A friend and I have been talking on-and-off about using propane (or was it diesel?) injection on the hot-side of a turbocharger to spool it up faster or maintain compressor speed during off-throttle periods. My understanding is the misfire systems work very well, but are hard on the turbine. I would imagine this is the same reason compressors have blowoff valves – the shock waves of the air being stuck in the intake manifold hurts the compressor. The other reason of course is you're blowing up fuel in the turbine, which is not a very subtle effect.
The more I think about it, though, the more I think you wouldn't need to have fuel burning actually in the turbine. If you instead had fuel injected directly south of the turbine, the combustion would bring more fuel and oxygen into the hot side, where, hopefully, it would burn of its own volition. Behold, the pulsejet:
Here's a rough diagram, chopped to look more like a turbocharger:
For our purposes, this would look like:
In the above picture, you see a dark tube flowing into the combustor (bright orange), with a little blue flame coming out one end. That's the intake, the "cold side". The principle is the flowing of exhaust (the expanded, heated air) out the hot end causes more air to be pulled in, and it pulls fuel with it, as you would have in a carburettor. Because of this, there are in principle no moving parts. Just a tube with a hot end and a cold end.
How does this help a turbocharger? Well, if you have a turbocharger on the hot side of your reciprocating engine, it's only able to get new air when there is positive pressure at the turbine inlet (the turbine inlet pressure increases with engine RPM and compressor pressure at the throttle body; essentially the compressor feeds the turbine, which spools the compressor, and so on).
I've been thinking that the problem with injecting fuel into the turbine side is the above-mentioned problem with turbine life. However, it seems that if you could get combustion happening at parts south of the turbine, what you'd have is a combustion reaction that sucks new air into the turbine either through the engine (and again, spooling the turbine spools the compressor, which adds pressure to the turbine inlet) or a "fresh air" pipe on the hot side (which would probably need a valve) without actually heating it any more than it would be by the reciprocating engine.
The first thing that probably comes to mind here is, wait, you want to burn the exhaust? Well, yeah. It turns out that piston engines are not especially efficient at combustion. Usually with piston engines, you're looking at 15-25% fuel burn efficiency. This is why tricks like CVCC, TGV's and even PAIR work:
The tumble generator valve is installed on the engine intake manifold and sends a strong vertical swirl of intake air into the cylinder to improve engine performance, particularly during ignition and starting. In addition, it provides a stable, lean burn even at engine start, thereby significantly reducing hydro-carbon emissions immediately after engine start.
The other thing that comes to mind is you could have a turbine spun by a pulsejet that in turn spun a compressor which was not coupled to the exhaust, giving you an on/off switch for (constant-pressure) boost, without a restriction on either the exhaust or intake.
Anyone actually seen or heard of anything like this?
Anyone experienced with gas turbines and care to comment on how sturdy your average centrifugal flow turbine (not axial flow) is?
Suggestions on fuel? Seems just about any liquid fuel would work okay (diesel and kerosene readily and have more energy but are dirty; alcohol is cheap and cleaner, but doesn't have nearly the energy of dino fuels) whereas propane is much cleaner but more flammable and, oh yeah, is a gas at room temperature. (frown) Hydrogen would of course work, but I think I'd rather have a bucket of propane than one of hydrogen. Perhaps a mixture of a fuel and nitrous oxide would work better in the exhaust (it seems to be working well enough for Rutan – boy that Branson is one flaming dude).
Anyone want to sound like a V-1 driving to work? (german)
And, uh, no, this probably doesn't fall under the "reliable" or even "practical" category. However, it would be a lot of fun to take e.g., your average $1500 FR junker and, uh, adapt it.