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Turbojet

From Academic Kids

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Jet engine diagram

Turbojets are the simplest and oldest kind of jet engine. The design was arrived at the same time by two different engineers, Frank Whittle in Britain and Hans von Ohain in Germany.

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, with annular predominating in larger more modern engines.

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 auxillary propulsion units and small propulsion turbines today); modern jet engines have compression ratios as high as 44:1 when operating at high altitude. 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, achieved 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 brought 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 piston engines and jet engines is that the peak flame temperature in a piston 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 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 efficiency 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.

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