What are aircraft engines like?

The reciprocating gasoline engine was the only option available during the early decades of aviation and still powers the vast majority of trainer and touring light aircraft. The most common are four- or six-cylinder horizontally opposed, air-cooled engines with dual ignition for safety and outputs of 75 to 300 kW (100 to 400 hp). They drive a fixed- or variable-pitch propeller and burn leaded aviation gasoline, although the shift toward unleaded fuels and diesel engines that burn kerosene is advancing. They are simple and relatively cheap, but lose power with altitude and their moving parts limit their reliability compared with turbines.

All aviation turbines share a core: a compressor takes in air and compresses it, a combustion chamber mixes it with kerosene and burns it at constant pressure, and a turbine extracts energy from the hot gases to drive the compressor. What changes is how the leftover energy is used. In the pure turbojet, all the gas exits through a nozzle at high speed: it's efficient at very high speeds and noisy, and today it's practically limited to older military applications and missiles. In the turbofan, the turbine also drives a large front fan that pushes a much greater mass of air around the core; in modern engines, most of the thrust comes from the fan, with bypass ratios of 9:1 to 12:1. It is the engine of every current airliner because of its moderate fuel consumption and contained noise. In the turboprop, the turbine drives a propeller through a reduction gearbox: it's the most efficient solution below about 650 km/h and powers regional and cargo aircraft. The turboshaft is the variant that delivers power through a shaft, for helicopters.

A current turbofan measures up to 3.4 m in fan diameter, like the GE9X on the Boeing 777X, and generates more than 450 kN of thrust. Its turbine blades operate in gases hotter than the melting point of their alloy thanks to internal cooling channels and ceramic coatings. Composite materials in the fan, additive manufacturing, and gearboxes between fan and turbine, as in the Pratt & Whitney PW1000G, have cut fuel consumption per seat by around 15 percent compared with the previous generation. At the other end of the spectrum are electric motors, still limited to small aircraft and demonstrators because of battery energy density, and hybrid projects that combine a turbine with a generator.