The schematic diagram below shows the process through which a jet engine converts the energy in fuel into kinetic energy that makes a plane soar through the air. (Source : Ian Schoeneberg courtesy of US Navy):
1) For a jet going slower than the speed of sound, the engine is moving through the air at about 1000 km/h (600 mph). We can think of the engine as being stationary and the cold air moving toward it at this speed.
2) 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).
3) A second fan called a compressor squeezes the air (increases its pressure) by about eight times, and this dramatically increases its temperature.
4) Kerosene (liquid fuel) is squirted into the engine from a fuel tank in the plane's wing.
5) 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).
6) 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.
7) 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.
8) 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 after burner 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, it can be seen that each main part of the engine does a different thing to the air or fuel mixture passing through:
1) Compressor: Dramatically increases the pressure of the air (and, to a lesser extent) its temperature.
2) Combustion chamber: Dramatically increases the temperature of the air-fuel mixture by releasing heat energy from the fuel.
3) Exhaust nozzle: Dramatically increases the velocity of the exhaust gases, so powering the plane.
What do jet engines look like in reality? The figure below shows a photoshoot of a real turbofan engine, opened up and undergoing maintenance.
Photo: A Pratt Whitney F117 PW-100 jet engine from a US Air Force C-17 Globemaster plane, undergoing maintenance. Photo by Joshua J. Seybert courtesy of US Air Force.
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.
But there the similarities end. Different types of engines have extra components (driven by the turbine), the inlets work in different ways, there may be more than one combustion chamber, there might be two or more compressors and multiple turbines. And the application (the job the engine has to do) is also very important. Aerospace engines are designed through meticulously engineered compromise: they need to produce maximum power from minimum fuel (with maximum efficiency, in other words) while being as small, light, and quiet as possible.
Photo: Early Turbojet engines on a Boeing B-52A Stratofortress plane, pictured in 1954. The B-52A had eight Pratt and Whitney J-57 turbojets, each of which could produce about 10,000 pounds of thrust. Picture courtesy of US Air Force.
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.
Turbojets are basic, general-purpose jet engines that produce steady amounts of power all the time, so they're suitable for small, low-speed jet planes that don't have to do anything particularly remarkable (like accelerating suddenly or carrying enormous amounts of cargo).
Photo: A turboprop engine uses a jet engine to power a propeller. Photo by Eduardo Zaragoza courtesy of US Navy.
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.
Photo: A turbofan engine produces more thrust using an inner fan and an outer bypass (the smaller ring you can see between the inner fan and the outer case). Each one of these engines produces 43,000 pounds of thrust (almost 4.5 times more than the Strato fortress engines up above)! Photo by Lance Cheung courtesy of US Air Force.
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. A measurement called the bypass ratio tells you how much air (by weight) goes through the engine core or around it; in a high-bypass engine, the ratio might be 10:1, which means 10 times more air passes around than through the core. 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.