Model airplane engines
(glow plug or nitro)

When talking about model airplane engines by far the most common type is the glow plug engine, often also called a gas or nitro engine, the latter name being a reference to nitromethane, an ingredient of rc glow fuel. The name gas is a generic one often applied to all types of IC (internal combustion) even though in some parts of the world 'gas' means gasoline (petrol).

Just like standard gasoline powered engines, there are 2 stroke and 4 stroke model airplane engines, also sometimes referred to as 2 cycle and 4 cycle. The primary difference between the two types is that a 2 stroke engine fires per single revolution whereas a 4 stroke engine fires once per two revolutions.
There is also a distinct difference in physical appearance, as you can see from the picture below (2 stroke left, 4 stroke right). The primary difference is the presence of two pushrods on the 4 stroke engine...

Browse some 2 & 4 stroke model airplane engines.

2 stroke model airplane engines produce more power for their size and are generally more 'user-friendly'. They are cheaper to buy and easier to run, and the majority of gas rc training airplanes will be designed around using a 2 stroke engine rather than a four.

A 4 stroke engine produces more torque at lower revs and also produces less noise at a lower frequency. Because of this more realistic sound, it is quite normal for 4 stroke engines to be used in scale model airplanes.

Model airplane engine sizes

All model aero engines, whether 2 stroke or 4 stroke, are expressed as a certain size. This size refers to the engine's capacity in terms of 1/100th of a cubic inch but is usually expressed as just the number eg a .40cu.in. engine would be referred to as a 40 and a .60cu.in. engine would be called a 60 etc.
It's also worth noting that the airplane itself may also be referred to with the same number ie a '40' size trainer would be a model that has been designed to take a 40 engine.

Glow plugs

Where a gasoline internal combustion engine uses a spark plug to ignite the fuel/air mixture within the compression chamber, so a glow or nitro engine uses a small glow plug.

Although the plug filament needs to be heated up initially using a glow plug igniter, once the engine is running the heat generated within the combustion chamber keeps the filament glowing continuously and is thus able to ignite the fuel/air mixture on each revolution, so long as the filament doesn't fail.

Glow plugs can fail at anytime, so it's always a good idea to have a selection of spare ones with you whenever you fly. More information on glow plugs for rc aircraft.

Carburation

Throttle assembly of a glow plug motor Model airplane engines have a very simple carburetor. It's usually located on the front of the engine body, although newer engines have the carb at the rear for safer adjusting. The carb consists of a venturi where the fuel and air mix, idle screw to adjust the idle speed of the engine, mixture screw, or 'needle valve', to allow fine tuning of the fuel/air mixture and hence the high-end RPM and a rotating barrel.

This rotating barrel is linked to the throttle control servo and rotates in response to your input at the transmitter. As you push up the throttle stick on the tx, the barrel turns opening the venturi to let more air and fuel into the combustion chamber. As you shut down the throttle, so the barrel rotates back again and the mixture is reduced by the closing of the venturi.

As the fuel passes through the carburetor it mixes with air being sucked in through the venturi, then passes into the combustion chamber. This fuel/air mixture is forced to the top of the chamber, via transfer ports, every time the piston goes through a downward stroke.
Once inside the chamber the mixture gets compressed by the piston as the piston rises back up. This rapid compression greatly increases both the temperature and the pressure of the fuel/air mixture, and the red hot filament of the glow plug ignites the mixture. The resulting explosion forces the piston back down again. As the piston moves downwards, the burned gases are expelled through the exhaust ports, and the cycle begins again.

Model airplane engines can be very temperamental sometimes and require what seems to be endless adjustments to keep them running smoothly. It is perfectly normal, however, for the needle valve to require regular adjustment before, and even during, each flying session.
Cold and damp weather can cause starting problems for glow plug engines, but the biggest cause of a non-starting engine is simply a burnt out plug!

Browse a good selection of glow plug engines.

Other model airplane engines

While glow plug engines are by far the most common type found on the flying field, they are not the only type.

A twin cylinder gasoline model aero engine Gasoline (petrol) engines can be found in larger rc airplanes and these differ to glow engines greatly in terms of physical appearance, design, size and power output.

Both 2 stroke and 4 stroke gasoline airplane engines are available, and they vary in levels of complexity. For example, some use a traditional magneto ignition system while others might have electronic ignition, and there are even liquid cooled engines for ultimate performance.

Gasoline model airplane engines aren't cheap - even a smaller bore one can cost you a few hundred dollars and the larger ones can run into many hundreds, even thousands of dollars. Having said that, there are manufacturers that are converting their more popular glow plug engines to gasoline and these tend to be more favorably priced simply because they're smaller.

A typical rc airplane that might use a large gasoline engine would be a large scale aerobatic model, such as a 1/4 scale 'Extra', for example. Of course, there are no limits to the type of model that will take such an engine, and gasoline engines are becoming a more popular choice with many modelers; although the initial purchase cost may be greater, the ongoing running costs (ordinary gasoline compared to glow fuel) are significantly less.

A diesel motor from PAW Diesel engines are another choice but are the least common of all IC model aero engines.

Diesel model airplane engines, such as the PAW shown right, are typically used in vintage rc airplanes to keep the model as authentic as possible; such engines were being manufactured long before glow plug technology and so were the first IC engines to be used in radio control flying, having seen great success in free flight models.

Diesel engines do not use any form of plug for ignition but instead rely on the fuel/air mixture inside the combustion chamber to ignite from adiabatic heating as the piston moves up and down.
The pressure inside the chamber (compression) can be increased or decreased by turning a threaded screw on top of the cylinder head; increasing pressure aids ignition of the fuel/air mixture. The fuel is a special mix that has a high ether content, which is easily ignited.

You can view a selection of glow plug model airplane engines here.

Model airplane engine reading

'2-Stroke Glow Engines For R/C Aircraft' is a very informative book on the subject and is an excellent guide to understanding model airplane glow engines.
"This book is undoubtedly one of the best (if not THE best) texts on the subject I've encountered. The section on correct running-in of engines should be included in the box with each and every 2-stroke R/C engine sold...(customer review extract) Read more.