Cylinder (locomotive)

The cylinders of a steam locomotive are the components that convert the power stored in the steam into motion.

Cylinders may be arranged in several different ways.

On early locomotives, such as Puffing Billy, the cylinders were often set vertical and the motion was transmitted through beams, as in a beam engine.

The next stage, for example Stephenson's Rocket, was to drive the wheels directly from steeply inclined cylinders placed at the back of the locomotive. Direct drive became the standard arrangement, but the cylinders were moved to the front and placed either horizontal or nearly horizontal.

The front-mounted cylinders could be placed either inside (between the frames) or outside. Examples:

In the 19th and early 20th centuries, inside cylinders were widely used in the UK, but outside cylinders were more common in Continental Europe and the United States. The reason for this difference is unclear. From about 1920, outside cylinders became more common in the UK but many inside-cylinder engines continued to be built.

Inside cylinders give a more stable ride with less yaw or "nosing" but access for maintenance is more difficult. Some designers used inside cylinders for aesthetic reasons.

The demand for more power led to the development of engines with three cylinders (two outside and one inside) or four cylinders (two outside and two inside). Examples:

On a two-cylinder engine the cranks, whether inside or outside, are set at 90 degrees. As the cylinders are double-acting (i.e. fed with steam alternately at each end) this gives four impulses per revolution and ensures that there are no dead centres.

On a three-cylinder engine, two arrangements are possible:

Two arrangements are also possible on a four-cylinder engine:

The valve chests or steam chests which contain the slide valves or piston valves may be located in various positions.

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