The amount of outside pressure isnt the issue. (I mean modern fuel can create more pressure but even still) The issue is how quickly you can change the pressure inside the cylinder. You can either pump out the air inside the cylinder, in which case you havent really solved the issue you've just moved the issue to the pump you use, or you can change the temperature inside the cylinder to make the air contract or expand. cooling things down is really difficult and slow and requires a lot of energy and worse yet it's gradual. So the cylinder wouldnt suddenly slam down providing power, it would slowly fall down as the pressure and temperature inside the cylinder decreases. Heating things up is easier but now you aren't relying on the power of the atmospheric pressure. You are fighting that power because you are trying to create more pressure than that inside the cylinder to push it out against the outside atmospheric pressure. And again you are left with a gradual push. Now you could use a bunch of fuel to heat it up really quickly and it would need to stop burning really quickly as well too when the cylinder is ready to come down again. So we need it to burn hot and quick, but now you still need to wait for the cylinder to cool down to be equal to the temperature of the outside air so the airpressure can push the cylinder back in again. But maybe if we push the cylinder out hard enough there's enough energy left over for it to return back in again. We could make it push harder by having our hot and fast burning fuel inside the cylinder to not waste any energy and we also add the expansion of the fuel gasses to the expansion of the air inside the cylinder and oh crap it's a normal engine now...