Quote:
Originally Posted by Varyon
A quick look online indicates some modern smokeless powders have energy densities a bit north of what gasoline does, if you need to bring oxidizer with you for the gasoline (gunpowders typically contain their own oxidizer).
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In order to get an explosion your fuel needs to contain (or be very thoroughly mixed with) the oxidizer - if you have to wait for an oxidizer to penetrate from the outside of the "explosive" it can't do so fast enough for the thing to actually explode. Anything that contains oxidizers in the same molecule as the fuel is effectively partly pre-burnt - it's necessarily lower energy than burning whatever the analogous molecule without the oxidizer in the pure oxidizer (oxygen, or nitric oxide or whatever) would give you - which rules out quite a broad range of explosives from being really good fuels. To find the ones that do OK, look up "monopropellants". Some of those can work in combustion-like engines - some torpedoes have been designed to use turbines driven by stuff like that, but generally nobody sane would consider them for an engine you expect to operate in air, there's no way they are going to compete with atmospheric oxygen.
Seriously, it would be easier to design an internal combustion engine that ran on finely powdered charcoal dust and air than one that ran on gunpowder even ignoring the fouling problems.
The Stirling engines, well, to match the thermodynamic efficiency of a steam engine, you need to take the air through the same temperature changes as the steam. Given that the heat capacity of air isn't as good, and you can't take advantage of a phase change to get a big volume reduction, your cylinders and radiator are necessarily going to be as big or bigger than the steam engine of the same efficiency, so it's pretty hard to come out smaller or lighter than a steam engine.
Modern high efficiency ones tend to be closed cycle, using a refrigerant (essentially a heat pump run in reverse) or a liquid metal (like mercury or molten sodium) rather than air. I guess nothing prevents you from building a Stirling engine using steam as a working fluid too - I suspect it'll be essentially equivalent to a (single expansion) steam engine of the same TL.