A jet engine is a type of reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. In modern aviation jet engines play a crucial role. Almost one in every two aircraft flying today is jet-powered.
In today’s article, we are going to discuss the various types of the jet engine which are in use with the armed forces.
Types of Jet Engine
All the jet engines that are in active use are airbreathing Jet Engines. These can be further classified into the following subcategories:-
- Turbofan Jet Engine
- Turbojet Jet Engine
- Turboprop Jet Engine
- Turboshaft Jet Engine
Turbojet Jet Engine
The turbojet is an airbreathing jet engine, typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine (that drives the compressor). The compressed air from the compressor is heated by burning 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. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany developed the concept independently into practical engines during the late 1930s.
History of Development
- The first patent for using a gas turbine to power an aircraft was filed in 1921 by Frenchman Maxime Guillaume. His engine was to be an axial-flow turbojet but was never constructed
- In 1928, British RAF College Cranwell cadet Frank Whittle formally submitted his ideas for a turbojet to his superiors. In October 1929 he developed his ideas further.
- On 16 January 1930 in England, Whittle submitted his first patent (granted in 1932). The patent showed a two-stage axial compressor feeding a single-sided centrifugal compressor. In Germany, Hans von Ohain patented a similar engine in 1935.
- On 27 August 1939, the Heinkel He 178 became the world’s first aircraft to fly under turbojet power, with test pilot Erich Warsitz at the controls, thus becoming the first practical jet plane. The Gloster E.28/39 was the first British jet-engined aircraft to fly. It was designed to test the Whittle jet engine in flight, leading to the development of the Gloster Meteor.
While the turbojet was the first form of gas turbine powerplant for aviation, it has largely been replaced in use by other developments of the original concept. Turbojets have been replaced in slower aircraft by turboprops because they have better specific fuel consumption. At medium speeds, where the propeller is no longer efficient, turboprops have been replaced by turbofans. The turbofan is quieter and has better range-specific fuel consumption than the turbojet. Turbojets can be highly efficient for supersonic aircraft.
Turbojets have poor efficiency at low vehicle speeds. Thus Turbojets were used on Concorde and the longer-range versions of the TU-144 which were required to spend a long period travelling supersonically. Turbojets are still common in medium-range cruise missiles, due to their high exhaust speed, small frontal area, and relative simplicity. They are also still used on some supersonic fighters such as the MiG-25,
Turbofan Jet Engine
A turbofan engine, also known as a fanjet or bypass engine, is a type jet engine which produces thrust using a combination of jet core efflux and bypass air accelarated by a ducted fan that is driven by the jet core.
The ratio of the mass of air bypassing the engine core versus the mass of the air going through the core is referred to as the bypass ratio.
A turbofan engine which derives most of its thrust from the jet engine core efflux called as low bypass engine whereas an engine that derives most of its thrust from the fan is called as a high bypass engine. In general, low bypass engines are most commonly found in military aircrafts and may be equipped with an afterburner, whereas high bypass turbofan engines are the prevalent design in today’s commercial aviation jet engines.
An afterburner is an additional component present in some jet engines, mostly military supersonic fighter jets. Its purpose is to provide an increase in thrust, usually for supersonic flight, takeoff and for combat situations. Afterburning is achieved by injecting additional fuel into the jet pipe downstream of the turbine. The advantage of afterburning is significantly increased thrust; the disadvantage is its very high fuel consumption and very poor efficiency, though this is often regarded as acceptable for the short periods during which it is usually used.
Difference between turbojet and turbofan
|A turbojet, or “straight jet” engine, consists of the four stages of thrust generation arranged in a straight line within a tube.||A turbofan consists of a turbojet engine (the “core”) surrounded by a larger tube (the “bypass”).|
|A Turbojet engine produces entire thrust by expanding the turbine exhaust through the nozzle by accelerating the gases(which has undergone combustion) to high speed.||A Turbofan has two types of thrust, fan thrust(cold thrust) and core thrust(hot thrust). Most of the modern turbofans produces majority thrust(80%) by the fan.|
|Turbojets do not have any bypass mechanism. All the air that enters the intake passes through the combustion chamber.||Turbofan have large bypass ratio. Only 20 to 40% of the intake air passes through combustion chamber.|
|Turbojets are efficient at very high speed. Usually optimum efficiency is attained at supersonic speeds.||Turbofans are efficient at medium speed (transonic speed). Supersonic speeds is attained by highly inefficient afterburners.|
Turboprop Jet Engine
A turboprop engine is a type of a jet engine that has been optimised to drive a propeller. Turboprop equipped aircraft are very efficient at lower flight speeds (less than Mach 0.6), burning less fuel per seat-mile and requiring significantly less runway for takeoff and landing than a turbojet or turbofan-powered aircraft of the same size. When the aircraft is used over relatively short distances, these cost and performance benefits offset the lower speed making turboprops the engine of choice for most commuter and frieght carrier aircraft. Examples of turboprop-powered aircraft include the C130J Super Hercules, HS 748 Avro etc
A turboprop engine employs the same principles as a turbojet to produce energy, that is, it uses a compressor, combustor and turbine within the gas generator of the engine. The primary difference between the turboprop and the turbojet is that additional turbines, a power shaft and a reduction gearbox have been incorporated into the design to drive the propeller. The gearbox may be driven by the same turbines and shaft that drive the engine compressor, mechanically linking the propeller and the engine, or the turbines may be separate with the power turbine driving a concentric, mechanically isolated shaft to power the gearbox. The turbines extract almost all of the energy from the exhaust stream using some of it to power the engine compressor and the rest to drive the propeller.
Turboshaft Jet Engine
A turboshaft engine is a variant of a jet engine that has been optimised to produce shaft power to drive machinery instead of producing thrust. Turboshaft engines are most commonly employed in applications that require a small, but powerful, lightweight engine, inclusive of helicopters and auxiliary power units (APU often mounted at the tail of aircraft)
A turboshaft engine uses the same principles as a turbojet to produce energy, that is, it uses a compressor, combustor and turbine within the gas generator of the engine. The primary difference between the turboshaft and the turbojet is that an additional power section, consisting of turbines and an output shaft, has been incorporated into the design.
A turboshaft engine is quite similar to a turboprop. The principal difference between the two is that the turboprop version must be designed to support loads of the attached propeller whereas a turboshaft engine need not be as robust as it normally drives a transmission which is structurally supported by the vehicle and not by the engine itself.