Fig. 18.—Two-Cycle Engine.
In order that this result may be attained, the construction of the engine is changed, and, as will be seen in Fig. 18, the crank case is utilized as a receiver for the mixture before it passes to the combustion space. The valves are replaced by ports, which are openings into the combustion space that are covered and uncovered by the piston as it slides in the cylinder. The inlet port is uncovered when the piston is at the inmost point of its stroke (Fig. 18, A), and then admits the mixture to the crank case; the by-pass port and the exhaust port are uncovered when the piston is at the outmost point of its stroke (Fig. 18, B), the former then permitting the mixture to pass from the crank case to the combustion space, and the latter is that through which the burned gases escape after combustion has taken place.
During an inward stroke, the pressure in the crank case is reduced as the piston slides away from it, and fresh mixture is forced into it by the higher atmospheric pressure as soon as the inlet port is uncovered. This port is covered when the piston makes an outward stroke, and the mixture, not being able to escape, is compressed. Its tendency to expand causes it to flow to the combustion space when the by-pass port is uncovered, and in entering it strikes a ledge on the piston so that it is deflected to the top of the combustion space instead of being able to shoot across the cylinder and out the open exhaust port. The inward stroke of the piston covers these two ports and compresses the mixture, ignition occurring in the regular manner. The pressure developed by the combustion drives the piston outward, and as soon as the exhaust port is uncovered (which is slightly before the uncovering of the by-pass port), the gases, which are still expanding, begin to escape, and are further expelled by the fresh charge that enters and drives them before it. Thus the five events of the cycle are performed during an inward and an outward stroke of the piston, the crank case end of the piston drawing a charge of fresh mixture into the crank case and forcing it into the combustion space, and the combustion chamber end compressing it and being acted on by the pressure from the combustion.
At slow speeds, two-cycle engines have advantages over the four-cycle type in the production of a power stroke at every revolution of the crank shaft, and the absence of valves and valve mechanism with their weight and possibility of giving trouble. This simplicity makes the two-cycle engine popular for motor boats, where they are run at slow and constant speed, but for higher and changing speeds these advantages are outweighed by disadvantages that show little sign of being overcome.
With the engine running at a thousand revolutions a minute, it can be understood that the ports will be open for only a brief period during each stroke, and that the faster the engine runs the shorter will be the period during which the gases may enter or leave the combustion space. The inefficiency of two-cycle engines as compared with engines of the four-cycle type is due entirely to the fact that the burned gases have not sufficient time in which to escape from the combustion space, nor the fresh charge time to enter. The fresh charge that does enter being incomplete, and being contaminated by the portion of the burned gases that has not been able to escape, result in the “choking up” of the engine, and in the production of lower power than the dimensions and weight of the engine should warrant.
CHAPTER V
CARBURETION AND GASOLINE FEEDS
Pure gasoline vapor will not burn, and in order to render it inflammable it must be combined with oxygen. The simplest manner of effecting this is to mix air with it, and when the correct proportions are obtained, the oxygen supplied by the air will be sufficient to result in the complete combustion of the gasoline vapor, without a surplus of either of the ingredients. This mixing is called carburetion, the air being said to be carbureted. A correct proportion of gasoline vapor and air results in rapid combustion; an excess of air makes combustion slower, and excess of gasoline vapor prevents the combustion from being complete, a residue of carbon remaining. The correct proportions of air and gasoline vapor are obtained by the use of a device called a carburetor, which is connected to the combustion chamber by the inlet pipe, and in such a manner that everything entering the combustion space by the inlet valve must first pass through it.
Liquid gasoline is led to the carburetor from the supply tank, and the air enters it when the pressure in the combustion space is reduced by the piston in making the inlet stroke. The speed with which the air flows through the carburetor depends on the extent to which the pressure is reduced, and the gasoline vapor that is required to form a mixture of the correct proportion must be maintained in accordance with it. While there are various classes of carburetors, practically all that are used for automobile engines are of the float-feed type; that is, the supply of gasoline is maintained by a float, just as water tanks are kept filled to a desired depth by a hollow metal ball that floats on the liquid and controls the valve by which the water enters.
