A compound locomotive is a type of steam locomotive where steam is passed that has already passed through one cylinder is then passed through another; i.e. the cylinders are in "series" as opposed to the normal arrangement of a simple locomotive in which the cylinders are in parallel.
Compounding is more efficient at using the engergy in the steam pressure. However, they also have higher maintenance costs and are more difficult to drive. For this reason, compounding remained uncommon.
Configurations
There are various configurations
- 1 high pressure, 1 low pressure (Von Borries )
- 2 high pressure, 1 low pressure (Francis William Webb)
- 1 high pressure, 2 low pressure (Smith-Deeley)
- 2 high pressure, 2 low pressure (de Glehn)
Mallet articulated locomotives
With well designed valve gear it is possible use the steam expansively without compounding, but the benefit is that the temperature drop in each of the two sets of cylinders is reduced, resulting in increased efficiency as the heat in the steam is not wasted in warming up cold cylinders.
History
The principle became popular in railway locomotives from about 1880. Many compound locomotives were constructed in France and Germany, famous examples being the Maffei 4-6-2 locomotives constructed in 1908, and locomotives by the French engineer André Chapelon. In Britain, compound locomotives were constructed in large numbers for the London and North Western Railway under its Chief Mechanical Engineer Francis William Webb. These had fundamental design flaws, in particular, in the earlier versions, the two driving axles were uncoupled, with the centre cylinder with a slip-eccentric valve gear driving the leading axle. This valve gear continued to run in the direction it had been running before coming to a stop, thus if the locomotive had backed onto a train, on re-starting, the two sets of driving wheels were liable to rotate in opposite directions!
A more successful type was constructed for the Midland Railway. These were based on a North Eastern Railway prototype built in 1898, designed by Frederick George Smith . The first five Midland compounds were built in 1900 under Samuel W. Johnson, with a further 40 locomotives with slight modifications from 1905 onwards under Richard Deeley. On the formation of the London, Midland and Scottish Railway in 1923, the class was considerably expanded, with a total of 245 locomotives of the type being constructed up to 1932. A further 5 similar but larger locomotives were built by Beyer-Peacock for the Great Northern Railway of Ireland in 1932 for the Dublin-Belfast expresses. The LMS locomotives did good work, particularly in Scotland. Preserved examples are the prototype Midland Compound, 1000 (BR 41000), and GNR(I) no 87 Kestrel.
Probably the most important of the compound locomotives are those built in France and rebuilt under Chapelon.
The 4-cylinder de Glehn compounds of 4-4-2 and 4-6-0 wheel arrangements were built in large numbers for service in France, the 4-6-0 type continuing in service until the late 1950s. Three of the 4-4-2 type were purchased around 1905 by the Great Western Railway under its Chief Mechanical Engineer George Jackson Churchward for use in comparative experiments and were tested against his own designs as follows: 4-6-0 versus 4-4-2; 2 cylinders versus 4 cylinders;
4 cylinder compound versus 4 cylinder simple. For the latter of these three comparisons, the 4 cylinder 4-4-2 locomotive no 4000 North Star was specially built.
The experiments, which were carried out around 1906/1907 concluded that there was no significant advantage in compounding, also that the 4-6-0 wheel arrangement was preferable to 4-4-2. Two cylinder locomotives had no clear efficiency advantage but four cylinders gave smoother running.
Following these experiments, standard designs of 4 cylinder 4-6-0 express locomotives of the Star class were constructed, and of 2 cylinder 4-6-0 (Saint class) and 2-6-0 types for secondary services. The 4 cylinder class was enlarged, first as the Castle class in 1923 and the King in 1927, construction of Castle class locomotives continuing until 1950. The 2 cylinder design was modified with smaller wheels to become the Hall and Grange mixed traffic classes, and similar locomotives were built for the London Midland and Scottish (the class 5), London and North Eastern (B1 class) and British Railways. The same design is being adopted by David Wardale as the basis for new steam locomotives for tourist routes. Thus the effect of the Great Western experiments was far-reaching, since it was to settle the design of medium sized steam locomotives, particularly in Europe, until the mid 20th century when construction ceased in favour of other methods of traction.
In 1978, the Argentinian engineer [L D Porta]http://www.trainweb.org/tusp/porta.html proposed a "Third Generation" steam locomotive which would incorporate advanced concepts requiring significant research and development. One such design was a three-cylinder compound locomotive of the 2-10-0 wheel arrangement for fast freight work in the US. It was claimed that with proper maintenance and operating techniques, such locomotives could compete with modern forms of traction.