How Jet Engines Work
How jet engines turn fuel into thrust — the Brayton cycle, turbofan anatomy with compressors and turbines, bypass ratio, and what limits efficiency.
A jet engine produces thrust by sucking in air, compressing it, mixing it with fuel and burning it, then expelling the hot exhaust at high speed. Newton's third law does the rest: the fast-moving exhaust pushes backward, the engine and aircraft are pushed forward, sustaining flight at high speed and altitude.
The Brayton cycle: suck, squeeze, bang, blow
All air-breathing jet engines run the same thermodynamic cycle, nicknamed "suck, squeeze, bang, blow": intake air is compressed, fuel is added and burned at roughly constant pressure, and the hot gas expands through a turbine and nozzle to produce a high-velocity exhaust jet. Compression ratios in modern engines exceed 40:1, and combustor temperatures approach the melting points of the materials involved — hotter combustion means more of the fuel's energy becomes useful work, a direct consequence of the second law of thermodynamics. Unlike a piston engine, which pulses through discrete strokes as in the four-stroke engine, a jet runs this cycle continuously: compression, combustion, and expansion happen simultaneously in different parts of the machine, with air flowing steadily through.
Anatomy of a turbofan engine
The airliner engine is a turbofan: a giant front fan, driven by a shaft from the rear turbine, accelerates a large mass of air, most of which bypasses the hot core entirely. Inside the core, rows of compressor blades squeeze the air stage by stage; fuel injectors and flame holders burn it in the combustor; turbine blades extract just enough energy from the exhaust to drive the fan and compressor; the nozzle shapes the remaining flow into thrust. Fan and core streams both contribute, and the whole rotating assembly spins at thousands of revolutions per minute on precision bearings. Turbine blades endure extreme centrifugal loads and temperatures, making them masterpieces of materials science — see stress and strain for the mechanics they must survive.
Bypass ratio, efficiency, and speed limits
Propulsive efficiency favors accelerating a lot of air a little rather than a little air a lot, which is why bypass ratios have grown: modern wide-body engines pass ten or more times as much air around the core as through it, making them quieter and far more fuel-efficient than early turbojets. The tradeoff is speed — high-bypass fans lose efficiency approaching the speed of sound, capping airliners below Mach 1, while military turbojets and ramjets sacrifice efficiency for supersonic flight. Overall efficiency also depends on thermal efficiency (hotter cores) and component efficiency (better blades), and geared-fan designs let fan and turbine each spin at their ideal speed.
Tags
aviation jet engines propulsion thermodynamics