[ QUOTE ]
Wasn't Honda or Toyota working on a better GA piston engine? Also, I'm no gearhead, but why is it that so few GA engines run on deisel fuel? Wouldn't deisel make sense from a cost perspective?
[/ QUOTE ]
Isn't the Diamond twin diesel powered, with a ridiculously good burn rate? I think I saw somewhere that it cruises at 200+ knots while burning only about 10 gph. Wow.
A scramjet is a ramjet in which the combustion process takes place at supersonic velocity. Like a ramjet, it has no moving parts in the flowpath (well, at least no turbomachinery).
A PDE is like a pulse-jet which produces thrust through a series of repeated detonations of a fuel-air mixture.
Although it could be stand-alone like a scramjet, a PDE can also be used as a "replacement" for a gas turbine combustor. Because it the process is discontinuous and produces a pressure rise in the combustor, it has the potential to alllow more mechanical work to be recovered in the turbine for the same average material temperature (like a reciprocating engine).
It will be interesting to see how the new diesel engines work out. There have been some used in the past. Packard built a diesel radial in the '30s that was used on a Bellanca to set some time/distance records, but I have heard that it was pretty rough running. The Germans also tried some in the '30s and '40s. The only major use of diesel aero engines I know of was on rigid airships.
The traditional problem with a diesel recip. is high weight and low power. Diesels are more fuel efficient due to their high compression ratios which extract more work from the fuel. Adding a turbocharger or supercharger only makes them more efficient. They also use excess air for cooling instead of excess fuel like gas engines. Diesel engines do not have a 'throttle' in a traditional sense. They vary power output by varying the amount of fuel injected into the cylinders, not by controlling the amount of air entering the engine.
Since they have very high compression ratios and corresponding high cylinder pressures, the engines must be more 'heavy duty' and are traditionally much heavier than a gas engine of similar power. They also usually operate at a lower rpm. BMW now has some high speed diesel car engines that, while less powerful than a similar gas engine, still make alot more power than a traditional diesel.
Much of the advantage of the diesel aircraft engine comes from simple fuel availability. In most of the world, general avation simply does not exist as we know it. Jet A is the only fuel available. In Europe diesel is also cheaper than gas due to taxes. This could easily change. I would think that a computer controlled gas engine with fuel injection and variable ignition timing could be made to run on high octane auto fuel, while being lighter and cheaper than a diesel. With FADEC they can easily run lean of peak for a measurable gain in fuel efficiency.