A jet engine is a machine that converts energy-rich, liquid fuel into a powerful pushing force called thrust. The thrust from one or more engines pushes a plane forward, forcing air past its scientifically shaped wings to create an upward force called lift that powers it into the sky.
That, in short, is how planes work—but how do jet engines work?
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How does a jet engine work?
The jet engine converts the energy in fuel into kinetic energy that makes a plane soar through the air. A fan at the front sucks the cold air into the engine and forces it through the inlet. This slows the air down by about 60 percent and its speed is now about 400 km/h (240 mph).
A second fan called a compressor squeezes the air (increases its pressure) by about eight times, and this dramatically increases its temperature.
Kerosene (liquid fuel) is squirted into the engine from a fuel tank in the plane’s wing. In the combustion chamber, just behind the compressor, the kerosene mixes with the compressed air and burns fiercely, giving off hot exhaust gases and producing a huge increase in temperature. The burning mixture reaches a temperature of around 900°C (1650°F).
The exhaust gases rush past a set of turbine blades, spinning them like a windmill. Since the turbine gains energy, the gases must lose the same amount of energy—and they do so by cooling down slightly and losing pressure.
The turbine blades are connected to a long axle (represented by the middle gray line) that runs the length of the engine. The compressor and the fan are also connected to this axle. So, as the turbine blades spin, they also turn the compressor and the fan.
The hot exhaust gases exit the engine through a tapering exhaust nozzle. Just as water squeezed through a narrow pipe accelerates dramatically into a fast jet (think of what happens in a water pistol), the tapering design of the exhaust nozzle helps to accelerate the gases to a speed of over 2100 km/h (1300 mph). So the hot air leaving the engine at the back is traveling over twice the speed of the cold air entering it at the front—and that’s what powers the plane. Military jets often have an afterburner that squirts fuel into the exhaust jet to produce extra thrust. The backward-moving exhaust gases power the jet forward. Because the plane is much bigger and heavier than the exhaust gases it produces, the exhaust gases have to zoom backward much faster than the plane’s own speed.
In brief, you can see that each main part of the engine does a different thing to the air or fuel mixture passing through:
Compressor: Dramatically increases the pressure of the air (and, to a lesser extent) its temperature.
Combustion chamber: Dramatically increases the temperature of the air-fuel mixture by releasing heat energy from the fuel.
Exhaust nozzle: Dramatically increases the velocity of the exhaust gases, so powering the plane.
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Types of jet engines
All jet engines and gas turbines work in broadly the same way (pulling air through an inlet, compressing it, combusting it with fuel, and allowing the exhaust to expand through a turbine), so they all share five key components: an inlet, a compressor, a combustion chamber, and a turbine (arranged in exactly that sequence) with a driveshaft running through them.
Here are Types of the Jet engine
Whittle’s original design was called a turbojet and it’s still widely used in airplanes today. A turbojet is the simplest kind of jet engine based on a gas turbine: it’s a basic “rocket” jet that moves a plane forward by firing a hot jet of exhaust backward. The exhaust leaving the engine is much faster than the cold air entering it—and that’s how a turbojet makes its thrust. In a turbojet, all the turbine has to do is power the compressor, so it takes relatively little energy away from the exhaust jet.
A turboshaft is very different from a turbojet because the exhaust gas produces relatively little thrust. Instead, the turbine in a turbojet captures most of the power and the driveshaft running through it turns a transmission and one or more gearboxes that spin the rotors. Apart from helicopters, you’ll also find turboshaft engines in trains, tanks, and boats. Gas turbine engines mounted in things like power plants are also turboshafts.
A modern plane with a propeller typically uses a turboprop engine. It’s similar to the turboshaft in a helicopter but, instead of powering an overhead rotor, the turbine inside it spins a propeller mounted on the front that pushes the plane forward. Unlike a turboshaft, a turboprop does produce some forward thrust from its exhaust gas, but the majority of the thrust comes from the propeller. Since propeller-driven planes fly more slowly, they waste less energy fighting drag (air resistance), and that makes them very efficient for use in workhorse cargo planes and other small, light aircraft. However, propellers themselves create a lot of air resistance, which is one reason why turbofans were developed.
Giant passenger jets have huge fans mounted on the front, which work like super-efficient propellers. The fans work in two ways. They slightly increase the air that flows through the center (core) of the engine, producing more thrust with the same fuel (which makes them more efficient). They also blow some of their air around the outside of the main engine, “bypassing” the core completely and producing a backdraft of air like a propeller. In other words, a turbofan produces thrust partly like a turbojet and partly like a turboprop. Low-bypass turbofans send virtually all their air through the core, while high-bypass ones send more air around it. Impressive power and efficiency make turbofans the engines of choice on everything from passenger jets (typically using high-bypass) to jet fighters (low-bypass). The bypass design also cools a jet engine and makes it quieter.
5. Ramjets and scramjets
Jet engines scoop air in at speed so, in theory, if you designed the inlet as a rapidly tapering nozzle, you could make it compress the incoming air automatically, without either a compressor or a turbine to power it. Engines that work this way are called ramjets, and since they need the air to be traveling fast, are really suitable only for supersonic and hypersonic (faster-than-sound) planes. Air moving faster than sound as it enters the engine is compressed and slowed down dramatically, to subsonic speeds, mixed with fuel, and ignited by a device called a flame holder, producing a rocket-like exhaust similar to that made by a classic turbojet. Ramjets tend to be used on rocket and missile engines but since they “breathe” air, they cannot be used in space. Scramjets are similar, except that the supersonic air doesn’t slow down anything like as much as it speeds through the engine. By remaining supersonic, the air exits at much higher speed, allowing the plane to go considerably faster than one powered by a ramjet (theoretically, up to Mach 15, or 15 times the speed of sound—in the “high hypersonic” region).