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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.
This friction meant that if the Johnson bar is unlatched while the engine is operating under high steam pressure (wide regulator openings and high cut-off) or at high speeds, the forces that are supposed to act on the slide valves can instead be transmitted back through the linkage to the now-free reversing lever.
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).
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 ...
Subclass I1sa increased maximum steam cut-off to admit steam for 78% of the piston stroke (rather than the original 50%), boosting low speed tractive effort from 90,000 to 96,000 pounds-force (400 to 430 kN). There was no obvious external difference, except for a revised builders' plate and combination lever.
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 ...
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(On some engines the die block was in the top of the link in forward gear. This type was generally used on tank engines, which worked in forward and reverse equally. [6]) Once the locomotive has accelerated the driver can adjust the reverser toward the mid-gear position, decreasing cut-off to give a more economical use of steam.