There are three kinds of engines that power most aircraft: piston engines, jet engines, and rocket engines. Each of these have the same basic principles; the engine mixes fuel with an oxidizer in a combustion chamber, the mixture is ignited, the burning mixture creates hot, expanding gases, and these gases will either produce thrust directly or are used to push a piston or drive a turbine. There are different variations of a jet engine, also known as a gas turbine. Most have the same five key parts: an inlet, a compressor, a combustion chamber, and a turbine with a driveshaft running through them.
Turbojets are the basic jet engine. They produce a steady amount of power and are useful for low-speed jet planes that do not have high payloads. Air passes through an inlet, is compressed to 3 to 12 times its existing pressure, fuel is added and ignited in a combustion chamber, and the hot air then passes through a turbine and is expelled past a nozzle. The turbine extracts energy and powers the compressor. An afterburner may be used to increase the temperature of the gas ahead of the nozzle, which results in the capacity for higher speeds.
Turboprop engines have the same components but transfer energy from the gas to the turbine which will then turn a shaft that drives a propeller. Thrust is produced from the rotating propeller instead of expelled gas. It has better propulsion efficiencies at speeds below 500 mph. Turboshafts are similar to turboprops, but instead of driving a propeller, it provides power to a helicopter rotor.
Most modern airliners use turbofans because they are quieter and have better fuel efficiency. They operate the same way as the previous two engines however, they have a large fan at the front of the engine. A portion of the air will pass through the gas generator and the remainder passes through the fan and is ejected directly into the jet stream or mixed with the gas-generator exhaust. This is called a bypass system and increases thrust without increasing fuel consumption.
Ramjets are in the shape of a rapidly tapering nozzle. This shape naturally compresses the incoming air, so ramjets do not have compressors or turbines. They require an assisted takeoff and are used for guided-missile systems and space vehicles. The difference between a ramjet and a scramjet is that a ramjet compresses the air and reduces it to subsonic speeds while a ramjet allows the airflow to remain supersonic and the plane can go much faster.
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