TRANSPORTATION ON LAND 



295 



Exercise. In the older type cars where a vacuum tank 

 as shoivn in Figure 489 was used in the fuel system, a 

 small hole was left in the cap of the gasoline tank. Infer 

 a reason for this. 



The gasoline engine used in the modern automobile 

 is made up of four or more cylinders. Each cylinder 

 is a unit for harnessing the energy of the explosion 

 of the air-gas mixture that takes place inside. With- 

 in each cylinder is a tight-fitting piston which moves 

 up and down and which is connected to a crank shaft 

 by a connecting rod. 



If you understand the construction of the engine 

 you are now ready to learn what happens inside it. 

 The air-gas mixture gets into the part of the cylinder 



Courtesy Standard Oil Co. of New York 



FIG. 489. COMPLETE FUEL SYSTEM OF 

 AUTOMOBILE 



above the piston through the intake valve, and the 

 burned gases are pushed out through the exhaust 

 valve. Follow the diagram (Fig. 490) which illustrates 

 the four strokes through which each cylinder passes 

 every time power is delivered to the crank shaft. 



As the piston goes down in the cylinder on the first 

 stroke, a low pressure is created. The intake valve 

 opens, and the air-gas mixture is pushed in from the 

 carburetor by pressure. This is called the intake stroke. 

 On the next stroke both valves are closed, and as the 

 piston comes up the air-gas mixture is compressed. 

 This is the second or compression stroke. Just as the 

 piston reaches the top of the compression stroke an 

 electric spark is sent across the spark plug and the pis- 

 ton is driven down by the force of the explosion. This 

 is the power stroke. Notice that both valves are still 

 closed. On the fourth stroke the exhaust valve opens, 



and as the piston comes up the burned gases are 

 forced out into the exhaust pipe and to the open air. 

 This is called the exhaust stroke. As the piston again 

 goes down new gas is drawn into the cylinder and the 

 whole cycle is again repeated. The explosion of the 

 gas develops a very high temperature in the cylinder, 

 and it is therefore necessary to cool the cylinder wall? 



INTAKE STROKE 







MTAHC VAlvf 

 ^ 0hS TO ADMIT 



POWER STROKE 



EXHAUST STROKE 



L oititesy Standard Oil Co. of New York 

 FIG. 490. FOUR STROKES OF THE GASOLINE ENGINE 



with water. The water is kept cool by being pumped 

 through the radiator where cool air takes the heat 

 away. 



Exercise. Study Figure 491 and see whether you can 

 explain hozv the cooling is accomplished. 



Exercise. Carefully study Figure 490 and explain 

 what is done when the automobile mechanic grinds the 

 I'ah'cs of the car. ll'hy must valves be ground? What 

 would result if they were not ground? 



