What about... Steam turbine.
You don't necessarily need solid fuels to generate steam.
Apparently ... a steam-powered version of the Messerschmitt Me 264 was hypothesized but never constructed - in 1944. It was meant to be powered by a steam turbine developing over 6,000 horsepower (4,500 kW), fueled with a mixture of powdered coal and petroleum.
It's a scientific fact that any and all external combustion cycles are significantly less power dense than the equivalent power internal combustion cycle. In addition, you need large amounts of water to generate the steam and then large heat exchangers to dump the heat into the air to get your water back. If you use some of the waste heat to warm up the water & increase efficiency, oyu increase weight again to account for the regenerators. Now, the 264 was a large bomber and the ability to use other fuels than scarce petrolium may have suggested a steam turbine to someone, but the hit it would have taken on payload/performance would have been truly brutal.
I've been under the impression that steam engine for tanks will have rather much to do with the development of steam cars.
And the use of steam engines for cars is primarily to permit the use of alternate fuels. Performance
will suffer dramatically.
A liquid-fueled high-pressure boiler feeding a steam turbine/generator set and the tank run by electricity.
Still massively inefficient and low powered. There is a fundamental problem with and, repeat, any application of external combustion. They are inefficient and low power. There is no way to get more out of a steam system thatn an equivalent mass internal combustion system. There is a reason that even the largest ships are now using diesels rather than any sort of steam. They are better. On the most fundament level, just simply better. If a diesel exists no-one would ever replace it with a steam engine of any type. If diesels are not available then any vehicle powered by steam will have significantly lower performance on all levels.
This has the advantage of reducing the mechanical complexity of the system
Without trying to be mean, where would you get this idea? The Steam engine alone is significantly more complex than the diesel as it sits. Adding the electrical generating system and the motors just adds to it. It would be a mechanical nightmare. Even with an internal combustion engine, Dr. Ferdinand Porche couldn't make it work properly on the Ferdinand.
No steam engine buildable using the laws of this universe can be made that will propel a vehicle even 1/2 the weight of an M48 any distance at all and still fit within even twice the volume of the Patton's engine bay.
There's a reason we don't use 'em anymore, y'know! :) :)
Paul