Murdoch's D Slide Valve
The D slide valve, or more specifically Long D slide valve, is a form of slide valve, invented by William Murdoch and patented in 1799. It is named after the hollow central D-sectioned piston.
This valve worked by "connecting the upper and lower valves so as to be worked by one rod or spindle, and in making the stem or tube which connects them hollow, so as to serve for an induction pipe to the upper end of the cylinder." This allowed two valves to do the work of four.
The above description (referring to upper and lower valves) clearly relates to an engine with a vertical cylinder, such as a beam engine. Where the cylinders are horizontal, as in a steam locomotive, the valves would be side-by-side.
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