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A jet engine is a type of air-breathing turbine engine, often used on aircraft. The principle of all jet engines is essentially the same. The engine draws air in at the front and compresses it. The air is combined with fuel, typically ignited by flame in the eddy of a flame holder, and burned as an atomized mixture. The combustion greatly increases the energy of the gases which are then exhausted out of the rear of the engine. The process is similar to a four-stroke cycle, with induction, compression, ignition and exhaust taking place continuously. The engine generates thrust because of the acceleration of the air through it - the equal and opposite force this acceleration produces (Newton's third law) is thrust. A jet engine takes a relatively small mass of air and accelerates it by a large amount, whereas a propeller takes a large mass of air and accelerates it by a small amount. The efficiency of the process, like any heat engine, is determined by the ratio of the compressed air's volume to the exhaust volume. The compression of the air passing into the ignition chamber prevents backflow from it and thus makes possible the continuous burn and propulsion process.
The advantage of the jet engine is its efficiency at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft, a propeller (powered by a gas turbine), commonly known as a Turboprop, is more common. Very small aircraft generally use conventional piston engines to drive a propeller.
The earliest attempts at jet engines were hybrid designs, in which an external power source supplied the compression. In this system (called a thermojet by Secondo Campini) the air is first compressed by a fan driven by a conventional gasoline engine, mixed with fuel, and then burned for jet thrust. Three known examples of this type of design were the Henri Coanda's Coanda-1910 aircraft, the much later Campini Caproni CC.2, and the Japanese Tsu-11 engine intended to power Ohka kamikaze planes towards the end of World War II. None were entirely successful, and the CC.2 ended up being slower than a traditional design with the same engine.
The key to the useful jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. Work on such a "self-contained" design started in England in 1930 when Frank Whittle submitted patents for such an engine (granted in 1932) using a single turbine stage in the exhaust to drive a centrifugal compressor. In 1935 Hans von Ohain started work on a similar design in Germany, seemingly unaware of Whittle's work.
Ohain approached Ernst Heinkel, one of the larger aircraft industrialists of the day, who immediately saw the promise of the design. Heinkel had recently purchased the Hirth engine company, and Ohain and his master machinist Max Hahn were set up there as a new division of the Hirth company. They had their first HeS 1 engine running by 1937. Unlike Whittle's design, Ohain used hydrogen as fuel, which he credits for the early success. Their subsequent designs culminated in the HeS 3 of 1,100 lb (5 kN), which was fitted to Heinkel's simple He 178 airframe and flew in August 1939, an impressively short time for development. The He 178 was the world's first jetplane.
In England, Whittle had significant problems in finding funding for research, and the Air Ministry largely ignored it while they concentrated on more pressing issues. Using private funds he was able to get a test engine running in 1937, but this was very large and unsuitable for use in an aircraft. By 1939 work had progressed to the point where the engine was starting to look useful, and Whittle's Power Jets Ltd. started receiving Air Ministry money. In 1941 a flyable version of the engine called the W.1, capable of 1000 lb (4 kN) of thrust, was fitted to the Gloster E28/39 airframe, and flew in May 1941.
One problem with both of these early designs, which are called centrifugal-flow engines, was that the compressor works by "throwing" air outward from the intake to the sides of the engine, where the air is then compressed by being "crushed" up against the side. This leads to a very large cross section for the engine, as well as having the air flowing the wrong way after compression - it has to be collected up and "bent" to flow to the rear of the engine where the turbine is located.
German Anselm Franz of Junkers' engine division (Junkers Motoren, or Jumo) addressed this problem with the introduction of the axial-flow compressor. Essentially, this is a turbine in reverse. Air coming in the front of the engine is blown to the rear of the engine by a fan, where it is crushed against a set of non-rotating blades called stators. The process is nowhere near as powerful as the centrifugal compressor, so a number of these pairs of fans and stators are placed in series to get the needed compression. Even with all the added complexity, the resulting engine is much smaller. Jumo was assigned the next engine number, 4, and the result was the Jumo 004 engine. After many teething troubles, mass production of this engine started in 1944 as a powerplant for the world's first jet-fighter aircraft, the Messerschmitt Me 262. The Me 262 came too late to decisively impact Germany's position in World War II, but it will be remembered as the first use of jet engines in service. After the end of the war, the German Me 262 aircraft were extensively studied by the victorious allies, and contributed to work on early Soviet and US jet fighters.
British engines also were licensed widely in the US. American designs wouldn't come fully into their own until the 1960s. Their most famous design, the Turbojet
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Whittle's and von Ohain's designs are now classified as turbojets, mostly to distinguish them from some of the types outlined above. Generally turbojets are arranged around a central shaft running the length of the engine, with the compressor and turbine connected to the shaft at either end. In the middle is a combustion area, typically in the form of a number of individual "flame cans" which are used to stabilize the combustion.
Like all heat engines, the efficiency of a jet engine is strongly dependent upon the temperature of the exhaust gas -- a higher temperature means more energy from the fuel. As shown by the ideal gas law, temperature and pressure in a gas are inversely related. A simplification is to compare the pressure of gas taken in to when it is burned, the so-called compression ratio. Early jet engines had compression ratios as low as 5:1, compared to an Otto cycle engine at anywhere from 6:1 to 9:1. The limiting factor is the temperature at the front of the turbine; increasing the compression ratio means that there is considerably more fuel/air mixture (the charge) burning in the flame cans, and a higher temperature. If the temperature in the engine gets too high, it can melt or burn the materials used to build the engine. This can be a problem when taking off; as the aircraft climbs, the ambient pressure (and temperature) drops and the compressor can be run at higher ratios.
German engines had serious problems in this regard. Their early engines averaged only 10 hours of operation before failing--often with chunks of metal flying out the back of the engine when the turbine overheated. British engines tended to fare much better due to better metals. For a time some US jet engines included the ability to inject water onto the engine to cool the exhaust in these cases. This was particularly notable because of the huge amounts of water vapor that would pour out of the engine when it was turned on.
Today these problems are much better handled, but temperature still limits airspeeds in supersonic flight. At the very highest speeds the compression of the air raises the temperature to the point that fan blades will melt. At lower speeds, better materials have increased the critical temperature, and automatic throttle controls have made it nearly impossible to overheat the engine. In addition, an effective solution to blade heating has been to bleed off some of the air from the compressor, run it down the shaft, and blow it through expensively shaped, hollow turbine blades. The blades remain below their melting point, even when the gas around them is considerably hotter. The quality of bleed systems has also continued to improve to the point where the latest Rolls-Royce Trent designs operate at a compression ratio of 44:1, considerably better than piston engines.
On the downside, the compressor uses up about 60 to 65% of all of the power generated by a jet engine. This helps to explain why they aren't used in cars, the engine would be burning a huge quantity of fuel even while sitting at a red light. Even in aircraft, every bit of efficiency in running the compressor is needed. One common design technique is to use more than one turbine to drive the compressors at various speeds. Most such designs that use two stages are known as "two spool" engines. A few have used three stages.
Given that 60% of the engine's power is being used for driving the compressor, one option for better efficiency is to do less compression - that is, make a smaller engine. This seems self-defeating, but it's not the case. If the engine uses some of that energy to push the air, rather than compress it, it can generate the same thrust with less compression.
If the propeller is better at low speeds, and the turbojet is better at high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Turbofans essentially increase the size of the first-stage compressor to the point where they act as a ducted propeller (or fan) blowing air past the "core" of the engine.
This type of engine runs best from about 250 to 650 mph (400 to 1,000 km/h), which is why the turbofan is by far the most used type of engine for aviation use.
The bypass ratio (the ratio of bypassed air mass to combustor air mass) is an important parameter for turbofans. Early turbofans (and most modern jet fighter engines) are low-bypass turbofans with bypass ratios less than 1. However, the "large mouthed" engines on almost all modern civilian jet aircraft are high-bypass turbofans which generally have bypass ratios of 3 or more.
Turbofans (especially high bypass engines) are fairly quiet. The noise of a jet engine is strongly related to the temperature of the air coming out the back. In the turbofan this hot air is mixed with the cold air bypassing the engine, so the result is a much lower temperature. Jet aircraft are often considered loud, but a conventional piston engine delivering the same power would be much louder.
The components of a jet engine are standard across the different types of engines (noted above). The parts include:
The various components named above have constraints on how they are put together to generate the most efficiency or performance. Important here is air intake design, overall size, number of compressor stages (sets of blades), fuel type, number of exhaust stages, metallurgy of components, amount of bypass air used, where the bypass air is introduced, and many other factors. For instance, let's consider design of the air intake.
For aircraft travelling at supersonic speeds, a design complexity arises, since the air ingested by the engine must be below supersonic speed, otherwise the engine will "choke" and cease working. This subsonic air speed is achieved by passing the approaching air through a deliberately-generated shock wave (since one characteristic of a shock wave is that the air flowing through it is slowed). Therefore some means is needed to create a shockwave ahead of the intake.
The earliest types of supersonic aircraft featured a central shock cone used to form the shock wave. This type of shock cone is clearly seen on the English Electric Lightning and MiG-21 aircraft, for example. The same approach can be used for air intakes mounted at the side of the fuselage, where a half cone serves the same purpose with a semicircular air intake, as seen on the F-104 Starfighter and BAC TSR-2. A more sophisticated approach is to angle the intake so that one of its edges forms a leading blade. A shockwave will form at this blade, and the air ingested by the engine will be behind the shockwave and hence subsonic. The Century series of US jets featured a number of variations on this approach, usually with the leading blade at the outer vertical edge of the intake which was then angled back inwards towards the fuselage. Typical examples include the Republic F-105 Thunderchief and F-4 Phantom. Later this evolved so that the leading edge was at the top horizontal edge rather than the outer vertical edge, with a pronounced angle downwards and rearwards. This approach simplified the construction of the intakes and permitted the use of variable ramps to control the airflow into the engine. Most designs since the early 1960s now feature this style of intake, for example the F-14 Tomcat, Panavia Tornado and Concorde.
In one unusual instance (the SR-71), a variable air intake design was used to convert the engine from a turbojet to a ramjet, in flight. The Pratt & Whitney J58 could move a conical spike fore and aft within the engine nacelle, to control the placement of the supersonic shock wave. In this manner, airflow within the engine was kept subsonic at all times. At higher speeds, the spike allowed the J58 to obtain 80% of it's thrust, versus 20% through the turbine itself, allowing the engine to operate as a ramjet.
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