If a volume of gasoline vapor be thoroughly mixed with an equal volume of air, the mixture will not explode. Only when there is an excess of air will combustion take place, and the most intensive explosion takes place when there are nine parts of air to one part of gasoline vapor. Where gasoline is vaporized in a carburetor the best mixture is one part of gasoline-saturated air to eight parts of pure air. Hence it is mainly air that is burned in an automobile engine. The energy of combustion is much greater when the mixture is compressed. The particles of gasoline and air are forced into more intimate contact by the compression. The same thing is true of gunpowder. If gunpowder is ignited in the open air, it will burn quickly, but not with explosive violence. If, on the other hand, the powder is compressed in a cartridge and is then ignited, an explosion takes place. Nearly twice as much gas was required in the early noncompression-type motors as is required to-day in the compression motors of the same power. In an internal-combustion engine the cylinder serves as a furnace. This “furnace” must be charged with fuel, and after the fuel has been burned the “ashes”--i. e., the products of combustion--must be removed. The piston serves as the furnace stoker. In the ordinary four-cycle engine the action of the piston is as follows: On the first or down stroke of the piston a mixture of air and gasoline vapor is drawn into the cylinder; on the next or rising stroke the charge is compressed. Then the charge is ignited by an electric spark, and the rapid combustion of the charge produces gases which drive the piston down. On the fourth stroke the piston rises again and pushes the burnt gases out of the cylinder. The piston receives energy intermittently, or only once out of four strokes, and a flywheel has to keep it going the rest of the time. It is just like propelling a bicycle with a single pedal and pushing the pedal every other time it comes out. It can be done as long as the wheel is moving fast enough to carry itself along between power strokes. Naturally a single-cylinder motor cannot be slowed down very much without stalling, and it will not start of itself because it needs outside help in stoking its furnace before it acquires the power to do this job alone. [Illustration: FIG. 51--SECTIONAL VIEW OF A TWO-CYCLE ENGINE]