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Aircraft engine

An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines.
Turbine-powered
Turboprop

A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller. The engine'sexhaust gases do not contain enough energy, compared to a jet engine, to create significant thrust 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.
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. In its simplest form 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 thencombusts. 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 through the reduction gearing 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.
Turboprops are very efficient at flight speeds below 725 km/h (450 mph; 390 knots) because the jet velocity of the propeller (and exhaust) is relatively low.

Turboshaft

A turboshaft engine is a form of gas turbine which is optimized to produce shaft power rather than jet thrust.In concept, turboshaft engines are very similar to turbojets, with additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. They are even more similar to turboprops, with only minor differences, and a single engine is often sold in both forms.Turboshaft engines are commonly used in applications that 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.A turboshaft engine may be made up of two major parts assemblies: the 'gas generator' and the 'power section'. The gas generator consists of the compressor, combustion chambers with ignitors and fuel nozzles, and one or more stages of turbine. The power section consists of additional stages of turbines, a gear reduction system, and the shaft output. The gas generator creates the hot expanding gases to drive the power section. Depending on the design, the engine accessories may be driven either by the gas generator or by the power section.
TURBOJET
The turbojet is a kind of general-purpose airbreathing jet engine. Two engineers, Hans von Ohain in Germany and Frank Whittle in theUnited Kingdom, developed the concept independently into practical engines during the late 1930s.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. Compared to turbofans, turbojets are quite inefficient if flown below about Mach 2 and are very noisy. Turbojet efficiency only comes into play at supersonic Mach numbers and high altitudes where small frontal area is optimal, and large fan blades are inefficient. Few aircraft cruise in this expensive regime, so most modern aircraft use turbofans instead for fuel economy and low altitude performance. However, turbojets are still common in medium range cruise missiles, due to their high exhaust speed, small frontal area, and relative simplicity.
Air intake
Preceding the compressor is the air intake (or inlet). It is designed to be as efficient as possible at recovering the ram pressure of the air stream tube approaching the intake. The air leaving the intake then enters the compressor. The stators (stationary blades) guide the airflow of the compressed gases.
Compressor
The compressor is driven by the turbine. The compressor rotates at a very high speed, adding energy to the airflow and at the same time squeezing (compressing) it into a smaller space. Compressing the air increases its pressure and temperature.Several types of compressors are used in turbojets and gas turbines in general: axial, centrifugal, axial-centrifugal, double-centrifugal, etc.Early turbojet compressors had overall pressure ratios as low as 5:1 After leaving the compressor section, the compressed air enters the combustion chamber.
Combustion chamber
In a turbojet the air and fuel mixture passes unconfined through thecombustion chamber. As the mixture burns its temperature increases dramatically, but the pressure actually decreases a few percent.
The fuel-air mixture must be brought almost to a stop so that a stable flame can be maintained. This occurs just after the start of the combustion chamber. The aft part of this flame front is allowed to progress rearward. This ensures that all of the fuel is burned, as the flame becomes hotter when it leans out, and because of the shape of the combustion chamber the flow is accelerated rearwards. Some pressure drop is required, as it is the reason why the expanding gases travel out the rear of the engine rather than out the front. Less than 25% of the air is involved in combustion, in some engines as little as 12%, the rest acting as a reservoir to absorb the heating effects of the burning fuel.
A central core of the flow (primary airflow) is mixed with enough fuel to burn readily. The cans are carefully shaped to maintain a layer of fresh unburned air between the metal surfaces and the central core. This unburned air (secondary airflow) mixes into the burned gases to bring the temperature down to something a turbine can tolerate.
Turbine[edit]
Hot gases leaving the combustor are allowed to expand through the turbine. Turbines are usually made up of metals such as inconel or Nimonic[6] to resist the high temperature, and frequently have built-in cooling channels.
The turbine's rotational energy is used primarily to drive the compressor. Some shaft power is extracted to drive accessories, like fuel, oil, and hydraulic pumps. Because of its significantly higher entry temperature, the turbine pressure ratio is much lower than that of the compressor. In a turbojet almost two-thirds of all the power generated by burning fuel is used by the compressor to compress the air for the engine.
Nozzle
After the turbine, the gases are allowed to expand through the exhaust nozzle to atmospheric pressure, producing a high velocity jet in the exhaust plume. In a convergent nozzle, the ducting narrows progressively to a throat. The nozzle pressure ratio on a turbojet is usually high enough for the expanding gases to reach Mach 1.0 and choke the throat. Normally, the flow will go supersonic in the exhaust plume outside the engine.
If, however, a convergent-divergent de Laval nozzle is fitted, the divergent (increasing flow area) section allows the gases to reach supersonic velocity within the nozzle itself. This is slightly more efficient on thrust than using a convergent nozzle. There is, however, the added weight and complexity since the convergent-divergent nozzle must be fully variable in its shape to cope with changes in gas flow caused by engine throttling.
Thrust Augmentation
Thrust can be increased by injecting additional fluids. It is then called wet thrust.Early and nonafterburning engines use water injection to temporarily increase thrust. Water is injected at the compressor air inlet or the diffuser to cool the compressing air which permits an increase in pressure for a higher burning. A 10-30% additional thrust can be gained. Examples being the Pratt & Whitney J-57 and derivatives still in use today.
Afterburner
An afterburner or "reheat jetpipe" is a device added to the rear of the jet engine. It provides a means of spraying fuel directly into the hot exhaust, where it ignites and boosts available thrust significantly; a drawback is its very high fuel consumption rate. Afterburners are used almost exclusively on supersonic aircraft – most of these are military aircraft. The two supersonic civilian transports, Concorde and the TU-144, also used afterburners but these two have now been retired from service. Scaled Composites White Knight, a carrier aircraft for the experimentalSpaceShipOne suborbital spacecraft, also uses an afterburner.
Propfan

A propfan is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The engine uses a gas turbine to drive an unshielded propeller like a turboprop, but the propeller itself is designed with a large number of short, highly twisted blades, similar to a turbofan's bypass compressor (the "fan" itself). For this reason, the propfan has been variously described as an "unducted fan" or an "ultra-high-bypass (UHB) turbofan". In technical papers it is described as "a small diameter, highly loaded multiple bladed variable pitch propulsor having swept blades with thin advanced airfoil sections, integrated with a nacelle contoured to retard the airflow through the blades thereby reducing compressibility losses and designed to operate with a turbine engine and using a single stage reduction gear resulting in high performance." The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. The propfan concept was first revealed by Carl Rohrbach and Bruce Metzger of the Hamilton Standard Division of United Technologies in 1975[1] and was patented by Robert Cornell and Carl Rohrbach of Hamilton Standard in 1979.[2] Later work by General Electric on similar propulsors was done under the name unducted fan, which was a modified turbofanengine, with the fan placed outside the engine nacelle on the same axis as the compressor blades.

Aircraft with propfans
Antonov An-70
Beriev A-40
EcoJet
McDonnell Douglas MD-94X

Pulsejet
A pulse jet engine (or pulsejet) is a type of jet engine in which combustion occurs in pulses. Pulsejet engines can be made with few[1]or no moving parts and are capable of running statically.Pulse jet engines are a lightweight form of jet propulsion, but usually have a poor compression ratio, and hence give a low specific impulse.One notable line of research of pulsejet engines includes the pulse detonation engine which involves repeated detonations in the engine, and which can potentially give high compression and good efficiency.

Pulsejet engines are characterized by simplicity, low cost of construction, and high noise levels. While the thrust-to-weight ratio is excellent, thrust specific fuel consumptionis generally very poor. The pulsejet uses the Lenoir cycle which lacking an external compressive driver such as the Otto cycle's piston, or the Brayton cycle's compression turbine, drives compression with acoustic resonance in a tube. Pulsejets have been used to power experimental helicopters, the engines being attached to the ends of the rotor blades. Another feature of pulsejet engines is that their thrust can be increased by a specially shaped duct placed behind the engine. The combustion cycle comprises five or six phases: Induction, Compression, (in some engines) Fuel Injection, Ignition, Combustion, and Exhaust.Starting with ignition within the combustion chamber, a high pressure is raised by the combustion of the fuel-air mixture. The pressurized gas from combustion cannot exit forward through the one-way intake valve and so exits only to the rear through the exhaust tube.The inertial reaction of this gas flow causes the engine to provide thrust, this force being used to propel an airframe or a rotor blade. The inertia of the traveling exhaust gas causes a low pressure in the combustion chamber. This pressure is less than the inlet pressure (upstream of the one-way valve), and so the induction phase of the cycle begins.

RAMJET
A ramjet, sometimes referred to as a flying stovepipe, or an athodyd which is an abbreviation of Aero thermodynamic duct, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed, thus they cannot move an aircraft from a standstill. Ramjets therefore require assisted take off likeJATO to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at supersonic speeds around Mach 3. This type of engine can operate up to speeds of Mach 6.
Ramjets can be particularly useful in applications requiring a small and simple mechanism for high-speed use, such as missiles or artillery shells. Weapon designers are looking to use ramjet technology in artillery shells to give added range; a 120 mm mortar shell, if assisted by a ramjet, is thought to be able to attain a range of 22 mi (35 km).[1] They have also been used successfully, though not efficiently, as tip jets on the end of helicopter rotors.

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