CHAPTER XVI 

 THE GAS ENGINE 



Although not strictly a factor in plant growth, the gas 

 engine has become a very important factor in farm life. 

 For running tractors in plowing large farms rapidly (Fig. 73), 

 in furnishing power for pumping, silage cutting, cream 

 separating, and in moving products to market, as well as 

 providing rapid means of locomotion for the modern farmer, 

 the gas engine today occupies an almost essential place on 

 many farms. On this account a short discussion of the prin- 

 ciples of combustion, products obtained, fuels, and lubricants, 

 should fit in at this point. The subject of gas engines is too 

 large a one to treat from any standpoint except the chemical 

 one and should include only such other details as may be 

 necessary to fully understand the chemistry involved. 



219. The Gas Engine. — The gas engine is an appliance for 

 making use of the energy developed when a mixture of a 

 combustible gas and air explodes. When gases explode a 

 sudden pressure is produced. This pressure is directed against 

 a piston-head and the movement of the piston is trans- 

 mitted to a wheel by a connecting rod. In a steam engine, 

 steam is admitted first to one end and then to the other 

 end of the cylinder. This moves the piston back and forth 

 and by means of the connecting rod a wheel rotates. In a 

 gas engine a mixture of gas and air is admitted to one end of 

 the cylinder and at the proper moment is exploded, usually 

 by an electric spark. The pressure thus developed drives 

 the piston to the other end of the cylinder. Gas is not 

 exploded on the other end of the piston to drive it back, 

 but the momentum of a heavy fly-wheel attached to the 

 main shaft carries the piston back again. In one type of 

 engine, the so-called "four-cycle" type, the explosion comes 

 at every other inward movement of the piston, or once every 

 two revolutions of the fly-wheel. It is called four-cycle 

 because there are four strokes which complete the cycle : 



