Robin's Facebook post reminded me to finish this post... Have been busy with other things recently, apols!
I have mentioned these contraptions a few times so some more explanation is due. A Wankel engine is an internal combustion engine which basically has a triangle-shaped rotating piston inside a cavity instead of the up-and-down cylinder pistons you might be used to. A Wankel engine has less moving parts and inherently rotational motion, so no need for a crankshaft, vs its upright cylinder cousins. Mazda is keen on them for instance but they haven't caught on that much elsewhere.
Why my interest? I think there is no way we will have enough power for everything using batteries or solar cells, although perhaps I will be pleasantly surprised.
So, I was wondering: There is still some air at the altitude we have in mind; about 0.01bar to be precise. So can we still use an internal combustion engine, Wankel or otherwise? These have a much higher power density per unit weight than a battery.
I happened to have a Wankel in mind since there is a Wankel popular model aircraft engine, putting out a horsepower or so - I.e about 750W.
Let's imagine we rig something like this to work with ambient air. Now it can only burn about 1% as much fuel as usual (since there's only 1% as much oxygen), so let's say about 7.5W. Still pretty good though.
Any holes in this reasoning?
Obviously you'd need fuel injection, not a carburettor, since you'd need to constantly match the available air and fuel (propane, say?) stoichiometrically (sic?).
At some point the mechanical friction in the moving parts would be more than the pressure the exploding gases could generate. Who knows? Would it work?
Can you in fact compress the air to get a higher power density from the engine and still come out on top? (I.e more power out than used in the compression)
Better to take your own oxygen?
Better to use something exotic like a fuel cell?