Concept
A Cornish engine is a type of steam engine developed in Cornwall, England, mainly for pumping water from a mine. It is a form of beam engine that uses steam at a higher pressure than the earlier engines designed by James Watt. The engines were also used for powering man engines to assist the underground miners' journeys to and from their working levels, for winching materials into and out of the mine, and for powering on-site ore stamping machinery. Cornwall has long had tin, copper and other metal ore mines, but if mining is to take place at greater depths, a means to dewater the mine must be found. Lifting the weight of water up from the depths requires great amounts of work input. This energy may be weakly supplied by horse power or a waterwheel to operate pumps, but horses have limited power and waterwheels need a suitable stream of water. Accordingly, the innovation of coal-fired steam power to work pumps was more versatile and effective to the mining industry than primitive means. The mine Wheal Vor had one of the earliest Newcomen engines (in-cylinder condensing engines, utilising sub-atmospheric pressure) before 1714, but Cornwall has no coalfield and coal imported from south Wales was expensive. The cost of fuel for pumping was thus a significant part of mining costs. Later, many of the more efficient early Watt engines (using an external condenser) were erected by Boulton and Watt in Cornwall. They charged the mine owners a royalty based on a share of the fuel saving. The fuel efficiency of an engine was measured by its "duty", expressed in the work (in foot-pounds) generated by a bushel () of coal. Early Watt engines had a duty of 20 million, and later ones over 30 million. The Cornish cycle operates as follows. Starting from a condition during operation with the piston at the top of the cylinder, the cylinder below the piston full of steam from the previous stroke, the boiler at normal working pressure, and the condenser at normal working vacuum, The pressurized steam inlet valve and low-pressure steam exhaust valves are opened.