Compound locomotive
Large numbers were constructed, mostly two- and four-cylinder compounds, in Germany, Austria, Hungary, and the United States.
French compounding of railway engines became so highly developed, eventually incorporating reheaters between the high and low pressure stages as well as the initial use of superheaters, that France achieved the highest power to weight ratio and the highest horsepower to fire grate-area ratio of any steam locomotives ever built.
[5][6] The main benefits sought from compounding are reduced fuel and water consumption plus higher power/weight ratio due to more expansion in the cylinder before the exhaust valve opens, which gives a higher efficiency; additional advantages include smoother torque and in many cases, superior riding qualities with consequent less wear on the track and running gear.
Where heavy grades and low axle loads were combined, the compound locomotive was often deemed to be the most viable solution.
In rebuilding older locomotives from 1929 onwards, André Chapelon was able to inexpensively obtain what seemed almost "magical" improvements in power and economy by improving flow through the steam circuit, at the same time putting in a larger superheater to increase the initial steam temperature and delay condensation in the LP cylinders.
[11]: 12 Another was by Anatole Mallet who introduced in 1876 a series of small 2-cylinder compound 0-4-2 tank locomotives for the Bayonne-Anglet-Biarritz Railway.
Mallet also worked out schemes for compounds with independent divided drive for HP and LP, some with a single rigid chassis that were never built, others with a rigid rear chassis on which the HP cylinders were mounted and an articulated LP front engine unit.
The first application was a series of 600 mm gauge locomotives specially built by the Decauville Company for the Paris Exposition of 1889; the design was introduced to the North American railroading in 1900 with B&O No.
Mallet's aforementioned rigid wheelbase divided-drive schemes, although never actually applied, may have inspired Francis Webb in Britain.
224 of the North British Railway which was built in 1871 as a 4-4-0 simple-expansion locomotive, being the pioneer of the 224 Class; it was converted to a tandem compound in 1885, but reverted to simple in 1887.
[16][17][18] Tandem compound locomotives were very common in the United States prior to WW1, with some railroads such as the Santa Fe having large numbers in several wheel arrangements.
In 1891 two production locomotives, Nord 2.121 and 2.122, were placed in service with the cylinder positions inverted at du Bousquet's insistence, that is outside HP and inside LP, one of which initially had uncoupled driving axles as before but this arrangement proved inferior to the coupled version.
The type was greatly improved by du Bousquet who refined the layout of rods and valve gear along the inside of the frames for easy access.
Many gave long service: a 4-6-0 230.D [fr] locomotive introduced 1909, stationed at Creil could often still be seen at the Gare du Nord, Paris in the late 1960s.
Three of the 4-4-2 type were purchased by the Great Western Railway, one in 1903 and two slightly larger ones in 1905 under its Locomotive Superintendent George Jackson Churchward for use in comparative trials and were tested against his own designs.
Although a number of items of French practice were adopted by the Great Western as a result of these trials, the de Glehn compound system was not one of them.
In 1904 The Pennsylvania railroad ordered a copy of the Nord Atlantic called "the French aristocrat" on the Pennsy, but too light which made her underpowered due to low traction.
The North British Locomotive Company of Glasgow built de Glehn compounds for the Bengal Nagpur Railway of India in 1906, which were very successful and economical on water.
In 1900 the Italian engineer Enrico Plancher developed a new and curious design of compound engine, which first appeared on the Rete Adriatica 500 class express locomotive; it was notable for being an asymmetrical four-cylinder design, in which the two HP and the two LP cylinders were grouped together, with each couple being served by a single piston valve which admitted steam simultaneously to the opposite ends of the two cylinders.
The prototype of the class was presented at the International Railway Congress of 1900 in Paris and was looked at with interest, while not meeting with outstanding success; however, on the long run the asymmetrical design, while simple, proved to be rather awkward, as it was difficult to equalize the work of each side of the locomotive and this caused hunting.
Chapelon, along with other French engineers such as Gaston du Bousquet, and Marc de Caso brought these locomotives to their highest pinnacles of performance.
In spite of a sweeping standardisation policy by the Reichsbahn imposing simple expansion, a small but consequent number of Maffei Pacifics of a design dating from 1908 were nevertheless considered indispensable for hilly routes with severe axle load limitations and were built new as late as 1931.
Livio Dante Porta in 1948 drew inspiration from Chapelon's 4700/240P rebuilds for "Argentina"; his first production, a 4-cylinder compound rebuilt from an old British-built metre-gauge Pacific into a futuristic 4-8-0.
1619 (NER Class 3CC) with this same layout to the design of Walter Mackersie Smith (this itself being rebuilt from an earlier Worsdell/Von Borries 2-cylinder compound prototype of 1893).
These were followed from 1905 onwards by 40 of an enlarged production version where all the Smith fittings were replaced by a simplified starting arrangement incorporated into the regulator; this to the design of Johnson's successor, Richard Deeley.
Preserved examples are the rebuilt prototype Midland Compound, 1000 (BR 41000), and Great Northern Railway (Ireland) no.
From 1896, Weymann introduced a 3-cylinder 2-6-0 type with divided drive and cranks at 120° for service on the heavily graded Swiss Jura-Simplon routes; eventually they numbered 147 units.
242A 1 was probably the most important compound locomotive of all time, capable of developing a remarkable 5,300 cylinder horsepower (4,000 kW) for an engine unit weighing just 145.6 metric tons.
These included locomotives of the 2-10-0 wheel arrangement, one of which was intended for fast freight work in the US, this being a high-pressure triple-expansion machine.
Strange as this layout may seem, it had a number of advantages from the point of view of equalising piston thrusts and arrangement of steam passages.
