rapid series of explosions. It has been shown that with the fourstroke engine three operations are necessary to obtain a useful power impulse. The first downward movement of the piston draws in the gas. The following upward movement compresses the mixture. The second down movement of the piston takes place during the expansion or explosion stroke, and is the third and power-producing part of the cycle. Following this operation the remaining stroke of the piston, which is upward, clears the cylinder of burned gas.

Obviously all the work is done at the top of the piston, and the differing functions take place in the combustion chamber or that portion of the cylinder above the piston. With a two-cycle engine all this work can be accomplished in half the number of strokes, and it is possible to secure an explosion stroke after each idle stroke. There is a power impulse every revolution of the crank shaft for each cylinder instead of every two turns, and energy is created every two strokes of the piston. To accomplish this, work is performed on both sides of the piston, as the crank case is utilized as well as the part of the cylinder above the piston. The top of this member compresses the charge and receives the pressure of the explosion, as these two essential functions can only take place in that portion of the cylinder above the piston. The crank case performs the duty of the inspiration stroke, as the mixture is drawn into the engine base by the upwardly moving piston. This is done at the same time that the piston is moving up to compress the gas above it, previously taken in. The views at Fig. 48 show the principles of operation of two-port, two-cycle engines, while those at Fig. 49 define the fundamentals of three-port two-cycle engine operation. The principle in the main is the same. for both types, the only difference being in the method of introducing. the gas to the crank case.

It will be seen that such engines are very simple, and that the pistons, connecting rods, and crank shaft are the only moving parts. Instead of valves and operating mechanism to control the gas flow by ports, or passages which are cast in the cylinder walls about halfway down their length are used. The gas is taken into the engine base, which is air tight, and of small capacity; in one case through an automatic check valve fitting to which the carburetor is attached. In the three-port form this valve is eliminated and the carburetor is coupled

directly to the cylinder. The gas is transferred from the crank case to the cylinder through a bypass, or a passage leading from the engine base to the intake port.

Considering first the action of the two-port, two-cycle type it will be evident that if the base is air tight an upward movement of the piston will produce a suction in the engine base, drawing the inlet valve down from its seat and inspiring a mixture of gasoline and air from the carburetor. Thus when the piston moves up, the engine base fills with vapor. On the down movement of the piston the gas in the crank case is compressed, and when the top of the piston registers with the inlet port in the side of the cylinder the mixture will transfer to the cylinder above the piston because of its pressure. The intake port is opened as the piston side uncovers it. The compression in the engine base is light compared to that above the piston. While the compression pressure before ignition may be as high as eighty pounds per square inch, that in the engine base necessary to insure prompt transfer of the charge seldom exceeds ten pounds. The operation of this type engine is not difficult to understand.

Referring to Fig. 48, A, we will assume that there is a compressed charge of gas above the top of the piston, and that the crank case is full of mixture. The spark occurs at the spark plug and the resulting explosion forces the piston down on its power stroke, this movement also compressing the gas already in the engine base. When the piston uncovers the exhaust port it begins to open before the intake port is uncovered and the burned gases escape to the air because of their pressure. After the exhaust port has been open for a small period the inlet port is uncovered by the piston which is still on its downward movement. The compressed gas in the case flows through the bypass and into the cylinder. It cannot escape out of the open exhaust port opposite, because the deflector directs the gases toward the top of the cylinder. The fresh gas coming in tends to force out any of the burned residue which tends to remain. As the piston rises on the return stroke both ports are closed and the gas in the cylinder is compressed. Simultaneously with the gas compression the partial vacuum created in the engine base draws in new mixture from the carburetor through the open check valve.

Three-port engine operation, as shown at Fig. 49, is very similar

to that previously described, except that the intake port in the cylinder to which the carburetor is coupled is uncovered by the piston only when it has traveled up in the cylinder. There is a pronounced vacuum in the crank case when this port is uncovered and the rapid rush of gas insures quick charging. The three-port engine is considerably faster than the two-port type, and is more popular for motorcar service. In the three-port form of engine the mixture rushes in to fill the engine base as a body, whereas in the two-port type it is drawn in gradually during the whole upstroke of the piston.

If the reader compares the views of the simple two-cycle engines shown, with the four-cycle types previously described, it will be seen · that the former are simpler, because many of the small parts necessary for the successful action of a four-stroke motor are not needed in the two-stroke motor. The valves and operating mechanism are dispensed with, which means elimination of the cam shafts, driving gearing, cams, and push rods, incidental to valve operation. As the cylinder of the two-cycle engine is charged and cleaned through simple ports which are but orifices left after cutting away part of the cylinder wall, the gas can be introduced and discharged with much less mechanism. The ports are covered by the piston until the time comes when they are needed. In the two-port form both openings are opened fully only when the piston reaches the end of the downstroke, but in the three-port motor the remaining opening through which the crank case is charged is uncovered when the piston reaches the end of its upward movement.

The simple forms of two-cycle motors described have been replaced in some cases by types in which the preliminary compression of gas in the crank case has been eliminated. This is accomplished by the use of a two-diameter piston, as shown at Fig. 50. The cylinder proper consists of an upper and lower chamber. The small portion of the piston fits the upper cylinder while the enlarged part fits the lower end. Instead of the gas being inspired into the engine base it is drawn into the enlarged portion of the cylinder. It has been difficult to maintain regularity of two-cycle motor action when crank case. compression was used, because any leakage at the bearings, or packings in the crank case, meant a reduction in vacuum and made for uncertain charging.