Valve gear
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Walschearts_valve_gear.jpg
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit high-pressure steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It is sometimes referred to as the "motion".
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 throughout the stroke) while peak efficiency is achieved by only having the inlet valve open for a short time and then letting the steam expand in the cylinder (expansive working).
The point at which steam stops being admitted to the cylinder is known as the cutoff and the optimal position for this varies depending on the work being done and the tradeoff desired between power and efficiency. Steam engines are fitted with throttles (regulators in British parlance) to vary the restriction on steam flow, but controlling the power via the cutoff setting is generally preferable since it makes for more efficient use of boiler steam.
In the internal combustion engine, this task is performed by cams on a camshaft driving poppet valves, but this arrangement is not commonly used with steam engines, partly because achieving variable engine timing using cams is complicated. Instead, a system of eccentrics, cranks and levers is generally used to derive the motion of a slide valve or piston valve from the motion of the main crank/big end. A variety of these systems have been devised over the years, with varying success.
A key flaw in all of these valve gears is that the same valve motion operates the exhaust valve as well as the inlet valve, making it much harder to achieve the desirable valve events on both the inlet and the exhaust at the same time. As the cutoff is reduced, the exhaust valve starts being opened for a shorter period also, reducing efficiency drastically. Thus, with traditional piston or slide valves, there is a limit to how far the cutoff can be reduced before the engine is choked by its inability to exhaust the steam. This, above all else, was what drove the widespread experimentation in poppet valve gears for locomotives, although few locomotives overall ever were fitted with poppet valves.
Valve gear designs
Valve gear was a fertile field of invention, with probably several hundred variations devised over the years. However, only a small number of these saw any widespread use. They can be divided into those that drove the standard reciprocating valves (whether piston valves or slide valves), and poppet valve gear.
Reciprocating valve gears
- Stephenson valve gear - most common valve gear on early locomotives, normally inside the locomotive frame.
- Walschaert valve gear - most common valve gear on later locomotives, normally externally mounted.
- Baker valve gear - fairly common in the United States, notable for using no slides and thus could use efficient, easy to maintain needle bearings for all motion.
- Young valve gear - used the piston rod motion on one side of the locomotive to drive the valve gear on the other side.
- Southern valve gear - somewhat related to the Baker.