posted on Mar, 19 2007 @ 09:58 PM
The fuel to air ratio is basically determined by the throttle setting, and it basically works the same in conventional engines, ramjets, scramjets,
and PDEs.
The big problem that is different between subsonic burners and supersonic burners is mixing the fuel and air before combustion. Techniques that work
quite well in subsonic burners are stupendously bad in supersonic burners, and much research has been done on different techniques for scramjet or
dual-cycle applications.
If you are asking how they run ramjets or scramjets on the ground (or anytime at relatively slow speeds) well they typically just don't. They are
horribly inefficient so some other propulsion system must be used to get them up to speed first. For testing purposes they can pump large amounts of
air through them on the ground, but they don't have a magical way of making them work at slow speeds, they just have to design around this major
drawback of those particular types of engines.
Pulse Detonation Engines (PDEs) don't have this particular limitation and that's why they are of major interest to propulsion engineers. In theory,
a PDE can operate from standing still all the way up to hypersonic speeds, but of course there are many design challenges along the way.