[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.