The basic components of an air-breathing (jet) 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. Some engine designs even leave out one or more of these components (see next section). But these are the basic building blocks of an engine. The figure below shows a typical jet engine design and its components.

Inlet:

The design of the inlet, or air intake, helps determine the amount of air flow into an engine. After deciding the cruise speed of the aircraft, engineers design the inlet to suck in as much of the air coming toward it as needed. Subsonic, supersonic and hypersonic cruise speeds each require a different inlet design. Inside the engine the next component, the compressor, works much, much better when the air enters fairly slowly, (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 squeeze the air, or to increase the pressure of the air flow. This is vital to creating thrust. Using a balloon, as an example: as more air is blown into the balloon the pressure increases. Increased pressure will produce increased thrust. To increase the pressure you must use power (your lungs). The purpose of the compressor is to increase the pressure of the incoming air (power). 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 to find the compression ratio that fits the specifications of the airplane being built.

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 leaving the burner 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 they are not melted or destroyed. The combustion engineer works with the mixture of fuel and air to get just the right combination for a good, hot burn. Too little fuel and the mixture doesn't burn hot enough and the resulting thrust is lower. Too much fuel and the mixture doesn't burn completely; you may be getting enough thrust, but the engine is wasting fuel. Sometimes a second burner is used after the turbine. The second burner reheats the gases to a higher temperature just as they exit the engine into the much cooler outside air, the velocity of the gases increase, generating more thrust.

Turbine:

The very high temperature, high pressure gases are released from the burner and passed into the turbine (engine) where the local pressures are much lower. The high pressure gases begin to drop in pressure. As the pressure drops, the velocity of the flow, of exhaust gases, increases. As these gases leave the engine (turbine) they generate thrust. Part of this flow may also be used to power (run) the compressor. Although this decreases overall thrust, it is more efficient than having a separate power source for the compressor.

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 even break off and destroy the rest of the turbine.

Some engines use an afterburner. Remember, the afterburner does a second burn on the lower pressure gases coming from the turbine. Some of the gas flow is used to run the compressor, the afterburner reheats the gases which increases their velocity, thereby increasing thrust. Without the afterburner the additional thrust would not be there.

Nozzle:

The inside walls of the exit nozzle are shaped so that the exhaust gases continue to increase their velocity as they travel out of the engine. The higher the exit velocity of the gases, the more thrust that can be generated. Some fighter aircraft have adjustable nozzles, allowing the pilot to adjust thrust as needed. Other nozzles are of a fixed design because conditions do not change enough to need an adjustable nozzle. Again, the engineer must be concerned with the temperatures of the exhaust gases in the exit nozzle, expecially if there is an afterburner. If the walls on the inside of the nozzle melt and change, then the exhaust velocities and thrust may not be correct.

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Last modified: Sat Jul 25 21:59:04 PDT 1998

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