Why do pulse jets lose power at high speed?

[GasSavers_maximilian]

Not about cars, but it is an engine at least! I have always been interested in valved pulse jets and was curious if anyone knew the theoretical reasons they lose power at higher speeds. I once saw an old book that showed a design that got around this by enclosing the engine in a shroud that had a diffuser to slow the incoming air down. They didn't discuss why it was a problem though, which was very frustrating.

I have my own theory, but would love to be sanity checked. I think it's because the valves rely on the pressure of the explosion to close them. They cannot move instantly, so a small amount of combustion pressure leaks out before they close. At higher speeds the ram air pressure is greater, causing the valves to take longer and longer to act, thus losing progressively more power.

[JanGeo]

Yeah look at RAM JET engines for higher speeds and SCRAM JET engines.

[kamesama980]

I'd guess you're right about that, in short anything with moving parts, especially reciprocating parts, is going to have sweet spots for economy, power, mechcanical limit (where it falls apart) and motive limit (yea made that one up. basically where it can't move any faster).

[shatto]

Originally Posted by kamesama980

I'd guess you're right about that, in short anything with moving parts, especially reciprocating parts, is going to have sweet spots for economy, power, mechcanical limit (where it falls apart) and motive limit (yea made that one up. basically where it can't move any faster).

To expand;

The opening and closing of the vanes is limited by physics.

[GasSavers_maximilian]

Originally Posted by shatto

To expand;

Good one.

[Snax]

It seems to me that one way of enhancing response would be to increase the number of valves/decreasing their size, weight, and response times.