I think you will have to clarify your question to get meaningful responses.
In a diesel engine, the diesel doesn't explode, it burns. The pressure generated depends on the pressure at time of ignition and the amount of fuel injected. In a piston steam engine, the steam will always expand at least as fast as the piston moves (within reason at least). If you release steam into space or into the atmosphere, the speed at which it expands would depend on the pressure and temperature.
If you're trying to compare a steam engine and diesel engine in some way, or trying to turn one into the other, let us know more details.