THE INTERNAL -COMBUSTION TRACTOR 81 



nor is the heart of the engine, then the carbureter may be said 

 to be the lungs, which bring the life-blood and oxygen into the 

 necessary contact. Of the three general classes of carbureter, 

 only one is used to any extent upon tractors. This is the spray 

 type, which divides the fuel into a fine mist rather than a true 

 gas. The carbureter must deliver the fuel to the cylinder in 

 proper proportions, at any speed or load, regardless of varia- 

 tions in the temperature of fuel and air, the difference in 

 density in the air at different times and altitudes, and the 

 wide extremes in the volatility of fuels secured from different 

 sources. In order to meet all the varying requirements, many 

 carbureters are so complicated and delicate that they are 

 not suitable for the rough work which a tractor is called upon 

 to do. 



In brief, the spray carbureter delivers the fuel through a small 

 opening or needle valve, situated in a passage through which 

 air rushes, in obedience to the difference in pressure between 

 the outside air and the contents of the cylinder on the suction 

 stroke. The fuel is atomized by the air current and turned 

 into gas by the heat of the cylinder. Gasoline is sufficiently 

 volatile to give an explosive mixture in a cold cylinder. 

 Kerosene requires more heat for its evaporation; hence it is 

 usually necessary to start on gasoline or alcohol and switch to 

 kerosene after a half minute's run. Sometimes two car- 

 bureters are provided for starting, but a simpler way is to add 

 a gasoline compartment to the kerosene carbureter. 



The majority of carbureters are designed to overcome the 

 fluctuation in outside conditions by keeping the air and fuel at 

 a constant temperature, through heat from the water jacket 

 or exhaust pipe. All are provided with a wide range of adjust- 

 ment, and some meet the many different conditions auto- 

 matically. Most carbureters provide for a constant quality 

 of mixture i. e., a fixed proportion of fuel and air. However, 

 on a throttling governed engine, running at high speed and tak- 

 ing full charges, the compression is considerably increased 



