ig6 OUR PHYSICAL WORLD 



sends a current to the battery to replace the electricity used 

 constantly at the spark plugs and in the lights. 



The current from the induction coil to the spark plugs must 

 also pass through the distributor and the timer. The distributor 

 sends the current first to one cylinder then to another and so on, 

 so they will be fired in the proper order. This is usually not the 

 order in which the cylinders stand in their row. The firing order 

 in a four-cylinder engine may be cylinder one, then three, 

 then four, and, finally, two, rather than one, two, three, four, 

 for the vibration of the engine is usually less when the order is 

 not in the regular succession. The timer determines the exact 

 moment at which the spark fires the mixture with reference to 

 the position of the piston. When the engine is running slowly, 

 firing can come at the moment of greatest compression as the 

 piston head has reached the top of its stroke and is just about to 

 begin the descent. But when the engine is running rapidly, the 

 spark must come slightly sooner else the piston head will be well 

 on its down stroke before the gases will develop their maximum 

 pressure. The timer in most machines is now automatic in this 

 adjustment, but a lever is put on the steering wheel to advance 

 or retard the spark when speeds are very extreme. 



When the valves are opened to let the gases out of the 

 cylinders after the gasoline is burned, they are still under high 

 pressure, and if discharged directly into the air they would come 

 out with a noise like that of a pistol shot. They are therefore 

 discharged through a muffler, a long tube of increasing diameter 

 with numerous incomplete cross-partitions. The gases go into a 

 succession of constantly enlarging chambers, and thus expand 

 gradually instead of suddenly. When the muffler is not in use 

 the "cut out" is said to be "open," and the exhaust is noisy. 



As explained, the piston rods are so attached to the crank 

 shaft as to make it turn around. On the rear end of this crank 

 shaft is a heavy flywheel which helps to keep the engine running 

 smoothly and which also serves to transmit the engine's power to 

 the rear wheels of the car. Through a device known as a clutch 



