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Components The basic components of an air-breathing engine are the inlet, a compressor or fan, the combustor (burner), a turbine, and an exit nozzle. Different engines will use these components in various combinations, and some will exclude one or more components (see next section). But these are the basic building blocks of an engine. The figure below shows a typical jet engine design illustrating the different components. Inlet: The design of the inlet (sometimes called the air intake) helps determine the amount of air flow into an engine. Engineers take into account the desired cruise speed of the aircraft and design the inlet to suck in as much of the air coming towards it as possible. Different designs are needed for supersonic and hypersonic engine inlets than for subsonic engine inlets because of shocks and other aerodynamic properties. Inside the engine the next component, the compressor, works much, much better when the air entering it is fairly slow (usually much slower than cruise velocity), so the inner walls of the inlet are designed to slow the velocity of the air stream as it comes to the compressor. Compressor: The compressor is used to TsqueezeU the air, or to increase the pressure of the air flow. Just as in the balloon where the escaping high pressure air generates a high velocity and therefore thrust, the high pressure air coming out of the compressor is a factor in the generation of thrust by the engine. Unfortunately, to raise the pressure of the air takes power, since increasing pressure is not a natural occurrence. Using the balloon as an example again, it is natural for the pressure in a balloon to decrease over time as the individual molecules of air escape through the rubber molecules of the balloon (density drops so the pressure has to drop). It is natural for pressures to decrease. The balloon can be refilled by making the effort to blow air back into the balloon and increase the pressure inside. But the added air comes from an outside agency; the balloon will not refill on its own. Typical compressors increase the pressure of the air by 15 to 30 times the original pressure. Usually, an engine designer will choose among different compressors with different pressure differences and evaluate the resulting effect on the thrust on the way to the final design to meet the airplane specifications. Combustor: The slow moving, high pressure air from the compressor is fed into the combustor or burner where it is mixed with a highly flammable fuel and ignited. The very hot, high pressure air exiting the burner becomes the primary source of the exhaust gases that will be used to generate the thrust. These gases are very, very hot, and the engineer must be careful designing the components that come after the burner so that they are not melted or destroyed. The temperatures coming out of the burner become a design limitation for the next components. The combustion engineer works with the mixture of fuel to air to get just the right combination for a good, hot burn. Too little fuel and the mixture doesnUt burn hot enough and the resulting thrust is lower. Too much fuel and the mixture doesn't burn completely; the gases are swept out of the burner before they can burn up and the resulting exhaust still has unburned fuel in it. While the gases are at the desired hot temperature, the engine is wasting fuel; it is not fuel efficient. A better mixture would yield the same thrust for less fuel. Sometimes an additional secondary burner is used in an engine design. The second burner is called an afterburner. It usually follows behind the turbine. An afterburner is used to reheat the gases to a higher temperature after the turbine. As the high temperature exhaust gases exit the engine into the much cooler outside air, the velocity of the gases is increased, generating more thrust. Turbine: The very high temperature, high pressure gases are released from the burner and passed into the turbine where the local pressures are much lower. It is natural for high pressure gases to decrease or drop in pressure when the local pressure is lower. As the pressure drops, the velocity of the flow increases, and the high velocity flow eventually exits the engine to generate thrust. Some of the energy in the higher speed air flow can be used to generate power to run the compressor as well. Usually, only a small percentage of the high speed flow is used to generate the power necessary to run the compressor. An airplane engine could run without a turbine, and it would generate more thrust, but it would have to have a power source somewhere on the plane for the compressor. It is likely that this alternate power source would increase the weight and possibly the size of the airplane. So, more lift would be needed to counteract the weight, and more drag would be generated to offset the thrust. Overall, in spite of the increase in pure thrust, the net effect on the airplane would be a decrease in range (the distance it can fly). All in all, the compressor-turbine combination is one of the most efficient uses of power in the engine. An engineer must be very careful in the design of a turbine because of the high temperature of the gases coming from the burner. If the materials in the turbine blades are not chosen well, the blades can melt and deform and be less efficient, or they might break off and destroy the rest of the turbine. Some engines use the afterburner discussed above to do a second burn on the now lower pressure gases to increase the temperature again after the turbine has been used to power the compressor. Even though the pressure is lower, the increased temperature yields an increase in velocity of the exhaust gases for higher thrust. That way, they can lower the temperature coming out of the primary burner into the turbine, get the power to run the compressor from the pressure change, do a second burn in the afterburner to increase the temperatures, and get a higher velocity for the exhaust gases than if there was no afterburner and thus higher thrust. Nozzle: The inside walls of the exit nozzle are shaped so that the exhaust gases continue to increase their velocity as they travel to the exit of the engine. The higher the exit velocity of the gases, the more thrust that can be generated. Some nozzles on fighter aircraft are designed to be adjustable; the shape can change depending on the conditions of the engine and the flight needs. Other nozzles are of a fixed design because the conditions donUt change enough to warrant movable parts that may break down. The nozzle engineer must also be concerned with the temperatures of the exhaust gases, especially if there is an afterburner. If the walls on the inside of the nozzle melt and change, then the exhaust velocities are not being increased properly and the thrust generated is not the desired amount.
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