Fleasel engine

I hypothesise that jumping fleas will act very much like gas molecules. This assumes that they will jump into things repeatedly.

Take a large diesel engine and remove the valves (and the holes that they were stopping up too - there is no inlet or exhaust involved, except of gasses to keep the cylinder at constant pressure). Put lots of fleas into the cylinder and bolt the cylinder head on. Don't forget to reconnect the fuel injector.

The cycle works as follows, starting at bottom dead centre:

The piston moves up from BDC, excess gas is vented around the base of the cylinder head. At TDC, there is only just enough room to fit all the fleas in the cylinder, and at this point, a fine mist of sugar solution with caffeine* is injected. The fleas will start to jump, and thus produce a downward force on the piston, just as gas molecules do in an engine where a traditional gas is the working fluid, and this will turn the engine to the bottom again. Gas is re-admitted to keep the cylinder at a constant pressure. A high concentration of oxygen might increase the engine's power and speed. A very large flywheel is used to move back up to TDC as this will, presumably, be quite a slow engine.

*I couldn't find any information about the effect of caffeine on fleas, however there is plenty to say that it increases the heart rate of water fleas (daphnia). A pheromone can be mixed in if caffeine alone is not enough to start them jumping.