Question

Which of the following process(es) is/are involved in the compression of air in an ideal ramjet engine?

• A. oblique shock
• B. mechanical compression
• C. normal shock
• D. subsonic diffusion

Hint:

Remember: an ideal ramjet has no compressor rotor. All compression comes from decelerating the air through oblique shocks, a normal shock, and subsonic diffusion.

Answer 1

The Correct Option is A, C, D

Step 1: Understand how a ramjet compresses air.
A ramjet has no compressor blades or turbine. All of the pressure rise needed before combustion comes purely from slowing down the incoming high speed air, which is called ram compression. The whole job is done by the shape of the inlet/diffuser duct.

Step 2: Compression at supersonic flight speed.
When the ramjet flies supersonically, the air entering the intake is still supersonic. An efficient supersonic diffuser first slows this air down using one or more oblique shocks. Oblique shocks give a smaller loss in stagnation pressure than a single strong shock, so real ramjet inlets are shaped to create a system of oblique shocks ahead of the throat.

Step 3: The final shock down to subsonic flow.
After the oblique shocks reduce the Mach number, the flow is still supersonic (just closer to Mach 1). A normal shock, placed near the inlet throat, then completes the job of dropping the flow from supersonic to subsonic.

Step 4: Subsonic diffusion after the shock system.
Once the flow is subsonic, it still carries more kinetic energy than the combustor can use efficiently. The diverging (widening) duct downstream of the normal shock acts as a subsonic diffuser, slowing the flow further and converting that leftover kinetic energy into an additional rise in static pressure, before the air reaches the combustion chamber.

Step 5: Why mechanical compression is excluded.
Mechanical compression means using rotating compressor blades (as in a turbojet or turbofan) to add work to the air. A ramjet, by definition, has no compressor rotor and no turbine. It relies only on the ram effect of the incoming air, so mechanical compression plays no part in an ideal ramjet.

Final Answer:
The compression in an ideal ramjet is carried out by oblique shocks, a normal shock, and subsonic diffusion; mechanical compression is not present. \[ \boxed{\text{A, C, D}} \]