39 2 FARM MOTORS 



advantages to be gained from a double-cylinder engine 

 are: (i) being able to start without turning off dead 

 center by hand ; (2) being able to start with a heavy load ; 

 and (3) being able to move slowly with a heavy load. 

 If the cranks were set in line with each other, these ad- 

 vantages would not be gained. The disadvantages of a 

 double engine are : more moving parts, greater chances 

 for steam to leak about the cylinder and the piston, and 

 more cooling surface, hence greater condensation during 

 the working stroke. Although a double engine is more 

 easily handled than a single one, there are only a few in- 

 stances, such as plowing and heavy traction work, where 

 its use is recommended for farm work. 



540. Compound engines. The purpose of compound 

 engines is not to give a greater expansion. This could 

 be accomplished with the low-pressure cylinder and early 

 cut-off. The real purpose is (i) to keep the cylinders as 

 nearly as possible at the temperature of the entering 

 steam, preventing losses by condensation; (2) to reduce 

 the surface exposed to the high-pressure steam to a mini- 

 mum ; (3) to use the high-pressure steam in a small 

 cylinder, hence requiring less material to make it suffi- 

 ciently strong. 



The first cylinder, known as the high-pressure cylin- 

 der, expands the steam partly ; then the second, or low- 

 pressure, receives it and expands it further. Since the 

 steam as it enters the high-pressure cylinder is under a 

 higher pressure than when it enters the low-pressure 

 cylinder, xhe latter cylinder must be larger than the 

 former to accommodate' the increased volume of the 

 steam. Where steam is expanded in two cylinders the 

 engine is known as a double-expansion compound ; where 

 it is expanded in three cylinders it is known as triple- 

 expansion, and in four cylinders it is known as quadruple- 



