As can be seen from the figure, the intake valve and exhaust valve are closed, and the piston moves downward, so it is a power stroke; At this time, the high-temperature gas inside the internal combustion engine rapidly expands to do work outside, converting internal energy into mechanical energy, so the internal energy of the gas is reduced.

Because the specific heat capacity of water is relatively large, the same mass can absorb more heat.

So the answer is: do work; Reduce; The specific heat capacity of water is relatively large.