The piston valve releases incoming compressed steam into the piston at either end of its stroke, closes to allow expansion in the cylinder, and reopens to exhaust the spent steam into the blast pipe. The piston valve, one of a number of steam- and motion-control alternatives tested, proved to be the most durable under demanding conditions, in the desert West, in the semi-tropical South, and in the impossibly-cold North.
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First use in England
First use in United States