THE STEAM TRACTOR 55 



part of the engine frame. A removable cover usually excludes 

 dust and grit from the cross-head and guide. 



Control of the Engine 



If steam were fed at boiler pressure during the entire stroke 

 it would convert only about 8 per cent, of its energy into work. 

 For the sake of economy it is therefore necessary to admit 

 steam during only a part of the stroke and allow it to expand 

 during the remainder. An engine working at its greatest 

 economy will cut off the admission of steam at from two fifths 

 to one half its stroke. The throw of the valve which accom- 

 plishes the variation in cut-off is under easy control of the 

 operator. Where greater power is needed, the steam can be 

 fed at practically boiler pressure the whole length of the stroke, 

 and an enormous increase in power may be had, provided the 

 boiler will continue to generate sufficient steam. This accounts 

 for the great elasticity of steam for plowing and traction pur- 

 poses as compared to the internal combustion engine, which 

 will be discussed later. 



The motion of the valve must be controlled in order to 

 reverse or stop the engine and to vary the point of cut-off. The 

 means employed to reverse the valve motion can be used in 

 intermediate positions to control the throw, and in neutral 

 position to stop the action of the valve entirely. Since a 

 separate crank for operating the valve is out of the question, 

 an eccentric takes its place. This is simply a sort of crank 

 formed by setting a disk off centre upon the crankshaft. The 

 throw, or eccentricity, of the disk is twice the distance from 

 the centre of the crankshaft to the centre of the disk. The 

 valve push rod is driven by a strap which fits around the 

 eccentric. It is evident that if the disk could be rotated about 

 the shaft it would give the valve more or less throw, and event- 

 ually cause it to move in the opposite direction. However, 

 the setting of a valve is rather a delicate operation, and since 

 the average operator is none too well qualified, a great many 



