398 PRODUCTIVE FARMING 



opposite end of the cylinder. This performs the work. If 

 the spark fails the power fails. The firing or lighting of the gas 

 is now usually done, in American engines, by means of an 

 electric spark from a spark coil which receives its current 

 either from a battery or from a magneto (dynamo). 



The make=and=break method uses a simple coil and low 

 tension current. The circuit is closed arid opened at the 

 proper time, automatically, by a device for that purpose. 

 This plan is common on stationary engines. 



FIG. 251. Tractor used in harvesting oats with two self-binders. (International 



Harvester Co.) 



The jump=spark system uses a high tension induction 

 coil, and the current when interrupted will jump a small 

 gap of a spark plug. This spark ignites the compressed gas 

 in the cylinder. There are no movable parts within the 

 cylinder as in the other system. The jump-spark may usually 

 be controlled by changing the time of the circuit breaker. 

 This is called advancing or retarding the spark. 



Proper valve action is essentially the careful timing of the 

 valves which open to let the gases into the cylinder and 

 close it for the power stroke. Exhaust valves open just 

 before the end of the expansion stroke to let the burned 

 gases escape. Inlet valves are self-timing on many station- 

 ary engines, as the suction stroke of the piston opens them. 



Directions for the regulation of the valves of each engine 



