Abstract
The problem of estimating with reasonable accuracy the amount of tractive force exerted by a locomotive at running speeds enters every tonnage rating, economy study, and locomotive-operating investigation. Its importance is recognized, and many investigators have approached a rational solution by determining the hourly steam supply and the steam consumption per horsepower-hour, and from these the horsepower and tractive force. The steam rates assumed have been generally correlated with the speed only. Indirectly this requires the defining of the resulting speed-tractive-effort relation as “maximum tractive effort”, “all-day tractive effort”, or similar terms. This paper points out that the variation of the steam rate with the cut-off is of equal importance, but that a chart showing the variation of steam rate with the two factors, speed and cut-off cannot be used directly to obtain horsepower without an additional process. A more direct attack is made by (a) estimating the actual weight of steam used per revolution in cylinders of various sizes, with varying speeds, cut-offs, and working pressure; and (b) by correlating the speed, cut-off, and mean effective pressure, from which the tractive force corresponding to the cylinder horsepower is directly calculated. Each relation calculated gives the four values of speed, tractive effort, cut-off, and steam consumption for a given operating condition, and the calculation of a number of selected points results in a chart presenting a set of curves showing the tractive effort which may be expected at various speeds for specified conditions of boiler output and cut-off. This type of chart permits the user to see the economy to be expected at any condition within the probable working range of the locomotive and to determine the validity of any single speed-pull curve, involving a specified program of cut-off and boiler output for each speed.
