The various events which are governed by the D slide valve of a steam engine are as follows:
The live steam period is that during which the steam is admitted from the steam chest into the cylinder and the steam admitted during this period is termed live steam.
The point of cut off is that at which the valve closes the steam port, and the admission of steam into the cylinder is stopped, hence the point of cut off is at the end of the live steam period.
The period of expansion is that during which the steam is allowed to expand in the cylinder, and therefore begins at the point of cut off, and ends at the point of release.
The point of release is that at which the valve opens the port and permits the steam to escape.
The point of compression is that at which the exhaust port is closed, which occurs before the piston has reached the end of its stroke; the steam that has not passed out of the cylinder is therefore compressed, the compression continuing until the valve opens for the lead.
The lead of the valve is the amount the port is open to the live steam when the crank is on the dead centre.
The point of admission is that at which the port opens for the live steam to enter, and it follows that the lead and compression both act as a cushion, arresting the motion of the piston when it reaches the end of the stroke.
Cushioning begins, however, at the time the exhaust port is closed enough to arrest the escape of the steam, while compression begins when the valve has closed the exhaust port.