MECHANICAL POWER EOR THE FARM 453 



the ignition of the explosive mixture heats the cylinder, some 

 method of cooling the cylinder walls must be used. Cooling is 

 necessary to facilitate oiling, to prevent the moving parts from 

 being heated and twisted, and to avoid the ignition of the ex- 

 plosive mixture at the wrong time by the heat of the cylinder 

 walls. One method of cooling is to jacket the cylinder so as to 

 allow water to pass through the jacketed space. In Fig. 226 

 the water contained in the hopper H is heated by contact with 

 the hot cylinder walls, rises, and is replaced by cooler water. 

 Some small engines are air-cooled. 



The exhaust muffler M is to decrease the noise made by 

 the burned gases as they leave the engine cylinder. Four com- 

 plete strokes of the piston, or two revolutions, are essential for 

 carrying out the various processes in the type of gas engine 

 shown in the figure. The name four-cycle, or four-stroke, en- 

 gine is given to this form of gas engine. Some engines, called 

 two-cycle engines, carry out the above operations in two strokes, 

 but the four-cycle type of engine is usually preferred on account 

 of its greater reliability and better fuel economy. 



593. Selecting and installing an oil engine. An oil engine 

 should be large enough to do the required work, as it will stand 

 little overload. On the other hand, an engine too large for the 

 work it has to do will give poor fuel economy. 



A two-horse-power or three-horse-power engine is usually 

 large enough to do the work about the house, dairy, and barn. 

 F*or feed grinding and for threshing and wood sawing a five- 

 horse-power engine, or even a larger size, may be needed. 



Engines of five horse power and larger should be operated 

 on kerosene or on a still heavier petroleum fuel, as the cost of 

 gasoline is high in comparison with these fuels. Most manu- 

 facturers build engines which are well adapted to the cheaper 

 fuels as well as to gasoline. 



It is best to locate a gasoline engine in a separate room. 

 The room should be well lighted and ventilated, free from dirt 

 and dust, and should be large enough so that there will be 

 sufficient space for easy access to any part of the engine. 



