Many of you might have flown in an aircraft at least once in your lifetime and even if you haven’t, you might have at least seen one in a picture. You might have noticed that there are different types of aircraft for different purposes.
Some are small and can accommodate only 5 people while some are huge and can carry more than 600 people, some are used for dropping bombs while some others are used for rescuing people during disasters.
One of the most essential parts of any of these aircrafts is the engine. The engine provides the thrust required to take the aircraft to the sky. Now, all aircraft cannot have the same type and size of engines.
The size and type of the engine depend upon various factors, a major one being the uses of the aircraft. Let us now go through some of these different types in detail.
Reciprocating (Piston) Engines
A turboprop consists of an intake, compressor, combustor, turbine, and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture is burnt.
The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop.
The major portion of the thrust is generated by the propeller. These are used in small aircraft used to transport a few passengers over short distances. The ATR 72-500 uses turboprop engines.
A turbojet consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine.
The compressed air from the compressor is heated by the fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust.
These are not used anymore as turbofan engines are more efficient than these. The Concorde was a supersonic jet which used turbojet engines.
Engines used in modern day airliners are of this type. They consist of a massive fan at the front which draws in the air. Most of this air passes bypasses the turbine and is directly sent to the nozzle to provide thrust.
Some of the air drawn in by the fan is sent to the compressor which compresses the air after which combustion occurs. The exhaust is used to drive a turbine which powers the fan in the front and the compressors. The exhaust gasses are then sent through a nozzle to provide thrust.
The ratio of mass flow of air bypassing the engine core to the mass flow of air entering the engine core is called bypass ratio. Modern day airliners use engines with a high bypass ratio as they are more efficient.
Ramjets are engines that do not have a moving compressor. They use the forward motion of the engine to push the air through the compressor in the engine. Because of this they require some other form of thrust at lower speeds. Therefore they are not useful in slow moving aircraft.
The compressor is a geometrical feature of the engine. It has a lower area of cross section than the inlet. Therefore the air passing through that area gets compressed. After compression the fuel is injected and combustion occurs. The exhaust gasses are then directed into a nozzle which allows for expansion into the atmosphere and provides thrust. These are mainly used in missiles. The Brahmos missile jointly developed by India and Russia uses ramjet engines.
Many more types of engines exist. The ones mentioned here are just to give a basic idea. Continuous research is being done to further improve the efficiency of the modern day engines and to develop new engines which are more efficient. We have come a long way from the early days of air travel when planes could carry very few people and required a lot of fuel. There is still a long way to go to improve the efficiency of engines and make planes lighter.