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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.
By allowing the inlet-valves to open quickly, full boiler-pressure can be established in the cylinder early in the stroke. [2] Due to how quickly the inlet-valve closes, the area of the work-diagram is increased. This is due to a sharp corner at the cut-off point rather than a rounded curve.
The valve gear, actuated by connection to the driving wheels, ensures that steam is delivered to the piston with precision. Types are slide valves, piston valves or poppet valves. [2] [3]: 62 Valve chest / Steam chest Valve chamber next to the cylinder (24) containing passageways to distribute steam to the cylinders. [6]: 41 [3]: 75 Firebox
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 four-way valve or four-way cock is a fluid control valve whose body has four ports equally spaced round the valve chamber and the plug has two passages to connect adjacent ports. The plug may be cylindrical or tapered, or a ball.
Diagram showing lap and lead and their relation to valve travel When on the move, a steam locomotive requires steam to enter the cylinder at precise times relative to the piston's position. [ 3 ] This entails controlling the admission and exhaust of steam to and from the cylinders with a valve linked to the motion of the piston.
Control is by a small three-way steam valve (“forward”, “stop”, “back”) and a separate indicator showing the position of the rod and thus the percentage of cutoff in use. When the steam valve is at “stop”, an oil cock connecting the two ends of the locking piston is also closed, thus holding the mechanism in position.
The Hackworth valve gear is a design of valve gear used to regulate the flow of steam to the pistons in steam engines. It is a radial gear , with an actuating lever driven from the crank. The drive may be taken directly from the crank or indirectly via a return crank (as in all pictures).