Jet engines are a type of aircraft engine that generates thrust through jet propulsion. While the term can be more general, it is typically used to refer to internal combustion engines that are used for aircraft. While coming in a variety of forms, most jet engines utilize a rotating air compressor that is driven by a turbine while exhaust air and leftover power is used by the propelling nozzle to generate thrust. In this blog, we will provide a brief overview of the common types of jet engines, allowing you to better understand their differences, uses, and applications.
The turbojet is a jet engine type that comes in the form of a cylinder that accelerates a mass of air through itself. Once air enters the intake point, it will be forced through a series of compressors that increase air pressure. Through compression, the mass of air will increase in temperature before it reaches the combustion chamber. Vaporized fuel is then mixed with the compressed air before being ignited. With the intense heat and exhaust resulting from combustion, the large mass of air will push its way through the turbine and nozzle before exiting the assembly.
The turbine assembly of the turbojet engine is similar to the compressors, albeit not used to increase air pressure or speed. Instead, the airflow is used to drive the blades of the assembly through collision, resulting in the rotation of the turbine. As the turbine is connected to the compressor, the rotation of the turbine will also drive the compressor assembly. Once past the turbine, the hot air will then be forced through a nozzle with a small diameter so that it will accelerate. This ensures that the air exits the engine at a speed higher than the entrance speed, causing increased thrust.
While the turbojet engine compresses all of its intake air for standard operations, the turbofan engine will separate air into two different flows. A typical geared turbofan engine will have two drive shafts, those of which are the high-pressure shaft and low pressure-shaft. With the low-pressure shaft, power from the turbine is used to drive a large ducted fan located near the initial compressors. Meanwhile, the high-pressure shaft utilizes power from turbines situated behind the combustion engine in order to rotate high-pressure compressors placed after the initial compressors.
As air enters the assembly, the fan will accelerate cold and hot airflows. The cold airflow is routed around the compressors and turbines through the venturi effect, later exiting the nozzle so that it may surround the hot air. Meanwhile, the hot air will be directed through the engine as normal to be mixed with fuel for combustion, drive assemblies, and more. As turbines and compressors will absorb high amounts of mechanical energy from air, the cold air can assist in propulsion and ensure that the noise of the exit air is mitigated.
While the turboprop engine may be considered a type of propeller engine, such types have functions that are closer to a conventional turbofan. Instead of using a large forward fan, however, the turboprop has a variable pitch propeller that is connected to the assembly with a reduction gear. As is standard with many jet engines, the intake air will be compressed, heated, and used to drive turbines. As the internal engine airflow is not required for thrust, the nozzle will not accelerate air and it may be turned outwards with an exhaust thrust value below 5%. While such engine types are incapable of reaching transonic or supersonic speeds, the pitch and aerodynamic design of the propeller assembly ensures an optimal performance output. Generally, such types of jet engines are most often found on regional transport aircraft.
The ramjet engine is a more specialized type, devoid of rotating elements. Instead, the assembly is static and consists of a tube. As such engines are incapable of generating thrust while at a standstill speed or while traveling at subsonic speed, they are most efficient between Mach 3 and Mach 5. In order for the ramjet engine to function correctly, it needs to have intake air traveling at supersonic speeds. As the airflow enters the engine, it will be compressed to a subsonic speed while fuel is mixed for ignition. With the use of a divergent nozzle, the combusted exhaust air is accelerated to a supersonic speed once again, allowing the jet engine to produce large amounts of thrust. As mentioned prior, such engines cannot function properly at slow speeds, thus must be accelerated before ignition. This can be done through having the aircraft launched from an aircraft carrier or through other various methods.
Also called a reheat, afterburners are used to significantly accelerate an aircraft. To achieve such acceleration, a large mass of fuel is injected at an area behind the nozzle. When coming into contact with extremely hot exhaust gases, the mixture will immediately ignite and produce temperatures upwards of 1800 degrees Celsius. This creates a massive volume increase, resulting in exhaust gases that can be propelled from the engine for increased thrust. Afterburners are most commonly used on military aircraft, allowing for them to conduct interceptions, rapid climbs, escapes, or for maintaining equal energy while traveling at a high angle of attack. As of the present, nearly all fighter jets will have an engine that is capable of being augmented with an afterburner.
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