THE INTERNAL -COMBUSTION TRACTOR 83 



height of the oil in the tank. Plunger, centrifugal, and ro- 

 tary gear pumps are used successfully to deliver fuel. Owing 

 to the rapid deterioration of leather when exposed to gasoline, 

 the pumps are usually without leather valves. 



IGNITION 



Having secured a proper proportion of fuel and air in the 

 cylinder, the next problem is to ignite it at such a point during 

 the compression stroke as to allow the charge to be fully ignited 

 as soon as the piston starts upon its third, or working, stroke. 

 This point, in a high-speed engine, or one using a slow-burning 

 mixture, may be as much as one eighth of a revolution of the 

 flywheel before the end of the compression stroke; or, as more 

 often stated, 45 degrees before dead centre. Kerosene 

 engines and those using a "lean" mixture (one with a low 

 proportion of gasoline to air) are ignited early. 



Of the many types of ignition only two, both electric, are 

 used to any extent on tractors. These are the jump spark 

 and the make-and-break systems. In the former a high tension 

 current is employed. A spark coil and con- 

 denser are used to increase the tension and a 

 vibrator rapidly opens and closes the circuit, 

 causing the electric current to leap across the 

 gap between the ignited points, thus forming a 

 spark which explodes the mixture. This system 

 is free from the difficulty of using delicate mov- 

 ing parts on an engine which is exposed to very 

 severe conditions. On the other hand it involves 

 more complicated wiring and greater danger of 

 short circuit, besides being less easily compre- 

 hended by the average farmer. 



The make-and-break system commonly uses - ^ 

 a low tension current which passes through a pair of 

 electrodes fitted to a plug inserted in the cylinder. One 



