Editing Ramjet (section)

===Diffuser===
The diffuser converts the high velocity of the air approaching the intake into high (static) pressure required for combustion. High combustion pressures minimise entropy rise during heat addition,<ref>{{cite book |last=Builder |first=C. |title=1st Annual Meeting |date=1964-06-29 |publisher=American Institute of Aeronautics and Astronautics |page=2 |language=en |chapter=On the thermodynamic spectrum of airbreathing propulsion |doi=10.2514/6.1964-243 |chapter-url=https://arc.aiaa.org/doi/10.2514/6.1964-243}}</ref> this minimising wasted thermal energy in the exhaust gases<ref>{{cite journal |title=Propulsive Efficiency from an Energy Utilization Standpoint |journal=Journal of Aircraft |volume=13 |issue=4 |date=April 1976 |publisher=American Institute of Aeronautics and Astronautics |via=Internet Archive |language=English |url=http://archive.org/details/sim_journal-of-aircraft_1976-04_13_4}}</ref>

Subsonic and low-supersonic ramjets use a [[pitot tube|pitot]]-type opening for the inlet. This is followed by a widening internal passage (subsonic diffuser) to achieve a lower subsonic velocity that is required at the combustor. At low supersonic speeds a normal (planar) shock wave forms in front of the inlet.

For higher supersonic speeds the pressure loss through the shock wave becomes prohibitive and a protruding spike or cone is used to produce oblique shock waves in front of a final normal shock that occurs at the inlet entrance lip. The diffuser in this case consists of two parts, the supersonic diffuser, with shock waves external to the inlet, followed by the internal subsonic diffuser.

At higher speeds still, part of the supersonic diffusion has to take place internally, requiring external and internal oblique shock waves. The final normal shock has to occur in the vicinity of a minimum flow area known as the throat, which is followed by the subsonic diffuser.