How Does a Jet Engine Work? Suck, Squeeze, Bang, Blow
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The engine under an airliner's wing takes in about a tonne of air every second, sets fire to it, and turns the result into a push of some 40 tonnes, and it does that for tens of thousands of hours between overhauls with parts spinning at speeds that would tear an ordinary metal to pieces. The principle was patented by a 23-year-old RAF officer in 1930 and dismissed by the Air Ministry as impracticable, and it is now the only kind of engine on any large aircraft. Pilots summarise it in four words: suck, squeeze, bang, blow.
The four stages
A jet engine is a tube through which air flows continuously, and along the tube four things happen to it:
- •Suck: air enters the front through the intake and the fan
- •Squeeze: a compressor of many rows of small blades, spinning at thousands of revolutions a minute, packs the air to thirty or forty times atmospheric pressure, which heats it to several hundred degrees
- •Bang: in the combustion chamber, fuel is sprayed into the compressed air and burns continuously, raising the temperature to around 1,500 degrees and making the gas expand violently
- •Blow: the hot gas rushes out through a turbine, a set of blades that it spins as it passes, and then through the nozzle at the back at high speed, and the reaction to that jet pushes the engine forward
The turbine pays for the compressor
The trick that makes the engine self-sustaining is that the turbine at the back is on the same shaft as the compressor at the front. The hot gas leaving the combustor spins the turbine, the turbine spins the compressor, and the compressor supplies the air that feeds the combustor, so that once the engine is started, by an electric or air-driven motor spinning it up, it keeps itself running as long as fuel is supplied. About two thirds of the energy released by the fuel goes into driving the compressor; only the remainder is available as thrust. The engine is, in effect, a gas turbine, and the same machine with the exhaust used to turn a generator shaft instead of to push is what runs a gas-fired power station or a warship.
The fan does most of the pushing
The pure jet of Whittle's day, in which all the air goes through the core and out the nozzle, is efficient only at very high speed and is deafening. Every modern airliner engine is a turbofan, and the large fan at the front, three metres across on the biggest, is the main source of thrust. The core drives the fan through the shaft, and most of the air the fan moves, ten or twelve times as much as goes through the core, passes around the outside of the engine and out the back without being burned at all. Pushing a large mass of air moderately fast is more efficient than pushing a small mass very fast, for the same reason a long wing beats a short one, and it is far quieter. A modern turbofan gets about eighty percent of its thrust from the fan and twenty from the jet, and the ratio of bypass air to core air, which was zero in 1950, is now above ten.
How it survives
The turbine blades sit in gas hotter than the melting point of the metal they are made of, and they survive by three means. They are cast as single crystals of nickel superalloy, with no grain boundaries for cracks to start at; they are riddled with tiny passages through which cooler air from the compressor flows and leaks out over the surface as a protective film; and they are coated with ceramic that insulates the metal beneath. Each blade, the size of a hand, extracts about as much power as a family car's engine and is subject to a centrifugal load of several tonnes. The fan blades at the front are hollow titanium or woven carbon fibre, and the whole engine is designed to contain the loss of one of them: a test in which a blade is deliberately blown off at full speed, and the casing must hold the debris, is required before any engine is certified. Bird strikes are tested by firing carcasses into a running engine.
Whittle and von Ohain
Frank Whittle, an RAF cadet, saw in 1928 that a piston engine and propeller could not push an aircraft much past 800 kilometres an hour and that a turbine driving a jet could; he patented the engine in 1930, let the patent lapse when he could not afford the renewal, and ran the first one on a test bench in April 1937, with the throttle running away and everyone but Whittle leaving the room. In Germany, Hans von Ohain reached the same design independently, and his engine flew first, in a Heinkel in August 1939. The British Gloster Meteor and the German Messerschmitt 262 were both in service by 1944. The first jet airliner, the Comet, entered service in 1952 and was grounded by metal fatigue in its fuselage, not its engines; the Boeing 707 in 1958 made the jet age general, and the turbofan of the 1960s made it affordable. Fuel burn per passenger has fallen by about eighty percent since the Comet, and the engine on a new airliner is roughly the same weight as the one on a 707 and produces four times the thrust.
The takeaway
A jet engine sucks in air, squeezes it in a compressor, burns fuel in it, and blows the hot gas out through a turbine and nozzle, with the turbine driving the compressor so that the engine sustains itself. Modern turbofans get most of their thrust from a large fan that pushes air around the core rather than through it, which is more efficient and quieter, and the turbine blades survive gas hotter than their melting point through single-crystal alloys, internal cooling and ceramic coats.