Valve gear

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The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit 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".

1 Valve gear designs
- 1.1 Reciprocating valve gears
- 1.2 Poppet valve gears
2 See also
3 External links

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.

A further benefit may be obtained by admitting the steam to the cylinder slightly before front or back dead-center. This advanced admission (also known as lead steam) assists in cushioning the inertia of the motion at high speed.

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. Generally, two simple harmonic motions with different fixed phase angles are added in varing proportions to provide an output motion that is variable in phase and amplitude. A variety of such mechanisms have been devised over the years, with varying success.

Both slide and piston valves have the limitation that intake and exhaust events are fixed in relation to each other and cannot be independently optimised. Lap is provided on steam edges of the valve, so that although the valve stroke reduces as cutoff is advanced, the valve is always fully opened to exhaust. However, as cutoff is shortened, the exhaust events also advance. The exhaust release point occurs earlier in the power stroke and compression earlier in the exhaust stroke. Early release wastes some energy in the steam, and early closure also wastes energy in compressing an otherwise unnecessarily large quantity of steam. Another effect of early cutoff is that the valve is moving quite slowly at the cutoff point, and this causes 'wire drawing' of the steam, another wasteful thermodynamic effect visible on an indicator diagram.

These inefficiencies drove the widespread experimentation in poppet valve gears for locomotives. Intake and exhaust poppet valves could be moved and controlled independently of each other, allowing for better control of the cycle. In the end, few locomotives overall ever were fitted with poppet valves. A very late British design, the SR Leader Class, used sleeve valves adapted from internal combustion engines, but this class was not a success.

In stationary, traction engine and marine engine practice, the shortcomings of valves and valve gears were among the factors that lead to compound expansion. In stationary engines trip valves were also extensively used.

[edit] 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 those which were used with poppet valves.

[edit] Reciprocating valve gears

Gab or hook gear - used on earliest locomotives. Allowed reversing but no control of cutoff.
William T. James valve gear 1832, used first by Baltimore and Ohio Railroad, USA
Stephenson valve gear - most common valve gear in 1800s, normally inside the locomotive frame.
Walschaerts or Heusinger valve gear - most common valve gear on later locomotives, normally externally mounted.
Gresley conjugated valve gear - Used on 3-cylinder locomotives. Walschaert gear was usually used for the outside cylinders. Two rods connected to the outside cylinders drove the valve for the inside cylinder.
Baker valve gear - fairly common in the United States, it had no sliding parts.
Southern valve gear - Briefly popular in the United States around 1920. It combined elements of the Walschaert and Baker patterns.
Young valve gear - used the piston rod motion on one side of the locomotive to drive the valve gear on the other side.
Joy valve gear - a design used extensively on the L&YR and LNWR in England, and elsewhere.
Baguley valve gear - used by W.G. Bagnall
Bagnall-Price valve gear - a variation of Walschaert used by W.G. Bagnall
Allan valve gear
Gooch valve gear
Hackworth valve gear
Marshall valve gear