THE AUTOMOBILE 275 



applied to the piston. In order to keep the piston running 

 during the other three strokes, a heavy flywheel is provided, 

 whose momentum keeps the axle rotating until the next 

 explosion. 



In order to give a more even application of power, auto- 

 mobiles are now furnished with engines of four, six, eight, or 

 even more cylinders. A four-cylinder engine would be so 

 arranged that the driving gear would receive an impetus 

 four times as frequently as it would from an engine of one 

 cylinder. 



Starting. In order to start the engine, some power must 

 be used to give the piston a few strokes till an explosion 

 occurs. In some cars this is done by hand, by means of a 

 crank attached to the front of the car. Other cars have 

 electric starters, in which the power to start the piston is 

 furnished by a motor, driven by electricity from a storage 

 battery. This is controlled by pressing a button or lever. 



Comparison with steam engine. The gas engine differs 

 from the steam engine in a number of ways. In the first 

 place the fuel is burned inside of the cylinder instead of in 

 a fire box. In the four-cycle engine the propelling force is 

 applied only once in four strokes, and on only one side of 

 the piston ; while in the steam engine it is applied at every 

 stroke, alternating on the two sides of the piston. A steam 

 engine will start as soon as the steam is admitted to the 

 cylinder; while in order to start a gas engine, the crank 

 shaft must first be rotated by some external means. A 

 steam engine can begin to do work at once as soon as the 

 steam moves the piston head, while a gas engine must be 

 started first without connection with the axle of the auto- 

 mobile, and then the load is gradually applied by means of a 

 clutch. 



Carburetion. Before the gasoline is allowed to enter the 

 cylinder, it is changed to a gas and mixed with air, so that 

 it will explode in the cylinder. The machinery which does 



