86 POWER AND THE PLOW 



of a cylinder full of gas. The inlet valve is frequently auto- 

 matic i. e., opened by the suction of the piston but in the 

 majority of higher priced tractors both inlet and exhaust valves 

 are mechanically operated. The controlling mechanism usually 

 consists of a rocker arm and push rod, driven by an eccentric 

 or cam. The camshaft is driven by gears from the crankshaft, 

 but at a lower speed than the latter. In case of valves set in 

 an offset chamber, the rocker arms are dispensed with and the 

 push rods act directly upon the valves. 



Some engines are provided with an auxiliary or relief exhaust. 

 This consists simply of a port, which, at the will of the operator, 

 is uncovered at the end of the outward stroke of the piston, 

 thus allowing part of the products of combustion to pass out 

 immediately into the exhaust pipes. The hottest gases are 

 thus removed from the vicinity of the exhaust valve, and 

 danger of corroding and sticking the valve stem is thus les- 

 sened. The use of this feature is advisable at heavy, con- 

 tinuous loads, and in very hot weather. 



PISTON 



The piston is of the trunk type commonly used on single- 

 acting engines. The explosion acts on one end of the piston, 

 the other being open to receive the connecting rod. The 

 piston is of cast iron turned to size. In order to allow free 

 movement it is made slightly smaller than the bore of the 

 cylinder, usually one thousandth of an inch less for each inch 

 in diameter of the latter. In order to retain the compression 

 the piston is encircled by expanding rings set in grooves 

 machined out of the piston. There are usually three or four 

 rings at the head of the piston and sometimes one ring near 

 the open end. This latter ring is sometimes used for the 

 purpose of distributing the lubricating oil and sometimes its 

 main purpose is to assist in preventing the loss of compression. 

 In other cases it is to prevent the wearing of the cylinder wall 



