The Cornish Cycle
Both of the engines at Crofton employ the Cornish Cycle, so called because it was developed by Cornish engineers for use in their tin mine pumping engines. The Cornish Cycle combines the use of steam pressure to push upon the piston and the vacuum produced when steam is condensed in an enclosed space to pull the piston. James Watt used both steam pressure and vacuum, but the arrangement of Watt valve gear and condenser was not as efficient as that employed by the Cornish engineers and, strictly speaking, engines built to the Watt design were not Cornish Cycle Engines. When originally installed at Crofton, both the 1809 and the 1812 engines employed Watt valve gear, but the 1812 engine was converted to the Cornish Cycle by Harvey’s of Hayle in 1843 when they replaced the boilers and changed the valve gear to cope with the increased steam pressure. At this time the 1809 engine was also replaced.
This is how the Cornish Cycle works
- When the engine is at rest, the pump side of the beam is down, and the steam piston is at the top of its stroke. The inlet and exhaust valves are open, and the equilibrium valve is closed. When the driver opens the throttle valve, steam is admitted above the piston and forces the piston down, thus lifting the water pump plunger. At Crofton, at one half stroke, the inlet valve closes, and the steam continues to expand.
- At the bottom of the stroke, the exhaust valve closes, and the equilibrium valve opens, permitting the steam that is above the piston to pass into the cylinder below the piston. The piston then rises under the weight of the pump plunger on the other end of the beam, displacing the steam into the void under the piston.
- At the top of the stroke, the equilibrium valve closes, and the exhaust valve opens. At the same time, the inlet valve opens to permit admission of another shot of steam. When the exhaust valve opens, a jet of cold water is injected into the condenser chamber which is connected to the exhaust system, causing the steam under the piston to condense, thus producing a vacuum.
- On the next power stroke, the difference of pressure between the steam above, and the vacuum below causes the piston to descend. In this way, the steam injected into the cylinder is used twice and is then recoverable as clean, warm water to replenish the boiler.
The condensed water in the condenser chamber is extracted by means of the Air Pump (so called because there is also air in the condenser and the pump extracts this as well). The Air Pump discharges the condensed water into the Hot Well which is the boiler feed reservoir.
It should be noted that the vacuum produced by these engines provides a significant proportion of their power. Both engines at Crofton run at a vacuum of -85 kPa (25 inches Mercury), compared to a boiler pressure of 138 kPa (20 psi) which probably equates to a cylinder pressure of no more than 62 kPa (9 psi). Certainly, when starting these engines, the driver is only too aware of the vacuum developing as they ‘come alive’ when this happens.