There are two main propulsion systems used by R/C models today.
The internal combustion systems (glow engines) and the electric motors.
Combustion engines' energy source has so far a higher energy/weight ratio
than the batteries used to power the electrics.
However, the combustion engines are usually more noisy and more prone to
oil spillage than the electric motors.
There are two
types of glow engines.
The four-stroke and the two-stroke.
Two-stroke engines are the most used, mainly because they are
simple made, light, easy to operate, easy to maintain, and are
usually inexpensive. Two-stroke engines operate at a high
RPM and therefore can be quite noisy without a good silencer.
Nevertheless, the four-stroke engines also enjoy some
popularity, mainly because they produce a lower, more scale-like
sound and consume less fuel. They have lower power/weight
ratio and lower RPM, but provide more torque (use larger
propellers) than theirs two-stroke counter-parts.
However, since the four-stroke engines require high precision
engineering and more parts to manufacture, they are usually more
expensive. They also need more maintenance and adjustment than the
two-stroke, yet they are not too difficult to operate and maintain.
A glow engine consists basically of :
- which is the main body of the engine and houses the internal parts.
Head - mounted on the top of crankcase. It has fins to provide
Muffler - damps the exhaust noise as it exits the combustion
Carburetor - to control the amount of fuel and air that enters
Prop Shaft - is a part of the Crankshaft that protrudes from the
Crankshaft - transforms the movements of the Piston into
The Piston has a cylindrical form
and operates by an up/down movement (assuming the engine is
viewed upright) inside a sleeve, which is called Cylinder.
motor's Carburetor consists basically of :
Rotating barrel which controls the amount of fuel/air
mixture going to the combustion chamber.
Throttle arm connected to the barrel, which enables the
engine's speed to be controlled by a servo.
Idle Stop Screw to adjust how far the throttle barrel
Idle Mixture Screw to adjust the amount of fuel entering the
carburetor while the engine is idling.
Needle Valve to adjust the amount of fuel entering the
carburetor during medium and high-speed operation.
engines require a special fuel, called "glow fuel." It consists of
methanol as base, with some amount of nitro methane to increase the
energy and pre-mixed oil into the fuel, which lubricates and protects
the engine parts.
Two-stroke engines operate by igniting the fuel in its
combustion chamber once every turn of its crankshaft.
The fuel is mixed with air at the carburetor and forced into the
cylinder during the down movement of the piston (1st stroke).
While the piston moves up, the mixture is compressed and when
the piston reaches the top, the glow plug ignites the compressed
gases, forcing the piston down(2nd stroke).
On the way down exhaust gases escape through the exhaust port
while the fuel mixture enters the cylinder again.
In a four-stroke engine the fuel/air mixture enters the combustion
chamber during the down movement of the piston through a valve operated
by the camshaft (1st stroke).
When the piston moves up, the valve closes and the mixture is compressed
When the piston reaches the top, the glow plug ignites forcing the
piston down (3rd stroke).
On the next up movement of the piston, a second valve opens and allows
the exhaust gases to escape (4th stroke).
The piston moves down and the fuel mixture enters the combustion chamber
again, repeating the 1st stroke.
The glow engines usually have a simple ignition system based on a glow plug made up of a little coil of platinum wire rather
than a spark plug. A 1.5V battery is used to heat the glow plug only during the
starting procedure and is removed when the motor reaches a certain rpm. This is
possible because the glow plug keeps glowing by the heat produced during the
compression and combustion without needing the battery.
There are two
lengths of glow plugs available.
ones are normally used on engines smaller than 2.5cc (.15cu in).
Some have a metal bar across the bottom of the plug called for Idle Bar,
which prevents raw fuel from dousing the heat from the element during
There are also the so-called "hot" and "cold" glow plugs, which refer to
their effective coil operating temperature. The glow plug's temperature
depends on several factors, such as the coil's alloy, thickness and
length, the size of the hole in which the coil is located as well as
which material the glow plug's body is made of.
Usually smaller engines and those that run on less nitro prefer hotter
plugs. In case of doubt just follow the engine manufacturer's
Turbo glow plugs have a chamfered end that matches the threaded hole on
the engine's head. It is claimed to give less compression leakage
around the glow plug and less disruption of the combustion chamber.
Also the hole in the cylinder head, which exposes the glow plug to the
air/fuel mixture in the cylinder is much smaller, resulting in fewer
rough edges that could create unwanted hot spots.
The turbo plug is shown on the left of the picture below.
Glow engines may have plain bushed supported crankshaft or ball
bearings. Ball bearing engines usually have a better performance,
run smoother, and last longer but are more expensive than those with
The model engines' piston and cylinders construction are usually based
in two methods: Ringed engines or ABC. Ringed engines have been
the main method of construction until recently. It consists of an
aluminum or iron piston with a ring moving in an iron sleeve. The
ring provides the compression when operating. Ringed engines are
inexpensive to restore its compression after long usage by simply
replacing a ring, and are generally slightly cheaper. They require
an extended break-in period where the motor is run very rich to provide
lots of lubrication while the ring fits itself to the cylinder. They are
also more easily damaged if the engine is run too lean.
A more recent method is the ABC, which stands for Aluminum, Brass,
Chrome where an aluminum piston runs in a chrome plated brass sleeve.
The piston and cylinder are matched at the factory to give a perfect fit
compression. ABC engines start easily by hand, give more power
than the ringed engines, have a good life-span and are less prone to
damage with a lean run.
Schnuerle ported engines have several fuel inlet ports on three sides of
the cylinder allowing more fuel to flow to the combustion chamber.
This gives somewhat more power than with standard porting, which has
only one fuel inlet port on the side of the cylinder opposite the
exhaust outlet. A Schnuerle ported engine is usually slightly more
expensive due to higher manufacturing costs involved.
The fuel tank size and location affects the engine operation during the
flight. A typical tank placement is shown on the picture below.
engine is in the upright position, the fuel tank's centreline should be
at the same level as the needle valve or no lower than 1cm, (3/8in) to
insure proper fuel flow. A too large fuel tank may cause the motor
to run "lean" during a steep climb and "rich" during a steep dive.
Normal tank size for engines between 3.5cc (.21) and 6.5cc (.40) is 150
In order to emulate the full-size aircraft jet-power systems, it is
often used the so-called Ducted Fan, there a glow-engine drives a fan
fitted inside the model.
There are also glow engines specially designed for Ducted
Fans, which have a special
shaped head, also having the
exhaust port facing towards
the rear of the model.
are often equipped with a tuned pipe exhaust in order to improve their
efficiency at high rpm.
Since a special method to start is often required due to the reduced
access to the engine, this arrangement is not recommended for beginners.