So today I am going to write on a fascinating topic that has enthralled me since a long time : Jet Engines! Let’s get right into it!
So what exactly is a Jet Engine? What does it do? Why do you use a Jet Engine?
A jet engine is a form of the power producing device known as a Gas Turbine. The Gas Turbine uses the controlled combustion of an air fuel mixture to develop mechanical rotational energy at the shaft. Besides being used in airplanes, other uses of gas turbines are in Helicopters, Military Tanks and in Power-plants for generation of electricity.
A jet engine is basically an “air-breathing” Engine that uses incoming air to produce acceleration by adding a small amount of fuel and igniting the resulting mixture. A jet engine is used primarily in airplanes to generate the pushing force called thrust, which, on interaction with the incoming air results in the production of a lifting force that allows aircraft to rise. Let’s see how it does this!
There are five major parts of a Jet engine : Fan, Compressor, Combustion Chamber, Turbine and the Nozzle.
- The Fan is the part which pulls incoming air into the engine. The fan is the part generally seen at the front end of the engine.
- The Compressor comes next into the picture. It comprises of a large number of fan blades attached to a common shaft. As the name suggests, the compressor blades compress the incoming air by pushing it into successively smaller spaces. This can be seen in the above figure as well. The main purpose of the compressor is to compress as much air as possible to provide adequate oxygen for the combustion process. The ratio of the incoming air pressure to the outgoing air pressure is generally over 30.
- In the Combustion Chamber, the incoming air is mixed with a fine mist of fuel. This fuel is injected into the combustion chamber by around 10-20 nozzles. The resulting air-fuel mixture is then ignited by a spark plug similiar to that in a car engine. The amount of fuel injected directly depends on the aircraft speed required. More the thrust required, more the fuel injected. The resulting gases formed flow out through the combustion chamber and onto the turbine blades. Since these gases are produced very rapidly, the result is a very high pressure and high velocity gas stream.
- The Turbine blades are rotated by this high velocity stream of gases and this rotates the main shaft. It should be noted that all the rotating components of the engine are mounted on this very shaft. The rotation of the turbine blades generates the rotational energy required by the compressor to compress the incoming air.
- Finally, the gas stream is expelled out of the engine through the exit nozzle. The shape of the nozzle causes the expulsion of the gas stream at high speeds. The high speed rearward exit of the gas stream generates a forward pushing force as per Newton’s Third Law of motion.
Types of Jet Engines: According to the various purposes for which a jet engine is designed and the major components, the broad classification is as follows.
- Turbojet : The turbojet engine is the most basic engine as explained above. In a turbojet, all the air sucked in is expelled from the rear nozzle. Turbojet engines are used commonly in civil aviation.
- Turboprop/Turboshaft :In a turboprop jet engine, the major components are the same except for the inclusion of a large propeller blade at the front. The propeller serves to increase the amount of thrust developed. The turboprop engine is used commonly on smaller aircraft. The main advantage of the propeller is that it provides greater efficiency than turbojets at low flying speeds. Turboshaft Engines are also used in Helicopters. Turbofans are quite noisier than other jet engines.
Airplane With a Turboprop engine
- Turbofan: Turbofan engines contain a larger fan at the entrance of the jet engine. The larger fan draws in a lot of air, but, most of the air that is sucked, bypasses the combustion chambers and is sent out directly from the exhaust. This produces an exhaust stream as a mixture of cold and hot jets. The major benefit of such a system is that it increases the mass flow of the air that is expelled out of the exit nozzle, thus directly increasing the thrust. This is the major reason that today’s aircraft can travel larger distances with a lower number of engines. The turbofan engines can produce a larger amount of thrust for the same amount of fuel consumption. Also, since turbofan engines suck in a larger amount of air, they are generally larger in size and weight.
The Rolls Royce Trent900 Turbofan Engine of an Airbus A380
- Jet Engine with Afterburners: In some aircraft,mostly military, additional thrust is obtained by using afterburners. Afterburners involve injecting more fuel directly into the high velocity jet that comes out of the turbine. The injection of the fuel into the high temperature exhaust gas stream causes it to spontaneously ignite, producing even more gas. However, the use of this technique is not very efficient and is thus, not used in civil aviation.
Afterburner on a Fighter Jet
- Ramjets : The Ramjet is a type of jet engine which has no moving parts. The ramjet involves just the inlet, combustion chamber and the exhaust. The ramjet develops thrust only by combustion of the high speed air incoming into the engine. This engine works only at very high speeds, above Mach 5 or 5 times the speed of sound. The use of this engine is limited to missiles and requires another power source until a high cruise velocity is reached.
Okay, so a few other things to note:
- Jet engines are incredibly noisy. They can produce up to 140dB of noise at takeoff.
- Jet engines typically weigh anywhere between 5000-8000 kg. A typical jet engine can produce about 400KiloNewtons of Thrust at full capacity.
- The turbines in a jet engine can produce about 30,000 HP of power. However, most aircraft do not utilise this power to develop electricity. That is done by an Auxillary Power Unit.
- General Electric’s GE90-115B, with a diameter of 3.25 metres, generating 115,000 pounds of thrust, is currently the biggest in the world. You’ll find these big boys hanging under the wings of KLM’s Boeing 777-300s.
Alright that’s it for this post! Hope you enjoyed it!
Until next time!