Turbines
Another well-known propulsion system is the gas turbine or jet engine. There are several types of gas turbine engines, but the simplest ones are the so-called turbojets.
These engines are shaped like a cylinder containing several parts inside, which rotates on a central shaft. An auxiliary electric motor is needed to start the turbine engines.
The outside air enters the engine through the inlet into the compressor, which consists of one set of fixed blades (stator) and another of rotating blades (rotor). The air is then compressed at the compressor section and enters thereafter the burner where the fuel/air mixture is ignited. This creates a hot gas passing through the turbine and out the nozzle, which is shaped to accelerate the hot exhaust. The turbine uses the energy from the hot exhaust to rotate and since the turbine is linked to the compressor by the central shaft, it will also keep the compressor rotating, thus no longer needing the electric motor.
Normally the model aircraft turbines use propane/butane gas along with a glow plug to start the ignition and rise the burner's temperature above 100oC before liquid fuel is injected through small holes into the burner. Once the combustion gets started, the glow plug is no longer needed.
The combustion process may be controlled or stopped by regulating the amount of the fuel available, the amount of oxygen available or the source of heat.
Unlike the conventional combustion/piston engines, the jet engines don't have a natural limitation of the rpm. This means that the rpm will keep rising as more fuel is fed to the engine until the materials no longer withstand the high temperature and/or the high rpm and will breakdown. Therefore, an Electronic Control Unit (ECU) is required to limit the max fuel flow. The max value is set by using an external device called Ground Support Unit (GSU).
Since model aircraft powered by gas turbines flies very fast, with speeds up to 500Km/h (312mph), these type of engines are definitely not recommended for the beginners. Besides, operating gas turbines also involves some risks.
For further info about the function of other turbine engine types such as Turbofan, Turboprop and Ramjet, check here.