1 94 OUR PHYSICAL WORLD 



moves in to compress the mixture after which the explosion 

 occurs and the cycle begins over again. 



Evidently there must be valves arranged so as to open and 

 let out the burned gas, others to let in the fresh mixture of air 

 and gasoline, and these must open and close at just the right 

 times. These valves are usually opened by rods that are raised 

 and lowered by eccentrically placed disks called cams revolving 

 on a cam shaft (see Fig. 78$). The valves are closed by springs. 

 In some engines the valves operate by means of a rotating sleeve 

 that fits inside the cylinder with holes in the sleeve and in the 

 cylinders that coincide when gases are to enter or leave, but are 

 closed at other times. 



The continued burning of gasoline in the cylinders would 

 naturally keep them very hot. They are cooled either by a draft 

 of air or more often by a jacket of water that is forced to circulate 

 in the spaces about them. This water is kept cool by circulating 

 also in the radiator, a honeycomb metal device with water in the 

 hollow comb and air drawn through its holes by a fan operated 

 by a belt or chain drive to the crank shaft. 



The carburetor is very variable in different makes (one is 

 diagrammed here, Fig. 79), but its purpose is the same in 

 all, namely, to saturate partially the air with gasoline vapor 

 before it is drawn into the cylinders. Gasoline is either 

 carried to the carburetor by gravity from the gasoline tank or 

 pumped up to it. Usually there is a " choke" attached to the 

 carburetor, a sort of damper which regulates the air intake. 

 When it is wide open, the air goes in rapidly, and is not as com- 

 pletely filled with gasoline vapor as it is when it is closed so the 

 air enters slowly. In the former case the mixture is said to be 

 lean, in the latter rich. In starting the engine a rich mixture 

 is used. After it has been running a short time and the cylinders 

 get heated, the mixture becomes hot also, and will fire even if it is 

 lean. When one "steps on the gas," a throttle in the pipe 

 between the cylinder and the carburetor is opened, thus allowing 

 more of the mixture to flow in and make the explosions more 



