Although there are many types of model airplane engines, the 2 stroke glow-fuel powered is the most common. Sizes available for the sport pilot range from .010 cubic inch to 1.50 cubic inch displacement and higher.  The plastic, control line airplanes you see in most toy stores are usually powered by .049 engines and are considered quite small by most modeler's standards.  .25 to .60 sized aircraft are the most popular.  

How does it work?

The glow engine is unique in that it doesn't use a standard spark plug to ignite the fuel air mixtue in the combustion chamber. Instead it uses a glow plug with a special element that contains platinum.  To start a glow engine a battery is attached to the glow plug to heat the element up red hot.  The engine is then started and the battery removed.  The engine can continue to run without the starter battery because the glow plug then continues to "glow".  Hence the name.  It doesn't glow just from the heat of running though.  The heat of compression along with a chemical reaction between the platinum element and the methenol based fuel causes ignition.  It actually operates more like a diesel engine than a spark ignition engine.

What's in the fuel?

Model engine glow fuel consists of at least two ingredients,  Methanol and Oil.  A common recipe would be 80% methanol and 20% castor oil.  Most fuels in the United States contain a third ingredient called Nitro-Methane.  This gives the engine a better idle and more power.  The amount of "Nitro" in fuel ranges anywhere from 5% for sport flying to 50% for high performance racing needs.  Hence you might have some fuel that has 60% mehnanol, 20% nitro-methane, and 20% oil.  Nitro-Methane can be very expensive and therefore most people in the USA will use 5% to 10% for everyday sport flying.  In other countries the price may be even higher, so no-nitro fuels are common.  Sometimes manufacturers will add other ingredients such as anti-foaming compounds and cleaners but these are in very small amounts.  

Although castor oil is the traditional oil of choice, many fuels now contain synthetic oils or maybe a blend of castor and synthetic.  Castor oil is very good at protecting a model engine but over time can leave a build-up in the engine called varnish.  It also gets baked on the outside of the engine leaving a brown coating that is difficult to clean off. Synthetic oils overcome these disadvantages but may lack in protecting the engine at extreme temperatures due to the fact that they can vaporize.  A castor/synthetic blend is a good way to get the best of both worlds.

What's an ABC engine?

Model airplane engines many times come in ringed and ABC versions.  The ringed engine has a piston ring to create the piston-cylinder seal that is needed for compression.  An ABC engine has no ring.  The ABC stands for Aluminum, Brass, Chrome.  It means the engine has an aluminum piston that is running inside of a Brass cylinder that is chrome plated.  The ABC engine has the potential for more power and requires very little break-in.  It is, however, less tolerant of dirt or dust that may be ingested by the engine.

What is meant by Schnuerle Porting?

Some engines are advertised as being schnuerle ported.  This means that there are multiple (usually three) bypass ports in the cylinder which allow fuel up into the combustion chamber.  On a non schnuerle ported or "loop scavanged" engine there is only one bypass port which is opposite the exhaust port.  This may result in some of the fuel passing over the piston and right on out the exhaust port.  With schnuerle porting the fuel arrives from different directions and converges at the center of the cylinder which allows more fuel to be burned, hence more power.  Most newer engines use schnuerle porting.