The Cornish engine was a most unusual machine. It had neither connecting rod, crank or flywheel, and operated on a single acting cycle which dated back to Watt's first involvement with the steam engine, a cycle which remained unchanged as long as these engines operated. It reached a peak of efficiency in the 1840s not achieved by the normal reciprocating engine for years and yet it was almost exclusively limited in application to water works and mining installations. It was also big; 9' and 10' strokes with bores up to 90' or 100' were common by the middle of the 19th century.
We can examine its decendency from the atmospheric engine in order to better understand its development and operation.
Although the shaft is no longer used for pumping, it served until recently as a ventilating shaft, and air ducts are located behind the concrete walls.
This engine is now preserved and open to the public. Notice the resemblence to the engine house model shown in the July/August issue of IMA.
The windows indicate the three floor levels of these engine houses. The lower one is level with the cylinder bottom; the middle several feet below the cylinder top, and the upper level with the walking beam.
The beam bearings rested on the wall of the engine house itself, and as a result the shaft side wall was heavily built, often being three feet thick.
The first engines built by Newcomen were single cylinder machines set up next to mine shafts for the purpose of removing ground water. The cylinder was vertical, often mounted directly on top of a round boiler, in appearance much like a brewery still.
The piston was connected to one end of a wooden beam, the other end of which was over the mine shaft and connected to the pumps down in the mine. The beam was usually mounted on a solid mass of cut stone which supported the middle pivot bearing.
The engine was so balanced that when it was at rest the weight of the pump gear pulled the piston to the top of the cylinder.
To start the engine steam at low pressure, only 1 or 2 p.s.i. was admitted into the cylinder till all the air was blown out. The admission valve was then closed and a jet of cold water sprayed into the cylinder which condensed the steam forming a vacuum, and also cooling the cylinder at the same time. Atmospheric pressure then furled the piston down raising the pump gear through the medium of the walking beam. At the bottom of the piston stroke the steam valve opened to repeat the cycle.
Watt's first and major contribution was to relocate the condensing part of the cycle to a separate con-densor thus materially improving the engine's thermal efficiency.
He also went a step further and covered the open top of the cylinder to keep cold air out. By substituting a piston rod and packing gland for the chains of Newcomen he gave the steam engine its potential for becoming double acting.
Initially, however, Newcomen's cycle was followed except that steam was allowed to flow into the top of the cylinder, at first merely for the purpose of keeping cold air out.
When the piston reached the bottom of its stroke the steam above the piston was admitted below the piston, breaking the vacuum and equalizing the pressure in both ends of the cylinder so that the piston could be drawn up by the pump rods. At the top of its stroke an exhaust valve opened in the bottom of the cylinder and an admission valve let steam into the top and the cycle began anew.
At this point it was obvious that by increasing the boiler pressure additional work could be gained from the engine as steam pressure now assisted the vacuum. This primitive cycle endured, as long as Cornish engines, until the 1950s in Great Britain.
In the early part of the 19th century, the southwestern part of Great Britain was in the midst of a mining boom and bringing up thousands of tons of tin and copper ore. Good pumping engines for the mines were needed and as the engine's major developments took place there the name 'Cornish engine' was coined. Engine houses by this time had progressed from the simple crude wooden shacks of Newcomen's day to massive stone buildings three stories high.
The engine house was built on the lip of the mine shaft and one wall now took the place of the cut stone beam supports. This wall was invariably thicker than the other three and had an opening built into it for the end of the beam to pass through. These by now were of cast iron and usually consisted of two flat oval shaped beams side by side. The piston rod was connected to one end of the beam by Watt's parallel motion. The other, 'outdoor,' end was usually simply connected to the uppr end of the pump rods and these were long enough that the arc described by the beam end as it moved up and down was absorbed by the spring of the wooden pump rods.
These latter were massive affairs; in some cases 18' or 24' square at the top end, usually of baltic pine or from the west coast of the United States. (Many old Cornish records refer to shipments of 'Oregon pine', but the exact nature of this type of wood seems to be in doubt. An English friend of mine has asked about 'Oregon pine' while on a business visit to the West Coast and been told no such designation exists today. Can some reader offer information on what 'Oregon pine' was or what it is called today?)
These rods ran to the bottom of the pit, regardless of how deep it was, and pump rods several thousand feet long were not unknown.
It was the great weight of these rods which actually pumped the water, for except for the lowest pump in the sump which was of the piston in cylinder type, the pumps were single acting outside packed plumber units. The vertical plumbers were mounted on offset arms bolted to the pump rods and of course had the same stroke as the rods. They worked in 'lifts' of 150' or 200' thus minimizing the strains on the cast iron piping. The pump rods were lifted by the steam cylinder pulling the 'indoor' end of the beam which was the suction stroke. As the rods descended the water was pumped up to the next pump's sump and so on until it was discharged from the mine. It was necessary to carefully balance the weight of the pump rods so that they would not descend too fast and 'balance boxes' were connected to the rods. These were simply a pivoted horizontal beam with one end connected to the pump rod and a box, loaded with cast iron or rocks at the other end. One of these shows clearly in the photo of a Cornish engine model on page 2 of the July/August '81 issue of I.M.A. It is the slightly tilted box to the left bottom of the photo.
The large frame over the shaft mouth was for the purpose of pulling up the pump rods for maintenance work.
The high efficiency of the Cornish engine appealed to the engineers who began to lay out the early water works system for English cities and it also found a ready field for its use in that capacity. There were slight differences of construction. The main difference was that the entire engine was within the pump house including the pumps. Without the weight of the mine shaft pump rods, the engine was useless, so an artificial load was added. This consisted of a cast iron case, usually with fluting or some other ornamentation hung on the pump end of the beam. Tons of cast iron would be packed inside this case and the pump plunger was fastened underneath it. Now, the steam cylinder lifted this weight, and the pump made its suction stroke. As the cylinder pressure on both sides of the piston equalized, the massive cast iron weight descended and the pump operated on its discharge stroke. The beam center supports were no longer mounted on a masonry wall but instead on a massive cast iron framing which was built into the walls of the engine house. For water works engines, both steam piston rod and pump rods were guided by parallel motion. The condensors, usually of the jet type, were under the floor; condensate and boiler feed pumps were also operated by links from the beam.
'Outdoor' end of the engine. The two platforms would be used at the engine 'driver' to lubricate or maintain the upper pump rod bearing, or 'nose bearing' as it was known. The iron plates which reinforced the massive wood pump rod can be seen.
The date 1924, when this engine was installed in its present location, is just visible in the keystone over the door.
As these engines had no crank shafts there were several unique problems to be faced in their operation.
The first and perhaps most dangerous was that there was no crank to limit piston movement at the end of the stroke. This was controlled solely by the valve gear; should the engine overstroke, the piston would hit the cylinder head. In an effort to keep the engine from smashing itself to pieces, (and more than one did), massive timbers were bolted across the ends of the beam, at right angles. These were supposed to strike 'stops' or catch beams built into the engine framing and so limit piston travel. In some cases the wood was replaced by forked iron 'ears.' When the engine was at rest the pump end of the beam rested on its stops. It may seem to us to be an incredible way to build a machine, yet the Victorian engineers had a direct approach to solving problems!
Some of the engines preserved today still show traces of accidents, either in repairs to cylinders or other metal parts or extra timbers and strapping to repair cracked catch beams.
The biggest dangers the engines faced was a bursting of the discharge water mains which allowed the pump end to descend too fast. It was said that when a big Cornish engine hit its stops, anyone within a couple of blocks of the engine house knew it!
Another peculiarity of the Cornish engine was its speed control. It was not regulated by throttle opening but by stalling it between strokes! With everything else about the engine being oddball, why not the valve gear as well?
As there was no eccentric the valve gear was operated by a 'plug rod' which hung from the walking beam between the center bearing and the cylinder. The cylinder itself had four valves, three on top, one at the bottom.
The upper valve chest housed throttle, admission, and equilibrium valves; the bottom valve chest had the exhaust valve. The cylinder had a framing on the side facing the beam center which mounted several horizontal shafts about 4 to 6 feet above floor level. These were connected to the valves by offset arms and links so that as the horizontal shafts or 'arbors' as they were called rotated slightly in their supports, the valves opened or closed. Each arbor also had a large handle fixed to it which served a dual purpose. The handles were struck by pins connected to the plug rod as it rose and fell thus opening and closing the valves, but they were also used by the engine driver when starting an engine, to open and close the valves until the engine was self acting.
Cylinder or 'indoor' end of the beam. Notice the massive construction of the beam and the links which connect it to the piston rod cap, the latter being visible between the links.
The 'ears' which prevent the engine from overstroking are fitted across the upper end of the beam. The timbers on which they strike are located just below the wooden fence. An interesting point is that the upper link pin, in the beam eye, works only in a half bearing. There is no top brass as the strain is always in one directiondown.
The wire cable holds up this end of the beam as the pump rods in the shaft have been removed, removing the weight which would normally keep the cylinder end up.
The valve action wasn't quite as simple as this, however, for gravity also played a key part. To better understand the system, let's follow the whole cycle starting with the piston at the top of the cylinder.
The steam and exhaust valves open which puts boiler pressure above, and vacuum below, the piston. The piston descends, raising the pump weight or shaft rods. The plug rod also descends, and at a predetermined point an adjustable bar on it strikes the admission valve handle closing the admission valve and holding it shut as the bar continues to slide down along the handle. The bar's position on the plug rod is adjustable by means of a screw so that the point at which it hits the handle and cuts off the steam can be set to suit operating conditions.
When the piston reaches the bottom of its stroke, another plug rod pin strikes and closes the exhaust valve. This valve is automatically 'latched' shut by a catch device on the arbor and as the exhaust valve arbor rotates, it in turn releases a catch which has locked the equilibrium valve shut allowing it to pop open. The opening of the equilibrium valve allows the steam from above the piston to flow under the piston thus equalizing pressure in both ends of the cylinder.
The equilibrium arbor's rotation also locked the admission valve in its closed position against the force of a counterweight which would otherwise open it. The plug rod on reaching the bottom of its stroke lifted the cataract, still another part of the valve gear which we will come to presently.
With cylinder pressure equalized, the pump end of the beam descended, the discharge stroke took place and the piston neared the top of its stroke. The plug rod was rising as well, and just before the end of the stroke it hit the handle on the equilibrium valve arbor and closed it, forming a steam cushion to bring
This machine was built by Boulton and Watt in 1S20. In 1840 it was installed in its present location and in 1848 the valve sear was rebuilt into its present style. This engine, with a 64' bore ran in regular service until 1943. Today it is in operation every weekend at the Kew Bridge Steam Museum, London.
The main steam line conies in at the middle right of the photo.
This engine is unusual in that one arbor (horizontal valve sear rock shaft) is mounted just below the upper valve chest. Usual practice had all three arbors at about chest level.
The two fluted columns are side pipes, one of which carries the steam from the upper valve chest to the bottom of the cylinder.
The vertical rod with a crank at the bottom is the throttle. The larger rod running top to bottom just to the left of center is the plus rod.
The beam center bearing and pumps are to the extreme left, out of the photo.
Now springs into play the cataract! This is simply a weighted hydraulic plunger which is lifted about 6-10' by the plug rod and whose rate of descent is controlled by a needle valve. When the cataract nears the bottom of its stroke, a linnale trips the catches holding exhaust and steam valves shut. These are both counter balanced with weights and as the catches are released the arbors rotate, the handles fly up and the valves open beginning the cycle.
It was this pause, controlled by an adjustable cataract, that determined actual total gallonage output of a Cornish engine, and they could be adjusted by it to run from 2 or 3 strokes per minute to 12 or 14. The actual 'stroke', that is one down and up of the piston, took the same length of time regardless of how many strokes per minute the engine was set for.
The arbors had several interlocking features built into them to prevent the possibility of the wrong valve dropping open at the wrong time. All in all it is an ingenious valve gear and especially unusual to see in operation, especially when the engine stops, and then suddenly almost as if at their own volition the valves suddenly pop open and the engine starts again.
Yes, I did say 'see in operation' for there are a number of preserved Cornish engines in Great Britain, and two locations still operate them.
The oldest of these is the Crofton Pump house on the Kennet and Avon Canal in Wiltshire. (British Rail to Great Bedwyn, pleasant hour walk past thatched roof farms to pump house) and this location has two engines, the no. 1, 42' bore built 1812, and no. 2 built in 1846 also 42' bore. These engines are steamed several times a year and the no. 1 engine is the oldest known steam engine in the world which is still on its original site and handling its original load. (Water for the summit level of the canal.)
The other site to visit is in the city of London itself. It is the 'Kew Bridge Steam Museum' and has engines which have to be seen to be believed. There is an excellent collection of engines which have been moved to the site from other areas but the 'originals' consist of 5 Cornish engines. Two of these, a Boulton and Watt built in 1820 with a 64' dia cylinder, and a 90' bore engine built by Sandys, Carne and Vivian, have been restored to operation and are run, together with the other aforementioned engines every weekend.
The Boulton and Watt engine, with a 8' stroke pumped 2 million gallons a day from 1820 to 1943, an incredible service life of 123 years.
The '90' as the Sandys engine is known as an 11-foot stroke with a capacity of 6 million gallons a day. It ran only from 1846 to 1943 so missing the century mark by 3 years! The station also has a 100' x 11' Cornish engine which is not yet restored to service. The cast iron beam on this monster weighs almost 50 tons!
These engines, Crofton, and Kew, are all of the 'Water works' pattern, where the entire engine is within the house. There are several preserved Cornish engines on old mine sites which show the type illustrated by the photo in I.M.A. July/August '81.
One site is the Preston Grange Mining Museum near Edinburgh, Scotland. Several others are in Cornwall but not all are accessible to the general public as some are located on still active sites and require permission to visit.
The Cornish working cycle described in this article is a typical one and as with all other forms of engines there were minor variations. Of the 26 preserved engines listed above there are probably close to a dozen variations in the engine 'gear work' as the old Cornish men called it.
Cornish engines were used in this country as well but never to the extent that the British used them.
Of all forms of the steam engines made by man, the Cornish engine must certainly rate as one of the more unusual types and without doubt impressive in concept, finish and operation.
For those wishing additional information on location of other Cornish pumping engines, send 50 to: Copies, Stemgas, P.O. Box 328, Lancaster, PA 17603, for a 3-page Xerox copy of a list published in February 1981 by the Trevethick Society Newsletter. Listed are all known Cornish pumping engines still in existence throughout the world.