshafts.
The induction/exhaust cycle is similar
to that in your automobile’s engine. In
theory, the cycle begins with the piston at
the top of its stroke, called Top Dead
Center (TDC). The intake valve opens as
the piston begins its first downward stroke
(stroke 1). This creates a low-pressure area
in the combustion chamber above the
piston.
A fuel/air mixture from the carburetor is
pushed into the intake manifold through the
open intake valve and into the combustion
chamber by the greater atmospheric
pressure trying to fill the internal lowpressure
area. After the fuel/air mix is in
place, the intake valve closes and the piston
starts its upward stroke (stroke 2).
Again, in theory, the piston compresses
the fuel/air mix until it reaches TDC. The
intense pressure, plus the catalytic effect
from the hot glow-plug element, ignites the
mixture. This controlled burning, called
combustion, forces the piston onto a
downward stroke (stroke 3), producing
power and turning the propeller that is
connected to the rotating crankshaft.
Once the piston reaches Bottom Dead
Center (BDC) again, the exhaust valve
opens and rotational momentum of all the
moving parts causes the piston onto
another upward stroke (stroke 4). As it
moves upward, the piston pushes the
burned gases out the exhaust port. The
exhaust valve closes and the cycle repeats.
Four piston strokes produce one power
stroke. The three other piston strokes are
required to get the cycle to repeat. As I
have discussed in previous articles in this
series, a model two-stroke produces one
power stroke with just one additional
stroke required for operation. In theory, the
two-stroke should produce twice the power
of an equivalent-size four-stroke. In
56 MODEL AVIATION
So Different,
Yet So Familiar by Frank Granelli
O.S. 120 valve and rocker arm assembly is
visible with valve cover removed. Thin
tubes in front house pushrods that
operate valves.
Camshaft determining engine’s timing is
located in round housing just under
pushrods. “Upside-down” carburetor is
connected to intake manifold that leads to
intake valve.
10sig2.QXD 7/23/04 11:46 am Page 56
practice, it is not that simple.
Two-stroke engines have their own
inherent inefficiencies that rob power. In
addition, what extra power two-strokes
have is often unusable by the modeler
because it occurs at high engine speeds
(rpm) that are difficult to reach in sport
models running on sport fuels.
In reality, even the actual four-stroke
cycle is more complex than I have
described. The operations described do not
occur in the simple order pictured. Many of
the operations overlap; the intake valve
begins to open before the piston first
reaches TDC. Why?
Since the piston slows its normally
rapid motion as it nears the top of each
stroke, it creates a slight area of negative
pressure just above itself. This happens
because the gases being pushed by the
piston are moving at the piston’s rapid
speed, and their inertia carries them away
from the piston, and through the exhaust
valve, as the piston suddenly slows.
The advanced intake-valve opening uses
this sudden negative pressure to begin
accelerating the fresh intake gases into the
chamber even before the piston begins
traveling on its downward, intake stroke.
This “advance timing” also allows the
intake valve enough time to open and the
fuel/air mix in the carburetor more time to
begin to move, or accelerate, through the
intake manifold and the open intake valve.
The intake gases have inertia and cannot
instantly move at top speed. At this point,
the exhaust gases from the previous cycle
are still quickly exiting the chamber. The
extra low pressure their exit creates also
helps overcome the intake gases’ inertia.
The intake valve remains open even
after the piston reaches BDC and starts
upward again, to allow the quickly moving
intake gases more time to “pack” as much
gas into the chamber as possible. Again,
this extra movement is caused by the gases’
inertia—this time, fast-moving inertia. The
intake valve only begins to close after the
piston has completed roughly 25% of its
upward travel and is fully closed before the
piston reaches the 50% point.
The exhaust valve actually opens before
the piston reaches BDC after the power
stroke. The burning gases still have extra
pressure at this point, which helps
accelerate the exhaust gases through the
opening, but not yet fully open, exhaust
valve.
Once the piston starts up on its “exhaust
stroke,” the spent gases are already on their
way out of the chamber and the exhaust
valve is fully opened. The exhaust valve
only begins to close after TDC to allow
extra time for the exhaust gases to escape.
As the exhaust gases escape the chamber,
they help create the initial low-pressure
area that begins to move the fresh intake
fuel/air mix.
As I mentioned, the intake valve also
starts to open as the piston nears the top of
the exhaust stroke. This means that for a
brief moment both valves are open at the
same time. This is called “valve overlap”
and is important for producing maximum
power. The amount of overlap and its
relationship to the actual combustion event
is called the engine’s “timing.”
Sport engines designed for good power
and good fuel economy usually have
“mild” timing and overlap, meaning that
although there is some overlap, it is not
excessive and will not waste fuel out of
open exhaust ports. High-performance
engines use more overlap to produce extra
power, but they lose fuel economy as some
unburnt fuel escapes through the exhaust
port or some exhaust gases may actually
enter the intake area.
Settings: Despite all the complex timing
and extra parts, the model pilot operates the
four-stroke exactly as if it were a twostroke.
The carburetor has the same low-
October 2004 57
Intake valve in open position as seen from inside head. Exhaust
valve next to it is closed. Intake and exhaust manifolds are
attached directly to head and lead to their respective valves.
Two- and four-stroke carburetors are nearly identical. Both have
external high-speed needle valves. Two-stroke Webra .61 (R) has
external idle needle; O.S. 120 hides idle-adjustment needle inside
throttle arm.
These parts are expensive—a good reason to never run fourstrokes
too lean. A second, safety nut prevented propeller and
other parts from being thrown from model. Always use it.
Right thrust washer shows detonation damage that can occur
when too-lean mixture causes engine to backfire. Left washer is
worn nearly smooth from 250 flights of normal four-stroke wear.
Photos by the author
10sig2.QXD 7/23/04 11:47 am Page 57
and high-speed needle valves that work the same way. Adjust the
high-speed needle valve until the engine runs 400-500 rpm less
than maximum. Adjust the slow-speed needle valve until the
engine maintains a constant 2,200-2,400 rpm idle.
If the idle slows, the idle mixture is too rich; there is too much
fuel and too little air. If the idle speeds up, the mixture is too lean;
there is too much air and too little fuel. If the engine quits when
the throttle is quickly opened, the idle mixture is too lean. If it
stumbles during acceleration, the idle mixture is too rich. A toolean
idle can also lead to detonation during throttle-up that could
cause propeller throwing.
Because model four-strokes do not have accelerator pumps, the
idle must be set slightly rich. The same is true of a two-stroke but
nowhere as critical. They are simple, easy adjustments to make,
just as they are on any two-stroke.
However, the four-stroke engine is intolerant of lean highspeed
mixtures. Although two-strokes may run with a slightly lean
mixture, four-strokes will not. A lean mixture usually causes the
engine to experience detonation; the piston actually stops its
upward travel because combustion occurs too soon.
This sudden reversal can cause the propeller to loosen or even
separate from the aircraft. Just one such detonation can be
expensive. Never lean a four-stroke to peak rpm, and always
operate at least 400 rpm less than peak—more if the weather is dry
and cool.
Even when run at normal mixture settings, four-strokes tend to
loosen propellers. Four-stroke acceleration is not always smooth.
There is much change in the amount of torque the engine delivers
during speed-up and slow-down. This happens because the ignition
and valve timing is mechanically fixed—not variable as in a car
engine.
Timing can only be optimized for one rpm range. Therefore,
the engine torque varies, as does its power output, as its speeds
change. These sudden changes in the amount of acceleration or
deceleration eventually cause the propeller to loosen.
It is a good idea to tighten the propeller before flying each day.
Eventually the engine’s thrust washer will wear out and need
replaced. Most four-strokes are supplied with two propeller nuts;
one tightens against the propeller and the other locks the first in
place. Never use just one propeller nut on a four-stroke. If you do,
detonation will cause the propeller to leave the aircraft while still
rotating. Anything or anyone it hits will come out on the losing
end.
Light the Fire: Besides detonation, a four-stroke-exclusive factor
is glow-plug choice. Since combustion occurs only once during
four piston movements, the glow plug must be designed to stay hot
during all that “spare” time. Regular glow plugs will not work.
The first model four-stroke used a special O.S. “F”-type glow
plug. It extends deep into the combustion chamber to capture as
much combustion heat as possible as quickly as possible. The
extra length also helps keep the element hot during the lengthy
noncombustion period. Several other manufacturers have begun
making this style of glow plug. Check the instructions that come
with your engine, but the F plug or equivalent is basically all that
is used in four-strokes.
If you are flying with the larger two-stroke engines—1.20 cu.
in. and bigger—try the F plug if you are experiencing problems
accelerating from idle to full speed. It works well in this
environment and could solve such transition difficulties. Precision
Aerobatics (Pattern) pilots use Fs in larger engines—two- or fourstrokes—
for extra reliability during transition.
Do not use the F plug in smaller two-strokes; it could cause
detonation or physically strike the piston.
Fill ’er up With? When four-stroke model engines came onto the
scene, much attention was paid to fuel selection. Many
manufacturers offered special fuels with reduced oil content
designed exclusively for four-strokes. Since oil is the poorestburning
ingredient in model fuel, less oil content made the early
four-strokes run more consistently. Today, low oil content is not
only unnecessary, but is probably a negative. Most engine
58 MODEL AVIATION
Original O.S. Max 60 four-stroke nestled in Sig Kadet Senior’s
nose (perfect airframe/engine match). Rocker arms, pushrods are
exposed. Producing roughly the same power as a .35 two-stroke,
the 60 was still able to use a larger propeller.
A 120-size four-stroke muffler looks tiny next to 120-160 twostroke
muffler. Both have pressure taps to ensure even fuel
delivery.
Adjusting valves takes a few minutes and should be done after
first two hours of operation. After that, frequent checks keep
engine operating at peak power.
10sig2.QXD 7/23/04 11:49 am Page 58
manufacturers recommend at least 16-18%
oil; high-performance engines demand 20%
or more.
The myth that four-strokes require lowoil-
content fuels started because early
modelers used regular two-stroke glow
plugs. Now that four-stroke glow plugs are
available, the oil’s heat-removing ability is
a benefit—not a problem.
Although four-stroke sport engines run
cooler than equivalent two-strokes, cylinder
pressures are much higher. The extra oil
helps protect parts such as the ring, cylinder
lining, and wrist pin that are exposed to this
higher pressure. As I have discussed, model
fuel cools the engine by lubricating it and
carrying away excess “top end” heat as
unburnt oil exits the exhaust.
Most model fuels use a mixture of
synthetic oil and castor oil. Except for
high-performance, supercharged fourstrokes
that require synthetic oil only,
approximately 5% castor oil is a good
amount for two- and four-strokes. The total
recommended oil content is the same as for
two-strokes: 18-20% minimum. This
provides a small error margin during
extreme operation.
Unlike in a two-stroke, there is no
refrigeration cooling of the four-stroke’s
lower crankcase since the fuel never gets
there in quantity. Many Pattern
competition pilots have learned that
providing extra cooling air to a fourstroke’s
lower crankcase area is beneficial.
It provides extra cooling, but then the
60 MODEL AVIATION
cooler air flows past the crankcase and into
the “upside-down”-mounted carburetor,
making the entire fuel/air mix denser for
extra power.
Make sure the lower crankcase receives
cooling air when you install any fourstroke.
Regardless of the power
advantages, having a cool lower end
prolongs bearing life.
What about nitromethane content?
Since four-strokes have just one power
stroke per two crankshaft revolutions,
nitromethane content less than 10% makes
it harder to keep the glow plug operating at
peak efficiency. In most sport four-strokes,
nitromethane contents higher than 25% can
result in extra detonation and thrust-washer
and spinner-backplate wear unless
everything is set perfectly. Even highperformance,
supercharged four-strokes
experience problems when nitromethane
content exceeds 35%.
For sport use, consider 15%
nitromethane content when flying at lower
than 5,000-foot density altitudes and in
temperatures lower than 95°. Consider 20%
nitromethane content if conditions exceed
these figures.
Sport four-strokes actually burn less
fuel than equivalent-size two-strokes. This
is partly because of their better combustion
efficiency and higher internal pressures,
but mostly because fuel is burned only on
every other piston stroke.
However, four-strokes do no get twice
the “mileage” of two-strokes. At best, sport
four-strokes enjoy 20-40% better fuel
economy. Since they use less fuel, it is
easier to feed them higher nitromethaneand
oil-content fuels that might cost
slightly more.
Propellers: Propeller choices for fourstrokes
may be slightly different than for
two-strokes. Both produce roughly the
same torque (twisting force) for a given
displacement engine size. Two-strokes still
develop more horsepower, but it is usually
at high rpm (exceeding 13,000) that most
sport fliers at club fields cannot readily use.
The noise is excessive, the propellers must
be small, and high-nitromethane-content
fuels must be used. Besides, turning so fast
prematurely wears out most sport engines.
Four-strokes have horsepower peaks in
the 9,000-11,000 rpm range. Sport fliers
find it easier to choose a propeller that
allows the engine to operate in this range.
Only sport fuels are required, and
everything is quieter and easier to set up at
these low rpm.
The four-stroke’s power curve makes it
possible for Sport Scale fliers to use
larger-diameter propellers and still reach
their engine’s peak ratings. Biggerdiameter
propellers are more efficient if
big obstructions such as scale cowls or
wide fuselages are located just to the
propeller’s rear. The more the propeller’s
swept area that is located outside the
obstruction, the less interference the
propeller receives from deflected airflow.
10sig2.QXD 7/23/04 11:49 am Page 60
Through the years, four-strokes earned a
reputation for having more torque and
therefore being able to turn larger-diameter
propellers with higher pitches. After
extensive research by modeling’s Engine
Gurus, we know that this is untrue and that
four-strokes have nearly the same peak
torque as two-strokes. Yet four-strokes
seem to have more torque because all that
they do have is fully available.
If two-strokes’ peak torque could be
reached at 8,000 rpm, they could use the
same larger-diameter propellers. But the
torque peak is higher in the rpm range, and
they can’t.
However, the rules for choosing a
propeller are the same for four-strokes as
they were from last month for two-strokes.
Pick the largest-diameter propeller, with
sufficient pitch to fly at the speed you want,
which allows the engine to turn
approximately 1,000 rpm higher than the
engine’s peak torque rpm. Make fuel and
glow-plug choices first—they affect an
engine’s top rpm ability—and then choose
the propeller.
Sound: A four-stroke’s exhaust note has a
lower pitch than a two-stroke’s, probably
because its noise-making power stroke
occurs on every other crankshaft
revolution. Many times the four-stroke is
also turning at a lower rpm and is therefore
not producing the high-pitched scream that
is so common with the two-strokes. This
lower-pitched noise may seem quieter, but
it is not.
Without a muffler, .45 cu. in. two- and
four-strokes make roughly the same amount
of noise: approximately 108 decibels (dB)
measured 9 feet from the engine. That is
loud. With factory mufflers, both engines
usually produce 100-102 dB, which is still
loud but more common and therefore
seldom intolerable to most clubs.
Four-stroke mufflers are smaller than
two-strokes’ since the four-stroke exhaust
outlet is smaller. Scale modelers like the
smaller muffler because its diminutive size
is less objectionable and easier to work into
their realistic airplanes.
Another commonality is that two- and
four-stroke engines usually require muffler
pressure to the fuel tank. Some highperformance
four-strokes are equipped with
fuel pumps or engine-driven fuelpressurization
systems that do not
necessitate muffler pressure, but most sport
four-strokes are not so equipped. Use
muffler pressure at all times on these
engines.
Maintenance: Two- and four-strokes need
regular attention to keep everything
working well, but four-strokes require a bit
more. The main difference is that the valveto-
pushrod clearance must be adjusted. You
must do this before first running the engine,
and then again after the first two hours of
run time. Check the clearances every 10
running hours for the next 50 hours or so; if
there is no change, it is usually safe to
extend inspection times to 50 hours.
As does a two-stroke, a four-stroke
“stores” a great deal of unburnt fuel inside
the engine after it is shut down. You must
run the engine dry of this fuel at the end of
each day. There are two techniques to
accomplish this.
Some engine experts favor keeping the
glow plug connected and going to full
throttle while the fuel line is disconnected,
allowing the engine to run dry. Others
prefer the same procedure but use a high
idle instead of full speed. This is safer and
quieter. If you use the idle method, try to
restart the engine after it first quits in case
residual fuel remains. But do not overdo it;
the engine has little or no internal
lubrication at this point since most of the
fuel is gone.
After-run oil is essential rust protection
for a four-stroke. Many good kinds are
available at hobby shops. Some experts
prefer Marvel Mystery Oil, automatic
transmission fluid, or a 50/50 mixture of
the two. Others like air-tool oil.
You must be careful; the petroleum
distillates in these products could damage
the fuel-pump diaphragms or carburetor Orings
in some engines. O.S. specifically
warns against using petroleum products in
some of its carburetors.
Pattern pilots fly more in one year than
most sport pilots fly in several years. Based
on their extensive engine use, most use
Mobil 1 Synthetic Engine Oil or equivalent
as their after-run oil. The synthetic oil has
no petroleum content, will not thicken with
time, and seems to prevent rust better than
most other choices, even though it contains
no specific rust inhibitors as far as is
known.
Whichever oil you choose, use a “glue
syringe” (available at most hobby shops) to
inject approximately 10 drops into the
crankcase breather fitting, and put the same
amount in the glow-plug hole. Rotate the
engine several times and replace the glow
plug. A few more rotations with the glow
plug in place couldn’t hurt.
You should be doing this with all of
your two-strokes as well, so this is not extra
four-stroke maintenance. The only
difference is that the oil is dropped into the
wide-open carburetor of a two-stroke
instead of into the breather fitting.
If YS made your four-stroke, it will not
have a breather fitting because the
crankcase is pressurized. In this case, drop
the oil into the glow-plug hole and the
carburetor. With the carburetor facing
upward, rotate the engine as described.
Never use petroleum distillate oil in these
high-performance “wonder” engines.
I had hoped to discuss fuel-tank styles and
location but ran out of room. I will cover
tank selection and placement next month,
along with several other engine tools and
accessories. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
62 MODEL AVIATION
10sig2.QXD 7/23/04 11:50 am Page 62
Edition: Model Aviation - 2004/10
Page Numbers: 56,57,58,60,62
Edition: Model Aviation - 2004/10
Page Numbers: 56,57,58,60,62
IT LOOKS DIFFERENT
from the .40 cu. in. engine
that came with your RTF
trainer. There is a “bump”
on top, and the glow plug
points from the head at an
angle. The carburetor
appears to be upside down,
and the throttle arm is on the wrong side.
The sound is also different from your
engine; it’s lower in pitch with a “crack” to
it. The owner calls it a “four-stroke” and
says he wouldn’t fly with anything else.
Despite its different appearance and
sound, the model four-stroke engine is
identical to its two-stroke cousin except for
the manner in which the fuel/air mixture
enters the combustion chamber and the
way in which the burnt gases escape the
chamber after combustion.
The four-stroke is fuel and air cooled, is
fuel lubricated, runs on alcohol-based fuel,
uses glow catalytic ignition, usually has
carburetor induction, and relies on fixed,
mechanical timing for operation—just like
a two-stroke engine.
Operationally, there is no difference in
user technique or equipment between a
two- and four-stroke, with the possible
exception of fuel and glow plugs. This
commonality makes it easy for the newer
model pilot to enjoy both types of power
plants without learning new techniques or
buying additional field equipment.
So then, why the different name and
appearance?
The induction/exhaust characteristics
that differentiate a four-stroke from a twostroke
do have some effect after all.
Although they do not change the way the
engine is used, they do change almost
everything else. The label “four-stroke” is
derived from these differences.
Unlike an engine that produces power
on every up and every down piston
stroke—two strokes—the manner in which
the gases enter and leave the combustion
chamber in a four-stroke requires that it
produce power only on every other up and
down piston stroke, which is four strokes.
How Those Parts Work Together: To
understand why this happens, let’s look
closer at four-stroke operation. As we do,
keep in mind that the engines being
discussed are normally aspirated sport
engines intended for sport, high-drag
models.
Similar to the engine in your
automobile, except for rotary-powered
cars, the model four-stroke uses intake and
exhaust valves driven by a camshaft. Most
four-strokes also use pushrods from the
camshaft to move the valves, but a few use
belt-driven overhead camshafts.
The induction/exhaust cycle is similar
to that in your automobile’s engine. In
theory, the cycle begins with the piston at
the top of its stroke, called Top Dead
Center (TDC). The intake valve opens as
the piston begins its first downward stroke
(stroke 1). This creates a low-pressure area
in the combustion chamber above the
piston.
A fuel/air mixture from the carburetor is
pushed into the intake manifold through the
open intake valve and into the combustion
chamber by the greater atmospheric
pressure trying to fill the internal lowpressure
area. After the fuel/air mix is in
place, the intake valve closes and the piston
starts its upward stroke (stroke 2).
Again, in theory, the piston compresses
the fuel/air mix until it reaches TDC. The
intense pressure, plus the catalytic effect
from the hot glow-plug element, ignites the
mixture. This controlled burning, called
combustion, forces the piston onto a
downward stroke (stroke 3), producing
power and turning the propeller that is
connected to the rotating crankshaft.
Once the piston reaches Bottom Dead
Center (BDC) again, the exhaust valve
opens and rotational momentum of all the
moving parts causes the piston onto
another upward stroke (stroke 4). As it
moves upward, the piston pushes the
burned gases out the exhaust port. The
exhaust valve closes and the cycle repeats.
Four piston strokes produce one power
stroke. The three other piston strokes are
required to get the cycle to repeat. As I
have discussed in previous articles in this
series, a model two-stroke produces one
power stroke with just one additional
stroke required for operation. In theory, the
two-stroke should produce twice the power
of an equivalent-size four-stroke. In
56 MODEL AVIATION
So Different,
Yet So Familiar by Frank Granelli
O.S. 120 valve and rocker arm assembly is
visible with valve cover removed. Thin
tubes in front house pushrods that
operate valves.
Camshaft determining engine’s timing is
located in round housing just under
pushrods. “Upside-down” carburetor is
connected to intake manifold that leads to
intake valve.
10sig2.QXD 7/23/04 11:46 am Page 56
practice, it is not that simple.
Two-stroke engines have their own
inherent inefficiencies that rob power. In
addition, what extra power two-strokes
have is often unusable by the modeler
because it occurs at high engine speeds
(rpm) that are difficult to reach in sport
models running on sport fuels.
In reality, even the actual four-stroke
cycle is more complex than I have
described. The operations described do not
occur in the simple order pictured. Many of
the operations overlap; the intake valve
begins to open before the piston first
reaches TDC. Why?
Since the piston slows its normally
rapid motion as it nears the top of each
stroke, it creates a slight area of negative
pressure just above itself. This happens
because the gases being pushed by the
piston are moving at the piston’s rapid
speed, and their inertia carries them away
from the piston, and through the exhaust
valve, as the piston suddenly slows.
The advanced intake-valve opening uses
this sudden negative pressure to begin
accelerating the fresh intake gases into the
chamber even before the piston begins
traveling on its downward, intake stroke.
This “advance timing” also allows the
intake valve enough time to open and the
fuel/air mix in the carburetor more time to
begin to move, or accelerate, through the
intake manifold and the open intake valve.
The intake gases have inertia and cannot
instantly move at top speed. At this point,
the exhaust gases from the previous cycle
are still quickly exiting the chamber. The
extra low pressure their exit creates also
helps overcome the intake gases’ inertia.
The intake valve remains open even
after the piston reaches BDC and starts
upward again, to allow the quickly moving
intake gases more time to “pack” as much
gas into the chamber as possible. Again,
this extra movement is caused by the gases’
inertia—this time, fast-moving inertia. The
intake valve only begins to close after the
piston has completed roughly 25% of its
upward travel and is fully closed before the
piston reaches the 50% point.
The exhaust valve actually opens before
the piston reaches BDC after the power
stroke. The burning gases still have extra
pressure at this point, which helps
accelerate the exhaust gases through the
opening, but not yet fully open, exhaust
valve.
Once the piston starts up on its “exhaust
stroke,” the spent gases are already on their
way out of the chamber and the exhaust
valve is fully opened. The exhaust valve
only begins to close after TDC to allow
extra time for the exhaust gases to escape.
As the exhaust gases escape the chamber,
they help create the initial low-pressure
area that begins to move the fresh intake
fuel/air mix.
As I mentioned, the intake valve also
starts to open as the piston nears the top of
the exhaust stroke. This means that for a
brief moment both valves are open at the
same time. This is called “valve overlap”
and is important for producing maximum
power. The amount of overlap and its
relationship to the actual combustion event
is called the engine’s “timing.”
Sport engines designed for good power
and good fuel economy usually have
“mild” timing and overlap, meaning that
although there is some overlap, it is not
excessive and will not waste fuel out of
open exhaust ports. High-performance
engines use more overlap to produce extra
power, but they lose fuel economy as some
unburnt fuel escapes through the exhaust
port or some exhaust gases may actually
enter the intake area.
Settings: Despite all the complex timing
and extra parts, the model pilot operates the
four-stroke exactly as if it were a twostroke.
The carburetor has the same low-
October 2004 57
Intake valve in open position as seen from inside head. Exhaust
valve next to it is closed. Intake and exhaust manifolds are
attached directly to head and lead to their respective valves.
Two- and four-stroke carburetors are nearly identical. Both have
external high-speed needle valves. Two-stroke Webra .61 (R) has
external idle needle; O.S. 120 hides idle-adjustment needle inside
throttle arm.
These parts are expensive—a good reason to never run fourstrokes
too lean. A second, safety nut prevented propeller and
other parts from being thrown from model. Always use it.
Right thrust washer shows detonation damage that can occur
when too-lean mixture causes engine to backfire. Left washer is
worn nearly smooth from 250 flights of normal four-stroke wear.
Photos by the author
10sig2.QXD 7/23/04 11:47 am Page 57
and high-speed needle valves that work the same way. Adjust the
high-speed needle valve until the engine runs 400-500 rpm less
than maximum. Adjust the slow-speed needle valve until the
engine maintains a constant 2,200-2,400 rpm idle.
If the idle slows, the idle mixture is too rich; there is too much
fuel and too little air. If the idle speeds up, the mixture is too lean;
there is too much air and too little fuel. If the engine quits when
the throttle is quickly opened, the idle mixture is too lean. If it
stumbles during acceleration, the idle mixture is too rich. A toolean
idle can also lead to detonation during throttle-up that could
cause propeller throwing.
Because model four-strokes do not have accelerator pumps, the
idle must be set slightly rich. The same is true of a two-stroke but
nowhere as critical. They are simple, easy adjustments to make,
just as they are on any two-stroke.
However, the four-stroke engine is intolerant of lean highspeed
mixtures. Although two-strokes may run with a slightly lean
mixture, four-strokes will not. A lean mixture usually causes the
engine to experience detonation; the piston actually stops its
upward travel because combustion occurs too soon.
This sudden reversal can cause the propeller to loosen or even
separate from the aircraft. Just one such detonation can be
expensive. Never lean a four-stroke to peak rpm, and always
operate at least 400 rpm less than peak—more if the weather is dry
and cool.
Even when run at normal mixture settings, four-strokes tend to
loosen propellers. Four-stroke acceleration is not always smooth.
There is much change in the amount of torque the engine delivers
during speed-up and slow-down. This happens because the ignition
and valve timing is mechanically fixed—not variable as in a car
engine.
Timing can only be optimized for one rpm range. Therefore,
the engine torque varies, as does its power output, as its speeds
change. These sudden changes in the amount of acceleration or
deceleration eventually cause the propeller to loosen.
It is a good idea to tighten the propeller before flying each day.
Eventually the engine’s thrust washer will wear out and need
replaced. Most four-strokes are supplied with two propeller nuts;
one tightens against the propeller and the other locks the first in
place. Never use just one propeller nut on a four-stroke. If you do,
detonation will cause the propeller to leave the aircraft while still
rotating. Anything or anyone it hits will come out on the losing
end.
Light the Fire: Besides detonation, a four-stroke-exclusive factor
is glow-plug choice. Since combustion occurs only once during
four piston movements, the glow plug must be designed to stay hot
during all that “spare” time. Regular glow plugs will not work.
The first model four-stroke used a special O.S. “F”-type glow
plug. It extends deep into the combustion chamber to capture as
much combustion heat as possible as quickly as possible. The
extra length also helps keep the element hot during the lengthy
noncombustion period. Several other manufacturers have begun
making this style of glow plug. Check the instructions that come
with your engine, but the F plug or equivalent is basically all that
is used in four-strokes.
If you are flying with the larger two-stroke engines—1.20 cu.
in. and bigger—try the F plug if you are experiencing problems
accelerating from idle to full speed. It works well in this
environment and could solve such transition difficulties. Precision
Aerobatics (Pattern) pilots use Fs in larger engines—two- or fourstrokes—
for extra reliability during transition.
Do not use the F plug in smaller two-strokes; it could cause
detonation or physically strike the piston.
Fill ’er up With? When four-stroke model engines came onto the
scene, much attention was paid to fuel selection. Many
manufacturers offered special fuels with reduced oil content
designed exclusively for four-strokes. Since oil is the poorestburning
ingredient in model fuel, less oil content made the early
four-strokes run more consistently. Today, low oil content is not
only unnecessary, but is probably a negative. Most engine
58 MODEL AVIATION
Original O.S. Max 60 four-stroke nestled in Sig Kadet Senior’s
nose (perfect airframe/engine match). Rocker arms, pushrods are
exposed. Producing roughly the same power as a .35 two-stroke,
the 60 was still able to use a larger propeller.
A 120-size four-stroke muffler looks tiny next to 120-160 twostroke
muffler. Both have pressure taps to ensure even fuel
delivery.
Adjusting valves takes a few minutes and should be done after
first two hours of operation. After that, frequent checks keep
engine operating at peak power.
10sig2.QXD 7/23/04 11:49 am Page 58
manufacturers recommend at least 16-18%
oil; high-performance engines demand 20%
or more.
The myth that four-strokes require lowoil-
content fuels started because early
modelers used regular two-stroke glow
plugs. Now that four-stroke glow plugs are
available, the oil’s heat-removing ability is
a benefit—not a problem.
Although four-stroke sport engines run
cooler than equivalent two-strokes, cylinder
pressures are much higher. The extra oil
helps protect parts such as the ring, cylinder
lining, and wrist pin that are exposed to this
higher pressure. As I have discussed, model
fuel cools the engine by lubricating it and
carrying away excess “top end” heat as
unburnt oil exits the exhaust.
Most model fuels use a mixture of
synthetic oil and castor oil. Except for
high-performance, supercharged fourstrokes
that require synthetic oil only,
approximately 5% castor oil is a good
amount for two- and four-strokes. The total
recommended oil content is the same as for
two-strokes: 18-20% minimum. This
provides a small error margin during
extreme operation.
Unlike in a two-stroke, there is no
refrigeration cooling of the four-stroke’s
lower crankcase since the fuel never gets
there in quantity. Many Pattern
competition pilots have learned that
providing extra cooling air to a fourstroke’s
lower crankcase area is beneficial.
It provides extra cooling, but then the
60 MODEL AVIATION
cooler air flows past the crankcase and into
the “upside-down”-mounted carburetor,
making the entire fuel/air mix denser for
extra power.
Make sure the lower crankcase receives
cooling air when you install any fourstroke.
Regardless of the power
advantages, having a cool lower end
prolongs bearing life.
What about nitromethane content?
Since four-strokes have just one power
stroke per two crankshaft revolutions,
nitromethane content less than 10% makes
it harder to keep the glow plug operating at
peak efficiency. In most sport four-strokes,
nitromethane contents higher than 25% can
result in extra detonation and thrust-washer
and spinner-backplate wear unless
everything is set perfectly. Even highperformance,
supercharged four-strokes
experience problems when nitromethane
content exceeds 35%.
For sport use, consider 15%
nitromethane content when flying at lower
than 5,000-foot density altitudes and in
temperatures lower than 95°. Consider 20%
nitromethane content if conditions exceed
these figures.
Sport four-strokes actually burn less
fuel than equivalent-size two-strokes. This
is partly because of their better combustion
efficiency and higher internal pressures,
but mostly because fuel is burned only on
every other piston stroke.
However, four-strokes do no get twice
the “mileage” of two-strokes. At best, sport
four-strokes enjoy 20-40% better fuel
economy. Since they use less fuel, it is
easier to feed them higher nitromethaneand
oil-content fuels that might cost
slightly more.
Propellers: Propeller choices for fourstrokes
may be slightly different than for
two-strokes. Both produce roughly the
same torque (twisting force) for a given
displacement engine size. Two-strokes still
develop more horsepower, but it is usually
at high rpm (exceeding 13,000) that most
sport fliers at club fields cannot readily use.
The noise is excessive, the propellers must
be small, and high-nitromethane-content
fuels must be used. Besides, turning so fast
prematurely wears out most sport engines.
Four-strokes have horsepower peaks in
the 9,000-11,000 rpm range. Sport fliers
find it easier to choose a propeller that
allows the engine to operate in this range.
Only sport fuels are required, and
everything is quieter and easier to set up at
these low rpm.
The four-stroke’s power curve makes it
possible for Sport Scale fliers to use
larger-diameter propellers and still reach
their engine’s peak ratings. Biggerdiameter
propellers are more efficient if
big obstructions such as scale cowls or
wide fuselages are located just to the
propeller’s rear. The more the propeller’s
swept area that is located outside the
obstruction, the less interference the
propeller receives from deflected airflow.
10sig2.QXD 7/23/04 11:49 am Page 60
Through the years, four-strokes earned a
reputation for having more torque and
therefore being able to turn larger-diameter
propellers with higher pitches. After
extensive research by modeling’s Engine
Gurus, we know that this is untrue and that
four-strokes have nearly the same peak
torque as two-strokes. Yet four-strokes
seem to have more torque because all that
they do have is fully available.
If two-strokes’ peak torque could be
reached at 8,000 rpm, they could use the
same larger-diameter propellers. But the
torque peak is higher in the rpm range, and
they can’t.
However, the rules for choosing a
propeller are the same for four-strokes as
they were from last month for two-strokes.
Pick the largest-diameter propeller, with
sufficient pitch to fly at the speed you want,
which allows the engine to turn
approximately 1,000 rpm higher than the
engine’s peak torque rpm. Make fuel and
glow-plug choices first—they affect an
engine’s top rpm ability—and then choose
the propeller.
Sound: A four-stroke’s exhaust note has a
lower pitch than a two-stroke’s, probably
because its noise-making power stroke
occurs on every other crankshaft
revolution. Many times the four-stroke is
also turning at a lower rpm and is therefore
not producing the high-pitched scream that
is so common with the two-strokes. This
lower-pitched noise may seem quieter, but
it is not.
Without a muffler, .45 cu. in. two- and
four-strokes make roughly the same amount
of noise: approximately 108 decibels (dB)
measured 9 feet from the engine. That is
loud. With factory mufflers, both engines
usually produce 100-102 dB, which is still
loud but more common and therefore
seldom intolerable to most clubs.
Four-stroke mufflers are smaller than
two-strokes’ since the four-stroke exhaust
outlet is smaller. Scale modelers like the
smaller muffler because its diminutive size
is less objectionable and easier to work into
their realistic airplanes.
Another commonality is that two- and
four-stroke engines usually require muffler
pressure to the fuel tank. Some highperformance
four-strokes are equipped with
fuel pumps or engine-driven fuelpressurization
systems that do not
necessitate muffler pressure, but most sport
four-strokes are not so equipped. Use
muffler pressure at all times on these
engines.
Maintenance: Two- and four-strokes need
regular attention to keep everything
working well, but four-strokes require a bit
more. The main difference is that the valveto-
pushrod clearance must be adjusted. You
must do this before first running the engine,
and then again after the first two hours of
run time. Check the clearances every 10
running hours for the next 50 hours or so; if
there is no change, it is usually safe to
extend inspection times to 50 hours.
As does a two-stroke, a four-stroke
“stores” a great deal of unburnt fuel inside
the engine after it is shut down. You must
run the engine dry of this fuel at the end of
each day. There are two techniques to
accomplish this.
Some engine experts favor keeping the
glow plug connected and going to full
throttle while the fuel line is disconnected,
allowing the engine to run dry. Others
prefer the same procedure but use a high
idle instead of full speed. This is safer and
quieter. If you use the idle method, try to
restart the engine after it first quits in case
residual fuel remains. But do not overdo it;
the engine has little or no internal
lubrication at this point since most of the
fuel is gone.
After-run oil is essential rust protection
for a four-stroke. Many good kinds are
available at hobby shops. Some experts
prefer Marvel Mystery Oil, automatic
transmission fluid, or a 50/50 mixture of
the two. Others like air-tool oil.
You must be careful; the petroleum
distillates in these products could damage
the fuel-pump diaphragms or carburetor Orings
in some engines. O.S. specifically
warns against using petroleum products in
some of its carburetors.
Pattern pilots fly more in one year than
most sport pilots fly in several years. Based
on their extensive engine use, most use
Mobil 1 Synthetic Engine Oil or equivalent
as their after-run oil. The synthetic oil has
no petroleum content, will not thicken with
time, and seems to prevent rust better than
most other choices, even though it contains
no specific rust inhibitors as far as is
known.
Whichever oil you choose, use a “glue
syringe” (available at most hobby shops) to
inject approximately 10 drops into the
crankcase breather fitting, and put the same
amount in the glow-plug hole. Rotate the
engine several times and replace the glow
plug. A few more rotations with the glow
plug in place couldn’t hurt.
You should be doing this with all of
your two-strokes as well, so this is not extra
four-stroke maintenance. The only
difference is that the oil is dropped into the
wide-open carburetor of a two-stroke
instead of into the breather fitting.
If YS made your four-stroke, it will not
have a breather fitting because the
crankcase is pressurized. In this case, drop
the oil into the glow-plug hole and the
carburetor. With the carburetor facing
upward, rotate the engine as described.
Never use petroleum distillate oil in these
high-performance “wonder” engines.
I had hoped to discuss fuel-tank styles and
location but ran out of room. I will cover
tank selection and placement next month,
along with several other engine tools and
accessories. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
62 MODEL AVIATION
10sig2.QXD 7/23/04 11:50 am Page 62
Edition: Model Aviation - 2004/10
Page Numbers: 56,57,58,60,62
IT LOOKS DIFFERENT
from the .40 cu. in. engine
that came with your RTF
trainer. There is a “bump”
on top, and the glow plug
points from the head at an
angle. The carburetor
appears to be upside down,
and the throttle arm is on the wrong side.
The sound is also different from your
engine; it’s lower in pitch with a “crack” to
it. The owner calls it a “four-stroke” and
says he wouldn’t fly with anything else.
Despite its different appearance and
sound, the model four-stroke engine is
identical to its two-stroke cousin except for
the manner in which the fuel/air mixture
enters the combustion chamber and the
way in which the burnt gases escape the
chamber after combustion.
The four-stroke is fuel and air cooled, is
fuel lubricated, runs on alcohol-based fuel,
uses glow catalytic ignition, usually has
carburetor induction, and relies on fixed,
mechanical timing for operation—just like
a two-stroke engine.
Operationally, there is no difference in
user technique or equipment between a
two- and four-stroke, with the possible
exception of fuel and glow plugs. This
commonality makes it easy for the newer
model pilot to enjoy both types of power
plants without learning new techniques or
buying additional field equipment.
So then, why the different name and
appearance?
The induction/exhaust characteristics
that differentiate a four-stroke from a twostroke
do have some effect after all.
Although they do not change the way the
engine is used, they do change almost
everything else. The label “four-stroke” is
derived from these differences.
Unlike an engine that produces power
on every up and every down piston
stroke—two strokes—the manner in which
the gases enter and leave the combustion
chamber in a four-stroke requires that it
produce power only on every other up and
down piston stroke, which is four strokes.
How Those Parts Work Together: To
understand why this happens, let’s look
closer at four-stroke operation. As we do,
keep in mind that the engines being
discussed are normally aspirated sport
engines intended for sport, high-drag
models.
Similar to the engine in your
automobile, except for rotary-powered
cars, the model four-stroke uses intake and
exhaust valves driven by a camshaft. Most
four-strokes also use pushrods from the
camshaft to move the valves, but a few use
belt-driven overhead camshafts.
The induction/exhaust cycle is similar
to that in your automobile’s engine. In
theory, the cycle begins with the piston at
the top of its stroke, called Top Dead
Center (TDC). The intake valve opens as
the piston begins its first downward stroke
(stroke 1). This creates a low-pressure area
in the combustion chamber above the
piston.
A fuel/air mixture from the carburetor is
pushed into the intake manifold through the
open intake valve and into the combustion
chamber by the greater atmospheric
pressure trying to fill the internal lowpressure
area. After the fuel/air mix is in
place, the intake valve closes and the piston
starts its upward stroke (stroke 2).
Again, in theory, the piston compresses
the fuel/air mix until it reaches TDC. The
intense pressure, plus the catalytic effect
from the hot glow-plug element, ignites the
mixture. This controlled burning, called
combustion, forces the piston onto a
downward stroke (stroke 3), producing
power and turning the propeller that is
connected to the rotating crankshaft.
Once the piston reaches Bottom Dead
Center (BDC) again, the exhaust valve
opens and rotational momentum of all the
moving parts causes the piston onto
another upward stroke (stroke 4). As it
moves upward, the piston pushes the
burned gases out the exhaust port. The
exhaust valve closes and the cycle repeats.
Four piston strokes produce one power
stroke. The three other piston strokes are
required to get the cycle to repeat. As I
have discussed in previous articles in this
series, a model two-stroke produces one
power stroke with just one additional
stroke required for operation. In theory, the
two-stroke should produce twice the power
of an equivalent-size four-stroke. In
56 MODEL AVIATION
So Different,
Yet So Familiar by Frank Granelli
O.S. 120 valve and rocker arm assembly is
visible with valve cover removed. Thin
tubes in front house pushrods that
operate valves.
Camshaft determining engine’s timing is
located in round housing just under
pushrods. “Upside-down” carburetor is
connected to intake manifold that leads to
intake valve.
10sig2.QXD 7/23/04 11:46 am Page 56
practice, it is not that simple.
Two-stroke engines have their own
inherent inefficiencies that rob power. In
addition, what extra power two-strokes
have is often unusable by the modeler
because it occurs at high engine speeds
(rpm) that are difficult to reach in sport
models running on sport fuels.
In reality, even the actual four-stroke
cycle is more complex than I have
described. The operations described do not
occur in the simple order pictured. Many of
the operations overlap; the intake valve
begins to open before the piston first
reaches TDC. Why?
Since the piston slows its normally
rapid motion as it nears the top of each
stroke, it creates a slight area of negative
pressure just above itself. This happens
because the gases being pushed by the
piston are moving at the piston’s rapid
speed, and their inertia carries them away
from the piston, and through the exhaust
valve, as the piston suddenly slows.
The advanced intake-valve opening uses
this sudden negative pressure to begin
accelerating the fresh intake gases into the
chamber even before the piston begins
traveling on its downward, intake stroke.
This “advance timing” also allows the
intake valve enough time to open and the
fuel/air mix in the carburetor more time to
begin to move, or accelerate, through the
intake manifold and the open intake valve.
The intake gases have inertia and cannot
instantly move at top speed. At this point,
the exhaust gases from the previous cycle
are still quickly exiting the chamber. The
extra low pressure their exit creates also
helps overcome the intake gases’ inertia.
The intake valve remains open even
after the piston reaches BDC and starts
upward again, to allow the quickly moving
intake gases more time to “pack” as much
gas into the chamber as possible. Again,
this extra movement is caused by the gases’
inertia—this time, fast-moving inertia. The
intake valve only begins to close after the
piston has completed roughly 25% of its
upward travel and is fully closed before the
piston reaches the 50% point.
The exhaust valve actually opens before
the piston reaches BDC after the power
stroke. The burning gases still have extra
pressure at this point, which helps
accelerate the exhaust gases through the
opening, but not yet fully open, exhaust
valve.
Once the piston starts up on its “exhaust
stroke,” the spent gases are already on their
way out of the chamber and the exhaust
valve is fully opened. The exhaust valve
only begins to close after TDC to allow
extra time for the exhaust gases to escape.
As the exhaust gases escape the chamber,
they help create the initial low-pressure
area that begins to move the fresh intake
fuel/air mix.
As I mentioned, the intake valve also
starts to open as the piston nears the top of
the exhaust stroke. This means that for a
brief moment both valves are open at the
same time. This is called “valve overlap”
and is important for producing maximum
power. The amount of overlap and its
relationship to the actual combustion event
is called the engine’s “timing.”
Sport engines designed for good power
and good fuel economy usually have
“mild” timing and overlap, meaning that
although there is some overlap, it is not
excessive and will not waste fuel out of
open exhaust ports. High-performance
engines use more overlap to produce extra
power, but they lose fuel economy as some
unburnt fuel escapes through the exhaust
port or some exhaust gases may actually
enter the intake area.
Settings: Despite all the complex timing
and extra parts, the model pilot operates the
four-stroke exactly as if it were a twostroke.
The carburetor has the same low-
October 2004 57
Intake valve in open position as seen from inside head. Exhaust
valve next to it is closed. Intake and exhaust manifolds are
attached directly to head and lead to their respective valves.
Two- and four-stroke carburetors are nearly identical. Both have
external high-speed needle valves. Two-stroke Webra .61 (R) has
external idle needle; O.S. 120 hides idle-adjustment needle inside
throttle arm.
These parts are expensive—a good reason to never run fourstrokes
too lean. A second, safety nut prevented propeller and
other parts from being thrown from model. Always use it.
Right thrust washer shows detonation damage that can occur
when too-lean mixture causes engine to backfire. Left washer is
worn nearly smooth from 250 flights of normal four-stroke wear.
Photos by the author
10sig2.QXD 7/23/04 11:47 am Page 57
and high-speed needle valves that work the same way. Adjust the
high-speed needle valve until the engine runs 400-500 rpm less
than maximum. Adjust the slow-speed needle valve until the
engine maintains a constant 2,200-2,400 rpm idle.
If the idle slows, the idle mixture is too rich; there is too much
fuel and too little air. If the idle speeds up, the mixture is too lean;
there is too much air and too little fuel. If the engine quits when
the throttle is quickly opened, the idle mixture is too lean. If it
stumbles during acceleration, the idle mixture is too rich. A toolean
idle can also lead to detonation during throttle-up that could
cause propeller throwing.
Because model four-strokes do not have accelerator pumps, the
idle must be set slightly rich. The same is true of a two-stroke but
nowhere as critical. They are simple, easy adjustments to make,
just as they are on any two-stroke.
However, the four-stroke engine is intolerant of lean highspeed
mixtures. Although two-strokes may run with a slightly lean
mixture, four-strokes will not. A lean mixture usually causes the
engine to experience detonation; the piston actually stops its
upward travel because combustion occurs too soon.
This sudden reversal can cause the propeller to loosen or even
separate from the aircraft. Just one such detonation can be
expensive. Never lean a four-stroke to peak rpm, and always
operate at least 400 rpm less than peak—more if the weather is dry
and cool.
Even when run at normal mixture settings, four-strokes tend to
loosen propellers. Four-stroke acceleration is not always smooth.
There is much change in the amount of torque the engine delivers
during speed-up and slow-down. This happens because the ignition
and valve timing is mechanically fixed—not variable as in a car
engine.
Timing can only be optimized for one rpm range. Therefore,
the engine torque varies, as does its power output, as its speeds
change. These sudden changes in the amount of acceleration or
deceleration eventually cause the propeller to loosen.
It is a good idea to tighten the propeller before flying each day.
Eventually the engine’s thrust washer will wear out and need
replaced. Most four-strokes are supplied with two propeller nuts;
one tightens against the propeller and the other locks the first in
place. Never use just one propeller nut on a four-stroke. If you do,
detonation will cause the propeller to leave the aircraft while still
rotating. Anything or anyone it hits will come out on the losing
end.
Light the Fire: Besides detonation, a four-stroke-exclusive factor
is glow-plug choice. Since combustion occurs only once during
four piston movements, the glow plug must be designed to stay hot
during all that “spare” time. Regular glow plugs will not work.
The first model four-stroke used a special O.S. “F”-type glow
plug. It extends deep into the combustion chamber to capture as
much combustion heat as possible as quickly as possible. The
extra length also helps keep the element hot during the lengthy
noncombustion period. Several other manufacturers have begun
making this style of glow plug. Check the instructions that come
with your engine, but the F plug or equivalent is basically all that
is used in four-strokes.
If you are flying with the larger two-stroke engines—1.20 cu.
in. and bigger—try the F plug if you are experiencing problems
accelerating from idle to full speed. It works well in this
environment and could solve such transition difficulties. Precision
Aerobatics (Pattern) pilots use Fs in larger engines—two- or fourstrokes—
for extra reliability during transition.
Do not use the F plug in smaller two-strokes; it could cause
detonation or physically strike the piston.
Fill ’er up With? When four-stroke model engines came onto the
scene, much attention was paid to fuel selection. Many
manufacturers offered special fuels with reduced oil content
designed exclusively for four-strokes. Since oil is the poorestburning
ingredient in model fuel, less oil content made the early
four-strokes run more consistently. Today, low oil content is not
only unnecessary, but is probably a negative. Most engine
58 MODEL AVIATION
Original O.S. Max 60 four-stroke nestled in Sig Kadet Senior’s
nose (perfect airframe/engine match). Rocker arms, pushrods are
exposed. Producing roughly the same power as a .35 two-stroke,
the 60 was still able to use a larger propeller.
A 120-size four-stroke muffler looks tiny next to 120-160 twostroke
muffler. Both have pressure taps to ensure even fuel
delivery.
Adjusting valves takes a few minutes and should be done after
first two hours of operation. After that, frequent checks keep
engine operating at peak power.
10sig2.QXD 7/23/04 11:49 am Page 58
manufacturers recommend at least 16-18%
oil; high-performance engines demand 20%
or more.
The myth that four-strokes require lowoil-
content fuels started because early
modelers used regular two-stroke glow
plugs. Now that four-stroke glow plugs are
available, the oil’s heat-removing ability is
a benefit—not a problem.
Although four-stroke sport engines run
cooler than equivalent two-strokes, cylinder
pressures are much higher. The extra oil
helps protect parts such as the ring, cylinder
lining, and wrist pin that are exposed to this
higher pressure. As I have discussed, model
fuel cools the engine by lubricating it and
carrying away excess “top end” heat as
unburnt oil exits the exhaust.
Most model fuels use a mixture of
synthetic oil and castor oil. Except for
high-performance, supercharged fourstrokes
that require synthetic oil only,
approximately 5% castor oil is a good
amount for two- and four-strokes. The total
recommended oil content is the same as for
two-strokes: 18-20% minimum. This
provides a small error margin during
extreme operation.
Unlike in a two-stroke, there is no
refrigeration cooling of the four-stroke’s
lower crankcase since the fuel never gets
there in quantity. Many Pattern
competition pilots have learned that
providing extra cooling air to a fourstroke’s
lower crankcase area is beneficial.
It provides extra cooling, but then the
60 MODEL AVIATION
cooler air flows past the crankcase and into
the “upside-down”-mounted carburetor,
making the entire fuel/air mix denser for
extra power.
Make sure the lower crankcase receives
cooling air when you install any fourstroke.
Regardless of the power
advantages, having a cool lower end
prolongs bearing life.
What about nitromethane content?
Since four-strokes have just one power
stroke per two crankshaft revolutions,
nitromethane content less than 10% makes
it harder to keep the glow plug operating at
peak efficiency. In most sport four-strokes,
nitromethane contents higher than 25% can
result in extra detonation and thrust-washer
and spinner-backplate wear unless
everything is set perfectly. Even highperformance,
supercharged four-strokes
experience problems when nitromethane
content exceeds 35%.
For sport use, consider 15%
nitromethane content when flying at lower
than 5,000-foot density altitudes and in
temperatures lower than 95°. Consider 20%
nitromethane content if conditions exceed
these figures.
Sport four-strokes actually burn less
fuel than equivalent-size two-strokes. This
is partly because of their better combustion
efficiency and higher internal pressures,
but mostly because fuel is burned only on
every other piston stroke.
However, four-strokes do no get twice
the “mileage” of two-strokes. At best, sport
four-strokes enjoy 20-40% better fuel
economy. Since they use less fuel, it is
easier to feed them higher nitromethaneand
oil-content fuels that might cost
slightly more.
Propellers: Propeller choices for fourstrokes
may be slightly different than for
two-strokes. Both produce roughly the
same torque (twisting force) for a given
displacement engine size. Two-strokes still
develop more horsepower, but it is usually
at high rpm (exceeding 13,000) that most
sport fliers at club fields cannot readily use.
The noise is excessive, the propellers must
be small, and high-nitromethane-content
fuels must be used. Besides, turning so fast
prematurely wears out most sport engines.
Four-strokes have horsepower peaks in
the 9,000-11,000 rpm range. Sport fliers
find it easier to choose a propeller that
allows the engine to operate in this range.
Only sport fuels are required, and
everything is quieter and easier to set up at
these low rpm.
The four-stroke’s power curve makes it
possible for Sport Scale fliers to use
larger-diameter propellers and still reach
their engine’s peak ratings. Biggerdiameter
propellers are more efficient if
big obstructions such as scale cowls or
wide fuselages are located just to the
propeller’s rear. The more the propeller’s
swept area that is located outside the
obstruction, the less interference the
propeller receives from deflected airflow.
10sig2.QXD 7/23/04 11:49 am Page 60
Through the years, four-strokes earned a
reputation for having more torque and
therefore being able to turn larger-diameter
propellers with higher pitches. After
extensive research by modeling’s Engine
Gurus, we know that this is untrue and that
four-strokes have nearly the same peak
torque as two-strokes. Yet four-strokes
seem to have more torque because all that
they do have is fully available.
If two-strokes’ peak torque could be
reached at 8,000 rpm, they could use the
same larger-diameter propellers. But the
torque peak is higher in the rpm range, and
they can’t.
However, the rules for choosing a
propeller are the same for four-strokes as
they were from last month for two-strokes.
Pick the largest-diameter propeller, with
sufficient pitch to fly at the speed you want,
which allows the engine to turn
approximately 1,000 rpm higher than the
engine’s peak torque rpm. Make fuel and
glow-plug choices first—they affect an
engine’s top rpm ability—and then choose
the propeller.
Sound: A four-stroke’s exhaust note has a
lower pitch than a two-stroke’s, probably
because its noise-making power stroke
occurs on every other crankshaft
revolution. Many times the four-stroke is
also turning at a lower rpm and is therefore
not producing the high-pitched scream that
is so common with the two-strokes. This
lower-pitched noise may seem quieter, but
it is not.
Without a muffler, .45 cu. in. two- and
four-strokes make roughly the same amount
of noise: approximately 108 decibels (dB)
measured 9 feet from the engine. That is
loud. With factory mufflers, both engines
usually produce 100-102 dB, which is still
loud but more common and therefore
seldom intolerable to most clubs.
Four-stroke mufflers are smaller than
two-strokes’ since the four-stroke exhaust
outlet is smaller. Scale modelers like the
smaller muffler because its diminutive size
is less objectionable and easier to work into
their realistic airplanes.
Another commonality is that two- and
four-stroke engines usually require muffler
pressure to the fuel tank. Some highperformance
four-strokes are equipped with
fuel pumps or engine-driven fuelpressurization
systems that do not
necessitate muffler pressure, but most sport
four-strokes are not so equipped. Use
muffler pressure at all times on these
engines.
Maintenance: Two- and four-strokes need
regular attention to keep everything
working well, but four-strokes require a bit
more. The main difference is that the valveto-
pushrod clearance must be adjusted. You
must do this before first running the engine,
and then again after the first two hours of
run time. Check the clearances every 10
running hours for the next 50 hours or so; if
there is no change, it is usually safe to
extend inspection times to 50 hours.
As does a two-stroke, a four-stroke
“stores” a great deal of unburnt fuel inside
the engine after it is shut down. You must
run the engine dry of this fuel at the end of
each day. There are two techniques to
accomplish this.
Some engine experts favor keeping the
glow plug connected and going to full
throttle while the fuel line is disconnected,
allowing the engine to run dry. Others
prefer the same procedure but use a high
idle instead of full speed. This is safer and
quieter. If you use the idle method, try to
restart the engine after it first quits in case
residual fuel remains. But do not overdo it;
the engine has little or no internal
lubrication at this point since most of the
fuel is gone.
After-run oil is essential rust protection
for a four-stroke. Many good kinds are
available at hobby shops. Some experts
prefer Marvel Mystery Oil, automatic
transmission fluid, or a 50/50 mixture of
the two. Others like air-tool oil.
You must be careful; the petroleum
distillates in these products could damage
the fuel-pump diaphragms or carburetor Orings
in some engines. O.S. specifically
warns against using petroleum products in
some of its carburetors.
Pattern pilots fly more in one year than
most sport pilots fly in several years. Based
on their extensive engine use, most use
Mobil 1 Synthetic Engine Oil or equivalent
as their after-run oil. The synthetic oil has
no petroleum content, will not thicken with
time, and seems to prevent rust better than
most other choices, even though it contains
no specific rust inhibitors as far as is
known.
Whichever oil you choose, use a “glue
syringe” (available at most hobby shops) to
inject approximately 10 drops into the
crankcase breather fitting, and put the same
amount in the glow-plug hole. Rotate the
engine several times and replace the glow
plug. A few more rotations with the glow
plug in place couldn’t hurt.
You should be doing this with all of
your two-strokes as well, so this is not extra
four-stroke maintenance. The only
difference is that the oil is dropped into the
wide-open carburetor of a two-stroke
instead of into the breather fitting.
If YS made your four-stroke, it will not
have a breather fitting because the
crankcase is pressurized. In this case, drop
the oil into the glow-plug hole and the
carburetor. With the carburetor facing
upward, rotate the engine as described.
Never use petroleum distillate oil in these
high-performance “wonder” engines.
I had hoped to discuss fuel-tank styles and
location but ran out of room. I will cover
tank selection and placement next month,
along with several other engine tools and
accessories. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
62 MODEL AVIATION
10sig2.QXD 7/23/04 11:50 am Page 62
Edition: Model Aviation - 2004/10
Page Numbers: 56,57,58,60,62
IT LOOKS DIFFERENT
from the .40 cu. in. engine
that came with your RTF
trainer. There is a “bump”
on top, and the glow plug
points from the head at an
angle. The carburetor
appears to be upside down,
and the throttle arm is on the wrong side.
The sound is also different from your
engine; it’s lower in pitch with a “crack” to
it. The owner calls it a “four-stroke” and
says he wouldn’t fly with anything else.
Despite its different appearance and
sound, the model four-stroke engine is
identical to its two-stroke cousin except for
the manner in which the fuel/air mixture
enters the combustion chamber and the
way in which the burnt gases escape the
chamber after combustion.
The four-stroke is fuel and air cooled, is
fuel lubricated, runs on alcohol-based fuel,
uses glow catalytic ignition, usually has
carburetor induction, and relies on fixed,
mechanical timing for operation—just like
a two-stroke engine.
Operationally, there is no difference in
user technique or equipment between a
two- and four-stroke, with the possible
exception of fuel and glow plugs. This
commonality makes it easy for the newer
model pilot to enjoy both types of power
plants without learning new techniques or
buying additional field equipment.
So then, why the different name and
appearance?
The induction/exhaust characteristics
that differentiate a four-stroke from a twostroke
do have some effect after all.
Although they do not change the way the
engine is used, they do change almost
everything else. The label “four-stroke” is
derived from these differences.
Unlike an engine that produces power
on every up and every down piston
stroke—two strokes—the manner in which
the gases enter and leave the combustion
chamber in a four-stroke requires that it
produce power only on every other up and
down piston stroke, which is four strokes.
How Those Parts Work Together: To
understand why this happens, let’s look
closer at four-stroke operation. As we do,
keep in mind that the engines being
discussed are normally aspirated sport
engines intended for sport, high-drag
models.
Similar to the engine in your
automobile, except for rotary-powered
cars, the model four-stroke uses intake and
exhaust valves driven by a camshaft. Most
four-strokes also use pushrods from the
camshaft to move the valves, but a few use
belt-driven overhead camshafts.
The induction/exhaust cycle is similar
to that in your automobile’s engine. In
theory, the cycle begins with the piston at
the top of its stroke, called Top Dead
Center (TDC). The intake valve opens as
the piston begins its first downward stroke
(stroke 1). This creates a low-pressure area
in the combustion chamber above the
piston.
A fuel/air mixture from the carburetor is
pushed into the intake manifold through the
open intake valve and into the combustion
chamber by the greater atmospheric
pressure trying to fill the internal lowpressure
area. After the fuel/air mix is in
place, the intake valve closes and the piston
starts its upward stroke (stroke 2).
Again, in theory, the piston compresses
the fuel/air mix until it reaches TDC. The
intense pressure, plus the catalytic effect
from the hot glow-plug element, ignites the
mixture. This controlled burning, called
combustion, forces the piston onto a
downward stroke (stroke 3), producing
power and turning the propeller that is
connected to the rotating crankshaft.
Once the piston reaches Bottom Dead
Center (BDC) again, the exhaust valve
opens and rotational momentum of all the
moving parts causes the piston onto
another upward stroke (stroke 4). As it
moves upward, the piston pushes the
burned gases out the exhaust port. The
exhaust valve closes and the cycle repeats.
Four piston strokes produce one power
stroke. The three other piston strokes are
required to get the cycle to repeat. As I
have discussed in previous articles in this
series, a model two-stroke produces one
power stroke with just one additional
stroke required for operation. In theory, the
two-stroke should produce twice the power
of an equivalent-size four-stroke. In
56 MODEL AVIATION
So Different,
Yet So Familiar by Frank Granelli
O.S. 120 valve and rocker arm assembly is
visible with valve cover removed. Thin
tubes in front house pushrods that
operate valves.
Camshaft determining engine’s timing is
located in round housing just under
pushrods. “Upside-down” carburetor is
connected to intake manifold that leads to
intake valve.
10sig2.QXD 7/23/04 11:46 am Page 56
practice, it is not that simple.
Two-stroke engines have their own
inherent inefficiencies that rob power. In
addition, what extra power two-strokes
have is often unusable by the modeler
because it occurs at high engine speeds
(rpm) that are difficult to reach in sport
models running on sport fuels.
In reality, even the actual four-stroke
cycle is more complex than I have
described. The operations described do not
occur in the simple order pictured. Many of
the operations overlap; the intake valve
begins to open before the piston first
reaches TDC. Why?
Since the piston slows its normally
rapid motion as it nears the top of each
stroke, it creates a slight area of negative
pressure just above itself. This happens
because the gases being pushed by the
piston are moving at the piston’s rapid
speed, and their inertia carries them away
from the piston, and through the exhaust
valve, as the piston suddenly slows.
The advanced intake-valve opening uses
this sudden negative pressure to begin
accelerating the fresh intake gases into the
chamber even before the piston begins
traveling on its downward, intake stroke.
This “advance timing” also allows the
intake valve enough time to open and the
fuel/air mix in the carburetor more time to
begin to move, or accelerate, through the
intake manifold and the open intake valve.
The intake gases have inertia and cannot
instantly move at top speed. At this point,
the exhaust gases from the previous cycle
are still quickly exiting the chamber. The
extra low pressure their exit creates also
helps overcome the intake gases’ inertia.
The intake valve remains open even
after the piston reaches BDC and starts
upward again, to allow the quickly moving
intake gases more time to “pack” as much
gas into the chamber as possible. Again,
this extra movement is caused by the gases’
inertia—this time, fast-moving inertia. The
intake valve only begins to close after the
piston has completed roughly 25% of its
upward travel and is fully closed before the
piston reaches the 50% point.
The exhaust valve actually opens before
the piston reaches BDC after the power
stroke. The burning gases still have extra
pressure at this point, which helps
accelerate the exhaust gases through the
opening, but not yet fully open, exhaust
valve.
Once the piston starts up on its “exhaust
stroke,” the spent gases are already on their
way out of the chamber and the exhaust
valve is fully opened. The exhaust valve
only begins to close after TDC to allow
extra time for the exhaust gases to escape.
As the exhaust gases escape the chamber,
they help create the initial low-pressure
area that begins to move the fresh intake
fuel/air mix.
As I mentioned, the intake valve also
starts to open as the piston nears the top of
the exhaust stroke. This means that for a
brief moment both valves are open at the
same time. This is called “valve overlap”
and is important for producing maximum
power. The amount of overlap and its
relationship to the actual combustion event
is called the engine’s “timing.”
Sport engines designed for good power
and good fuel economy usually have
“mild” timing and overlap, meaning that
although there is some overlap, it is not
excessive and will not waste fuel out of
open exhaust ports. High-performance
engines use more overlap to produce extra
power, but they lose fuel economy as some
unburnt fuel escapes through the exhaust
port or some exhaust gases may actually
enter the intake area.
Settings: Despite all the complex timing
and extra parts, the model pilot operates the
four-stroke exactly as if it were a twostroke.
The carburetor has the same low-
October 2004 57
Intake valve in open position as seen from inside head. Exhaust
valve next to it is closed. Intake and exhaust manifolds are
attached directly to head and lead to their respective valves.
Two- and four-stroke carburetors are nearly identical. Both have
external high-speed needle valves. Two-stroke Webra .61 (R) has
external idle needle; O.S. 120 hides idle-adjustment needle inside
throttle arm.
These parts are expensive—a good reason to never run fourstrokes
too lean. A second, safety nut prevented propeller and
other parts from being thrown from model. Always use it.
Right thrust washer shows detonation damage that can occur
when too-lean mixture causes engine to backfire. Left washer is
worn nearly smooth from 250 flights of normal four-stroke wear.
Photos by the author
10sig2.QXD 7/23/04 11:47 am Page 57
and high-speed needle valves that work the same way. Adjust the
high-speed needle valve until the engine runs 400-500 rpm less
than maximum. Adjust the slow-speed needle valve until the
engine maintains a constant 2,200-2,400 rpm idle.
If the idle slows, the idle mixture is too rich; there is too much
fuel and too little air. If the idle speeds up, the mixture is too lean;
there is too much air and too little fuel. If the engine quits when
the throttle is quickly opened, the idle mixture is too lean. If it
stumbles during acceleration, the idle mixture is too rich. A toolean
idle can also lead to detonation during throttle-up that could
cause propeller throwing.
Because model four-strokes do not have accelerator pumps, the
idle must be set slightly rich. The same is true of a two-stroke but
nowhere as critical. They are simple, easy adjustments to make,
just as they are on any two-stroke.
However, the four-stroke engine is intolerant of lean highspeed
mixtures. Although two-strokes may run with a slightly lean
mixture, four-strokes will not. A lean mixture usually causes the
engine to experience detonation; the piston actually stops its
upward travel because combustion occurs too soon.
This sudden reversal can cause the propeller to loosen or even
separate from the aircraft. Just one such detonation can be
expensive. Never lean a four-stroke to peak rpm, and always
operate at least 400 rpm less than peak—more if the weather is dry
and cool.
Even when run at normal mixture settings, four-strokes tend to
loosen propellers. Four-stroke acceleration is not always smooth.
There is much change in the amount of torque the engine delivers
during speed-up and slow-down. This happens because the ignition
and valve timing is mechanically fixed—not variable as in a car
engine.
Timing can only be optimized for one rpm range. Therefore,
the engine torque varies, as does its power output, as its speeds
change. These sudden changes in the amount of acceleration or
deceleration eventually cause the propeller to loosen.
It is a good idea to tighten the propeller before flying each day.
Eventually the engine’s thrust washer will wear out and need
replaced. Most four-strokes are supplied with two propeller nuts;
one tightens against the propeller and the other locks the first in
place. Never use just one propeller nut on a four-stroke. If you do,
detonation will cause the propeller to leave the aircraft while still
rotating. Anything or anyone it hits will come out on the losing
end.
Light the Fire: Besides detonation, a four-stroke-exclusive factor
is glow-plug choice. Since combustion occurs only once during
four piston movements, the glow plug must be designed to stay hot
during all that “spare” time. Regular glow plugs will not work.
The first model four-stroke used a special O.S. “F”-type glow
plug. It extends deep into the combustion chamber to capture as
much combustion heat as possible as quickly as possible. The
extra length also helps keep the element hot during the lengthy
noncombustion period. Several other manufacturers have begun
making this style of glow plug. Check the instructions that come
with your engine, but the F plug or equivalent is basically all that
is used in four-strokes.
If you are flying with the larger two-stroke engines—1.20 cu.
in. and bigger—try the F plug if you are experiencing problems
accelerating from idle to full speed. It works well in this
environment and could solve such transition difficulties. Precision
Aerobatics (Pattern) pilots use Fs in larger engines—two- or fourstrokes—
for extra reliability during transition.
Do not use the F plug in smaller two-strokes; it could cause
detonation or physically strike the piston.
Fill ’er up With? When four-stroke model engines came onto the
scene, much attention was paid to fuel selection. Many
manufacturers offered special fuels with reduced oil content
designed exclusively for four-strokes. Since oil is the poorestburning
ingredient in model fuel, less oil content made the early
four-strokes run more consistently. Today, low oil content is not
only unnecessary, but is probably a negative. Most engine
58 MODEL AVIATION
Original O.S. Max 60 four-stroke nestled in Sig Kadet Senior’s
nose (perfect airframe/engine match). Rocker arms, pushrods are
exposed. Producing roughly the same power as a .35 two-stroke,
the 60 was still able to use a larger propeller.
A 120-size four-stroke muffler looks tiny next to 120-160 twostroke
muffler. Both have pressure taps to ensure even fuel
delivery.
Adjusting valves takes a few minutes and should be done after
first two hours of operation. After that, frequent checks keep
engine operating at peak power.
10sig2.QXD 7/23/04 11:49 am Page 58
manufacturers recommend at least 16-18%
oil; high-performance engines demand 20%
or more.
The myth that four-strokes require lowoil-
content fuels started because early
modelers used regular two-stroke glow
plugs. Now that four-stroke glow plugs are
available, the oil’s heat-removing ability is
a benefit—not a problem.
Although four-stroke sport engines run
cooler than equivalent two-strokes, cylinder
pressures are much higher. The extra oil
helps protect parts such as the ring, cylinder
lining, and wrist pin that are exposed to this
higher pressure. As I have discussed, model
fuel cools the engine by lubricating it and
carrying away excess “top end” heat as
unburnt oil exits the exhaust.
Most model fuels use a mixture of
synthetic oil and castor oil. Except for
high-performance, supercharged fourstrokes
that require synthetic oil only,
approximately 5% castor oil is a good
amount for two- and four-strokes. The total
recommended oil content is the same as for
two-strokes: 18-20% minimum. This
provides a small error margin during
extreme operation.
Unlike in a two-stroke, there is no
refrigeration cooling of the four-stroke’s
lower crankcase since the fuel never gets
there in quantity. Many Pattern
competition pilots have learned that
providing extra cooling air to a fourstroke’s
lower crankcase area is beneficial.
It provides extra cooling, but then the
60 MODEL AVIATION
cooler air flows past the crankcase and into
the “upside-down”-mounted carburetor,
making the entire fuel/air mix denser for
extra power.
Make sure the lower crankcase receives
cooling air when you install any fourstroke.
Regardless of the power
advantages, having a cool lower end
prolongs bearing life.
What about nitromethane content?
Since four-strokes have just one power
stroke per two crankshaft revolutions,
nitromethane content less than 10% makes
it harder to keep the glow plug operating at
peak efficiency. In most sport four-strokes,
nitromethane contents higher than 25% can
result in extra detonation and thrust-washer
and spinner-backplate wear unless
everything is set perfectly. Even highperformance,
supercharged four-strokes
experience problems when nitromethane
content exceeds 35%.
For sport use, consider 15%
nitromethane content when flying at lower
than 5,000-foot density altitudes and in
temperatures lower than 95°. Consider 20%
nitromethane content if conditions exceed
these figures.
Sport four-strokes actually burn less
fuel than equivalent-size two-strokes. This
is partly because of their better combustion
efficiency and higher internal pressures,
but mostly because fuel is burned only on
every other piston stroke.
However, four-strokes do no get twice
the “mileage” of two-strokes. At best, sport
four-strokes enjoy 20-40% better fuel
economy. Since they use less fuel, it is
easier to feed them higher nitromethaneand
oil-content fuels that might cost
slightly more.
Propellers: Propeller choices for fourstrokes
may be slightly different than for
two-strokes. Both produce roughly the
same torque (twisting force) for a given
displacement engine size. Two-strokes still
develop more horsepower, but it is usually
at high rpm (exceeding 13,000) that most
sport fliers at club fields cannot readily use.
The noise is excessive, the propellers must
be small, and high-nitromethane-content
fuels must be used. Besides, turning so fast
prematurely wears out most sport engines.
Four-strokes have horsepower peaks in
the 9,000-11,000 rpm range. Sport fliers
find it easier to choose a propeller that
allows the engine to operate in this range.
Only sport fuels are required, and
everything is quieter and easier to set up at
these low rpm.
The four-stroke’s power curve makes it
possible for Sport Scale fliers to use
larger-diameter propellers and still reach
their engine’s peak ratings. Biggerdiameter
propellers are more efficient if
big obstructions such as scale cowls or
wide fuselages are located just to the
propeller’s rear. The more the propeller’s
swept area that is located outside the
obstruction, the less interference the
propeller receives from deflected airflow.
10sig2.QXD 7/23/04 11:49 am Page 60
Through the years, four-strokes earned a
reputation for having more torque and
therefore being able to turn larger-diameter
propellers with higher pitches. After
extensive research by modeling’s Engine
Gurus, we know that this is untrue and that
four-strokes have nearly the same peak
torque as two-strokes. Yet four-strokes
seem to have more torque because all that
they do have is fully available.
If two-strokes’ peak torque could be
reached at 8,000 rpm, they could use the
same larger-diameter propellers. But the
torque peak is higher in the rpm range, and
they can’t.
However, the rules for choosing a
propeller are the same for four-strokes as
they were from last month for two-strokes.
Pick the largest-diameter propeller, with
sufficient pitch to fly at the speed you want,
which allows the engine to turn
approximately 1,000 rpm higher than the
engine’s peak torque rpm. Make fuel and
glow-plug choices first—they affect an
engine’s top rpm ability—and then choose
the propeller.
Sound: A four-stroke’s exhaust note has a
lower pitch than a two-stroke’s, probably
because its noise-making power stroke
occurs on every other crankshaft
revolution. Many times the four-stroke is
also turning at a lower rpm and is therefore
not producing the high-pitched scream that
is so common with the two-strokes. This
lower-pitched noise may seem quieter, but
it is not.
Without a muffler, .45 cu. in. two- and
four-strokes make roughly the same amount
of noise: approximately 108 decibels (dB)
measured 9 feet from the engine. That is
loud. With factory mufflers, both engines
usually produce 100-102 dB, which is still
loud but more common and therefore
seldom intolerable to most clubs.
Four-stroke mufflers are smaller than
two-strokes’ since the four-stroke exhaust
outlet is smaller. Scale modelers like the
smaller muffler because its diminutive size
is less objectionable and easier to work into
their realistic airplanes.
Another commonality is that two- and
four-stroke engines usually require muffler
pressure to the fuel tank. Some highperformance
four-strokes are equipped with
fuel pumps or engine-driven fuelpressurization
systems that do not
necessitate muffler pressure, but most sport
four-strokes are not so equipped. Use
muffler pressure at all times on these
engines.
Maintenance: Two- and four-strokes need
regular attention to keep everything
working well, but four-strokes require a bit
more. The main difference is that the valveto-
pushrod clearance must be adjusted. You
must do this before first running the engine,
and then again after the first two hours of
run time. Check the clearances every 10
running hours for the next 50 hours or so; if
there is no change, it is usually safe to
extend inspection times to 50 hours.
As does a two-stroke, a four-stroke
“stores” a great deal of unburnt fuel inside
the engine after it is shut down. You must
run the engine dry of this fuel at the end of
each day. There are two techniques to
accomplish this.
Some engine experts favor keeping the
glow plug connected and going to full
throttle while the fuel line is disconnected,
allowing the engine to run dry. Others
prefer the same procedure but use a high
idle instead of full speed. This is safer and
quieter. If you use the idle method, try to
restart the engine after it first quits in case
residual fuel remains. But do not overdo it;
the engine has little or no internal
lubrication at this point since most of the
fuel is gone.
After-run oil is essential rust protection
for a four-stroke. Many good kinds are
available at hobby shops. Some experts
prefer Marvel Mystery Oil, automatic
transmission fluid, or a 50/50 mixture of
the two. Others like air-tool oil.
You must be careful; the petroleum
distillates in these products could damage
the fuel-pump diaphragms or carburetor Orings
in some engines. O.S. specifically
warns against using petroleum products in
some of its carburetors.
Pattern pilots fly more in one year than
most sport pilots fly in several years. Based
on their extensive engine use, most use
Mobil 1 Synthetic Engine Oil or equivalent
as their after-run oil. The synthetic oil has
no petroleum content, will not thicken with
time, and seems to prevent rust better than
most other choices, even though it contains
no specific rust inhibitors as far as is
known.
Whichever oil you choose, use a “glue
syringe” (available at most hobby shops) to
inject approximately 10 drops into the
crankcase breather fitting, and put the same
amount in the glow-plug hole. Rotate the
engine several times and replace the glow
plug. A few more rotations with the glow
plug in place couldn’t hurt.
You should be doing this with all of
your two-strokes as well, so this is not extra
four-stroke maintenance. The only
difference is that the oil is dropped into the
wide-open carburetor of a two-stroke
instead of into the breather fitting.
If YS made your four-stroke, it will not
have a breather fitting because the
crankcase is pressurized. In this case, drop
the oil into the glow-plug hole and the
carburetor. With the carburetor facing
upward, rotate the engine as described.
Never use petroleum distillate oil in these
high-performance “wonder” engines.
I had hoped to discuss fuel-tank styles and
location but ran out of room. I will cover
tank selection and placement next month,
along with several other engine tools and
accessories. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866
62 MODEL AVIATION
10sig2.QXD 7/23/04 11:50 am Page 62