THE INTERNAL -COMBUSTION TRACTOR 93 



dirty nature of the work prohibits the use of open water tanks. 

 Owing to the size of the motors employed, the air-cooled traction 

 engine is practically out of the question, and the cooling 

 medium is usually either water or oil. The water should be 

 as pure as possible, owing to the fact that mineral salts in 

 solution may be precipitated by heat at temperatures even 

 below the boiling point. The scale thus formed tends to im- 

 pede the circulation, and if deposited on the cylinder wall of 

 the water jacket will decrease its conducting capacity and 

 tend to produce overheating in the cylinder. The oil used 

 for cooling is of rather heavy mixture with a high fire test. Its 

 boiling point is considerably higher than that of water, hence 

 it does not evaporate and require frequent replacement. It is 

 not so rapid a conductor of heat, yet on the other hand it has 

 the advantage of not freezing or depositing scale. One man- 

 ufacturer recommends a mixture of kerosene and water to 

 prevent corrosion and scale. Others suggest the use in severe 

 weather of some anti-freezing compound, such as a solution 

 of calcium chloride (specific gravity 1.2) or of equal parts of 

 glycerine and water, or a mixture of alcohol and water. 



In engines of ordinary design the cooling medium should 

 not be heated above the boiling point, nor cooled so low as 

 to absorb heat from the gases before they have had time to 

 act. In practice the temperature ranges from 160 to 180 

 degrees F., varying considerably with the movement of the 

 tractor with or against the wind. 



The medium is commonly circulated by either a centrifugal 

 or a plunger pump, though, in rare instances, it circulates en- 

 tirely by the difference in specific gravity between the liquid 

 in the tank and the hotter liquid in the cylinder jacket. One 

 simple, popular and effective system of removing surplus heat 

 from the water, which is used in this case, is that of an open 

 evaporating radiator. The water is sprayed over a screen, 

 along which it passes back to the tank, the partial evaporation 

 absorbing enough heat to cool the remainder rapidly. One 



