The traditional version is based on compression-ignition technology, colloquially referred to as 'two stroke'. This covers both 'glow' and 'diesel' engines.
The vast majority of model engines are glow engines - so called because to ensure ignition each cycle, they are fitted with a glow-plug in the cylinder head. This has the appearance of a spark plug, but has a small coil fitted in it's centre instead of an electrode. To start the engine a low voltage power source is applied to the plug to make the coil glow red hot, this then ignites the fuel. Once the engine is running the residual heat from each explosion cycle, together with a chemical reaction initiated by the catalytic platinum plug element, ensures that the glow plug element remains hot enough to ignite the fuel on the next cycle and the power supply to the plug can be removed. The fuel is primarily methanol, with up to 25% oil for lubrication, and usually a smaller percentage (around 5% is typical, but it can be much higher for specialist applications) of nitromethane, a chemical additive which improves the combustion efficiency of the methanol.
The alternative type of 2-stroke is the diesel engine. This is outwardly similar, but instead of a glow plug, there is an adjuster which moves a 'contra-piston' in the cylinder head, altering the compression ratio of the engine. The fuel used is a mixture of ether, paraffin and oil: the ether component has a low flash point, and when compressed by the engine, the temperature rise caused by the compression is sufficent to cause ignition. The adjuster is used to vary the compression ratio until smooth running is achieved. This type of engine, because of the adjustable available, can be set up to turn a bigger prop than a similar capacity glow engine.
The other type of engine that is gaining in popularity is the four stroke. These engines are more complex to manufacture, hence more expensive, and have always had a lower specific power output, but because they tend to have a quieter exhaust note, and to most ears a more realistic sound, they are very popular in scale models. The disparity in power output is not as pronounced as it was and they are now a good choice for any type of model.
They work in the same way as the familiar car engine, complete with overhead valves, but tend to be pushrod operated not overhead cam. There are also variations with side valves, rotary valves, and a recent development is a rotary sleeve valve. The illustration shows a typical engine: the pushrods are at the front, operated by a cam driven by the crankshaft, and the carburettor is behind the cylinder. The flap underneath the carb. is a rod operated choke, often needed because the position of the carb. makes it difficult to choke the engine with the traditional finger! The exhaust exits from the cylinder head on the other side, and is sometimes just a plain header pipe, but more often incorporates a small silencer.
The engines illustrated are single cylinder, but there are multi cylinder versions of both types of engine. You may find in-line, horizontally opposed or radial versions available. These can be particularly effective when used in scale models.
Recent developments in engineering and materials technology has now made it possible to create a true jet engine in a size suitable for model use. They operate at extremely high revolutions and temperatures, using liquid or gaseous fuels such as propane. Currently they are definitely a specialist engine and are very highly priced, with specialist installation and operating skills required. No doubt as development continues they will become both cheaper and easier to operate, making them suitable for general use.
The huge advantage is the possibility of creating a true scale model of a Jet aircraft, complete with an authentic sound, and this attraction means that the number of model jet engines available and in use is increasing rapidly.
Finally you might be considering an electric powered aircraft. These motors have many advantages over the traditional powerplant, being quiet, clean, and compact. Modern materials mean that they are not disadvantaged in power to weight ratio compared to liquid fuelled motors, and most model designs can easily be converted to an electric powerplant. With the ever increasing sensitivity to noise of many flying sites, electric powered models provide a way of enabling flying to take place without disturbing the neighbours.
The development of brushless motors and Lithium battery packs has allowed electric motors to reach the point where they are now a serious replacement for an i/c engine and can now power a model with similar flight times to that which might be expected from a liquid fuelled motor, without the cost and mess of fuel.
One other advantage is the ability to stop and start electric motors at will, something which is much more difficult to achieve with glow motors. This ability makes them ideal as launching powerplants for gliders, replacing winch or bungee as a means of getting to altitude. Once at the required height, the motor can be stopped and the glider flown using thermals while still retaining the ability to climb back to altitude if lift cannot be found.
Information taken from BMFA.org