Working of stream engine:

The high pressure steam from boiler is supplied to the steam chest. The high pressure steam enters cylinder through inlet port and exerts pressure on piston and drives it towards the other end. At same time the steam on other side of the piston is exhausted through exhaust port. The D slide valve moves gradually in opposite direction. The D slide valve then closes inlet port and steam supply is cut off. The entrapped steam expands and works to push piston further. With further movement of the D slide valve, inlet port of other side is connected to steam inlet and hence high pressure steam enters other side of the piston which provides return stroke of piston.