What was a Cornish engine?

The Cornish engine was so named because it was a type of steam engine developed by Cornish engineers and enginemen and mainly used in Cornwall. From 1810 the efficiency of beam engines was steadily improved. These were used to pump the water out of mines. They also, somewhat later, raised ore and lowered materials (whim engines) and powered the stamps that crushed ore (stamping engines).

The operation of the Cornish engine basically followed the principles of Watt’s steam engine, introduced in 1775. This had added a separate condenser to cool the steam being ejected from the cylinder. This meant that the cylinder could be kept permanently hot while in operation and greatly improved the engine’s efficiency.

You can view the main components of these engines – the boiler, cylinder, condenser and valves – and their connection to the beam in a working diagram on this site.

Richard Trevithick had by 1812 improved on Watt’s engine. He did this first by introducing steam into the cylinder from the boiler at a higher pressure. This, arriving above the piston at the top of its stroke, drove it down. Then, a complicated set of valves ensured that the intake of high pressure steam stopped before the piston completed its downward stroke. The steam above the piston then gradually expanded while the ejected steam below cooled in the condenser, creating a vacuum in the lower part of the cylinder and helping to draw the piston down.

Cornish engines were housed in the iconic Cornish engine house

At the bottom of the stroke another valve was opened which allowed the steam above the piston to travel below as the weight of the pumping gear and rods drew the piston up. This cycle would then be repeated.

These innovations – high pressure steam plus the valve work – were further added to in the 1820s by better insulation, producing greater efficiency in the use of fuel, attaining its peak in the 1840s. It was this period, from the 1810s to the 1840s, when Cornwall and its engineers led the way in steam engine technology, genuinely ‘world-beating’.

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