Mechanics: The Science of Machinery

by A. Russell Bond

BURNING AIR IN THE AUTOMOBILE ENGINE

Owing to the widespread use of automobiles, the general public is better acquainted with internal-combustion engines than with steam engines. Nevertheless, there are many elementary facts in connection with gasoline engines that are not generally known. The average motorist probably does not realize that he burns far more air in his motor than gasoline, and he probably does not understand why it is that the mixture of gasoline vapor and air must be compressed before it is ignited, or why only one out of four strokes of the piston is a power stroke.

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.

FIG. 51—SECTIONAL VIEW OF A TWO-CYCLE ENGINE

There is a two-cycle type of motor in which the burnt gases are removed from the cylinder, not by the piston, but by the injection of the unburnt fuel mixture. On the upward or compression stroke the piston not only compresses the mixture in the cylinder, but draws in a fresh charge of fuel into the crank case, and when the piston is driven down by the combustion of the fuel in the cylinder, the charge in the crank case is compressed until, near the end of its stroke, the piston uncovers a port in the cylinder through which the fuel from the crank case is forced in. (See Figure 51.) Just before this occurs the piston uncovers an exhaust port in the opposite side of the cylinder, and the burnt gases start to flow out before the fresh charge of fuel pours into the cylinder. The incoming gas is directed upward so as to completely scavenge the cylinder of all burnt gases.

There are no bothersome poppet valves in this engine. The piston itself acts as a valve, opening and closing the inlet and exhaust ports as it slides by them. Every other stroke of the piston is a power stroke, so that the engine acts like a one-pedal bicycle, which receives a push every time the pedal comes around. Unfortunately, the burnt gases are never completely scavenged, and the fresh charge of fuel is always more or less diluted by the product of combustion remaining from the previous charge. This represents just so much loss of power, and may interfere with the ignition of the charge. For this reason the four-cycle engine is generally considered more reliable and efficient, and is far more generally used, particularly on motor vehicles.

To overcome the intermittent character of the internal combustion engine a number of cylinders are used, which come into play successively. In the four-cylinder motor one piston is always on the power stroke. In recent years the number of cylinders has progressively increased from four to six, eight, and twelve cylinders, while in racing power boats the number of cylinders has gone up as high as twenty-four.