39O FARM MOTORS 



hence when the steam is throttled down as in No. 3 (Fig. 

 273) it is not as economical as when used at full pressure. 

 To overcome this loss in throttling-governed engines, 

 automatic cut-off governors have been devised. These 

 governors act in such a manner that they do not throttle 

 the steam as it enters the engine, but change the point 

 of cut-off and by so doing permit steam to enter for a 

 shorter or longer part of the stroke (as the speed of the 

 engine requires) at boiler pressure and allow it to work 

 on expansion. Fig. 274 represents the outline of an auto- 

 matic cut-off governor. A is the flywheel which carries the 

 governor mechanism ; B, the governing mechanism ; C, 

 the eccentric sheave ; E, a slot in the eccentric sheave 

 within which the engine shaft revolves. As the speed of 

 rotation of the engine varies, the weight B will move 

 the sheave C backward or forward across the engine 



shaft. This change in the 

 position of the eccentric 

 sheave changes the throw 

 of the eccentric and conse- 

 quently the point of cut-off 

 of the valve. Fig. 275 shows 

 indicator diagrams with 

 varying loads. No. I is 



FIG. 275 



overload; No. 2, full load; 



No. 3, about half load, and No. 4, practically no load. 

 The steam line for all loads is the same, but the 

 point of cut-off varies, thus giving an increased or 

 reduced amount of energy exerted on the piston. 

 Diagram No. 4 shows how the steam has expanded 

 below atmospheric pressure, and when the exhaust 

 port is opened the pressure rises instead of falling. 

 The area below the atmospheric line is then negative 

 work. Instead of working a large engine on as light a 



