From the Ground Up - Engines 101 - 2004/04

BOB ABERLE
CONCEIVED the
“From the Ground
Up” article series to
enable all new pilots
to get the most from
their Radio Control
modeling experience.
His concept was
successful, and his
execution of the first
two parts—radios and electric power—was
so successful that it is a real challenge to
even try to continue the series. But try I
will, and I hope to continue the spirit and
detail of this series.
As Bob did so well in his radio
installments, “From the Ground Up” will
continue to include detailed coverage of all
aspects of becoming a successful,
experienced model-aviation pilot. Selecting
and building that first airplane, learning to
fly it, maintaining it, progressing beyond
the solo, and those first steps in
advancement after the trainer will be
discussed.
However, gaining experience as a pilot
first requires flying! For the pilot to get that
flying practice, the airplane has to leave the
ground, and the most common way to make
that happen is to drag it up there behind an
engine. That makes engines the next step in
this series.
Having the correct engine for the
airplane, making it easy to start, and
running it reliably and without excessive
wear are some of the most important
aspects of learning this hobby/sport.
Learning to fly is challenge enough without
having to worry about flights that end too
quickly and far too quietly.
If you are flying with an instructor
(always recommended), engine failure too
far from the runway is almost the only way
your trainer will be damaged during your
flight education (if you follow Bob’s great
radio advice, that is). Repairing your
airplane may improve your building skills
(and is covered later in the series), but it
does little to speed your flight training.
So my goal is to present engines,
installation, mufflers, break-in and tuning
techniques, fuel, fuel tanks and supply
lines, propellers, support equipment,
spinners, maintenance, minor repair, and,
above all, safety practices, in as much detail
as possible to help you forget about your
engine while you are learning to fly. Your
engine is an important tool, but it should
not be the center of your attention during
the early stages of your becoming a model
pilot.
What is this tool called an “engine”? It is
an air- and fuel-cooled, fuel-lubricated,
venturi-fed, glow-assisted machine
constructed from aluminum, with some
steel in high-stress areas. It is designed to
convert a fuel’s chemical energy into
52 MODEL AVIATION
by Frank Granelli
Engines come in bewildering series of variations; these are .60s. L-R: K&B .65 is ABC, non-Schnuerle; SuperTigre .61 is Schnuerle
ported, ABC; black-headed Rossi .61 is high rpm, high compression, rear exhaust; O.S. rear-exhaust, long-stroke .61 produces
enormous torque at lower rpm and has fuel pump.
something that will turn a propeller.
Considering each aspect of that boring
description helps the pilot understand and
avoid some of the most common modelengine
problems.
Having a machine convert fuel into
mechanical energy releases heat. This heat
has to be removed or the machine will
literally begin to melt and fuse its moving
parts. Engines remove this heat by directing
the propeller’s airflow over most of the
engine; they are air-cooled.
However, airflow is a poor means of
engine cooling; unlike water or glycol
(antifreeze), air is not the best “heat
exchanger.” Also, the air does not remain in
contact with the engine for long; therefore,
it does not have time to absorb much heat.
Unlike water-cooling, the air cannot
reach deep into the engine to cool the
moving parts directly. Model engines use
“fins” to increase the surface area that the
air contacts, but air-cooling remains a
surface-contact process so it is inherently
inefficient.
To help remove heat the air can’t get rid
of, model engines use some fuel cooling as
well. As the fuel is converted to an air-fuel
mix in the carburetor, its density, and
therefore its temperature, drops, helping to
cool the intake and lower crankcase areas.
In the engine diagrams, you can see why
this occurs. As the fuel is burned, any
unburnt fuel is ejected from the exhaust.
Since alcohol is a great heat exchanger, the
expelled fuel droplets carry away heat.
But most important, model fuel is the
engine’s sole lubrication source. It contains
oil that keeps the moving parts separated
from each other, reducing friction and
lowering the engine’s temperature.
Unlike most car engines, which have an
independent oil source, the amount of oil
applied to a model engine’s moving parts
depends entirely on the engine’s rate of fuel
supply, or “mixture setting.” The mixture
setting adjusts the amount of fuel that is
mixed with an engine’s incoming air supply.
An engine’s maximum air supply is
fixed by the diameter of the carburetor
opening and adjusted by the area opened by
the throttle barrel. (See photo.) The pilot
April 2004 53
The cylinder liner has been removed from the engine. It is
positioned to show the large exhaust port.
The liner has been positioned to illustrate the Schnuerle port
(center) and one of the two standard intake ports (lower right).
The dark line around the top of this Webra .91 engine’s piston is the ring. Also notice
the idle mixture-adjustment screw in the center of the throttle arm.
Photos courtesy the author
adjusts the amount of fuel mixed with that
incoming air supply using high- and lowspeed
fuel-metering devices known as
“needle valves” and/or “air bleed”
adjustment screws.
By properly adjusting these fuelmetering
tools, the pilot alone is responsible
for the engine’s operating temperature and,
as a result, its reliability and durability. This
is true no matter what type of engine is
used—a two-stroke or a four-stroke.
Although there are several other breeds
of model engines, such as gas ignition or
true diesel, the two-stroke and four-stroke,
alcohol-fueled kinds comprise the majority
of what new pilots use. The exploded
engine views and photos reveal the
differences between the two major engine
types.
Of the two-stroke and four-stroke engines,
the former are the simplest kind; they are
almost as basic as engines get. They are
shown in the diagrams of the O.S. Max .40
LA and Max .25 FX.
When you start an engine, the piston
moves downward, creating a vacuum in the
cylinder area above the piston, and then
uncovers the fuel-intake ports that are cut
into the side of the cylinder. (See photo.)
54 MODEL AVIATION
O.S. Max .40 LA is typical of today’s modern two-stroke engines,
using bronze bushing to support crankshaft. This is the least
complicated a .40-size engine can get. Diagram courtesy O.S.
Engines.
O.S. Max .25 FX uses twin ball bearings in place of bushing. Up
to .65 displacement, size does not determine type of crankshaft
suspension. Larger engines are usually ball-bearing suspended.
Diagram courtesy O.S. Engines.
ABC SuperTigre .61 has no ring. The thin line on the piston is actually a shallow score
mark caused by the smaller-diameter cylinder bore. It will disappear once this engine is
run.
The intake ports connect the area above the
piston to the lower crankcase area just
behind the crankshaft, which is the device
that turns the propeller. The crankshaft is
hollow and has a slot cut into it just below
the carburetor. If the throttle barrel is open,
the vacuum above the piston is directly
connected to the air-fuel mixture in the
carburetor and draws this mixture into the
cylinder area above the piston.
Exactly when the slot is uncovered and
for how long is the engine’s “timing.” If the
slot is open sooner (more advanced) and
longer (duration), the power increases, but
that can cause other problems such as hard
starting, preignition, and poor idling.
Certain timing settings allow an engine to
achieve more power using tuned exhaust
systems than do others (but most new pilots
do not need to worry about tuning the
exhaust since they will be using standard
mufflers).
The piston continues downward,
uncovering the exhaust port, but the
pressure of the unburnt fuel-air mixture that
has been drawn into the upper cylinder area
is too low for it to escape before the piston
begins its upward journey. This mixture is
trapped between the cylinder head and
upward-moving piston, and it is
compressed.
How much the fuel-air mixture is
compressed is the engine’s “compression
ratio,” which is the proportion of the entire
cylinder volume above the piston in its full
down position to the remaining cylinder
volume with the piston fully raised (the
cylinder’s “combustion area”).
For instance, if the volume above the
piston is 10 times larger when the piston is
down than the cylinder’s combustion area,
the compression ratio is 10:1. The higher the
compression ratio, the more power the final
fuel-air explosion produces. However,
compression ratios can be too high, causing
preignition, hot running, burnt glow plugs,
and piston damage. Most sport engines do not
have high compression.
As the piston moves upward, it blocks the
intake ports again, sealing the cylinder’s
combustion area above the piston. Once the intake ports are
blocked, the upward piston movement creates a vacuum in the
crankcase area that begins to draw the next fuel-air charge into the
crankcase area; that is a “venturi” suction effect that provides the
fuel-air mix for the next cycle.
As the piston continues upward, it compresses the explosive
fuel-air mixture against the extremely hot glow-plug element. (You
did remember to hook up the battery to the glow plug so that the
element is glowing now, right?) The next step is obvious; the
compressed, explosive mixture meets the hot glow-plug wire. The
explosion moves the piston rapidly downward as the burning gas
continues to expand.
The piston eventually uncovers the exhaust port and the hot
gases escape the cylinder. But even as the cylinder is moving
downward, the burnt gases are losing pressure because of the
increasing cylinder area and time since the explosion. This creates a
vacuum above the cylinder even before all of the gases escape. The
next fuel-air mixture enters the cylinder just before—or just as—
the burnt gases are escaping. How soon is the engine’s “timing
advance.”
Once the spent gases are exhausted, fresh fuel-air gas enters the
cylinder and the process repeats. The piston continues to move up
April 2004 55
The opposite side port cut into the cylinder sleeve is the intake boost port. The front
intake port is the light area toward the right. A rear intake port is visible if you have
great eyes.
The intake slot in the crankshaft is clearly visible. This SuperTigre engine’s crankshaft
is ball-bearing supported front and rear. Check out the “bulges” that house the
bearings.
The connecting rod connects the piston to the pin on the
crankshaft, converting the piston’s up-and-down motion into
rotational motion.
and down with a fuel-air explosion every
time the piston completes its upward
journey.
Since the piston is connected to the
crankshaft—by a device called, for some
strange reason, a “connecting rod”—the
piston’s up-and-down movement is
converted to a rotating crankshaft that turns
the propeller. Both sides of the connecting
rod use bronze bushings and must be well
lubricated. This is the part that breaks if you
use a propeller that is too large or too small.
When the “rod” breaks, the rest of the
engine is usually destroyed.
This process creates a great deal of heat
which can be used to keep the glow plug
glowing even if you disconnect the battery.
In effect, the engine is working as a diesel
so it requires no outside ignition energy to
continue the process. Two-stroke engines
are the epitome of power and reliability.
There are several styles of two-stroke
engines, and each has its strong and weak
areas. The new pilot should be familiar with
a few of these designs; it is important for
proper engine selection. Look at the photo
of the two .60 engines. These are called
“.60s” because the piston displaces .60
cubic inch as it travels. The total size of the
space—stroke length multiplied by the
cylinder’s area—that the piston occupies in
56 MODEL AVIATION
Look carefully at the sides of these engines. The SuperTigre on the right has a bulge (under the “S”); that is the Schneurle boost port.
The K&B engine in the foreground uses a bronze bushing for support. The front
housing does not have the bearing-support bulges as does the ball-bearing-supported
SuperTigre in the rear.
its travels is the engine’s “displacement.”
Most two-stroke sport engines used in trainers range in
displacement from .25 cubic inch to .61 cubic inch. The larger the
displacement, the more powerful the engine and the larger the
propeller it can use. However, larger-displacement engines use
more fuel per minute and cost more. Bigger engines also require
larger airframes that could be more expensive.
Look carefully at the two .60s’ sides in one of the photos. One
engine has a straight cylinder while one has a raised area (under the
“S”). That elevated area is the space for the “boost port” designed
by a German engineer. This “Schnuerle port” is nothing more than
an extra intake that permits more air-fuel mixture into the engine
per stroke.
Some engines have several extra Schnuerle boost ports, but
almost all sport engines made today have at least one. Schnuerle
engines use more fuel and are slightly harder to adjust at idle, but
they are more powerful than non-Schnuerle engines.
Another point to consider during engine selection is the
crankshaft’s support system. If you look at the O.S .25 FX diagram,
you’ll see a front and rear ball bearing supporting both crankshaft
ends. The O.S. .40 LA diagram does not include ball bearings;
instead, there is a bronze bushing pressed into the crankcase
housing. The crankshaft slides into, and is supported by, this
bushing.
Ball bearings provide more crankshaft support, but at additional
cost. They are also more susceptible to corrosion if they are not
properly maintained. Bronze bushings tend to wear sooner than ball
bearings and are more difficult to replace, but they do not rust or
corrode. Electric starting is less problematic with ball bearings;
bushed engines require a lighter touch when pressing the starter
against the propeller.
Both types of crankshaft supports develop approximately the
same amount of power if the engines are of the same design type,
and the two main types are ABC (AAC) and ringed piston. The
latter has one or more compression rings on the piston, as does an
auto engine. The rings provide drag and do not have as tight a seal
against the cylinder wall as the ABC variety do.
ABC types are nearly exclusive to model engines. The piston is
made from aluminum, and the cylinder is constructed from brass
plated with chrome. The piston actually has a larger diameter than
does the top of the cylinder. This size difference can be felt when
turning the propeller; its rotation tends to stick as the piston reaches
the top of its stroke and enters the smaller cylinder section.
How can this work? The varied construction materials expand at
different rates when the engine heats while running. The cylinder
expands more than the piston does. The final diameter of both
becomes equal when the engine runs; this makes for a tight
compression seal at the top end and produces more horsepower.
April 2004 57
Webra .91 (L) is a two-stroke engine. Small exhaust, valve rockerarm
covers, pushrod housings, inverted rear-mounted carburetor
identify O.S. 120 FX (R) as a four-stroke.
Shown are four- and two-stroke engines’ “heads” (tops of
combustion chambers). Tubes are four-stroke’s intake and
exhaust “manifolds.” Both heads are hemispherical (recessed
half spheres) in shape for more power, made popular by
Chrysler’s “Hemis” of the 1960s.
With many parts and more complicated than two-strokes, fourstrokes
have less horsepower but more torque, making them
ideal for some Scale and sport applications. Diagram courtesy
O.S. Engines.
However, ABC engines have slightly
lower torque than ringed since compression
drops slightly as the piston progresses
downward into the larger cylinder area.
This expansion difference also provides
protection if the engine overheats. When
overheating, the cylinder continues to
expand faster than the piston, providing
extra clearance that protects these vital
parts, but the engine still sags, reducing
power, while the destruction process
begins.
ABC engines work well in the smaller
sizes—up to roughly .61 cubic inch. Most
engines that size that are sold today are
ABC or AAC (aluminum piston, aluminum
cylinder, chrome plated), but several fine
ringed designs are manufactured. Most
engines larger than .61 cubic inch are
ringed.
Most new pilots start with two-stroke
engines because they are simple, reliable,
less expensive, easy to operate, and
incredibly powerful. To date, all of the
Ready-to-Fly (RTF) trainers on the market
use two-strokes (except for one electricpowered
version). RTF trainers usually
come with an installed engine, fuel system,
and radio and a prebuilt airframe that
requires no previous modeling experience
and less than two hours to complete.
Many new pilots prefer the realistic sound,
fuel efficiency, and higher torque of a fourstroke
engine, so they forego the
convenience of an RTF airframe and
purchase an Almost Ready-to-Fly (ARF)
trainer. ARFs require a bit of modeling
know-how (e.g., gluing wing halves
together) and approximately 20 hours to
complete. The pilot must also buy and
install the radio and engine. This is
currently the only way to have a four-stroke
on your trainer.
As I mentioned about all model engines,
four-strokes are air-cooled, glow-assisted
diesels. Most important, unlike larger fourstroke
engines, such as those used in
automobiles, most lawn mowers, or
generators, the model varieties remain fuel
lubricated. That is important to remember.
Except for needing fuel lubrication,
model four-strokes work much as shown in
those blue-and-red engine diagrams some
of us learned with in school. (Of course,
those diagrams never showed the black,
messy grease we had to wade through to get
to those “simple” parts.)
Instead of drawing the fuel-air mix
through a venturi-fed, hollow crankshaft
and then through cylinder wall ports into
the combustion chamber, model fourstrokes
feed this combustible gas mixture
into the upper cylinder area through an
intake valve. As can be seen in the diagram
and photos, this one change makes for an
engine that operates using an entirely
different process.
As a four-stroke starts, the fuel-air mix
is created inside a carburetor that is almost
identical to that of a two-stroke. But instead
of then being drawn into a hollow
crankshaft, the combustion area’s vacuum
caused by the downward-moving piston
draws the fuel-air mix from the carburetor
into an intake pipe (intake manifold) and
then through the open intake valve and into
the cylinder’s combustion area. The intake
valve has to be open—pushed away from
its base—for the gases to flow into the
cylinder.
Since the steel valve can’t figure out
when to open on its own, it is forced open
by a device known as a “camshaft,” which
is usually located near the engine’s
crankshaft. Exactly when the camshaft
opens the intake and exhaust valves, and for
how long, determines the four-stroke’s
timing.
The camshaft is connected to the
crankshaft by a timing gear and opens and
closes the valves using pushrods that ...
Well, don’t worry about the details now;
I’ll cover four-stroke operation and care
and feeding in my third article in this series.
If you can’t wait, you’ll find a great deal of
interesting information if you study the
diagram of the O.S. FS-70S.
The preceding brief description serves to
point out that four-strokes operate
differently from their two-stroke cousins. In
review, a two-stroke has a fuel-air
explosion every time the piston reaches the
top of its travel, whereas a four-stroke has
58 MODEL AVIATION
such a power-producing explosion every
other time the piston reaches the top.
Since a four-stroke has half the number
of explosions per series of revolutions, it
should produce only half the horsepower of
a two-stroke, and that was the case when
model four-strokes were introduced.
However, manufacturers have learned that
four-strokes are more tolerant of timing
advances, can have larger carburetor
openings and intake valves, and are easier
to “supercharge” than most two-strokes.
A modern four-stroke can produce only
75-80% of the horsepower that an
equivalent-displacement two-stroke
produces, but four-strokes do have one
advantage over two-strokes. For various
60 MODEL AVIATION
technical reasons, such as explosion
duration, timing, and the nature of valveintake
combustion, four-strokes produce
more twisting force, or torque, and are
easier to adjust.
The extra torque means that a fourstroke
can safely turn larger-diameter
propellers, with equivalent pitches, than a
comparable two-stroke. The four-strokes
are also quieter, and their exhaust note is
slightly lower pitched because of fewer
revolutions per minute (rpm).
As for two-stroke horsepower ratings,
they are of little practical use when you are
selecting an engine. Such ratings are
usually computed at extraordinarily high
rpm using small propellers. Although it is
possible to use the same small propeller on
your trainer, the average trainer airframe
has so much drag that the fast-turning
propeller will stall, and your model will
barely move through the sky.
What is propeller stall? How do you find
the right propeller for your engine/airframe
combination? What good is a spinner? As
this series progresses I’ll cover these
subjects and many more of the engines’
technical details, similarities and
differences between engine types, and their
care and feeding. I’ll also discuss field
accessories, tools, fuel types, and many
other subjects in great detail.
You may not become a model-engine
expert, but you will learn everything you
need to know to choose an engine, keep it
running reliably, make it last for years, and
get the best flying performance possible.
The next installment will cover setup,
installation, adjustment, propellers, and the
care and feeding of the popular .40-cubicinch,
two-stroke engine. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866

BOB ABERLE
CONCEIVED the
“From the Ground
Up” article series to
enable all new pilots
to get the most from
their Radio Control
modeling experience.
His concept was
successful, and his
execution of the first
two parts—radios and electric power—was
so successful that it is a real challenge to
even try to continue the series. But try I
will, and I hope to continue the spirit and
detail of this series.
As Bob did so well in his radio
installments, “From the Ground Up” will
continue to include detailed coverage of all
aspects of becoming a successful,
experienced model-aviation pilot. Selecting
and building that first airplane, learning to
fly it, maintaining it, progressing beyond
the solo, and those first steps in
advancement after the trainer will be
discussed.
However, gaining experience as a pilot
first requires flying! For the pilot to get that
flying practice, the airplane has to leave the
ground, and the most common way to make
that happen is to drag it up there behind an
engine. That makes engines the next step in
this series.
Having the correct engine for the
airplane, making it easy to start, and
running it reliably and without excessive
wear are some of the most important
aspects of learning this hobby/sport.
Learning to fly is challenge enough without
having to worry about flights that end too
quickly and far too quietly.
If you are flying with an instructor
(always recommended), engine failure too
far from the runway is almost the only way
your trainer will be damaged during your
flight education (if you follow Bob’s great
radio advice, that is). Repairing your
airplane may improve your building skills
(and is covered later in the series), but it
does little to speed your flight training.
So my goal is to present engines,
installation, mufflers, break-in and tuning
techniques, fuel, fuel tanks and supply
lines, propellers, support equipment,
spinners, maintenance, minor repair, and,
above all, safety practices, in as much detail
as possible to help you forget about your
engine while you are learning to fly. Your
engine is an important tool, but it should
not be the center of your attention during
the early stages of your becoming a model
pilot.
What is this tool called an “engine”? It is
an air- and fuel-cooled, fuel-lubricated,
venturi-fed, glow-assisted machine
constructed from aluminum, with some
steel in high-stress areas. It is designed to
convert a fuel’s chemical energy into
52 MODEL AVIATION
by Frank Granelli
Engines come in bewildering series of variations; these are .60s. L-R: K&B .65 is ABC, non-Schnuerle; SuperTigre .61 is Schnuerle
ported, ABC; black-headed Rossi .61 is high rpm, high compression, rear exhaust; O.S. rear-exhaust, long-stroke .61 produces
enormous torque at lower rpm and has fuel pump.
something that will turn a propeller.
Considering each aspect of that boring
description helps the pilot understand and
avoid some of the most common modelengine
problems.
Having a machine convert fuel into
mechanical energy releases heat. This heat
has to be removed or the machine will
literally begin to melt and fuse its moving
parts. Engines remove this heat by directing
the propeller’s airflow over most of the
engine; they are air-cooled.
However, airflow is a poor means of
engine cooling; unlike water or glycol
(antifreeze), air is not the best “heat
exchanger.” Also, the air does not remain in
contact with the engine for long; therefore,
it does not have time to absorb much heat.
Unlike water-cooling, the air cannot
reach deep into the engine to cool the
moving parts directly. Model engines use
“fins” to increase the surface area that the
air contacts, but air-cooling remains a
surface-contact process so it is inherently
inefficient.
To help remove heat the air can’t get rid
of, model engines use some fuel cooling as
well. As the fuel is converted to an air-fuel
mix in the carburetor, its density, and
therefore its temperature, drops, helping to
cool the intake and lower crankcase areas.
In the engine diagrams, you can see why
this occurs. As the fuel is burned, any
unburnt fuel is ejected from the exhaust.
Since alcohol is a great heat exchanger, the
expelled fuel droplets carry away heat.
But most important, model fuel is the
engine’s sole lubrication source. It contains
oil that keeps the moving parts separated
from each other, reducing friction and
lowering the engine’s temperature.
Unlike most car engines, which have an
independent oil source, the amount of oil
applied to a model engine’s moving parts
depends entirely on the engine’s rate of fuel
supply, or “mixture setting.” The mixture
setting adjusts the amount of fuel that is
mixed with an engine’s incoming air supply.
An engine’s maximum air supply is
fixed by the diameter of the carburetor
opening and adjusted by the area opened by
the throttle barrel. (See photo.) The pilot
April 2004 53
The cylinder liner has been removed from the engine. It is
positioned to show the large exhaust port.
The liner has been positioned to illustrate the Schnuerle port
(center) and one of the two standard intake ports (lower right).
The dark line around the top of this Webra .91 engine’s piston is the ring. Also notice
the idle mixture-adjustment screw in the center of the throttle arm.
Photos courtesy the author
adjusts the amount of fuel mixed with that
incoming air supply using high- and lowspeed
fuel-metering devices known as
“needle valves” and/or “air bleed”
adjustment screws.
By properly adjusting these fuelmetering
tools, the pilot alone is responsible
for the engine’s operating temperature and,
as a result, its reliability and durability. This
is true no matter what type of engine is
used—a two-stroke or a four-stroke.
Although there are several other breeds
of model engines, such as gas ignition or
true diesel, the two-stroke and four-stroke,
alcohol-fueled kinds comprise the majority
of what new pilots use. The exploded
engine views and photos reveal the
differences between the two major engine
types.
Of the two-stroke and four-stroke engines,
the former are the simplest kind; they are
almost as basic as engines get. They are
shown in the diagrams of the O.S. Max .40
LA and Max .25 FX.
When you start an engine, the piston
moves downward, creating a vacuum in the
cylinder area above the piston, and then
uncovers the fuel-intake ports that are cut
into the side of the cylinder. (See photo.)
54 MODEL AVIATION
O.S. Max .40 LA is typical of today’s modern two-stroke engines,
using bronze bushing to support crankshaft. This is the least
complicated a .40-size engine can get. Diagram courtesy O.S.
Engines.
O.S. Max .25 FX uses twin ball bearings in place of bushing. Up
to .65 displacement, size does not determine type of crankshaft
suspension. Larger engines are usually ball-bearing suspended.
Diagram courtesy O.S. Engines.
ABC SuperTigre .61 has no ring. The thin line on the piston is actually a shallow score
mark caused by the smaller-diameter cylinder bore. It will disappear once this engine is
run.
The intake ports connect the area above the
piston to the lower crankcase area just
behind the crankshaft, which is the device
that turns the propeller. The crankshaft is
hollow and has a slot cut into it just below
the carburetor. If the throttle barrel is open,
the vacuum above the piston is directly
connected to the air-fuel mixture in the
carburetor and draws this mixture into the
cylinder area above the piston.
Exactly when the slot is uncovered and
for how long is the engine’s “timing.” If the
slot is open sooner (more advanced) and
longer (duration), the power increases, but
that can cause other problems such as hard
starting, preignition, and poor idling.
Certain timing settings allow an engine to
achieve more power using tuned exhaust
systems than do others (but most new pilots
do not need to worry about tuning the
exhaust since they will be using standard
mufflers).
The piston continues downward,
uncovering the exhaust port, but the
pressure of the unburnt fuel-air mixture that
has been drawn into the upper cylinder area
is too low for it to escape before the piston
begins its upward journey. This mixture is
trapped between the cylinder head and
upward-moving piston, and it is
compressed.
How much the fuel-air mixture is
compressed is the engine’s “compression
ratio,” which is the proportion of the entire
cylinder volume above the piston in its full
down position to the remaining cylinder
volume with the piston fully raised (the
cylinder’s “combustion area”).
For instance, if the volume above the
piston is 10 times larger when the piston is
down than the cylinder’s combustion area,
the compression ratio is 10:1. The higher the
compression ratio, the more power the final
fuel-air explosion produces. However,
compression ratios can be too high, causing
preignition, hot running, burnt glow plugs,
and piston damage. Most sport engines do not
have high compression.
As the piston moves upward, it blocks the
intake ports again, sealing the cylinder’s
combustion area above the piston. Once the intake ports are
blocked, the upward piston movement creates a vacuum in the
crankcase area that begins to draw the next fuel-air charge into the
crankcase area; that is a “venturi” suction effect that provides the
fuel-air mix for the next cycle.
As the piston continues upward, it compresses the explosive
fuel-air mixture against the extremely hot glow-plug element. (You
did remember to hook up the battery to the glow plug so that the
element is glowing now, right?) The next step is obvious; the
compressed, explosive mixture meets the hot glow-plug wire. The
explosion moves the piston rapidly downward as the burning gas
continues to expand.
The piston eventually uncovers the exhaust port and the hot
gases escape the cylinder. But even as the cylinder is moving
downward, the burnt gases are losing pressure because of the
increasing cylinder area and time since the explosion. This creates a
vacuum above the cylinder even before all of the gases escape. The
next fuel-air mixture enters the cylinder just before—or just as—
the burnt gases are escaping. How soon is the engine’s “timing
advance.”
Once the spent gases are exhausted, fresh fuel-air gas enters the
cylinder and the process repeats. The piston continues to move up
April 2004 55
The opposite side port cut into the cylinder sleeve is the intake boost port. The front
intake port is the light area toward the right. A rear intake port is visible if you have
great eyes.
The intake slot in the crankshaft is clearly visible. This SuperTigre engine’s crankshaft
is ball-bearing supported front and rear. Check out the “bulges” that house the
bearings.
The connecting rod connects the piston to the pin on the
crankshaft, converting the piston’s up-and-down motion into
rotational motion.
and down with a fuel-air explosion every
time the piston completes its upward
journey.
Since the piston is connected to the
crankshaft—by a device called, for some
strange reason, a “connecting rod”—the
piston’s up-and-down movement is
converted to a rotating crankshaft that turns
the propeller. Both sides of the connecting
rod use bronze bushings and must be well
lubricated. This is the part that breaks if you
use a propeller that is too large or too small.
When the “rod” breaks, the rest of the
engine is usually destroyed.
This process creates a great deal of heat
which can be used to keep the glow plug
glowing even if you disconnect the battery.
In effect, the engine is working as a diesel
so it requires no outside ignition energy to
continue the process. Two-stroke engines
are the epitome of power and reliability.
There are several styles of two-stroke
engines, and each has its strong and weak
areas. The new pilot should be familiar with
a few of these designs; it is important for
proper engine selection. Look at the photo
of the two .60 engines. These are called
“.60s” because the piston displaces .60
cubic inch as it travels. The total size of the
space—stroke length multiplied by the
cylinder’s area—that the piston occupies in
56 MODEL AVIATION
Look carefully at the sides of these engines. The SuperTigre on the right has a bulge (under the “S”); that is the Schneurle boost port.
The K&B engine in the foreground uses a bronze bushing for support. The front
housing does not have the bearing-support bulges as does the ball-bearing-supported
SuperTigre in the rear.
its travels is the engine’s “displacement.”
Most two-stroke sport engines used in trainers range in
displacement from .25 cubic inch to .61 cubic inch. The larger the
displacement, the more powerful the engine and the larger the
propeller it can use. However, larger-displacement engines use
more fuel per minute and cost more. Bigger engines also require
larger airframes that could be more expensive.
Look carefully at the two .60s’ sides in one of the photos. One
engine has a straight cylinder while one has a raised area (under the
“S”). That elevated area is the space for the “boost port” designed
by a German engineer. This “Schnuerle port” is nothing more than
an extra intake that permits more air-fuel mixture into the engine
per stroke.
Some engines have several extra Schnuerle boost ports, but
almost all sport engines made today have at least one. Schnuerle
engines use more fuel and are slightly harder to adjust at idle, but
they are more powerful than non-Schnuerle engines.
Another point to consider during engine selection is the
crankshaft’s support system. If you look at the O.S .25 FX diagram,
you’ll see a front and rear ball bearing supporting both crankshaft
ends. The O.S. .40 LA diagram does not include ball bearings;
instead, there is a bronze bushing pressed into the crankcase
housing. The crankshaft slides into, and is supported by, this
bushing.
Ball bearings provide more crankshaft support, but at additional
cost. They are also more susceptible to corrosion if they are not
properly maintained. Bronze bushings tend to wear sooner than ball
bearings and are more difficult to replace, but they do not rust or
corrode. Electric starting is less problematic with ball bearings;
bushed engines require a lighter touch when pressing the starter
against the propeller.
Both types of crankshaft supports develop approximately the
same amount of power if the engines are of the same design type,
and the two main types are ABC (AAC) and ringed piston. The
latter has one or more compression rings on the piston, as does an
auto engine. The rings provide drag and do not have as tight a seal
against the cylinder wall as the ABC variety do.
ABC types are nearly exclusive to model engines. The piston is
made from aluminum, and the cylinder is constructed from brass
plated with chrome. The piston actually has a larger diameter than
does the top of the cylinder. This size difference can be felt when
turning the propeller; its rotation tends to stick as the piston reaches
the top of its stroke and enters the smaller cylinder section.
How can this work? The varied construction materials expand at
different rates when the engine heats while running. The cylinder
expands more than the piston does. The final diameter of both
becomes equal when the engine runs; this makes for a tight
compression seal at the top end and produces more horsepower.
April 2004 57
Webra .91 (L) is a two-stroke engine. Small exhaust, valve rockerarm
covers, pushrod housings, inverted rear-mounted carburetor
identify O.S. 120 FX (R) as a four-stroke.
Shown are four- and two-stroke engines’ “heads” (tops of
combustion chambers). Tubes are four-stroke’s intake and
exhaust “manifolds.” Both heads are hemispherical (recessed
half spheres) in shape for more power, made popular by
Chrysler’s “Hemis” of the 1960s.
With many parts and more complicated than two-strokes, fourstrokes
have less horsepower but more torque, making them
ideal for some Scale and sport applications. Diagram courtesy
O.S. Engines.
However, ABC engines have slightly
lower torque than ringed since compression
drops slightly as the piston progresses
downward into the larger cylinder area.
This expansion difference also provides
protection if the engine overheats. When
overheating, the cylinder continues to
expand faster than the piston, providing
extra clearance that protects these vital
parts, but the engine still sags, reducing
power, while the destruction process
begins.
ABC engines work well in the smaller
sizes—up to roughly .61 cubic inch. Most
engines that size that are sold today are
ABC or AAC (aluminum piston, aluminum
cylinder, chrome plated), but several fine
ringed designs are manufactured. Most
engines larger than .61 cubic inch are
ringed.
Most new pilots start with two-stroke
engines because they are simple, reliable,
less expensive, easy to operate, and
incredibly powerful. To date, all of the
Ready-to-Fly (RTF) trainers on the market
use two-strokes (except for one electricpowered
version). RTF trainers usually
come with an installed engine, fuel system,
and radio and a prebuilt airframe that
requires no previous modeling experience
and less than two hours to complete.
Many new pilots prefer the realistic sound,
fuel efficiency, and higher torque of a fourstroke
engine, so they forego the
convenience of an RTF airframe and
purchase an Almost Ready-to-Fly (ARF)
trainer. ARFs require a bit of modeling
know-how (e.g., gluing wing halves
together) and approximately 20 hours to
complete. The pilot must also buy and
install the radio and engine. This is
currently the only way to have a four-stroke
on your trainer.
As I mentioned about all model engines,
four-strokes are air-cooled, glow-assisted
diesels. Most important, unlike larger fourstroke
engines, such as those used in
automobiles, most lawn mowers, or
generators, the model varieties remain fuel
lubricated. That is important to remember.
Except for needing fuel lubrication,
model four-strokes work much as shown in
those blue-and-red engine diagrams some
of us learned with in school. (Of course,
those diagrams never showed the black,
messy grease we had to wade through to get
to those “simple” parts.)
Instead of drawing the fuel-air mix
through a venturi-fed, hollow crankshaft
and then through cylinder wall ports into
the combustion chamber, model fourstrokes
feed this combustible gas mixture
into the upper cylinder area through an
intake valve. As can be seen in the diagram
and photos, this one change makes for an
engine that operates using an entirely
different process.
As a four-stroke starts, the fuel-air mix
is created inside a carburetor that is almost
identical to that of a two-stroke. But instead
of then being drawn into a hollow
crankshaft, the combustion area’s vacuum
caused by the downward-moving piston
draws the fuel-air mix from the carburetor
into an intake pipe (intake manifold) and
then through the open intake valve and into
the cylinder’s combustion area. The intake
valve has to be open—pushed away from
its base—for the gases to flow into the
cylinder.
Since the steel valve can’t figure out
when to open on its own, it is forced open
by a device known as a “camshaft,” which
is usually located near the engine’s
crankshaft. Exactly when the camshaft
opens the intake and exhaust valves, and for
how long, determines the four-stroke’s
timing.
The camshaft is connected to the
crankshaft by a timing gear and opens and
closes the valves using pushrods that ...
Well, don’t worry about the details now;
I’ll cover four-stroke operation and care
and feeding in my third article in this series.
If you can’t wait, you’ll find a great deal of
interesting information if you study the
diagram of the O.S. FS-70S.
The preceding brief description serves to
point out that four-strokes operate
differently from their two-stroke cousins. In
review, a two-stroke has a fuel-air
explosion every time the piston reaches the
top of its travel, whereas a four-stroke has
58 MODEL AVIATION
such a power-producing explosion every
other time the piston reaches the top.
Since a four-stroke has half the number
of explosions per series of revolutions, it
should produce only half the horsepower of
a two-stroke, and that was the case when
model four-strokes were introduced.
However, manufacturers have learned that
four-strokes are more tolerant of timing
advances, can have larger carburetor
openings and intake valves, and are easier
to “supercharge” than most two-strokes.
A modern four-stroke can produce only
75-80% of the horsepower that an
equivalent-displacement two-stroke
produces, but four-strokes do have one
advantage over two-strokes. For various
60 MODEL AVIATION
technical reasons, such as explosion
duration, timing, and the nature of valveintake
combustion, four-strokes produce
more twisting force, or torque, and are
easier to adjust.
The extra torque means that a fourstroke
can safely turn larger-diameter
propellers, with equivalent pitches, than a
comparable two-stroke. The four-strokes
are also quieter, and their exhaust note is
slightly lower pitched because of fewer
revolutions per minute (rpm).
As for two-stroke horsepower ratings,
they are of little practical use when you are
selecting an engine. Such ratings are
usually computed at extraordinarily high
rpm using small propellers. Although it is
possible to use the same small propeller on
your trainer, the average trainer airframe
has so much drag that the fast-turning
propeller will stall, and your model will
barely move through the sky.
What is propeller stall? How do you find
the right propeller for your engine/airframe
combination? What good is a spinner? As
this series progresses I’ll cover these
subjects and many more of the engines’
technical details, similarities and
differences between engine types, and their
care and feeding. I’ll also discuss field
accessories, tools, fuel types, and many
other subjects in great detail.
You may not become a model-engine
expert, but you will learn everything you
need to know to choose an engine, keep it
running reliably, make it last for years, and
get the best flying performance possible.
The next installment will cover setup,
installation, adjustment, propellers, and the
care and feeding of the popular .40-cubicinch,
two-stroke engine. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866

BOB ABERLE
CONCEIVED the
“From the Ground
Up” article series to
enable all new pilots
to get the most from
their Radio Control
modeling experience.
His concept was
successful, and his
execution of the first
two parts—radios and electric power—was
so successful that it is a real challenge to
even try to continue the series. But try I
will, and I hope to continue the spirit and
detail of this series.
As Bob did so well in his radio
installments, “From the Ground Up” will
continue to include detailed coverage of all
aspects of becoming a successful,
experienced model-aviation pilot. Selecting
and building that first airplane, learning to
fly it, maintaining it, progressing beyond
the solo, and those first steps in
advancement after the trainer will be
discussed.
However, gaining experience as a pilot
first requires flying! For the pilot to get that
flying practice, the airplane has to leave the
ground, and the most common way to make
that happen is to drag it up there behind an
engine. That makes engines the next step in
this series.
Having the correct engine for the
airplane, making it easy to start, and
running it reliably and without excessive
wear are some of the most important
aspects of learning this hobby/sport.
Learning to fly is challenge enough without
having to worry about flights that end too
quickly and far too quietly.
If you are flying with an instructor
(always recommended), engine failure too
far from the runway is almost the only way
your trainer will be damaged during your
flight education (if you follow Bob’s great
radio advice, that is). Repairing your
airplane may improve your building skills
(and is covered later in the series), but it
does little to speed your flight training.
So my goal is to present engines,
installation, mufflers, break-in and tuning
techniques, fuel, fuel tanks and supply
lines, propellers, support equipment,
spinners, maintenance, minor repair, and,
above all, safety practices, in as much detail
as possible to help you forget about your
engine while you are learning to fly. Your
engine is an important tool, but it should
not be the center of your attention during
the early stages of your becoming a model
pilot.
What is this tool called an “engine”? It is
an air- and fuel-cooled, fuel-lubricated,
venturi-fed, glow-assisted machine
constructed from aluminum, with some
steel in high-stress areas. It is designed to
convert a fuel’s chemical energy into
52 MODEL AVIATION
by Frank Granelli
Engines come in bewildering series of variations; these are .60s. L-R: K&B .65 is ABC, non-Schnuerle; SuperTigre .61 is Schnuerle
ported, ABC; black-headed Rossi .61 is high rpm, high compression, rear exhaust; O.S. rear-exhaust, long-stroke .61 produces
enormous torque at lower rpm and has fuel pump.
something that will turn a propeller.
Considering each aspect of that boring
description helps the pilot understand and
avoid some of the most common modelengine
problems.
Having a machine convert fuel into
mechanical energy releases heat. This heat
has to be removed or the machine will
literally begin to melt and fuse its moving
parts. Engines remove this heat by directing
the propeller’s airflow over most of the
engine; they are air-cooled.
However, airflow is a poor means of
engine cooling; unlike water or glycol
(antifreeze), air is not the best “heat
exchanger.” Also, the air does not remain in
contact with the engine for long; therefore,
it does not have time to absorb much heat.
Unlike water-cooling, the air cannot
reach deep into the engine to cool the
moving parts directly. Model engines use
“fins” to increase the surface area that the
air contacts, but air-cooling remains a
surface-contact process so it is inherently
inefficient.
To help remove heat the air can’t get rid
of, model engines use some fuel cooling as
well. As the fuel is converted to an air-fuel
mix in the carburetor, its density, and
therefore its temperature, drops, helping to
cool the intake and lower crankcase areas.
In the engine diagrams, you can see why
this occurs. As the fuel is burned, any
unburnt fuel is ejected from the exhaust.
Since alcohol is a great heat exchanger, the
expelled fuel droplets carry away heat.
But most important, model fuel is the
engine’s sole lubrication source. It contains
oil that keeps the moving parts separated
from each other, reducing friction and
lowering the engine’s temperature.
Unlike most car engines, which have an
independent oil source, the amount of oil
applied to a model engine’s moving parts
depends entirely on the engine’s rate of fuel
supply, or “mixture setting.” The mixture
setting adjusts the amount of fuel that is
mixed with an engine’s incoming air supply.
An engine’s maximum air supply is
fixed by the diameter of the carburetor
opening and adjusted by the area opened by
the throttle barrel. (See photo.) The pilot
April 2004 53
The cylinder liner has been removed from the engine. It is
positioned to show the large exhaust port.
The liner has been positioned to illustrate the Schnuerle port
(center) and one of the two standard intake ports (lower right).
The dark line around the top of this Webra .91 engine’s piston is the ring. Also notice
the idle mixture-adjustment screw in the center of the throttle arm.
Photos courtesy the author
adjusts the amount of fuel mixed with that
incoming air supply using high- and lowspeed
fuel-metering devices known as
“needle valves” and/or “air bleed”
adjustment screws.
By properly adjusting these fuelmetering
tools, the pilot alone is responsible
for the engine’s operating temperature and,
as a result, its reliability and durability. This
is true no matter what type of engine is
used—a two-stroke or a four-stroke.
Although there are several other breeds
of model engines, such as gas ignition or
true diesel, the two-stroke and four-stroke,
alcohol-fueled kinds comprise the majority
of what new pilots use. The exploded
engine views and photos reveal the
differences between the two major engine
types.
Of the two-stroke and four-stroke engines,
the former are the simplest kind; they are
almost as basic as engines get. They are
shown in the diagrams of the O.S. Max .40
LA and Max .25 FX.
When you start an engine, the piston
moves downward, creating a vacuum in the
cylinder area above the piston, and then
uncovers the fuel-intake ports that are cut
into the side of the cylinder. (See photo.)
54 MODEL AVIATION
O.S. Max .40 LA is typical of today’s modern two-stroke engines,
using bronze bushing to support crankshaft. This is the least
complicated a .40-size engine can get. Diagram courtesy O.S.
Engines.
O.S. Max .25 FX uses twin ball bearings in place of bushing. Up
to .65 displacement, size does not determine type of crankshaft
suspension. Larger engines are usually ball-bearing suspended.
Diagram courtesy O.S. Engines.
ABC SuperTigre .61 has no ring. The thin line on the piston is actually a shallow score
mark caused by the smaller-diameter cylinder bore. It will disappear once this engine is
run.
The intake ports connect the area above the
piston to the lower crankcase area just
behind the crankshaft, which is the device
that turns the propeller. The crankshaft is
hollow and has a slot cut into it just below
the carburetor. If the throttle barrel is open,
the vacuum above the piston is directly
connected to the air-fuel mixture in the
carburetor and draws this mixture into the
cylinder area above the piston.
Exactly when the slot is uncovered and
for how long is the engine’s “timing.” If the
slot is open sooner (more advanced) and
longer (duration), the power increases, but
that can cause other problems such as hard
starting, preignition, and poor idling.
Certain timing settings allow an engine to
achieve more power using tuned exhaust
systems than do others (but most new pilots
do not need to worry about tuning the
exhaust since they will be using standard
mufflers).
The piston continues downward,
uncovering the exhaust port, but the
pressure of the unburnt fuel-air mixture that
has been drawn into the upper cylinder area
is too low for it to escape before the piston
begins its upward journey. This mixture is
trapped between the cylinder head and
upward-moving piston, and it is
compressed.
How much the fuel-air mixture is
compressed is the engine’s “compression
ratio,” which is the proportion of the entire
cylinder volume above the piston in its full
down position to the remaining cylinder
volume with the piston fully raised (the
cylinder’s “combustion area”).
For instance, if the volume above the
piston is 10 times larger when the piston is
down than the cylinder’s combustion area,
the compression ratio is 10:1. The higher the
compression ratio, the more power the final
fuel-air explosion produces. However,
compression ratios can be too high, causing
preignition, hot running, burnt glow plugs,
and piston damage. Most sport engines do not
have high compression.
As the piston moves upward, it blocks the
intake ports again, sealing the cylinder’s
combustion area above the piston. Once the intake ports are
blocked, the upward piston movement creates a vacuum in the
crankcase area that begins to draw the next fuel-air charge into the
crankcase area; that is a “venturi” suction effect that provides the
fuel-air mix for the next cycle.
As the piston continues upward, it compresses the explosive
fuel-air mixture against the extremely hot glow-plug element. (You
did remember to hook up the battery to the glow plug so that the
element is glowing now, right?) The next step is obvious; the
compressed, explosive mixture meets the hot glow-plug wire. The
explosion moves the piston rapidly downward as the burning gas
continues to expand.
The piston eventually uncovers the exhaust port and the hot
gases escape the cylinder. But even as the cylinder is moving
downward, the burnt gases are losing pressure because of the
increasing cylinder area and time since the explosion. This creates a
vacuum above the cylinder even before all of the gases escape. The
next fuel-air mixture enters the cylinder just before—or just as—
the burnt gases are escaping. How soon is the engine’s “timing
advance.”
Once the spent gases are exhausted, fresh fuel-air gas enters the
cylinder and the process repeats. The piston continues to move up
April 2004 55
The opposite side port cut into the cylinder sleeve is the intake boost port. The front
intake port is the light area toward the right. A rear intake port is visible if you have
great eyes.
The intake slot in the crankshaft is clearly visible. This SuperTigre engine’s crankshaft
is ball-bearing supported front and rear. Check out the “bulges” that house the
bearings.
The connecting rod connects the piston to the pin on the
crankshaft, converting the piston’s up-and-down motion into
rotational motion.
and down with a fuel-air explosion every
time the piston completes its upward
journey.
Since the piston is connected to the
crankshaft—by a device called, for some
strange reason, a “connecting rod”—the
piston’s up-and-down movement is
converted to a rotating crankshaft that turns
the propeller. Both sides of the connecting
rod use bronze bushings and must be well
lubricated. This is the part that breaks if you
use a propeller that is too large or too small.
When the “rod” breaks, the rest of the
engine is usually destroyed.
This process creates a great deal of heat
which can be used to keep the glow plug
glowing even if you disconnect the battery.
In effect, the engine is working as a diesel
so it requires no outside ignition energy to
continue the process. Two-stroke engines
are the epitome of power and reliability.
There are several styles of two-stroke
engines, and each has its strong and weak
areas. The new pilot should be familiar with
a few of these designs; it is important for
proper engine selection. Look at the photo
of the two .60 engines. These are called
“.60s” because the piston displaces .60
cubic inch as it travels. The total size of the
space—stroke length multiplied by the
cylinder’s area—that the piston occupies in
56 MODEL AVIATION
Look carefully at the sides of these engines. The SuperTigre on the right has a bulge (under the “S”); that is the Schneurle boost port.
The K&B engine in the foreground uses a bronze bushing for support. The front
housing does not have the bearing-support bulges as does the ball-bearing-supported
SuperTigre in the rear.
its travels is the engine’s “displacement.”
Most two-stroke sport engines used in trainers range in
displacement from .25 cubic inch to .61 cubic inch. The larger the
displacement, the more powerful the engine and the larger the
propeller it can use. However, larger-displacement engines use
more fuel per minute and cost more. Bigger engines also require
larger airframes that could be more expensive.
Look carefully at the two .60s’ sides in one of the photos. One
engine has a straight cylinder while one has a raised area (under the
“S”). That elevated area is the space for the “boost port” designed
by a German engineer. This “Schnuerle port” is nothing more than
an extra intake that permits more air-fuel mixture into the engine
per stroke.
Some engines have several extra Schnuerle boost ports, but
almost all sport engines made today have at least one. Schnuerle
engines use more fuel and are slightly harder to adjust at idle, but
they are more powerful than non-Schnuerle engines.
Another point to consider during engine selection is the
crankshaft’s support system. If you look at the O.S .25 FX diagram,
you’ll see a front and rear ball bearing supporting both crankshaft
ends. The O.S. .40 LA diagram does not include ball bearings;
instead, there is a bronze bushing pressed into the crankcase
housing. The crankshaft slides into, and is supported by, this
bushing.
Ball bearings provide more crankshaft support, but at additional
cost. They are also more susceptible to corrosion if they are not
properly maintained. Bronze bushings tend to wear sooner than ball
bearings and are more difficult to replace, but they do not rust or
corrode. Electric starting is less problematic with ball bearings;
bushed engines require a lighter touch when pressing the starter
against the propeller.
Both types of crankshaft supports develop approximately the
same amount of power if the engines are of the same design type,
and the two main types are ABC (AAC) and ringed piston. The
latter has one or more compression rings on the piston, as does an
auto engine. The rings provide drag and do not have as tight a seal
against the cylinder wall as the ABC variety do.
ABC types are nearly exclusive to model engines. The piston is
made from aluminum, and the cylinder is constructed from brass
plated with chrome. The piston actually has a larger diameter than
does the top of the cylinder. This size difference can be felt when
turning the propeller; its rotation tends to stick as the piston reaches
the top of its stroke and enters the smaller cylinder section.
How can this work? The varied construction materials expand at
different rates when the engine heats while running. The cylinder
expands more than the piston does. The final diameter of both
becomes equal when the engine runs; this makes for a tight
compression seal at the top end and produces more horsepower.
April 2004 57
Webra .91 (L) is a two-stroke engine. Small exhaust, valve rockerarm
covers, pushrod housings, inverted rear-mounted carburetor
identify O.S. 120 FX (R) as a four-stroke.
Shown are four- and two-stroke engines’ “heads” (tops of
combustion chambers). Tubes are four-stroke’s intake and
exhaust “manifolds.” Both heads are hemispherical (recessed
half spheres) in shape for more power, made popular by
Chrysler’s “Hemis” of the 1960s.
With many parts and more complicated than two-strokes, fourstrokes
have less horsepower but more torque, making them
ideal for some Scale and sport applications. Diagram courtesy
O.S. Engines.
However, ABC engines have slightly
lower torque than ringed since compression
drops slightly as the piston progresses
downward into the larger cylinder area.
This expansion difference also provides
protection if the engine overheats. When
overheating, the cylinder continues to
expand faster than the piston, providing
extra clearance that protects these vital
parts, but the engine still sags, reducing
power, while the destruction process
begins.
ABC engines work well in the smaller
sizes—up to roughly .61 cubic inch. Most
engines that size that are sold today are
ABC or AAC (aluminum piston, aluminum
cylinder, chrome plated), but several fine
ringed designs are manufactured. Most
engines larger than .61 cubic inch are
ringed.
Most new pilots start with two-stroke
engines because they are simple, reliable,
less expensive, easy to operate, and
incredibly powerful. To date, all of the
Ready-to-Fly (RTF) trainers on the market
use two-strokes (except for one electricpowered
version). RTF trainers usually
come with an installed engine, fuel system,
and radio and a prebuilt airframe that
requires no previous modeling experience
and less than two hours to complete.
Many new pilots prefer the realistic sound,
fuel efficiency, and higher torque of a fourstroke
engine, so they forego the
convenience of an RTF airframe and
purchase an Almost Ready-to-Fly (ARF)
trainer. ARFs require a bit of modeling
know-how (e.g., gluing wing halves
together) and approximately 20 hours to
complete. The pilot must also buy and
install the radio and engine. This is
currently the only way to have a four-stroke
on your trainer.
As I mentioned about all model engines,
four-strokes are air-cooled, glow-assisted
diesels. Most important, unlike larger fourstroke
engines, such as those used in
automobiles, most lawn mowers, or
generators, the model varieties remain fuel
lubricated. That is important to remember.
Except for needing fuel lubrication,
model four-strokes work much as shown in
those blue-and-red engine diagrams some
of us learned with in school. (Of course,
those diagrams never showed the black,
messy grease we had to wade through to get
to those “simple” parts.)
Instead of drawing the fuel-air mix
through a venturi-fed, hollow crankshaft
and then through cylinder wall ports into
the combustion chamber, model fourstrokes
feed this combustible gas mixture
into the upper cylinder area through an
intake valve. As can be seen in the diagram
and photos, this one change makes for an
engine that operates using an entirely
different process.
As a four-stroke starts, the fuel-air mix
is created inside a carburetor that is almost
identical to that of a two-stroke. But instead
of then being drawn into a hollow
crankshaft, the combustion area’s vacuum
caused by the downward-moving piston
draws the fuel-air mix from the carburetor
into an intake pipe (intake manifold) and
then through the open intake valve and into
the cylinder’s combustion area. The intake
valve has to be open—pushed away from
its base—for the gases to flow into the
cylinder.
Since the steel valve can’t figure out
when to open on its own, it is forced open
by a device known as a “camshaft,” which
is usually located near the engine’s
crankshaft. Exactly when the camshaft
opens the intake and exhaust valves, and for
how long, determines the four-stroke’s
timing.
The camshaft is connected to the
crankshaft by a timing gear and opens and
closes the valves using pushrods that ...
Well, don’t worry about the details now;
I’ll cover four-stroke operation and care
and feeding in my third article in this series.
If you can’t wait, you’ll find a great deal of
interesting information if you study the
diagram of the O.S. FS-70S.
The preceding brief description serves to
point out that four-strokes operate
differently from their two-stroke cousins. In
review, a two-stroke has a fuel-air
explosion every time the piston reaches the
top of its travel, whereas a four-stroke has
58 MODEL AVIATION
such a power-producing explosion every
other time the piston reaches the top.
Since a four-stroke has half the number
of explosions per series of revolutions, it
should produce only half the horsepower of
a two-stroke, and that was the case when
model four-strokes were introduced.
However, manufacturers have learned that
four-strokes are more tolerant of timing
advances, can have larger carburetor
openings and intake valves, and are easier
to “supercharge” than most two-strokes.
A modern four-stroke can produce only
75-80% of the horsepower that an
equivalent-displacement two-stroke
produces, but four-strokes do have one
advantage over two-strokes. For various
60 MODEL AVIATION
technical reasons, such as explosion
duration, timing, and the nature of valveintake
combustion, four-strokes produce
more twisting force, or torque, and are
easier to adjust.
The extra torque means that a fourstroke
can safely turn larger-diameter
propellers, with equivalent pitches, than a
comparable two-stroke. The four-strokes
are also quieter, and their exhaust note is
slightly lower pitched because of fewer
revolutions per minute (rpm).
As for two-stroke horsepower ratings,
they are of little practical use when you are
selecting an engine. Such ratings are
usually computed at extraordinarily high
rpm using small propellers. Although it is
possible to use the same small propeller on
your trainer, the average trainer airframe
has so much drag that the fast-turning
propeller will stall, and your model will
barely move through the sky.
What is propeller stall? How do you find
the right propeller for your engine/airframe
combination? What good is a spinner? As
this series progresses I’ll cover these
subjects and many more of the engines’
technical details, similarities and
differences between engine types, and their
care and feeding. I’ll also discuss field
accessories, tools, fuel types, and many
other subjects in great detail.
You may not become a model-engine
expert, but you will learn everything you
need to know to choose an engine, keep it
running reliably, make it last for years, and
get the best flying performance possible.
The next installment will cover setup,
installation, adjustment, propellers, and the
care and feeding of the popular .40-cubicinch,
two-stroke engine. MA
Frank Granelli
24 Old Middletown Rd.
Rockaway NJ 07866