Piston valve (steam engine)

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Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders, and its exhaustion from the cylinders after use, enabling a locomotive to move under its own power.



In the 19th century, steam locomotives used slide valves to control the flow of steam into and out of the cylinders. In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using superheated steam. There were two reasons for this:

  • It is difficult to lubricate slide valves adequately in the presence of superheated steam
  • With piston valves, the steam passages can be made shorter. This, particularly following the work of André Chapelon, reduces resistance to the flow of steam and improves efficiency

The usual locomotive valve gears such as Stephenson, Walschaerts, and Baker valve gear, can be used with either slide valves or piston valves. Where poppet valves are used, a different gear, such as Caprotti valve gear is needed.


File:SwanningtonEngine 01.jpg
The Swannington incline winding engine of 1833 incorporated a piston valve

The Swannington incline winding engine on the Leicester and Swannington Railway, manufactured by The Horsely Coal & Iron Company in 1833, shows a very early use of the piston valve.[1] Piston valves had been used a year or two previously in the horizontal engines manufactured by Taylor and Martineau of London, but did not become general for stationary or locomotive engines until the end of the 19th century.[2]

Design principles

When on the move, a steam locomotive requires steam to enter the piston at a controlled rate, in order for the locomotive to function efficiently.[3] This entails controlling the admission and exhaustion of steam to and from the cylinders.[3] Steam enters and leaves the valve through a steam port, which is located in the middle position of the piston valve.[3] Where the valve is in contact with the steam ports, a consideration of the "lap" and "lead" is required.


The "Lap" is the amount by which the valve overlaps each steam port at the middle position of each valve.[3] However, there are two different types of "Lap."

The first kind is the "steam lap," which is the amount by which the valve overlaps the port on the live steam side of the cylinder.[3] Secondly, there is the "exhaust lap," which is the amount by which the valve overlaps the port on the exhaust side of the cylinder. "Exhaust lap" is generally given to slow-running locomotives.[3] This is because it allows the steam to remain in the cylinder for the longest possible amount of time before being expended as exhaust, therefore increasing efficiency.[3] shunter locomotives tended to be equipped with this addition.

The "Negative exhaust lap", also commonly termed "exhaust clearance," is the amount the port is open to exhaust when the valve is in mid-position, and this is used on many fast-running locomotives to give a free exhaust.[3] The amount seldom exceeds 1/16 in. when exhaust clearance is given; the cylinder on both sides of the piston is open to exhaust at the same time when the valve is passing through the mid-position, which is only momentary when running.[3]


The "lead" of the valve is the amount by which the steam port is open when the piston is static at front or back dead centre.[3] Pre-admission of steam fills the clearance space between the cylinder and piston and ensures maximum cylinder pressure at the commencement of the stroke.[3] "Lead" is particularly necessary on locomotives designed for high speeds, under which conditions the valve events are taking place in rapid succession.[3]

Valve travel

Long-travel piston valves allow the use of large steam ports to ease the flow of steam in to, and out of, the cylinder.

See also


  1. Clinker, C.R. (1977) The Leicester & Swannington Railway Bristol: Avon Anglia Publications & Services. Reprinted from the Transactions of the Leicestershire Archaeological Society Volume XXX, 1954.
  2. Information plaque on the Swannington engine, National Railway Museum, York.
  3. 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 Garratt, C. & Wade-Matthews, M.: The Ultimate Encyclopedia of Steam & Rail Locomotives: Nearly 2 Centuries of Locomotive & Railway Development (London: Hermes Publishing Company, Ltd., 1998) ISBN 1840380888
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