I read about a government program in maybe the 70s (oil crisis?) where steam engines were reviewed to replace gas engines. The reason being is that burning fuel at atmospheric pressure means you burn it more cleanly and fully. This sounds like an efficiency balance at some level, certainly a pollution advantage.
A quick search brought up this:
https://en.wikipedia.org/wiki/Advanced_steam_technology#Auto...
Superheated steam or closed-cycle (eg. Stirling) engines can do a lot better than this - Stirling engines can reach 50% efficiency, and this is nearly matched by superheated steam engines as used in power plants. But then you run into weight & safety problems. Superheated steam is great in a nuclear reactor or battleship, but in a car where a crash could easily break the engine piping? You're turning survivable crashes into death traps.
Even if there are some efficiency limitations, a small steam generator (range extender) combined with an electric vehicle could be ideal.
I really don't think efficiency is the primary problem with steam/heat tech, I think it's politics and society and possibly greed.
Edit: a quick search, it seems the Carnot limit applies to ICE engines as well. So your argument seems to be contradictory. Can you clarify how ICE engine efficiency is different from steam, related to the Carnot limit?
No, that's not true at all. Big power plants stayed with steam for efficiency reasons, and they are pretty close to theoretical limits.
Cars and aeroplanes, and later ships and trains, went to internal combustion because size and weight are very real concerns for them, and this trade-off against pure efficiency (heat-to-torque) was worth making. In the name of overall efficiency, if you like -- smaller engines meant more cargo room, so in oil-to-cargo terms you could come out ahead.
More electrical now is gas-turbine, but again this is about trading efficiency for other things -- peak-hour electricity is worth much more than 3am electricity, etc.
