Real human output is about 600W at most.
I didn't think that one through. Maybe it could be implemented as a joke engine with power output increasing if the player has speed and/or jump boost.
Considered in the past but extremely difficult to prevent it from being easier than every other engine.
Perhaps have it so that you have to manually clean out cobblestone as it piles up. That way it forces the player to have to maintain it and also explains what happens to the spent lava.
Violates the first and second laws of thermodynamics.
Whoops.
So a standard steam engine.
A standard steam engine that runs on furnace fuel rather than fire/lava underneath.
How? Burning them? Then it is another steam engine.
It's just an upgrade of the furnace engine.
An internal redstone clock forces the redstone to flux, extracting its stored energy as heat... Now that you mention it, that does sound really silly.
Nitrogen is very difficult to extract and nitrous oxide is a different chemical entirely.
Maybe a different propellant could be used. I considered oxygen too but figured it wouldn't be powerful enough.
Suggested some time ago and liked but far too difficult to implement.
Okay i'll split this into chunks.
Rocket fuel production: Jet fuel is compressed and enhanced with additives and propellant. This causes a large reduction of liquid and requires a large amount of energy be put in through both the additives and shaft power.
Rocket Turbine: Creates entities that check for players up to 20 blocks away and deal damage accordingly based on distance. They will also heat up blocks this way too. On contact the entities have a plasma effect, melting blocks and instantly killing players. Each of these said entities has the mobspawner particle effect to give it a truly epic flamethrower effect.
Activation: First you put the jet fuel in and then you give yourself some nice distance and either spark, flame, or apply redstone to the engine to ignite the fuel. The reaction will go until the fuel runs out.
Cooling: You'll need a lot of liquid nitrogen to keep this thing cool. Just pipe it in from the bottom and make sure the supply keeps. Or else.