192 OUR PHYSICAL WORLD 



The gasoline engine consists essentially of at least two 

 cylinders in which the gas explosions occur alternately, the 

 pistons which connect by their rods with the crank shaft that 

 bears the flywheel, the spark plugs, one in the end of each cylinder 

 where occurs the electric spark that fires the gas, the carburetor 

 in which the gasoline vapor is mixed with air before it is drawn 

 into the cylinders, and a storage battery, or else a magneto, which 

 supplies the electric current to the spark plugs. There are 

 many accessory parts (Fig. 78). 



The gasoline engine is usually at least a two-cylinder engine, 

 the cylinders firing alternately, and in most automobile engines 

 the cylinders are still more numerous, four, six, or twelve. 

 Then they work in groups, the explosion and out stroke (or 

 power stroke) occurring in part of them, while in others the piston 

 head is moving in to compress the gases (compression stroke), in 

 still others to drive out the gases after burning (exhaust stroke). 

 The crank shaft to which one end of each piston rod attaches by 

 a movable joint is a forged and accurately turned steel shaft 

 with as many right-angled bends in it, like squares with one side 

 open, as there are piston rods. Each piston rod fastens loosely 

 to one bend, and helps to rotate the crank shaft as hand and arm 

 rotate the crank on a coffee mill. In a two-cylinder engine the 

 two bends are in the same plane but face in opposite directions. 

 In a four-cylinder engine the pairs of bends are similarly placed, 

 one pair facing one way, the other in the opposite direction. In 

 the six-cylinder engine there are three pairs of bends that lie 

 in three planes that are 120 apart. By such an arrangement the 

 crank shaft is rotated by a succession of thrusts of the piston rods 

 rather than having them all push at once, and so the engine runs 

 smoothly. 



There are really four phases to a complete cycle in any 

 cylinder. Beginning with the explosion: (i) the piston head 

 (and rod) moves out, then (2) it moves in to expel the gases 

 formed by the explosion, (3) it moves out to draw in the new 

 charge of gasoline vapor mixed with air, and finally (4) it 



