Get Primed! Glow engine essential
Gerry Yarrish, Model
Airplane News, 2/2007
Radio control model airplane
engines are marvelous pieces of engineering. Two-stroke and 4-stroke glow
engines are by far the most popular ways to power model airplanes, but if
you are making the transition from electric to glow power, you should
learn a few basics. Each type of engine has its benefits and drawbacks,
and the choice of which to use is up to you. Let’s take a closer look at
these impressive power systems.
Two-stroke engines are the most used engine types, and they have a good
power-to-weight ratio (you get a lot of power for each ounce of engine
weight). They have few moving parts and are relatively inexpensive.
Maintenance is very easy, and with proper break-in, a 2-stroke engine will
last many years.
These engines range from .010 cubic inch (ci) to over 3ci, but .40 to
.60ci are the most used sizes. Also very popular are engines with
displacements of: .049 (1/2A), .10, .15, .25, .32, .45, .46, .50 and
.90ci. All model airplanes have a recommended engine-size range, but most
perform best when powered by an engine that’s toward the top of that
range. Nothing is worse than having an under-powered model, especially
when you are learning how to fly.
You should know a few basic engine terms (see Figure 1).
to the materials used to make the engine: aluminum piston (A), fitted
inside a brass sleeve (B) that’s chrome-plated (C) [non-ringed engine].
main body of the engine.
connecting rod; the part that attaches the piston to the crankshaft. It
has a bushing at either end and is connected to the piston with a
wristpin and to the crankcase with the crankpin.
part on top of the engine; it’s usually bolted on with four or six
bolts. In its center is a threaded hole for the glow plug.
(openings) inside the engine case that transfer the fuel/air mixture
from the crankcase to the combustion chamber. The ports are opened and
closed by the piston’s upward and downward motions.
cylinder lining; it houses and guides the piston and is separate from
the engine case; it has openings (ports) cut in its side.
The engine case usually has three parts:
case that surrounds the crankshaft.
main case on which the cylinder head sits.
part that seals the back of the engine.
Some engines have a
two-piece case, but the “internals” of all 2-stroke engines are the same.
The crankshaft is supported within the front housing by ball bearings or
bushings, and it has a threaded front end. Bushings, like bearings, are
machined parts in which other parts turn, and they are often used in less
expensive engines. A prop nut and a prop washer secure the prop against
the thrust washer at the front of the engine. At its rear, the crankshaft
has a counterweight web and a crankpin that engages the bottom end of the
The connecting rod is attached to the piston with a wristpin. The piston
fits inside the engine’s sleeve, which fits into and is supported by the
engine case. The head sits atop the cylinder and the inner sleeve, and the
space between the top of the piston and the bottom of the head is called
the combustion chamber.
This Enya .50SS has a
2-needle carburetor. The large needle adjusts the high-end fuel
mixture, and the small screw (see arrow) adjusts the idle mixture.
This older Enya .29 has
an air-bleed hole (arrowed) at the front of the carb body and an
adjustment screw to control the idle mixture.
Four-stroke engines such
as this Saito FA-56 are very popular. Instead of intake and exhaust
ports, cam-driven valves and rocker arms control the fuel flow into
and out of the combustion chamber.
BASIC 2-STROKE OPERATION
A 2-stroke engine makes one revolution for every power cycle (see Figure
2). As the piston moves upward in the cylinder, it compresses a fresh fuel
charge. The fuel/air mixture heats up and is ignited by the glow plug. The
piston’s upward motion creates negative pressure inside the crankcase
below the piston, and this draws air and fuel in from the carb when the
intake valve opens. The combustion of the fuel/air mixture forces the
piston downward, and that compresses the fresh fuel charge. As the piston
travels down and the hollow crankshaft rotates, the intake valve closes
and the intake ports are opened. The compressed-fuel charge passes upward
through the ports and into the combustion chamber.
This happens just as the last of the spent fuel charge exits the
combustion chamber through the exhaust port. As the piston moves upward
again, it closes the exhaust port and opens the intake valve, and the
entire process is repeated.
Many modelers enjoy using 4-stroke engines because they have a wide
powerband (they provide more torque at lower rpm) and sound so nice while
they’re running. They are, however, somewhat more expensive and more
complicated than 2-strokes and require a bit more maintenance to operate
Instead of intake and exhaust ports, a 4-stroke engine has intake and
exhaust valves (see Figure 3). The crankshaft drives a cam assembly and
lifter rods, and tappets and valve springs open and close the valves at
the right times. Four-strokes produce a fair amount of power, but they’re
at their peak at a lower rpm range than 2-stroke engines of the same size.
In comparison, the power of a typical .90 4-stroke is roughly equivalent
to that of a .60 2-stroke engine.
A glow-powered (nitro) engine uses a glow plug to ignite the fuel. In the
center is a coil of platinum wire that glows bright red when it is
attached to a 1.5 to 2V glow driver. To start the engine, the glow plug is
energized with the glow driver; after the engine has started, the driver
is removed because the piston’s compression of the fuel charge in the
combustion chamber fires the glow plug to ignite the fuel mixture. If an
engine fails to start or begins to lose power, check the glow plug’s
condition and replace it if you need to (see the “Reading the glow plug”
"READING" THE GLOW PLUG
The pro’s monitor how well their engines are running by “reading” their
glow plugs; after they’ve flown their models, they examine the glow-plug’s
If an engine has been tuned properly and is running well, the coil will be
shiny or a light gray, and its shape will be uniform even after a hard
run. The leaner you tune your engine, the grayer and more deformed the
coil will be, and these signs warn that you’re running your engine too
lean! If that’s the case, replace the glow plug, richen the fuel mixture
immediately, and retune your engine for good performance.
Before you unthread a glow plug, you must clean the area around it. When
the engine has cooled, spray a generous shot of fuel around the glow plug
to rid the opening of dirt and debris. If you don’t do this, you risk
having this dirt fall into your engine as you remove the glow plug.
PLUG 1. TOO LEAN
This plug came from an engine that was tuned to run too lean. The housing
is slightly oil-stained, but the coil is clearly compressed into the
housing, so we know it got too hot and was nearly melted by combustion
heat. If the engine had continued to run in this condition, the element
would have melted and come out of the plug. At that point, you’d be lucky
if it just fell out through the exhaust port; it could easily cause some
type of internal engine damage.
PLUG 2. NORMAL
Here’s what a reasonably fresh plug from a well-tuned engine should look
like. The housing is relatively clean, so it hasn’t seen too much hard
duty, and the coil is shiny and hasn’t been deformed by excessive
combustion temperatures or hydraulic locking caused by being run with too
PLUG 3. TOO RICH
This was taken from an engine that had been tuned with an excessively rich
fuel mixture. The deposits on the plug housing and the coil were formed
because the plug couldn’t burn off all the excess fuel; this would
eventually make the engine difficult to start and to tune.
PLUG 4. OLD AND WORN
This plug is closer to ideal, but it’s old and may fail soon. The
lubricants in the fuel and the temperatures generated during combustion
lead to glow-plug-housing discoloration. Notice that on the first few
coils, the element is relatively clean and a white-ish color. This
indicates an optimum to slightly lean fuel mixture. An engine with this
plug makes good power, but it might be too lean if there’s a slight change
in the weather (ambient temperature).
Figure 2: Two-stroke engine operation
1. As the piston
reaches top dead center, a new charge of fuel/air mixture is drawn into
the crankcase because of the low pressure created by the piston’s upward
2. The current fuel/air mixture is then compressed by the piston in
the combustion chamber, and it heats up and is ignited by the glow plug.
This forces the piston downward.
3. As the piston comes down, it opens the exhaust port, and the
spent fuel charge begins to exit the combustion chamber. At the same time,
the piston compresses the new fuel/air mixture charge in the crankcase.
4. At bottom dead center, the piston opens the bypass port, and the
new fuel-mixture charge flows from the crankcase into the combustion
chamber as the last of the spent charge leaves.
5. The piston comes back up and seals the exhaust and bypass ports,
and the entire sequence begins again.
Figure 3: Four-stroke engine operation
1. Intake: the piston
comes down, the intake valve opens, and the fuel charge is drawn into the
2. Compression: the piston comes up as the intake valve closes and
compresses the fuel charge.
3. Ignition: when the piston is at top dead center, the glow plug
ignites the compressed fuel charge.
4. The fuel mixture expands rapidly and forces the piston downward.
5. Exhaust: the piston comes back up again while the exhaust valve
opens and the spent fuel charge is expelled.
6. The piston goes back down, and the entire sequence of events
STARTUP AND BREAK-IN
A just-out-of-the-box, brand-new engine needs special handling before it
can be run at peak output. You shouldn’t just bolt a new engine to your
model and go to the flying field. Some modelers often break in engines
while flying their models, but with your first engine, you should play it
safe and break it in at home where you have all your tools and supplies.
Short, well-lubricated runs break an engine in gently because this allows
the parts to fit together and seat gradually.
When you break in an engine, it wears all the parts slowly so that they
will match one another precisely. To operate properly, all engines must be
broken in, and some engines take longer than others. If you run your
engine without breaking it in, it will get very hot because of excessive
friction, and the localized heating will damage the internal
parts—primarily the piston/sleeve fit.
Model airplane engines
use glow plugs to ignite the fuel charge. Several kinds of plugs are
available, so check your engine’s instruction manual, and use the one
recommended by the manufacturer.
When you first get the
engine, avoid the temptation to turn the crankshaft by hand. Carefully
open the backplate, and look inside the crankcase first to make sure
that there isn’t any debris inside that could damage the engine.
First, install a new glow
plug. Gently snug it down; don’t overtighten it, or you could strip out
the aluminum threads in the engine head. Fill the fuel tank with a
2-stroke fuel that contains 5 to 10 percent nitromethane and 18 to 20
percent oil. Attach the fuel line to the needle-valve assembly; make sure
that the line doesn’t kink or touch the engine case, which will get very
hot. Attach a propeller of the recommended size, and snug the prop nut
down firmly with a 6-inch wrench. Don’t use pliers; they will damage the
Open the engine’s needle valve at least four full turns counterclockwise,
and open the throttle sleeve fully. Put your thumb over the air intake,
and flip the prop counterclockwise several times until you see fuel start
to flow through the fuel line and into the carb. When the fuel reaches the
carb, close the sleeve to about 1/4 throttle and attach the glow-plug
battery. Flip the prop over with a chicken stick or an electric starter
until the engine starts to run. When the engine has warmed up a little,
open the throttle all the way and let the engine run at a very rich, low
power setting. After about 10 minutes, stop the engine and let it cool
completely. Repeat this process several times, and gradually lean the fuel
mixture each time by turning the needle valve in (clockwise) a couple of
clicks. Don’t run the engine at high rpm and at a lean mixture setting
until you have run at least six or seven tanks of fuel through it.
You’ll know that the engine has been properly broken in when it runs
consistently without overheating and transitions smoothly from idle to
full throttle. Remember: it’s always better to operate your engine a few
clicks too rich than a few clicks too lean!
For your model to land safely, its engine must have a very reliable idle.
Some engines have a single-needle valve and a small air-bleed hole in the
carb body that is used for the low-end, or idle, fuel/air mixture setting.
Other engines have two needles: a large main needle for the high-end
setting and a smaller one for idle. Start with the engine’s factory
setting; it’s usually pretty close to the correct setting. If you bring an
engine to idle and it dies, then the low-end, fuel/ air-mixture setting is
set too lean; you must increase the amount of fuel being drawn into the
carb at idle. If the engine settles into an idle but then burbles or dies
when the throttle is opened, the low-end mixture is too rich, so you must
decrease the amount of fuel that enters the carb at idle. It’s a balancing
With a twin-needle-valve carb, the idle-needle valve is used to adjust how
much fuel enters the carb. In an air-bleed design, the air-bleed screw is
used to adjust how much air enters the carb during idle. Both types work
well, but the more powerful engines usually rely on the twin-needle-valve
CARE AND MAINTENANCE
If you take care of your engine properly, it will provide maximum power
and last for a very long time. This care should begin on the day that you
bring it home from the hobby shop. Most engines come with Allen wrenches;
keep them in a safe place, and if you lose one, replace it with one of the
An engine-maintenance check begins with the removal of the backplate and a
look inside the crankcase for metal shavings and other debris. Also remove
the head and check the combustion chamber. Squirt a little 3-In-One oil
into the engine, and turn the engine over to lubricate the bearings and
the conrod bushings. Make sure that the ports in the sleeve are aligned
with the ports in the case.
Reassemble the engine, and tighten the screws in a “crisscross” pattern.
Don’t use thread-locking compound on the engine-case screws or head
screws. It isn’t required and will make it more difficult to remove the
screws for future maintenance. It can also cause the threads in the screw
holes to strip. Just snug the screws down using Allen wrenches and two
fingers. Never force any part that won’t go on or move easily. An engine
is made mostly of aluminum, and it is very easy to strip the screw-hole
After the day’s last flight, drain the fuel out of the tank and run the
engine dry of fuel. Clean any debris from the engine, especially from the
cooling fins around the cylinder; if the fins are blocked, the engine
won’t cool properly when it is running. Squirt after-run oil into the carb
and glow-plug hole to coat the inside surfaces; this helps to prevent
corrosion. Alcohol-based fuels attract moisture, and unprotected engine
surfaces will corrode—especially the ball bearings. Oil is inexpensive
insurance for long engine life.
That’s about it! To keep your engine happy, don’t run it too lean. With
proper break in and maintenance, it will provide you with many hours of
carefree flight time, and that’s what the hobby is all about!