Taylor's engine, 1840. This was a very efficient Cornish engine. The cylinder's steam supply was shut off only one-tenth of the way into the stroke.
Image: Science Museum / SSPL

Expansive working

Steam engines also worked 'expansively'. The steam flow into the cylinder stops during the stroke and then expands, exerting a continuing but diminishing force on the piston. Less steam was used than by simple steam engines.

Cornish enginemen competed in making their expansive engines as economical as possible. Engine performance was recorded in a journal - the Engine Reporter - published for over a century.

By 1830, Cornish engines worked more than twice as efficiently as the best Boulton & Watt engine.