A Steam raising unit (Firebox+Boiler+injectors+blower+all those bits) that produces Steam that can then be moved by pipes to an engine (Piston+valve gear) that produces rotatory energy would be welcome.
The steam raiser would be operated by a dwarf who takes fuel from a stockpile and controls the boiler and tends to the fire. Early boilers where bulky and the dwarf needs a place to stand, So the normal 3x3 with a square of water under it (like how magma furnaces work).
Boilers have to be a bit of a pain, yearly blow downs to run efficiently(Cleaning out the gunk that is left when the water boils), etc.
The engine it self could be a simple expansion engine. which are simple, But hilariously inefficient, So you will be guzzling fuel to feed one. They also release Scalding hot steam into the air; Which is nice underground
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Of course who says Magma can't be used to boil the water? No operator as the magma could flow directly though the heat pipes in the boiler and then back into where it came from; Occasionally a massive cloud of steam might bellow out the boiler as the safety valve trips(Or if you don't like anything with "safety" in the name: the boiler could explode.)
No magma would be consumed, just cooled; which isn't in the game yet as far as I'm aware.
I don't like water reactors and i feel that this would fill that niche quiet nicely. And the FUN of dealing with scalding hot steam bellowing out of the engines deep underground sounds interesting.