Jet engines take in a great amount of surrounding air through their inlet. Behind the inlet is the compressor section. The compressor section, as you may have already guessed, compresses this incoming air. The compressor’s job is to increase the pressure of the incoming air before it reaches the combustion section.
There are two main types of compressors: axial and centrifugal. Many modern turbojet and turbofan engines have axial compressors because of their performance efficiency. Centrifugal compressors can increase the total pressure of the air by a factor of 4, and axial-flow compressors can only increase the pressure by a factor of 1.2. So, how do axial compressors become more efficient? They’re made to be multi staged, therefore, the pressure increase is multiplied row by row. Axial compressors have an advantage over centrifugal compressors because of their ability to have multiple stages. These advantages are ideal for an application where the thrust of the engine itself is the driving force of the aircraft, or in our case the driving force of the vehicle.
A J85 engine is made up of 8 stages, and uses an axial-flow compressor. If you multiply the 8 stages by the factor of 1.2, that makes for an overall pressure increase factor of 4.3. In an axial-flow compressor, the flow enters in an axial direction, which means it is parallel with the axis of rotation. This compressor first compresses the incoming fluid by accelerating it and then diffusing it, which creates that increase in pressure.
A fluid is anything that flows, so do not be confused with the term “fluid” in referral to air. Although liquids are most commonly known as fluids, anything that is loosely held together by gas particles is in fact a fluid. Since air is a gas, it does flow and it will take the shape of its container.
Figure: General Electric J85 Cutaway
“Stages” within the compressor refers to the rotors and stators. Rotors are the blades that rotate and accelerate fluid, and stators are stationary blades that do the diffusing. In axial flow compressors, the air flows from stage to stage. The pressure increases in the direction of flow, and the stages allow for incremental increase in pressure to eliminate the risk of the engine stalling. Incremental pressure increases also allow for higher engine efficiency. Throughout the compressor, the flow area decreases. The blades get smaller and smaller, and this compensates for the increase in fluid density, creating a constant axial velocity.
Through the front of the engine, at the inlet, air enters the compressor at about 14.69 psi, which is the standard atmospheric pressure. Standard atmospheric (air) pressure at sea level is 14.7 psi, give or take. Air pressure is simply the force exerted against a surface by the weight of the air. Once the air reaches the back of the compressor, its pressure can reach around 70 psi. This is a very high pressure for air that is coming through a wide open inlet and exiting through another wide open area. Along with a pressure increase of the air within the compressor, there is a velocity increase too. Air entering the inlet is flowing at speeds higher than 200 mph, and once it reaches the back of the compressor it can reach speeds close to 700 mph!
The back of the compressor leads to the combustion section, which will be covered in the next segment of the jet engine breakdown. Check back next time to see how the combustion section of the engine works!