THE STEAM LOCOMOTIVE 



depends entirely upon the weight on the driving wheels, 

 the cylinder dimensions being proportioned to this 

 weight, and, whether the locomotive is compound or 

 single expansion, no larger boiler can be provided, 

 after allowing for the wheels, frame and mechanism, 

 than the total limit of weight permits. The heating 

 surface and grate areas in both compound and single- 

 expansion locomotives of the same class are practically 

 the same, and the evaporative efficiency of both loco- 

 motives is chiefly determined by the action of the ex- 

 haust, which must be of sufficient intensity in both 

 cases to generate the amount of steam necessary for 

 utihzing to the best advantage the weight on the driv- 

 ing wheels. This is a feature that does not appear in 

 any stationary engine, so that the compound locomo- 

 tive cannot be judged by stationary standards, and the 

 only true comparison to be made is between locomo- 

 tives of similar construction and weight, equipped in 

 one case with compound and in the other with single- 

 expansion cylinders. 



"No locomotive, compound or single-expansion, will 

 haul more than its adhesion will allow. The weight 

 on driving wheels is the limiting factor in the problem 

 which confronts the locomotive engineer. Power can, 

 of course, be increased by building a larger engine and 

 augmenting this weight but in the present construction 

 of tracks and bridges the limit of driving wheel load 

 has almost been reached. Hence in modem locomo- 

 tive practice the goal before the designer and engineer 

 is to obtain maximum efficiency for the minimum 

 weight admissible. 



[139] 



