166 



USING MACHINES 



line. Of these, gasoline is most important because of 

 its use as gas engine fuel. 



Gasoline is a fuel which is easily vaporized. This 

 vapor, when mixed with the proper amount of air, 

 forms a very explosive mixture. The gasoline engine 

 draws such a mixture of gasoline vapor and air into 

 its cylinders and explodes it with an electric spark. 

 The force of the exploding gas pushes a piston down 

 with great force, thus harnessing the energy of the 

 gasoline. Figure 277 shows a cross section of one 



cylinder of a gasoline 

 engine. Another type 

 of oil engine called the 

 Diesel engine after its 

 inventor, Rudolph 

 Diesel, is being used 

 on some types of ships 

 and for turning elec- 

 tric generators. Re- 

 cently these engines 

 have been used in an 

 experimental way in 

 airplanes and auto- 

 mobiles. The chief ad- 

 vantage of the Diesel 

 engine over the gaso- 

 line engine is in the 

 fact that it can run on 

 a cheaper fuel. It can, 

 in fact, be run on 

 crude oil. The Diesel 

 engine does not re- 

 quire an electric spark 

 to explode the fuel, which is sprayed into its cylinder. 

 Air is first drawn into the cylinder and then com- 

 pressed under a very high pressure. This heats the air 

 to such a high temperature that when the fuel is 

 sprayed in, it immediately explodes. Because of the 

 high pressure the cylinder walls of the Diesel engine 

 must be thick and strong. This makes a very heavy 

 power unit and has prevented its adoption for auto- 

 mobiles and airplanes. The picture in Figure 278 

 shows a modern Diesel engine. 



How is the energy of falling water harnessed? For 

 centuries man has used falling water as a source of 

 energy. Sawmills, grist mills, and other larger in- 

 dustries were run by great wheels turned by the water 

 as it rushed past on its way to the ocean. Today water 

 power finds its chief use in generating electrical 

 energy which can be sent long distances over wires 

 with only a small loss. 



There are four types of water wheels. In the over- 

 shot wheel the water runs over the wheel and turns 

 it by the weight of the water as it falls. This type of 

 wheel is used in hilly country where there is a con- 



siderable fall but a comparatively small amount of 

 water. Its efficiency varies from 75 per cent to 85 

 per cent. 



Courtesy General Motors 



FIG. 277. MODEL OF A GAS ENGINE 

 CYLINDER 



Courtesy Busch-Sulaer Bros. 



FIG. 278. A MODERN DIESEL ENGINE 



The undershot water wheel is used when there is 

 a large volume of water with only a little fall. This 

 type of wheel is about twenty-five per cent efficient. 

 These wheels, extensively used in the past, have 

 gradually given way to two other modern types. The 

 Pelton wheel is a modification of the undershot wheel 



UNDERSHOT TYPE 



PELTON WATER WHEEL OVERSHOT TVPE 



FIG. 279. TYPES OF WATER WHEELS 



