Aeronautical and Aerospace industries are growing fast as the propulsion technology grows. Initially, aircraft flies propelled with piston engine which rotates the propeller. The need of higher flight altitude and faster vehicle pushed the propulsion technology development to make higher performance engine such as gas turbine propulsion which is widely used in commercial flight (boeing, airbus etc.) up to high performance jet fighter which flights in supersonic even hypersonic regime.
Then, how those engines work? To understand, first we will study the working principle of turbojet. Turbojet has four main components, there are inlet, compressor, combustion chamber (combustor), turbine and nozzle as shows in the picture below:
This is the explanations of how it works:
- Air form the front of the aircraft goes into the turbojet trough the inlet. This inlet act as diffuser, which is the part to slowing down the wind speed hence raising the pressure (Bernoulli equation). Need to know, the compressor works best when the wind comes into it at low speed, on the other hand high pressure is favorable for the combustion process.
- The low speed-high pressure air then goes into compressor to be compressed up to very high pressure. This pressure increase is very important for the combustion process in combustion chamber. The process of compression followed by temperature increase.
- High pressurized air then come into the combustion chamber, or combustor and mixed with fuel. Initially, this combustion process is triggered with spark plug and then continuously burned after it.
- This very high elevated temperature causes extremely high air expansion that could drive the turbine blade in the case of turbine gas propulsion which is turns with extremely high rpm. This turbine assembly is connected to compressor as explained above, hence make continuous energy extraction from the fuel to high speed gas.
- The high speed and high temperature gas from turbine then accelerated with nozzle by reducing its cross-section area. This accelerated gas creates the action and reaction forces called thrust to propel the engine or aircraft as a whole.
As for, to improve turbojet’s performance, some modifications are used such as:
- Afterburner
As explained above in the turbojet working principle, hot gas out from turbine has a lot of thermal energy to convert into kinetic energy (in the form of velocity) with the aid of nozzle. One mean to boost this thermal energy is to introduce the fuel after the gas pass through the turbine, and the combustion processes occur once more, also known as afterburner. Despite its effectiveness of increasing thrust energy, this way is wasting a lot of fuel.
- Turboprop
Other than to rotate the compressor, energy from turbine rotation sometimes extracted to rotate the propeller in front of the engine. This is done to improve engine efficiency for slow to medium speed flight. This kind of engine is used for medium speed and medium capacity aircraft.
- Turboshaft
Similar with turboprop, the turboshaft engine used the turbine rotation energy to rotates the shaft to generate the other forms of energy, such as electricity, actuators etc.
- Turbofan
This engine also similar with turboprop in principle. Fan is different with propeller in term of function, not just increases the wind pass through the engine, fan also increases air compression. On the other hand, the air pass through the fan divided by the air pass through the compressor also known as by-pass ratio. The Higher the by-pass ratio, the more efficient in the low speed the engine will be.
This kind of engine is widely used in commercial transport aircraft because it is ideal for medium speed with long range operation.
- Ramjet
Different from the engines stated above, ramjet does not have compressor-turbine coupling system. It’s just has inlet, combustion chamber and nozle. Ramjet commonly used for extremely high-speed flight, in such high speed, the very high-velocity air pass through the inlet converted into extremely high-pressure air which in turn can be used for combustion process without any means of compressor.
For the summary, there are no such “best engine” choices for all of the vehicle, but all are depending on its operating conditions.
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