A Jet Engine. Why does the air flowing through it have to be less than the speed of sound?

-A jet aircraft can fly faster than the speed of sound (Mach 1).

But in the engines, the air flow into them is deliberatly slowed by the inlet shape to a speed below that of Mach 1. Why cant the engines operate with an airflow through them that is greater than Mach 1?They could try, but the friction of the air against the sides of the engine would create shock waves to slow the air down again. If there is only one or two strong shock waves (as there would be at the first compressor stage), it would be very inefficient and possibly damaging. Also, normal jet fuel combustion cannot happen at supersonic velocities.



Instead, they deliberately slow the air down using a series of weak shock waves before it reaches the engine. This also make the engine design easier since all it has to deal with is subsonic flow.



As speeds get higher, it gets more difficult to slow the air down. That is why faster engines are typically ram-jets or bypass the compressors at high speeds (like the J58 on the SR-71). The next step up is the SCRAM jet that can sustain supersonic combustion.The transition from subsonic to supersonic speeds creates turbulence inside the engine that disrupts the smooth flow of air into the compressors, and could cause the engines to 'flame out' or stop producing power. To eliminate turbulence, the speed of air flowing through the engines is kept to subsonic.