Airplanes And Aircraft: The Turbofan Engine
If ever you've been on an aircraft of any substantial size you'll probably have noticed those big cylindrical lumps of metal hanging off the wings; well, those nifty little devices are called turbofan engines and are really quite useful! Turbofan engines first came about in the 1940s but the first few never actually made it onto any airplanes and were instead used more experiments; however, now they can be found on the vast majority of large passenger airplanes and several military ones. The engines themselves are a variant on the turbojet engine - as are turboprops - and are subsequently forms of gas turbine engine and it is here that it is hence most appropriate to start.
Gas turbine - or combustion - engines are a form of engine that extracts energy from a flow of gas. The three principle parts of a gas turbine engine are the compressor, combustion chamber and the turbine:
The compressor is comprised of a series of blades fixed onto a shaft which is driven by the turbine. The compressor spins at very high speeds and forces the air into a gradually decreasing space: increasing its energy, pressure and temperature. Some of the compressed air in the compressor is 'bled' off to be used to remove ice from the inlet, cool the engine and many other things. Compressors in turbofan engines compress air to over 44 times its initial pressure. Having been compressed, the air then passes into the combustion chamber.
In the combustion chamber the compressed air is mixed with fuel before being ignited in what's called a flame can. The subsequent combustion of the air-fuel mixture causes a large rise in temperature which causes a drop in pressure - this ensures the flame travels backwards rather than forwards. The mixing of the air and fuel is done in such a way that the stoichiometric (optimum) ratio between the two is not attained; if it were attained then the resultant flame would be hot enough to melt the flame can. Around the outside of the flame can there's a secondary air flow to prevent the other metal surfaces in the engine from overheating; it later mixes with the hot gasses from the primary air flow (directly from the combustion chamber) to cool it down enough so that the turbine can cope with it.
The turbine is related at the rear of the engine and is spun by the hot gasses from the combustion chamber. The turbine is made of high temperature material and is connected via a shaft to the compressor - through which the majority of the energy from the combustion of the air-fuel mix is used to drive the compressor; the rest is ejected out the back of the engine as exhaust. Some of the energy from the shaft is used to drive other mechanical devices such as various forms of pumps, and the turbine itself is often cooled by compressed air extracted from the compressor.
Identical to a gas turbine engine but with a nozzle after the turbine, turbojet engines are used on many aircraft and some other vehicles; most notably Concorde and Thrust2 (a car that held the land speed record for 14 years). The nozzle in the gas turbine engine allows the gases to expand to atmospheric pressure creating a high-speed jet of exhaust that propels the craft forwards.
In two-spool turbojet engines, the setup described above is known as the high-pressure (HP) spool and a second, low-pressure (LP) spool is used. The LP spool consists of a second compressor in front of the first and a second turbine after the first as well - with the shaft connecting them running inside the HP spool's shaft. What this setup does is use the exhaust gases passing out of the first turbine to drive the LP turbine to spin the LP compressor - essentially acting as a turbocharger for the high-pressure spool.
Another variant on the gas turbine engine is the turboprop; where a propeller blade is mounted on an extension of the main shaft in a single-spool engine at the compressor end to pull the vehicle forwards.
Turbofan Engines (Finally!)
The difference between a turbofan engine and a two-spool turbojet engine is that the LP compressor's first set of blades are roughly twice the diameter of the rest - this is called the fan. The fan blows air back over the 'core' of the engine (the core essentially being a two-spool turbojet engine) through the nacelle (the streamlined casing of the turbofan engine). The air that flows around the core is called bypass air and mixes with the faster-flowing air from the core to produce more thrust but at a lower specific fuel consumption as the same amount of fuel is burnt as would be without the nacelle, however, the nacelle enables the bypass air to be channelled - producing more thrust for the same amount of fuel: this makes turbofan engines much more efficient than turboprops.
- Engines: Turbocharging and Supercharging
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There are, in fact, many more variants of gas turbine, turbojet and turboprop engines; this has only been a brief introduction to the fundamental principles of the way they work as is neither as technical nor specific as it could have been so as to make it more easily understandable for those with little prior knowledge.
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