The Development of Locomotive Power at Speed

Abstract

The power developed per unit of cylinder volume of locomotives in the speed range from 250 to 400 r.p.m. has in recent years been doubled. Forty years ago it was often the case that the diminution of the mean effective pressure with increase of speed, rather than the evaporative capacity of the boiler, limited the power of the engine. The most advanced designs maintain the mean pressure at a high percentage of the calculated value to the highest speeds, and the effect of valve events and clearance volume on the calculated mean pressure thus becomes of practical importance, especially as designers are endeavouring to use considerably higher steam pressures in single-expansion cylinders. The first part of this paper sets out the effect of these factors over a wide range of steam pressures in the form of basic data graphs of mean pressure, relative efficiency, and steam consumption. It is shown that without the decrease of clearance volume and increase of expansion ratio which compound expansion affords, the improved thermal efficiency which higher steam pressure offers cannot be fully realized above about 250 lb. per sq. in. boiler pressure, though greater power can be obtained. The second part of the paper is devoted to an examination of the actual deviation of mean pressure with speed for a wide range of locomotives, including some of the most recent designs, and the reasons for the radical improvement are indicated. In 1905 the late Professor Dalby suggested a simple proportional relationship between mean pressure and speed. Such a relationship does not hold for modern locomotives, and at the end of the paper a simple exponential law is proposed with a single coefficient characteristic of the locomotive. This law may be used to estimate the power of a future design at any speed, or as a criterion for assessing the performance of an existing locomotive in respect of power developed, which is generally of more importance to railway companies than thermal efficiency.