how jet engines work


While the reciprocating (or piston) engines that power cars and most light airplanes have a few commonalities with gas turbine (jet) engines, the two types are very different in most respects.

recipengBoth piston (left) and turbine (below) engines have somewhat similar stages of operation: intake, compression, combustion, and exhaust. But the similarities largely end there, the biggest difference being that in reciprocating engines those stages happen one at a time, while in turbine engines they are continuous.

Instead of compressing intake air with a piston, turbine engines use a series of wheels at the front of the engine known as compressors. Another set of wheels, known as turbines, is driven by exhaust gases departing the combustion section. Both compressor and turbine wheels are essentially sophisticated “fans,” composed of high-tolerance blades spinning at very high speeds inside a tightly-ducted cowl.

turbengMuch like a turbocharger, a turbine engine’s compressor and turbine sections are mounted on a common shaft. Intake air is compressed by the compressors, and forced into the combustion chamber. Fuel is continuously sprayed into the combustion chamber and ignited, generating expanding exhaust gases that drive the turbines. The turbines, through their shafts, drive the compressors, sustaining the process. The turbines also harness energy to drive accessories such as electrical generators and hydraulic pumps. Finally, the exhausting gases are accelerated through a nozzle at the back of the engine, producing thrust somewhat like the way air escaping the “nozzle” of an untied balloon propels it across the room.

enginecomparisonfig-webIn piston-powered airplanes the engine always turns a propeller to pull it through the air, but turbine engines can either operate as jets or drive propellers themselves. A turbine engine driving a propeller is known as a turboprop.

The basic gas turbine engine we’ve described is sometimes called a “gas generator,” or “core turbine engine.” Depending on how the exhaust gases are harnessed, the core turbine engine may be applied to turbojet, turbofan, or turboprop engines.

While most reciprocating aircraft engines burn gasoline, turbine engines consume kerosene. Turbine engines produce much more power for their weight than piston engines, but they burn more fuel and are far more expensive to manufacture. To learn more about turbine aircraft, see Greg’s book, The Turbine Pilot’s Flight Manual©2012 Gregory N. Brown

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