Compound locomotive
A compound locomotive is a steam locomotive which is powered by a compound engine, a type of steam engine where steam is expanded in two or more stages. The locomotive was only one application of compounding. Two and three stages were used in ships, for example.
Compounding became popular for railway locomotives from the early 1880s and by the 1890s were becoming common. Large numbers were constructed, mostly two- and four-cylinder compounds, in Germany, Austria, Hungary, and the United States. It declined in popularity due to a perceived increased maintenance requirement. Nonetheless, compound Mallets were built by the Norfolk and Western Railway up to 1952 and continued to be designed and built in France until the end of steam in the 1970's. French compound engines became highly developed, eventually incorporating reheaters between the high and low pressure stages as well as the initial use of superheaters, ultimately achieving the highest power-to-weight ratio and highest horsepower to fire grate-area ratio of any steam locomotives ever built.
Introduction
In the usual arrangement for a compound engine the steam is first expanded in one or two high-pressure ' cylinders, then having given up some heat and lost some pressure, it exhausts into a larger-volume low-pressure ' cylinder,, thus extending the expansion part of the thermodynamic cycle. The cylinders can be said to work in "series" as opposed to the normal arrangement of a simple-expansion locomotive where they work in "parallel". In order to balance piston thrusts of a compound, the HP:LP cylinder volume ratio has to be carefully determined, usually by increasing the LP cylinder diameter and/or by lengthening the stroke. In non-condensing engines, the HP:LP volume ratio is usually 1:2.25. On geared locomotives, cylinder volumes can be kept more or less identical by increasing LP piston speed. Compound may refer to any multiple-expansion engine. Added insight comes with the terms double, triple, quadruple. At least three triple-expansion locomotives were built; A vertical boiler, high- pressure 0-4-0 made by Greenwood & Batley in 1878, a 2-2-2 converted from a double expansion locomotive by Francis Webb of the London and North Western Railway and an experimental 4-8-0, named the L.F.Loree, which was built by the American Locomotive Company and the Delaware & Hudson Railroad in 1933.Reasons for compounding
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.Even more than other locomotives, designing a compound locomotive demands a firm grasp of thermo- and fluid dynamics. A lack of these led to many less-than-optimal designs, especially in the early years of the 20th century. Compound designs' long steam cycle made them particularly sensitive to temperature-drop and condensation of the steam during its lengthy passage. 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.
To prevent severe condensation taking place, the L.N.E.R. applied resuperheat to their water-tube boilered No. 10,000 to make up for inadequate HP superheat. The Paris-Orleans Railway designed a demonstrator 2-12-0 locomotive, No. 160-A1, with resuperheat between HP and LP stages. They also fitted steam jackets to both HP/LP cylinders for what was believed by Chapelon to be the first time for a compound locomotive. Resuperheating was also by Porta on his prototype 4-8-0 rebuild: 'La Argentina'.
Proponents of simple expansion argue that use of early cut-off in the cylinder thus expanding small quantities of steam at each piston stroke obviates the need for the complication and initial expense of compounding and indeed multi-cylinder single expansion – this is an ongoing debate.
Compound configurations
There are many configurations, but two basic types can be defined, according to how HP and LP piston strokes are phased and hence whether the HP exhaust is able to pass directly from HP to LP or whether pressure fluctuations necessitate an intermediate "buffer" space in the form of a steam chest or pipe known as a receiver.The eternal problem with compounds is starting: for all cylinders to take their weight, it is advisable to have some way of short-circuiting the HP cylinders and getting steam at a reduced pressure directly to the LP cylinders; hence many of the patented compound systems are associated with particular starting arrangements. The de Glehn 4-cylinder system is probably still the most sophisticated of all with independent HP & LP cut-off and a rotary valve, called a lanterne allowing independent working or combinations of HP and LP groups. Most other systems employ starting valves of various kinds. Another criterion is whether the valve gears of the two groups are wholly independent or linked together in some way.
Configurations
Two-cylinder compound
- 2 cylinders, alternating high and low pressure - "continuous expansion locomotive"
- 1 high-pressure, 1 low-pressure - these are usually referred to as "cross-compound" designs, and there are many variations
Three-cylinder compound
- 2 high pressure, 1 low pressure
- 1 high pressure, 2 low pressure
Three-cylinder semi-compound
- 1 high pressure, 2 low pressure; HP from HP boiler superheated, LP from HP exhaust mixed with LP boiler superheated
Three-cylinder triple-expansion
- 1 high pressure, 1 medium pressure, 1 low pressure
Four-cylinder triple-expansion compound
- 1 high pressure, 1 intermediate pressure, 2 low pressure
Four-cylinder compound
- 2 high pressure, 2 low pressure
Six-cylinder compound
- 2 high pressure, 4 low pressure
History
Early experiments
, the grandson of one of Newcomen's engine erectors in Cornwall, patented a double-cylinder compound reciprocating beam engine in 1781. He was prevented from developing it further by James Watt, who claimed his own patents were infringed.A method to lessen the magnitude of the continual heating and cooling of a single-expansion steam engine that leads to inefficiency was invented in 1804 by British engineer Arthur Woolf. Woolf patented his stationary Woolf high-pressure compound engine in 1805.
Application to railway locomotives
The first design of a compound railway locomotive on record belongs to Thomas Craddock, who patented a design for a condensing compound locomotive in 1846.In 1850 United Kingdom patent number 13029 was awarded to James Samuel, the engineer of the Eastern Counties Railway, for a "continuous expansion locomotive", a method of steam locomotive compounding, although the idea appears to have come from one [|John Nicholson], a driver on the line. In this system, the two cylinders alternated as high and low pressure, with the change-over occurring halfway through each stroke. Two locomotives, one passenger and one goods, were converted to the system but no further examples followed.
Whether the above locomotive is, strictly speaking, a compound is subject to debate: the first recognisable compound application to a locomotive was on Erie Railway's No 122, an ordinary American type fitted in 1867 with tandem compound cylinders following J.F. Lay's patent no. 70341. Nothing is known of this locomotive's subsequent career and it does not appear to have been reproduced.
Cross Compound
The simplest form of the compound locomotive has two cylinders, a high-pressure cylinder on one side, and a low-pressure on the other side; the name reflects the steam flow from the HP to the LP across the locomotive. Most early attempts at compound locomotives were variations on the cross compound design, some notable ones being that of Baxter and Hudson. 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. These were entirely successful and worked for many years.Cross compound locomotives have a fundamental design issue, which is that if the HP cylinder is stopped "on center", the locomotive cannot start. To resolve this, all practical cross compound locomotives have some form of starting valve, which allows admission of HP steam into the LP cylinder on starting. In some cases this is manually operated by the locomotive driver, while in other cases it is automatic; in the latter case the valve is often referred to as an "intercepting valve". The primary difference between various forms of cross compound locomotives is in the design and operation of the intercepting valve.
A second design issue of cross compound locomotives is that, if the engine is worked on a short cutoff, the steam is fully expanded in the HP cylinder and does no work in the LP cylinder, which causes unequal stresses in the engine. This problem is avoided if the engine has 3 or 4 cylinders, which was an element driving the development of the following types.