The Willans engine or central valve engine was a high-speed stationary steam engine used mainly for electricity generation around the start of the 20th century. Willans' engine was one of the best-known examples of the steeple compound engine. These were double- or triple-expansion compound engines, with the unusual features of single-acting cylinders and a central spindle valve shared between all the cylinders of that spindle. The cylinders are arranged as tandem compounds, with high- and low-pressure cylinders mounted on the same vertical shaft. This vertical arrangement of the steeple compound gives a compact floor layout for an engine of such power.