How Jet Engines Work: Turning Fuel Into Thrust

Strip away the cowling and a jet engine is doing one thing: taking in a mass of air, accelerating it, and throwing it out the back faster than it came in. The reaction to that acceleration, by Newton's third law, is thrust pushing the aircraft forward. Everything else in the engine — the compressor stages, the combustor, the turbine — exists to make that acceleration happen efficiently and repeatably, tens of thousands of times per flight, for years without failure.

Four stages, one continuous process

A basic turbojet, the ancestor of every jet engine flying today, works through four stages that happen continuously and simultaneously rather than in sequence like a piston engine's strokes.

Air enters through the intake and hits a compressor: a series of spinning blade stages, each one squeezing the air into a smaller volume and raising its pressure and temperature. A large engine might compress incoming air by a factor of 40 or more before it reaches the combustor. That compressed air then mixes with atomized jet fuel and is ignited continuously in the combustion chamber, not in discrete explosions like a car engine's cylinders, but as a steady, controlled flame reaching temperatures well above 1,500°C — hotter than the melting point of the turbine blades it's about to pass over.

The hot, high-pressure gas then expands through the turbine, a second set of spinning blades that extracts enough energy from the gas flow to keep the compressor at the front turning, since the two are joined by a common shaft. Whatever energy remains in the gas after the turbine gets its share is expelled through the exhaust nozzle at high velocity, producing thrust.

Why turbofans dominate commercial aviation

Pure turbojets are loud, thirsty, and inefficient at the speeds airliners actually fly. Nearly every commercial jet today uses a turbofan instead, which adds a large-diameter fan at the very front of the engine, ahead of the compressor. Some of the air that fan moves goes through the engine core as before; the rest is bypassed around the core entirely and ejected out the back without being burned at all.

The ratio of bypassed air to core air is called the bypass ratio, and modern high-bypass engines route roughly 8 to 12 times more air around the core than through it. Moving a large volume of air a little bit faster turns out to be far more fuel-efficient than moving a small volume of air a lot faster, for the same amount of thrust — the same principle that makes a wide propeller more efficient than a narrow, fast-spinning one. High-bypass turbofans are also dramatically quieter, since the bypassed air partially shrouds and slows the hot, noisy core exhaust before it mixes with outside air.

The metallurgy problem nobody sees

The hardest engineering problem in a jet engine isn't the airflow, it's the metal. Combustion temperatures routinely exceed the melting point of the nickel-based superalloys used in turbine blades. Engines survive this through a combination of tricks: the blades are cast as single crystals rather than the usual polycrystalline metal, which removes the grain boundaries where cracks like to start; they're coated in ceramic thermal barrier layers a fraction of a millimeter thick; and they're hollow, with cooling air bled from the compressor threaded through internal channels and out through tiny holes across the blade surface, wrapping each blade in a thin protective film of cooler air. Without this cooling architecture, turbine blades in a modern high-thrust engine would fail within seconds of ignition.

Starting and controlling the engine

A jet engine can't bootstrap itself from a standstill, since the compressor needs to already be spinning to compress anything. Engines are started with a separate starter motor, often pneumatic, that spins the core up to a speed where fuel can be introduced and ignited; once combustion is self-sustaining and producing enough energy to drive the turbine, the starter disengages. In flight, engine output is governed almost entirely through fuel flow: more fuel means a hotter, faster-expanding gas stream, more energy delivered to the turbine, and more thrust out the nozzle, all managed today by a digital engine controller that constantly balances thrust demand against temperature and stress limits on the hardware. NASA's Glenn Research Center maintains a public beginner's guide to the underlying thermodynamic cycle that engineers use to model this balance.

Why altitude changes everything

Jet engines get more efficient at altitude, up to a point, which is why airliners climb to cruise around 35,000 to 40,000 feet rather than staying low. Air is thinner at altitude, which reduces aerodynamic drag on the airframe, and colder outside air improves the engine's thermodynamic efficiency, since the useful work an engine can extract depends on the temperature difference between the hot combustion gas and the surrounding air it's expelled into. Go too high, though, and the air becomes too thin for the compressor to gather enough mass flow, which is why every aircraft-engine combination has an optimal cruise altitude rather than an unlimited one.

Modern engine design has pushed relentlessly on one number: how much useful thrust you get per unit of fuel burned, since fuel is the largest single operating cost for an airline. That pressure is why bypass ratios keep climbing and why the fan diameters on new engines keep growing relative to the core, a trend visible just by looking at how much wider new engine nacelles are than the ones from twenty years ago on the same size of aircraft, a shift in geometry that follows directly from the compression and bypass principles above rather than from styling choices, closely related to the aerodynamics that let a wing generate lift in the first place.