THE INTERNAL -COMBUSTION TRACTOR 79 



cylinder engines range from about 300 to 400 r.p.m., the three- 

 cylinder a trifle higher, and the four-cylinder from 450 to 600 

 r.p.m., though these limits are by no means absolute. The 

 speed is based largely upon the piston travel, which ranges 

 from about 600 to 750 feet per minute. The multiple-cylinder 

 engines are usually smaller in diameter and stroke; hence 

 require a higher speed to accomplish the desired piston travel. 

 The cylinder, even on a low or medium speed tractor, is 

 seldom over twelve inches hi diameter, owing to the difficulty of 

 cooling larger sizes through the cylinder wall. The stroke of 

 the piston is usually longer in proportion to the diameter in 

 the lower speed engines, the ratio ranging from an average of 

 about 1.65 for the low speed single-cylinder tractors to 1.5 

 for the medium speed, and 1.15 for the four-cylinder high-speed 

 engines. 



GOVERNOR 



Excellent regulation is especially necessary on a gas engine, 

 where perfect combustion, clean cylinders, and economy of 

 fuel depend upon a good mixture at all times. Only two sys- 

 tems of governing are in common use on tractors. In one of 

 these a fuel charge is taken for each cycle until the speed of the 

 engine runs above a certain limit. Thereupon the explosions 

 are automatically cut out until the speed drops below normal. 

 This sequence of power cycles and idle cycles gives rise to the 

 name of "hit-and-miss" governing. This system is economical 

 of fuel, especially on small engines, but naturally is not adapted 

 for work requiring close regulation. It is better adapted to 

 plowing than to threshing, sawing, and other stationary work, 

 where a constant speed must be kept in spite of great variation 

 in the load. An ordinary fly-ball governor, driven in some 

 way by the flywheel or crankshaft, is commonly used. The 

 governor weights act through suitable push rods, which in turn 

 may lock the fuel valve so as to choke off the supply or else 

 will prevent a spark while retaining an unexploded charge in 

 the cylinder. 



