330 FORMS OF ENERGY By combining the tire pump and emery wheel we obtain a good air pump. Let us take this air pump and add the things necessary to change it into an engine. Additions must be made to the air pump to allow it to pump a mixture of air and gasoline. The engine must have doors which will open and close to allow the mixture of gasoline and air to enter the cylinder and the burned gases to pass Fig. lis. Fig. 116. out again at the proper time. The drafts on your stove or furnace control the intake of air and outlet of burned gases. Likewise, the valves in the engine let in or allow the gases to be expelled. In the furnace we can take our time about opening and closing the drafts, but in the engine the valves must open and close in less than one hundredth of a second. It is necessary to open and close the valve automatically at exactly the right time. The cams and shaft are provided to do this. A cam is a device to convert rotary motion into up-and-down motion of the valves. The cam works something like the off-center wheel on the wheelbarrow pictured in the illustration. The camshaft is made up The camshaft is driven by the of a series of cams, a cam for each valve. Fig. 117. crankshaft and rotates at exactly one-half the speed of the crankshaft. The automobile engine takes a mixture of gasoline and air in the correct proportions and burns it in the cylinder. To mix air properly with liquid gasoline requires the carburetor. The carburetor operates Fig. 118. Fig. 119. Fig. 120. much like the atomizer or like the fly-sprayer. Air, rushing past an open tube of the correct size, picks up the liquid and mixes it with the air. For every pound of gasoline, about fifteen pounds of air are normally necessary to obtain the correct proportions for a. good burnable mixture. The air contained in a room ten feet square and twelve feet high is barely enough for the combustion of each gallon of gasoline.