GAS ENGINES 519 



Air-cooled engines are not extensively used where a con- 

 tinued high duty is demanded. They work very satisfactorily 

 where the load is light or is intermittent. 



Water cooling. The most common system of cooling is 

 accomplished by means of a double-cylinder wall with the 

 space between filled with water. A reserve supply of water 

 is provided and may be contained in a hopper directly over 

 the cylinder or in a tank at some convenient place near or in 

 a radiator. The limit of water temperature is the boiling 

 point, or 212 F. In the hopper-cooled system the water 

 will come to the boiling point. Continued use will make it 

 necessary to replenish the cooling water. Where a barrel 

 or tank reserve supply is used, the circulation between the 

 engine jacket and the reserve may be made either by a 

 thermo-syphon or by a circulating pump. With the circu- 

 lating pump and with the radiator the reserve supply of water 

 may be greatly reduced. 



Oil cooling. Oil is used for cooling some large tractor 

 engines, which generally run a great deal warmer than the 

 water-cooled engines, as the oil may become much hotter 

 than the water without evaporating and is not as thin as 

 water, and in consequence circulates less rapidly. The oil 

 used is a black residue from the refinery. 



Cooling in winter. In the water-cooling system there is 

 danger of the cooling water becoming frozen in cold weather. 

 When the engine is not running, all chambers and connecting 

 pipes should be thoroughly drained or a non-freezing solu- 

 tion used. Various mixtures may be made or bought which 

 will accomplish the desired purpose, but care must be taken 

 not to use one which has corrosive or otherwise destructive 

 effect upon the water jacket, manifold, and radiator. A 

 solution which attacks these surfaces is bound to give trouble 

 sooner or later. The solution may be directly corrosive 

 in its nature, or it may be conducive to electrolytic action 

 just as soon as the ignition currents begin to run at random 

 through the metal parts of the engine. All ignition systems 



