394 



FARM MOTORS 



expansion. When one cylinder is in front of the other, 

 the engine is said to be a tandem compound, and when 

 the cylinders are side by side it is said to be a cross- 

 compound engine. Fig. 277 shows the Woolf tandem- 

 compound engine in common use in traction service. 



The arrows show the 

 direction of the steam as 

 it passes from cylinder 

 to cylinder. Fig. 278 il- 

 lustrates a cross-com- 

 pound engine, showing 

 how the steam passes 

 through a superheater as 

 it travels from the high- 

 pressure to the low- 

 pressure cylinder. It also 

 shows the relative sizes 

 of the two cylinders. 



541. Horse power of 

 steam engines. There 

 are three methods of rat- 



Ml 



FIG. 278 CROSS-COMPOUND ENGINE 



ing steam engines. One 

 method is by the indi- 

 cated horse power, 

 which is the total work 

 exerted by the steam in the cylinder; the second 

 method is the actual or brake horse power (see Chapter 

 I), which is the actual work delivered from the fly- 

 wheel of the engine; and the third is the commercial 

 rating. 



542. Commercial rating of steam engines. The com- 

 mercial rating of all stationary steam engines is about 

 their actual horse power, but the commercial rating of 

 traction steam engines is far below their actual horse 



