A few flow-charts to TRY and explain more clearly than the bits and pieces mentioned in earlier posts:
compressed air stored in tanks at 300 bars (could be more in near future)
>>> "engine" (no combustion but uses pistons-rods-valves-crankshaft) at 30 bars >> drives anything (gearbox, genset, prop,etc).
available now are 10kw and 25kw units. up to 200-300kw in 22-24 months.
mains (220-110) >> drive compressor-pistons of same "engine" via an electric motor to compress air >> air tanks.
to facilitate the filling of the expansion chamber at higher revs (comparable to combustion chamber in a classic engine), the piston is stopped at top during 40% of crankshaft turn (crankshaft keeps turning of course).
overall "cost" = 25% of comparable diesel engine (75% economy, exactly!)
no toxic emissions.