Just like full size Internal Combustion (IC) 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 complete revolution whereas a 4-stroke engine fires once per two revolutions. 2-stroke model airplane engines produce more power for their size and are generally more 'user-friendly' i.e. easier to maintain. They are cheaper to buy, too, and the majority of gas RC training airplanes will be designed around using a 2-stroke engine rather than a four. 4-stroke engines produces more torque at lower revs and also produce 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.