296 



TRANSPORTATION 



The electric spark which explodes the air-gas mix- 

 ture is formed by electricity from an electric generator. 

 The electricity is first run through a coil to raise its 

 voltage and is then sent to each spark plug at the exact 

 moment it is needed to explode the gas. This is ac- 

 complished by the distributor. Figure 492 shows the 

 parts of the ignition system of the car. 



Exercise. Carefully study Figure 492 and explain 

 how the distributor of an automobile works. Briefly ex- 





Courtesy Standard Oil Co. of New York 



FIG. 491. COOLING SYSTEM OF AUTOMOBILE 



plain the use of the distributor in the automobile engine. 



The power-transmission system. This part of the auto- 

 mobile transmits the power of the engine to the 

 rear or driving wheels. In the transmission system are 

 included the clutch, the gear shift mechanism, and the, 

 differential. 



Power is transmitted from the engine to the drive 

 wheels through the clutch, the gears, and the differ- 

 ential. The transmission or shifting gears, located just 

 back of the engine, are connected to the differential 

 in the rear axle by the drive shaft which runs under 

 the car. 



The clutch is a device used to disengage the en- 

 gine from the rest of the transmission system. It con- 



sists of two sets of friction plates, one set connected 

 to the engine and the other set to the drive shaft, a 

 part of the transmission system. These two sets of 

 plates, when engaged, are held tightly together by a 

 very strong spring. In this way the power of the en- 

 gine is transmitted to the drive wheels at the rear. 



Courtesy Standard Oil Co. of New York 

 FIG. 492. IGNITION SYSTEM 



When it is necessary to disengage the engine from the 

 rest of the system, the clutch pedal is pushed in, thus 

 separating the two sets of plates and permitting gears 

 to be shifted. Figure 493 shows a cross section of a 

 clutch. Can you find the plates and understand what 

 happens when the clutch lever of the car is pushed? 

 Exercise. Carefully study Figure 493. How does the 

 inset in the middle of the picture explain the operation 

 of the clutch? What principle of science is made use 



