How do Jet Engines Work
It’s one thing to wrap your head around the physics of the Wright brothers’ plane, but how do you generate enough power to sling a 350-ton Boeing 747 into the air and keep it cruising at 1,000-plus kilometres (640 miles) per hour? You strap yourself to four workhorse jet engines, that’s how.
The modern jet engine represents the 80-year evolution of the gas turbine. A turbine is any kind of rotating device that extracts energy from a fluid flow and converts it into work. A windmill is a turbine that extracts energy from the wind to turn a shaft that can be used to grind grain. Steam turbines heat water to create high-pressure jets of steam that spin turbines to generate electricity. The power of a turbine is a product of the total mass flow of fluid – whether air, steam or water, etc – through the system and the efficiency with which the turbine converts this into energy.
A gas turbine is more complicated than a windmill or steam turbine as it adds combustion into the mix. Jet engines are a form of ‘air-breathing’ gas turbine, where the fluid (air) is compressed, mixed with fuel and burned at high temperature and pressure to create a flow of hot gas that spins the turbine. That’s where the name ‘jet’ engine is derived from – the jet of hot gas that spins the turbine and streams out the back, creating a huge amount of thrust.
The mechanics and physics of a jet engine are both elegantly simple and bafflingly complex. The best way to explain how they work is to dissect an engine and show how each part contributes to the immense thrust. The most common jet engine for passenger airliners is the turbofan. These engines are encased in a tube-shaped shell that tapers from front to rear. The opening of the shell is called the inlet, or intake, where the free air stream enters.
Nothing in a jet engine is designed as an afterthought. The lip of the intake on a turbofan engine is thick. That’s because it must slow down the speed of the air stream when the plane is cruising. Think of a regular propeller, which is fully exposed to the free air stream. The propeller must work extra hard (and burn more fuel) to overcome fast-moving air as it rotates to create thrust. The large fan inside a turbofan engine works like a propeller, and the thick intake lip ensures the air enters at a constant speed. Supersonic jet engines are built with a long, sharp cone in front of the intake to ‘shock’ air to subsonic speeds before it enters.
The fan component of a turbofan engine employs 20 large blades turned by a central rotating shaft. The blades are airfoils like propeller blades, but curved into a scythe shape to maximize airflow. The role of the fan is to suck as much air as possible into the engine; the biggest jet fans spin at 5,000 rpm and could suck all the air from a large arena in seconds.
The air that is drawn into the engine is now compressed by a series of rotating discs with hundreds of small blades. The precision of these rotating discs is an engineering marvel. Again, each blade is a flawless airfoil, capitalizing on Bernoulli’s principle, which states that air passing below the blade has a higher pressure than air above the blade. As the incoming air flows from one whirling compressor stage to the next, the pressure mounts, squeezing an enormous volume of air into an increasingly smaller space.
According to the laws of thermodynamics, when a static volume of air increases in pressure, it also increases in temperature, so as the air moves through the compressor stage, it builds both in pressure and heat. Now it’s time to light the fuse. A jet engine’s immense power comes from the continuous combustion of an explosive mix of hot, pressurised air and jet fuel. The combustor itself is a doughnut-shaped tube with perforations to slow the flow of hot air. The combustor is ringed with a dozen or more fuel injectors that spray a precise mist of high-octane jet fuel. The fuel and hot air ignite at temperatures exceeding 815 degrees Celsius (1,500 degrees Fahrenheit), and the resulting superhot jet of exhaust gas runs smack into the turbines.
The job of the turbines in a turbofan engine is to convert some of the immense energy of combustion into mechanical rotary motion. Like the compressor, the turbines are arranged as multistage rotating discs fitted with hundreds of blades. Turbine blades need to withstand long periods exposed to extreme temperatures, so they are built from heat-resistant alloys and are perforated with tiny holes that channel cooler bypass air from the fan. The spinning turbines are connected by a central shaft to the compressor and fan components. In fact, it’s the rotary motion of the turbines that powers both the compressor and the fan, creating a highly efficient closed loop. Turbines can power more than compressors and fans though. In turboshaft jet engines, the turbines are connected to a secondary gearbox that powers a propeller blade – that’s how helicopters like the AH-64 Apache get their speed.
The turbines absorb some of the energy from the exhaust gas, but not all of it. The rest is directed through the rear nozzle. The tapered shape of the nozzle plays a critical role in producing thrust. The idea is to slightly restrict the flow of the exhaust gas, building up pressure before releasing it. When the highly pressurised air enters the free air stream, pressure drops steeply, which translates into high velocity. In compliance with Isaac Newton’s third law of motion -every action (force) in nature has an equal and opposite reaction – as high-velocity exhaust gas escapes from the back of the engine, it essentially pushes the plane forward.
Turbofan engines are so efficient because they get thrust from two sources: the exhaust gas and the bypass air stream. If you remember, the huge fan in the front of the engine only forces a portion of its air into the compressors. The rest – a 9:1 ratio on bigger engines – bypasses the engine core, flows through the shell and exits through a special double-barrel nozzle paired with the exhaust gas. It’s the combination of the huge fan – which acts like 20 propellers moving 1,088 kilograms (2,400 pounds) of cool air per second – and the hot exhaust gas that makes turbofans the top choice for long-haul passenger and cargo planes.
Fighter jets and other supersonic craft have engines that sacrifice fuel efficiency for raw power. When high-speed aircraft approach the speed of sound, drag increases significantly. To provide extra thrust, supersonic jet engines are armed with afterburners. The afterburner is a ring of fuel injectors located behind the turbines, directly in the hot exhaust stream. Afterburners combust the exhaust gases a second time, generating even higher exit velocities.
To exceed Mach 5, engineers are experimenting with ramjet and scramjets with no fans, compressors or turbines. Instead, air is forced into the cone-shaped intake by the speed of the craft and compressed. Fuel injectors combust this air and hot exhaust gas explodes out of a convergent-divergent nozzle. Ramjets and scramjets must be launched by rocket engines or released from other supersonic craft. NASA’s unmanned X-43 used a scramjet to reach Mach 9.6 (11,760 kilometres/7,310 miles per hour) in 2004, the fastest speed ever achieved by an air-breathing jet engine.
Fan and Compressor Blades
All fan and compressor blades in a turbofan engine are airfoils, meaning they have an elliptical leading edge like a conventional propeller blade. The tapered shape follows Bernoulli’s principle, forcing the air to move faster over the curved ‘top’ of the blade, reducing pressure and creating lift or thrust from ‘below’. Turbofan blades are also long and wide, giving them a large surface area. When 20 blades with a six-metre (20-foot) diameter are spinning at the same time, they can move around 1,100 kilograms (2,400 pounds) of air per second, producing significant thrust.
Turbofan blades are also ‘ducted fans’, which means the spinning blades are housed within a cylindrical duct rather than rotating freely. Ducted fans have the advantage of reducing a drag effect called wingtip vortices. When an elliptical wing cuts through the air, it leaves a spinning trail of air called a vortex. That vortex increases drag, vibration and noise. Ducted fans prevent this and, as a result, are quieter, run smoother and can create the same amount of thrust with shorter blades.
Is bigger always better?
A bigger engine is not necessarily more powerful. The thrust-to-weight ratio is a measurement of the power of a jet engine for its size. An engine with a high thrust-to-weight ratio produces a lot of thrust for its size, while an engine with a low thrust-to-weight ratio is generally less powerful, but not necessarily less efficient. What’s the difference? Well, for commercial airliners and cargo planes, the vast majority of the flight is spent in cruising mode. To maintain cruising speed, the engine needs to produce just enough thrust to overcome drag. Large turbofan engines, which have a low thrust-to-weight ratio, are the most efficient for this task, because the large fans burn less fuel while providing sufficient thrust. Fighter jets, on the other hand, need a high thrust-to-weight ratio in order to pull off high-speed manoeuvres and near-vertical climbs. The afterburners employed by fighter jet planes generate tremendous thrust, but consume bucketloads of fuel. In the turbofan example, a large mass of gas (air) is accelerated a small amount. In the fighter jet example, a relatively small amount of air is accelerated a large amount. Different engine designs can be tailored to other users’ requirements.