An aircraft engine must satisfy a number of major design requirements, including high reliability, long life, low weight, low fuel consumption, and low frontal area. The most important factor is reliability. Long life is mainly an economic consideration, of special interest in commercial aviation. The relative importance of the other three requirements depends upon the type of plane that the engine is intended to propel. Low weight and low fuel consumption are naturally interdependent because the fuel itself is a weight factor. Low frontal area is desirable as a means of minimizing the drag caused by the engine.

Piston Engines

The piston engine used in most propeller-driven aircraft is one of two types, the reciprocating engine and the rotary engine. In the reciprocating engine, heat energy is utilized to move pistons operating within cylinders. Cylinder arrangement is generally in-line, horizontal-opposed, or radial, and either air-cooling or liquid-cooling systems are used. Nearly all aircraft reciprocating engines are petrol operated. In general, the advantages of the reciprocating engine are reliability and fuel economy. The rotary engine replaces the pistons by a single rotating one and hence has fewer ports. It is claimed to produce lower vibration. Some engines of this type are becoming available for use in small aircraft.

Jet Engines

Most non-reciprocating aircraft engines operate on the principle of jet propulsion, and include the turbojet, the turboprop, the ramjet, and the rocket engine. The turbojet and its modifications, the turbofan and turboprop, are gas turbine engines, in which the air that enters the intake of the engine is first compressed in a compressor. Fuel is then added to burn with the oxygen in the air, increasing the gas temperature and volume. The high-pressure gases are then directed through a turbine, which drives the rotating assembly of the engine. In the case of the turbojet the expansion is partial and the residual gas, which is now at intermediate pressure, is accelerated by expansion through a rear-facing nozzle, to produce a high leaving velocity and, with it, the desired thrust.

Turboprop and turbofan engines extract most of the gas energy in the turbine, the residual jet thrust being of secondary magnitude. Turboprop engines are efficient for medium-sized planes at speeds up to about 480 to 640 km/h (300 to 400 mph). At higher subsonic speeds the turbofan is the preferred engine, as the performance of a propeller drops to a low level of efficiency. Turbofan engines use less fuel and are quieter than turbojets, but at higher, supersonic, speeds the high exhaust velocity of the turbojet is necessary.