FARM MOTORS 



351 



be about one part of gas or fuel vapor to four parts of air. 

 The same authority gives one part of fuel vapor to fourteen 

 parts of air as being the leanest mixture that will burn. 



It is to be remembered in this connection that of every 

 one hundred parts of air only 23 parts are oxygen; and it is 

 the oxygen that supports combustion. The largest constitu- 

 ent of air, nitrogen, composes about 77 parts of the one 

 hundred. Nitrogen is entirely inert, and 

 in the gasoline engine cylinder it occu- 

 pies space which would be more desir- 

 ably filled with gasoline and oxygen. 



In changing from a liquid to a vapor, 

 the fuel is increased in volume some 600 

 to 1000 times. This means that the 

 ratio of the volume of liquid fuel used to 

 that of air must vary from 1 to 8,000 up 

 to about 1 to 16,000. From this we see 

 why the carburetor of the gasoline engine 

 is such a sensitive affair. 



Not only must the ratio of fuel to air be quite constant, 

 but the difficulties encountered are magnified by the fact that 

 the mixing must take place between colorless gases and " sight 

 unseen" inside the engine cylinder. The gas engineer must 

 resort to tests that will show the condition of the mixture. 



If the mixture can be adjusted until it will burn, then the 

 adjustment for the proper mixture is easy. A too rich mix- 

 ture is indicated by black smoke from the exhaust; and one 

 too lean, by a sharp, prolonged exhaust, indicating a slowly 

 burning mixture. The smoke of a too rich mixture is black, 

 while that caused by too much lubricating oil is blue. 



When the engine is provided with a hit-or-miss governor, 

 the needle or supply valve should be adjusted to require the 

 least number of explosions necessary to furnish a given 



Fig. 223. An au- 

 tomatic carburetor, 

 shown in section, 

 which supplies addi- 

 tional air through an 

 auxiliary air valve 

 when the engine 

 runs at high speed. 



