Cochrane's Rotary Steam Engines.

Updated: 18 Nov 2007
With a guest appearance of Stephenson's Rocket.

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The name of Lord Dundonald is frequently mentioned in the General Meeting of the Institute of Engineers, and it sounded familiar, so I thought some investigation was called for. And remarkable the results were.

Rear Admiral Lord Thomas Cochrane, Tenth Earl of Dundonald (1775-1860), had an extraordinary career. He was the son of Archibald Cochrane, the ninth Earl of Dundonald. (1749-1831) The ninth Earl was an unsuccessful inventor who attempted amongst other things, new processes for alkali manufacture; the family was greatly impoverished due to losses over these schemes.

Left: Thomas Cochrane during his naval career.

Thomas Cochrane had a remarkable naval career, successfully attacking enemy vessels against outrageous odds. He was the prototype for C S Forester's Captain Hornblower, and more recently, Patrick O'Brian's Jack Aubrey. He later became an MP, was convicted of stock-exchange fraud in 1814 and imprisoned for a year, (naturally he escaped, but was recaptured) and expelled from The Order of The Bath. From 1817 he commanded the Chilean Navy, going on to command the Brazilian and Greek Navies, and returning home in 1828.
Cochrane was restored to the Order of The Bath in 1847, and re-employed by the Royal Navy; from 1848 to 1851 he commanded the North American and West India station.
He died in London on the 30th of October 1860, and was buried in Westminster Abbey.

At various periods of his life Cochrane, following in his father's inventive footsteps, took out patents for lamps to burn oil of tar, for the propulsion of ships at sea, for facilitating excavation, mining and sinking, and for other purposes; as early as 1843 he was an advocate of the use of steam and screw propellers in warships.

Cochrane became a strong advocate of rotary steam engines. He produced at least four rotary engine designs, none of which had any success. I am still in the process of sorting out the order in which they were built, patented, tested and abandoned, so for the moment I have just called them Type 1, etc.

In 1834 Cochrane had a pair of rotary engines built for locomotive work, probably by John Seaward & Co, of the Canal Ironworks, Limehouse, on the river Thames, who had built some of his earlier marine rotary engines. He contacted the London & Midland Railway, to arrange for tests, and the locomotive selected for modification was none other than Stephenson's Rocket.
It was available, and had straight axles unlike the cranked axles of the "Planet" class of locomotive, which made the modifications easier.

Left: Lord Dundonald's rotary engine; Type 1.

Not for the first time I wonder about an engine where the exhaust passages are the same size as the inlet passages. This does not sound like a good route to the expansive use of steam. So far as I can see there is no way to adjust admission cut-off, and it looks as though the steam would be recompressed in the "lunate space or crescent shaped channel" at the top of the cylinder. This might explain the apparent feebleness of the engine.

Left: Lord Dundonald's rotary engine; Type 1.

Note the two screws for adjusting the "packing plate" at the bottom. This sort of thing is common in rotary engines but never seen in reciprocating engines of the day, and gives a hint as to the difficulties in sealing.

Cochrane (now Lord Dundonald) told the LMR that he was so confident that the expense of the modifications would not exceed 30 that he would pay any additional cost himself. In the event the cost was nearer 80. A trial was made on or about the 22nd of October 1834, and it was this debacle that inspired George Stephenson's comment at the General Meeting of the Institute of Engineers, that "the engine could not be made to draw a train of empty carriages".

Left: Lord Dundonald's rotary engine applied to a locomotive. From a pamphlet he produced in 1833, before the Rocket modifications.

This fails to make clear why two engines are required on one solid axle. It also looks as though maintenance of the water-pump- a rotary type, one hopes- might be a bit tricky.

It is also slightly worrying that the wheels do not appear to have proper flanges; very likely the artist was not an expert on locomotives.

Here are some other Cochrane engines that have appeared in the literature. The dates of construction or testing are not known; the diagrams appeared in print decades afterwards.

Left: The Cochrane Rotary Engine; Type 2

"A wing piston rotating around the central axis of an outer shell or cylinder. A hollow cylinder of smaller diameter is pivoted eccentric to the wing axis to keep one side in contact with the shell. The steam pressure revolves the wing and shaft with a force due to the varying area of the wing outside of the inner cylinder."

This looks rather similiar to the much earlier Trotter engine. (1805)

From "Mechanical Movements, Devices and Appliances" by Gardner D Hiscox, published by Sampson Low, Marston & Co in 1899.

Left: The Cochrane Rotary Engine; Type 3

"The wing pistons d,d are packed in the eccentric inner cylinder by a slotted rocking cylinder and revolve concentric with the outer cylinder or shell. The inner cylinder is pivoted eccentric to the shell, making a tight joint at the bottom."

Cochrane (Lord Dundonald) strikes again. At first sight this is just a cruder drawing of the Type 1 engine illustrated above, but in this version there are two slotted cylinders inside the chamber, in an attempt to seal the vanes.

1 is the axis of the vanes, and 2 the axis of the eccentric inner cylinders.

From "Mechanical Movements, Devices and Appliances" by Hiscox.

Left: The Cochrane Rotary Engine; Type 3

This animation shows how the eccentic inner cylinders cause the vanes to form spaces that expanded and contracted.

This animation is kindly provided by Bill Todd

Left: The Cochrane Rotary Engine; Type 4. 1844

"An eccentric cylindrical piston rotating on an axis central to the shell. The vibrating wings pivoted in the outer shell form the steam abutment by closing against the eccentric revolving cylinder."

That word "abutment" again...

This version was patented in 1844.

From "Mechanical Movements, Devices and Appliances" by Hiscox.

Left: The Cochrane Rotary Engine; Type 4. 1844

This is a rather better drawing of the Type 4 engine. It shows a small feature on the eccentric rotor; from its position it is probably a seal that contacted the inside of the casing when the rotor was sweeping through its lower section. There are tilting shoes pivoted at the end of the two flaps, which at least promised an area rather than a line contact for sealing against the rotor.

It also shows what appears to be a fatal flaw in the geometry of this engine. With the rotor in the position shown, there is a direct path from steam inlet to exhaust, and this would seem to exist while the rotor was going through the top 90 degrees or so of its rotation. This would have had catastrophic effects on the economy of the engine.

From A Treatise on The Steam Engine in Its Application to Mines,Mills, steam navigation and Railways by the Artizan Society, ed John Bourne, pub 1853 Longmans

This is the form of engine that was tested in the steam frigate Janus.
"This contrivance has not yet realised any great success, and the prevailing opinion among engineers appears to be that it will not supercede ordinary engines. Similar engines have been tried on many former occasions, but they have always been found to involve either a ruinous amount of leakage, or such a degree of friction as to make the plan impossible in practice." said John Bourne in 1853.

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