Design Principles
When on the move, a steam locomotive requires steam to enter the piston at a controlled rate. This entails controlling the admission and exhaustion of steam to and from the cylinders. Steam enters and leaves the valve through a steam port, usually at the middle position of the piston valve. Where the valve is in contact with the steam ports, a consideration of the "lap" and "lead" is required.
Read more about this topic: Piston Valve (steam Engine)
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