Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(27) Three Port—Two Stroke Cycle Engine.

The principal difference between the three port and two port types of the two stroke cycle engine is in the manner in which the charge is admitted to the crank case for the initial compression. In the two port motor, as previously described, the check valve “V” opens to admit the charge, and closes during its compression in order to prevent its escape through the opening by which it was admitted to the cylinder. With the three port type there is no check valve in the crank case, the admission and the retention of the charge being controlled by the movement of the piston in practically the same way that the piston controls the opening and closing of the exhaust and transfer ports in the cylinder.

Fig. 6.

Fig. 7

Figs. 6–7. Diagram of Three Port—Two Stroke Cycle Engine in Two Positions.

By the piston control of the gases in the crank case, the valve is eliminated, which makes one less moving part to cause trouble and expense, and permits the use of the same type of carburetor that is used on the four stroke cycle engine. As the check valve opens and closes at a high speed, (twice that of the valves on a four stroke cycle engine), there is considerable wear on the valve seats due to the continuous banging, which results finally in a loss of the initial compression. When the initial compression is reduced in this way the engine loses power because of the reduction of the charge in the cylinder.

While the three port type is free from valve leakage troubles, it has a steady loss due to the high vacuum that exists in the crank chamber when the piston is on its upward stroke. This vacuum drags against the piston and absorbs a considerable amount of power until the piston reaches the upper end of the stroke. At this point the inlet port is opened and the vacuum is broken by the rush of the mixture through the inlet port. Besides the power loss, the vacuum has a bad effect on the lubrication of the main crank shaft bearing.

Elevation of Fairbanks-Morse Three-Port Two Stroke Marine Motor Showing Warming Device for Carburetor Air.

Described by strokes, the cycle of events in the three port, two stroke cycle engine is as follows:

STROKE 1. In Fig. 6, the piston is shown at the end of the compression stroke with ignition taking place in the combustion chamber C. The pressure due to the expansion drives the piston down on the working stroke at the same time causing the initial compression of the mixture in the crank case as shown by Fig. 7. The gas in the crank case cannot escape during compression as the inlet port A is covered by the piston.

(a) As the piston descends, its upper edge uncovers the exhaust port D, allowing the greater portion of the exhaust gases to escape and reduces the pressure in the cylinder to that of the atmosphere.

(b) Descending a little farther, the top of the piston uncovers the opening of the transfer port B, allowing the compressed gases in the crank case to enter the cylinder as shown by the arrows. These gases, guided by the deflector plate on the top of the piston are thrown upwardly, as shown by the arrows, and sweep the residual burnt gases before them through the exhaust port. The cylinder is now filled with the combustible mixture ready for compression.

STROKE 2. The piston now moves up on the compression stroke, compressing the charge in the cylinder and at the same time creates a vacuum in the crank-case. Just before the piston reaches the end of the exhaust stroke, the lower edge of the piston uncovers the inlet port A (See Fig. 7), which allows the mixture from the carburetor to flow into the partial vacuum and fill the crank case ready for the next initial compression. When the end of the stroke is reached, the charge in the combustion chamber C is fired and the cycle is repeated. It should be noted that the incoming gas and the initial compression are controlled entirely by the action of the lower edge of the piston on the inlet port A.