A jet engine is a type of air-breathing internal combustion engine often used on aircraft. The principle of all jet engines is essentially the same; they accelerate a mass (air and fuel residue) in one direction and, from Newton's third law, are forced in the opposite direction.
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 (by 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. In a turbine engine the compression of the air and the shape of the ducts passing into the ignition chamber prevents backflow from it and thus makes possible the continuous burning and propulsion process.
The advantage of the jet engine is its efficiency at high speeds (especially supersonic speeds) and high altitudes. On slower aircraft and those required to fly short stages, a gas turbine-powered propeller engine, commonly known as a turboprop, is more common and much more efficient. Very small aircraft generally use conventional piston engines to drive a propeller but small turboprops are getting smaller as engineering technology improves.
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 piston engine, then it is mixed with fuel and 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 the same design with a traditional engine and propeller combination.
The key to the useful jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. The first gas turbine to sucessfully run self-sustaining was built in 1903 by Norwegian engineer Aegidius Elling. In 1930 in England Frank Whittle submitted patents for his own design for a full-scale aircraft engine (granted in 1932). 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" (accelerating) air outward from the central intake to the outer periphery of the engine where the air is then compressed by a divergent duct setup—converting velocity into pressure. The advantage was that such compressor designs were well understood in centrifugal superchargers but this leads to a very large cross section for the engine at rotational speeds that were usable at the time. A disadvantage was that the air flow had to be "bent" to flow rearwards through the combustion section and to the turbine and tailpipe. With improvements to bearings the shaft speed of the engine would increase and the diameter of the centrifugal compressor would reduce greatly. The shortness of this engine is an advantage. The strength of this type of compressor is an advantage over the later axial flow compressors that are still liable to foreign object damage (FOD in military parlance).
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 stage (convergant ducts), where it is crushed against a set of non-rotating blades called stators (divergent ducts). 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 in diameter. 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. Because Hitler wanted a new bomber 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. Their most famous design, the Nene would also power the USSR's jet aircraft after a technology exchange. American designs wouldn't come fully into their own until the 1960s.
There are a number of types of jet engines, all of which are based on the principle that air is compressed and used as an oxidizer for the fuel. Some examples are as follows:
|Turbojet||generic term for simple turbine engine||simplicity of design||basic design, misses many improvements in efficiency and power|
|Turbofan||power tapped off exhaust used to drive bypass fan||quieter due to greater mass flow and lower total exhaust speed, more efficient for a useful range of subsonic airspeeds for same reason, less subject to FOD and ice damage||greater complexity (multiple shafts), large diameter engine, need to contain heavy blades|
|Ramjet||Intake air is compressed entirely by speed of oncoming air and duct shape (divergant)||very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all jets (thrust:weight ratio up to 30 at optimum speed)||must have a high initial speed to function, inefficient at slow speeds due to poor compression ratio, difficult to arrange shaft power for accessories|
|Turboprop (Turboshaft similar)||jet turbine engine used as powerplant to drive (propeller) shaft||high efficiency at lower subsonic airspeeds(300 knots plus), high shaft power to weight||limited top speed (aeroplanes), somewhat noisy, complexity of propellor drive, very large yaw (aeroplane) if engine fails|
|Unducted fan (UDF or Propfan)||turbojet engine drives a propeller, like a turbofan but without ductwork||higher fuel efficiency, some designs are less noisy than turbofans, could lead to higher-speed commercial aircraft, popular in the 1980s during fuel shortages,||development of UDF engines has been very limited, typically more noisy than turbofans, complexity|
|Pulsejet||Air enters a divergant-duct inlet, the front of the combustion area is shut, fuel injected into the air ignites, exhaust vents from other end of engine||Very simple design, commonly used on model aircraft||noisy, inefficient (low compression ratio), works best at small scale|
|Pulse detonation engine||Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves||Maximum theoretical engine efficiency||Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use|
|Integral rocket Ramjet||Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket||Mach 0 to Mach 5+ atmospheric no particular limit exoatmospheric, good efficiency at Mach 2 to 5||similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties|
|Scramjet||Intake air is compressed but not slowed to below supersonic, intake, combustion and exhaust occur in a single constricted tube||
can operate at very high Mach numbers (Mach 8 to 15)
||still in development stages, must have a very high initial speed to function (Mach >6!), cooling difficulties, inlet difficulties, testing difficulties|
|Turborocket||An additional oxidizer such as oxygen is added to the airstream to increase max altitude||Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed||Airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous|
Whittle's and von Ohain's designs are classified as turbojets, the simplest kind of jet engine. In a turbojet all of the air passing through the engine goes through the combustion chambers. Generally turbojets are arranged around a central shaft, running the length of the engine, with the compressor and turbine connected to the shaft at opposite ends. In the middle of the engine is a combustion area, typically in the form of a number of individual "flame tubes" or "cans". The combustion area is either annular or can-annular in large modern engines with annular predominating.
The compressor adds energy to the air flow, at the same time squeezing it into a smaller space (increasing its pressure), slowing it down, and increasing its temperature. Early jet engines had compression ratios as low as 5:1 (as do a lot of simple APUs and small propulsion turbines today); modern engines have compression ratios as high as 44:1, when operating at very high altitudes. These compression ratios are not comparable to those in a piston engine because the combustion process is continuous, as explained below. Higher compression ratios imply larger temperature rises; modern engines only achieve their high compression ratios at high altitude with very cold intake air (around –54 C). When taking off in warmer air they run at lower compression ratios to keep the temperature of the compressed air within turbine temperature limits by bleeding air away from the compressor stages and dumping it overboard. As a result the engines are much less efficient when running at low altitudes.
The burning process in the cans is significantly different from that in a piston engine. In the piston engine the burning gases are confined to a small volume and, as the fuel burns, the pressure increases dramatically. In a turbojet the air and fuel mixture passes, unconfined, through a can. As the mixture burns its volume increases dramatically and the pressure actually decreases (in the convergent duct) as the gases accelerate towards the rear of the engine.
In detail, the fuel-air mixture must be bought almost to a stop so that a stable flame can be maintained, this occurs just after the beginning of the combustion chamber. The aft part of this flame-front is allowed to progress rearward in the engine, this ensures that the rest 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. At the same time some pressure drop is unavoidable 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 soak up the heating effect of the fuel burning.
Another difference between four-cycle engines and jet engines is that the peak flame temperature in a four-cycle engine is experienced only momentarily, and for a small portion of the entire cycle. The can in a jet engine is exposed to the peak flame temperature continuously, and operates at a pressure high enough that a stoichiometric fuel-air ratio would melt the can and everything downstream. Instead, jet engines run a very lean mixture, so lean that it would not normally support combustion. A central core of the flow 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 mixes into the burned gases to bring the temperature down to something the turbine can tolerate.
After the cans the gases are allowed to expand through the turbine. In the first stage the turbine is largely a reaction turbine (similar to a pelton wheel) and rotates because of the impact of the hot gas stream, later stages are convergent ducts that accelerate the gas rearward and gain energy from that process. Pressure drops and energy is transferred into the shaft. The turbine's rotational energy is used to drive the compressor to compress the intake air and some shaft power is extracted to drive accessories like fuel, oil and hydraulic pumps. The pressure drop through the turbine is much lower than the pressure rise through the compressor because the flow volume in the turbine is so much higher (because of the added fuel), which in turn is due to the higher temperature. 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.
The efficiency of a jet engine is strongly dependent on the pressure drop through the turbine and nozzle. To achieve the largest possible drop, the engine operates at the highest possible compression ratio. Higher compression ratios imply higher compressor outlet temperatures and thus higher flame temperatures. The tolerable temperature limit is set by the turbine blades— usually the first stage. Modern turbine blades are single metal crystals with hollow interiors. Cooler air from the compressor is blown through the hollow interior of the blades. In a modern engine the turbine inlet temperature will typically be around 1700 C, higher than the melting temperature of the blade material (around 1600 C). Still higher temperature operation will require not only better materials but also some means of eliminating the oxides of nitrogen that form at such high combustion temperatures.
After the turbine, the gases are allowed to expand and accelerate further through the exhaust nozzle. In some turbojets the gases may actually transition to supersonic flow in the nozzle, in which case the nozzle will be a converging-diverging nozzle. A subsonic nozzle converges all the way to the end. Some supersonic military jets have variable nozzles that can change from subsonic to supersonic flow in different flight regimes.
Early German engines had serious problems controlling the turbine inlet temperature. 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. The Americans had the best materials because of their reliance on turbosupercharging in high altitude bombers of World War II. For a time some US jet engines included the ability to inject water into the engine to cool the compressed flow before combustion, usually during takeoff. The water would tend to prevent complete combustion and as a result the engine ran cooler again but the planes would take off leaving a huge plume of smoke.
Today these problems are much better handled but temperature still limits airspeeds in supersonic flight. At the very highest speeds the compression of the intake air raises the temperature to the point that the compressor blades will melt. At lower speeds better materials have increased the critical temperature and automatic fuel management controls have made it nearly impossible to overheat the engine.
Turbojets do not throttle efficiently. To operate well at all the compressor blades must turn at not less than 50 to 70% of the design maximum. At low throttle settings a great deal of power is wasted compressing a large fraction of the full-throttle airflow, only to expand it back again with relatively little temperature gain from the combustion chamber. Poor efficiency at low throttle settings helps to explain why turbines aren't used in cars—the engine would be burning a huge quantity of fuel even while sitting at a red light. 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 compressor stages at various speeds. Most such designs that use two stages are known as "two spool" engines. A few have used three stages with demonstrated efficiency gains. An airliner consuming 20 tons of fuel to fly from the east coast of the US to the west coast will gain much from a fractional increment in efficiecy gain. An airliner on an 11-hour trip across the Pacific Ocean could reach Australia rather than New Zealand on a very small efficiency gain.
Main article: Turbofan
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 common type of engine in aviation.
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:
- Air Induction
For subsonic aircraft, the air intake to a jet engine presents no special difficulties, and consists essentially of an opening which is designed to minimise drag, as with any other aircraft component. However, the air reaching the compressor of a normal (not scramjet) jet engine must be travelling below the speed of sound, even for supersonic craft.
- Compressor Fan
The compressor is the series of fans that are spaced very closely together. Each fan compresses the air a little more. Energy is derived from the exhaust fan (see below), passed along the shaft.
This carries power from the exhaust fan to the compressor, and runs most of the length of the engine. There may be as many as three concentric shafts, rotating at independent speeds, with as many sets of exhaust fans and compressors. Other services, like a bleed of cool air, may also run down the shaft.
- Flame Cans or Flameholders or combustion chambers
These are combustion chambers where fuel is continuously burned in the compressed air.
Turbine fans or Exhaust fans
These gather energy from the hot expanding air rushing out of the engine. This energy is used to drive the compressor through the shaft, or bypass fans, or props, or even (for a gas turbine-powered helicopter) converted entirely to rotational energy for use elsewhere.
- Afterburner (optional)
(mainly military) Produces extra thrust by burning extra fuel, usually inefficiently, at the exhaust.
Air, once cooled and expanded, is vented out the back of the engine. Exhausts are designed to maximize thrust, since venting hot air does not provide nearly as much thrust as venting fast-moving cool air.
The standard aerodynamic reference frame is attached to the aeroplane. Air must travel through the engine at subsonic speeds, to sustain operation of flow mechanics at the blades of the compressors and turbines. The supersonic nozzle is needed to convert the pressure and heat to velocity, and consequently momentum of the expelled air. A de Laval nozzle tapers down to a neck, accelerating the gas up to sonic speed, and then the nozzle opens out again. The hot gas thus expands and cools whilst pressing on the inside of the nozzle at a rearward facing angle. This accelerates the air even further; forming a powerful supersonic exhaust jet. The reaction on the inside of the nozzle multiplies the thrust up and accelerates the vehicle.
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.
Air intake design
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. To get good efficiency over a wide range of speeds the Pratt & Whitney J58 could move a conical spike fore and aft within the engine nacelle, to keep the supersonic shock wave just in front of the inlet. In this manner, the airflow behind the shock wave, and more importantly, through the engine, was kept subsonic at all times. Additionally, and unusually for this engine, at high mach, the compressor for the J58 was unable to carry the high air flow entering the inlet without stalling its blades, and so the engine directed the excess air through 6 bypass pipes straight to the afterburner. At high speeds the engine actually obtained 80% of its thrust, versus 20% through the turbines itself, in this way. Essentially this allowed the engine to operate as a ramjet, and actually improving specific impulse (fuel efficiency) by 10-15%.
- Gas Turbine Builders' Resources
- Journey through a jet engine(flash)