I coped this text from this website
This stroke is where the spark plug ignites the air/fuel mixture,
creating very high cylinder pressure which rise very quickly. Peak
cylinder pressures near TDC (where spark occurs) will be in the range
of 300 psi for engine's at light loads, to 1000 psi for production
engines at full power to 1500 psi or greater for race engines. This is
where the engine's power comes from, as it forces the piston down. As
the piston goes down, the cylinder volume increases which reduces the
cylinder pressure. When the piston gets to the bottom on the cylinder
(BDC) there may only be 100 to 500 psi in the cylinder.
So the engine is designed to take pressures around 1000 psi which will be at its highest around TDC.
If as you question, the piston couldn’t move anymore, I see the following things happening -
- The hot gasses will start to cool causing a pressure drop.
- The piston rings have a small gap around their circumference and do not perfectly seal against the piston, so gasses will slowly escape passed the rings.
- The inlet and exhaust valves will likely not provide a perfect gas tight seal, so gasses will slowly escape passed the valves.
The engine won’t explode due to it suddenly stopping, since it is designed to take the maximum possible pressure that occurs every revolution.
The pressure will slowly drop due to cooling and gas leaking away.
What is more likely to cause damage in a situation where the engine suddenly stops moving, is the rapid change in momentum. The crankshaft for example can snap or become bent due to rapid deceleration. Here I am talking about suddenly stopping like in your example of a lawnmower stopping after hitting a rock for example, not a car engine stalling when pressing the brakes.