In marine service where the loads are constant, and the speeds fairly uniform, there is but little trouble from the last mentioned source, and as the fuel is usually a smaller item than the repair bill, the simplicity of the small two stroke engine with its freedom from mechanical troubles usually gives satisfactory results in the hands of the novice. (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. [Illustration: Fig. 6. ] [Illustration: 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. [Illustration: 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: