5°° 



ENGINEERING ON THE FARM 



that it be possible to change slightly the time at which 

 ignition shall take place in order to avoid a kick backward 

 when the engine is turned over slowly (Fig. 389). The 



rapidity of burning is in- 

 fluenced by the shape of the 

 combustion chamber, the 

 timing of the ignition, the 

 degree of compression in 

 pounds per square inch, the 

 quality of the mixture, the 

 temperature of the cylinder 

 walls, the temperature of the 

 mixture entering the cylin- 

 der, the position of the spark 

 plug or ignitor in the com- 

 bustion chamber, and the 

 piston speed. If there were 



Fig. 389. Showing, the quadrants in 



which valves or spark may function to 



be designated as " early" and "late" 



no leakage of compression, 

 the flame wave would travel 

 across the combustion cham- 

 ber with equal rapidity at all 

 engine speeds. If an engine 

 is working at approximately 

 its maximum load, its tem- 

 perature will become constant, 

 its piston speed will remain 

 constant, and the temperature 

 of the incoming mixture will 

 also remain constant. Under 

 these conditions the position 

 of the ignition should remain 

 constant. If, however, any 

 of the above conditions are 

 changed, it will be found 



Carbon or poj/tire 

 (+) ferm/na/- 



Zinc or negative 

 (-J ferm/ffa/-> 



Fig. 390. 



Cross section of an ordi- 

 nary dry cell 



necessary to alter the ignition point to get the best results. 

 Types. The earliest types of ignition were: raw or 



