Compound Locomotive - The Thinking Behind Compounding

The Thinking Behind Compounding

For railway locomotive applications the main benefit sought from compounding is economy in fuel and water consumption plus high power/weight ratio due to temperature and pressure drop taking place over a longer cycle, this resulting in increased efficiency; additional perceived advantages include more even torque and in many cases, superior riding qualities with consequent less wear on the track. In situations where heavy grades and low axle loads were combined, the ordinary compound locomotive was often deemed to be the most viable solution. However, in the past, optimum performance demanded constant attention and expert handling, which usually made single-manning advisable, thus reducing a locomotive's availability. At the end of the steam age, this particular problem was being addressed by Andre Chapelon and Livio Dante Porta also actively pursued this line of research. Successful design of a compound locomotive demands a firm grasp of thermo- and fluid dynamics; that such has frequently not been the case is why many productions in the past have been far from optimal. This is especially true of locomotives built in the early years of the 20th Century. The problem not only affected compounds, but was dramatic in their case due to the long steam cycle which made them particularly sensitive to temperature-drop and condensation of the steam during its lengthy passage. In rebuilding older locomotives from 1929 onwards, 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 in order to increase the initial steam temperature so that cooling took longer. Subsequently, in order to maintain a more constant temperature throughout the cycle, Chapelon successfully applied re-superheating between HP and LP stages plus steam-jacketed cylinders to an a "test-bed" freight locomotive, 160 A1 (tested 1948-51). Resuperheating was also a feature of L.D. Porta's prototype 4-8-0 rebuild: 'Presidente Peron'/'Argentina' (tested around the same time in Argentina from 1949). 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 on-going debate.