AIRCRAFT JET ENGINE

AIRCRAFT JET ENGINE

AIRCRAFT JET ENGINE

An aircraft engine is a propulsion system for an aircraft. The key part of a jet engine is the exhaust nozzle. This is the part which produces thrust for the jet; the hot airflow from the engine is accelerated when exiting the nozzle, creating thrust, which, in conjunction with the pressures acting inside the engine which are maintained and increased by the constriction of the nozzle, pushes the aircraft forward.



The most common jet propulsion engines flown are turbojet, turbofan and rocket. Other types such as pulsejets, ramjets, scramjets and Pulse Detonation Engines have also flown.

TURBOPROP

Turboprop engines are a type of aircraft powerplant that use a gas turbine to drive a propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller. The engine's exhaust gases contain little energy compared to a jet engine and play a minor role in the propulsion of the aircraft.


The propeller is coupled to the turbine through a reduction gear that converts the high RPM, low torque output to low RPM, high torque. The propeller itself is normally a constant speed (variable pitch) type similar to that used with larger reciprocating aircraft engines.

Turboprop engines are generally used on small subsonic aircraft, but some aircraft outfitted with turboprops have cruising speeds in excess of 500 kt (926 km/h, 575 mph). Large military and civil aircraft, such as the Lockheed L-188 Electra and the Tupolev Tu-95, have also used turboprop power. The Airbus A400M is powered by four Europrop TP400 engines, which are the third most powerful turboprop engines ever produced, after the Kuznetsov NK-12 and Progress D-27.

Turboprops are very efficient at modest flight speeds (below 450 mph) because the jet velocity of the propeller (and exhaust) is relatively low. Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required. The most common application of turboprop engines in civilian aviation is in small commuter aircraft, where their greater reliability than reciprocating engines offsets their higher initial cost.

TURBOSHAFT

A turboshaft engine is a form of gas turbine which is optimized to produce shaft power, rather than jet thrust. In principle, a turboshaft engine is similar to a turbojet, except the former features additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power.


Turboshaft engines are commonly used in applications which require a sustained high power output, high reliability, small size and light weight. These include helicopters, auxiliary power units, boats and ships, tanks, hovercraft, and stationary equipment.

The general layout of a turboshaft is similar to that of a turboprop. The main difference is that a turboprop is structurally designed to support the loads created by a rotating propeller, as the propeller is not attached to anything but the engine itself. In contrast, turboshaft engines usually drive a transmission which is not structurally attached to the engine. The transmission is attached to the vehicle structure and supports the loads created instead of the engine. However, in practice many of the same engines are built in both turboprop and turboshaft versions, with only minor differences.

TURBOJET

Turbojets are the oldest kind of general-purpose jet engines. Turbojets consist of an air inlet, an air compressor, a combustion chamber, a gas turbine (that drives the air compressor) and a nozzle. The air is compressed into the chamber, heated and expanded by the fuel combustion and then allowed to expand out through the turbine into the nozzle where it is accelerated to high speed to provide propulsion.


Turbojets are quite inefficient (if flown below about Mach 2) and very noisy. Most modern aircraft use turbofans instead for economic reasons. Turbojets are still very common in medium range cruise missiles, due to their high exhaust speed, low frontal area and relative simplicity.

TURBOFAN

A turbofan is a type of aircraft gas turbine engine that provides thrust using a combination of a ducted fan and a jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. However, the rest of the air flow bypasses the engine core and mixes with the faster stream from the core, significantly reducing exhaust noise. The rather slower bypass airflow produces thrust more efficiently than the high-speed air from the core, and this reduces the specific fuel consumption.


A few designs work slightly differently and have the fan blades as a radial extension of an aft-mounted low-pressure turbine unit. Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above. However, the lower speed also reduces thrust at high speeds.

All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation.

PULSEJET

A pulse jet engine (or pulsejet) is a very simple type of jet engine in which combustion occurs in pulses. Pulsejet engines can be made with few or no moving parts, and are capable of running statically.


Pulsejet engines are a unique type of jet engine, able to operate statically with few or no moving parts. There are two main types of pulsejet engines, both types use resonant combustion and harness the expanding combustion products to form a pulsating exhaust jet, which produces thrust intermittently.

RAMJET

A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill.


Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3. This type of jet can operate up to speeds of at least Mach 6.

Ramjets can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles, while weapon designers are looking to use ramjet technology in artillery shells to give added range; it is anticipated that a 120-mm mortar shell, if assisted by a ramjet, could attain a range of 22 mi (35 km). They have also been used successfully, though not efficiently, as tip jets on helicopter rotors.

SCRAMJET

A scramjet (supersonic combustion ramjet) is a variation of a ramjet distinguished by supersonic combustion. A scramjet, like a ramjet, essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the exhaust jet leaves at higher speed than the inlet air.


Most jet engines use a fan-style compressor to squeeze air into the engine, then spray fuel into the compressed air and ignite it to produce thrust as it exits the engine through an expansion nozzle. A ramjet uses the speed of the aircraft to compress the air, so very few moving parts are needed to operate it. In particular there is no high-speed turbine, as in a turbofan or turbojet engine, that is expensive to produce and can be a major point of failure.

Most ramjets decelerate the incoming air to subsonic speeds (relative to the engine) so that combustion can be more easily sustained. However, higher vehicle speeds cause higher increases in air pressure and temperature during this deceleration at the air intake. The properties of known materials impose effective limits on what pressures and temperatures an engine can withstand, thus limiting ramjet airspeed to about Mach 5—if the air must be decelerated inside the engine to below Mach 1. If combustion can be sustained in supersonic air inside the engine, then the vehicle can fly at higher speeds while engine inlet air deceleration pressures and temperatures remain at tolerable levels.

In order to achieve sufficient internal air pressure and temperature to function, a ramjet must be accelerated by some other means of propulsion. A scramjet, designed for higher speeds, requires acceleration to hypersonic speed before it can become active. A scramjet requires supersonic airflow through the engine; thus, similar to a ramjet, scramjets have a minimum functional speed, about Mach 4.5 for current models.

Projections for the top speed of a scramjet engine (without additional oxidiser input) vary between Mach 12 and Mach 24 (orbital velocity).