[Vehicles] Gunpowder Engines and Stirling Engines

[Marasmusine]

The gunpowder-driven piston was experimented with in late TL4.
https://en.wikipedia.org/wiki/Gunpowder_engine
Historically, the atmospheric engine (steam engine) won out - but what if it didn't?
Putting aside the engineering problem of a somewhat rapid expansion, What would some ballpark GURPS Vehicles numbers be? In particular I'm wondering what the fuel consumption per kW might be, compared to gasoline, at TL 5.

A mythbuster's episode showed that Huygen's design could not have had tight enough seals to generate vacuum power from the explosion. I want to pretend that this design issue is resolved.

Similar question for the Stirling's "hot air engine", a TL 5 engine that did do actual work.
https://en.wikipedia.org/wiki/Stirling_engine

Thanks for any advice. This is for a TL4 clockpunk campaign but I'd like a realistic comparison with other early engines.

[Anthony]

The specific energy of gunpowder is 3 MJ/kg, as compared to 46.4 MJ/kg for gasoline (or 10.4 if you include oxidizer instead of using atmospheric oxygen), and it has a huge number of issues (expensive, solid fuels are hard to work with, engines don't generally like large quantities of soot, etc). You could use it to produce steam but it's harder and more expensive than just using the charcoal you need anyway directly.

[Rupert]

The problem with stirling engines at those tech levels is that their energy density compared to normal steam engines was low. They were fuel efficient, but their power/weight ratio was poor, so while useful in massive fixed installations they were not terribly useful for vehicles. This is the same reason it took decades before diesels replaced steam for ships - large diesels lacked the energy density of a steam turbine system

[Fred Brackin]

Originally Posted by Marasmusine

The gunpowder-driven piston was experimented with in late TL4.
https://en.wikipedia.org/wiki/Gunpowder_engine


Similar question for the Stirling's "hot air engine", a TL 5 engine that did do actual work.
https://en.wikipedia.org/wiki/Stirling_engine

The glib answer to the first question is "2d Cr and another 2D of fragmention" and I'm afraid that there are no reasonable stats. If you're using GM control over reality to make it work in your setting you're going to have to decided how well you want it to work.

If I was going to dress up my technobabble in such a stiuation I'd be waiving my hands and invoking the "pulse jet" used in the V1 as a similar device..

Contrariwise there were Ve2 stats for Stirling Engines in Vehicle Exansions 2. the TL5 stats were only a little better on weight than late TL5 double-expansion steam engines and actually poorer thna the very late quadruple expansion steam engines or the hypothetical sextuple exansion engines.

The stats for TL7 and 8 Stirling engines are much better but they use pressurized hydrogen or helium to get a better working fluid. I wouldn't climb into a steampunk vehicle using pressuried hydrogen though i might watch from a safe distance.

[Marasmusine]

Thank-you for your replies everyone.

[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). I suspect the issues involved in using it would overall still make a gasoline+oxidizer engine more mass-efficient.

[malloyd]

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).

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.

[Anthony]

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).

I suspect that depends on what you're using as an oxidizer. I can't find any evidence for smokeless powders (or, really, any conventional explosives at all) that beat the energy density of LOx/gasoline (it's significantly higher than octanitrocubane)

[Varyon]

Originally Posted by Anthony

I suspect that depends on what you're using as an oxidizer. I can't find any evidence for smokeless powders (or, really, any conventional explosives at all) that beat the energy density of LOx/gasoline (it's significantly higher than octanitrocubane)

I came across a value of 11.3 MJ/kg (compared to the 10.4 MJ/kg you indicated for LOx/gasoline) here. The value is claimed to have come from this paper, but I can't access it to confirm.

[Anthony]

Originally Posted by Varyon

I came across a value of 11.3 MJ/kg

I came across the same figure, but the reaction is not identified and no known reactions used for smokeless powder even reach half that value.