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In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed. On a steam locomotive , the cutoff is controlled by the reversing gear . The point at which the inlet valve closes and stops the entry of steam into the cylinder from the boiler plays a crucial role in the control of a steam engine.
The reversing lever has a catch mechanism which engages with a series of notches to hold the lever at the desired cut-off position. This means that the operator does not have a full choice of cut-off positions between maximum and mid-gear, but only those which correspond with the notches.
In the simple case, this can be a relatively simple task as in the internal combustion engine in which the valves always open and close at the same points. This is not the ideal arrangement for a steam engine, though, because greatest power is achieved by keeping the inlet valve open throughout the power stroke (thus having full boiler pressure, minus transmission losses, against the piston ...
The Pennsylvania Railroad (PRR) class I1s steam locomotives were the largest class of 2-10-0 "Decapods" in the United States. From 1916 to 1923, 598 locomotives were produced (123 at Altoona Works and 475 at Baldwin Locomotive Works). They were the dominant freight locomotive on the system until World War II and remained in service until 1957.
The first kind is the steam lap, which is the amount by which the valve overlaps the steam port on the live steam side of the piston or slide valve (i.e. the distance the valve needs to move to just begin to uncover the port).
The WHE-25 engine has a 34% cutoff. [4] This allows for the remaining 66% of the piston stroke to expand the steam, extracting work from it and causing the pressure to drop. The figure to the right shows how pressure in the cylinder of a steam engine drops after the cutoff point. The WHE-DR must have a much later cutoff to allow it to self-start.
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Cross compound engine, with an expansion valve (top) on the high-pressure cylinder. An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the ...