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Two-CYCLE OR TWO-STROKE MOTORS
THE two-cycle motor already described and figured in the preceding chapter is known as the Two-port motor from the fact that there are but two openings, or ports, from the cylinder. Another form of two-cycle motor in common use is known as the Three-port motor (Fig. 9). The operation of this motor is essentially the same as the two-port type, but in addition to the two ports this motor has a third port or opening, A, which is closed by the piston acting as a sliding valve. On the downward stroke of this motor the third port is closed as shown in the figure, while the gas in the base is forced up to the top of the cylinder through the by-pass B. On the upward stroke the gas is sucked into the cylinder through the port A, as in the two-port type. The only advantage of this type of motor over the two-port is that no check valve is required in the inlet for fuel at A. It is usually a high-speed engine, and as a partial vacuum is created in the base on the upward or suction stroke and a considerable pressure must be maintained on the downward stroke, tight bearings and joints are essential and these, especially in a marine engine, are hard to retain for any length of time. Many of the more recent types of twocycle engines are constructed so that either a two- or three-port system may be used as desired. A very successful motor of this design is the Gray Model “T,"
illustrated in Fig. IO. Almost any good three-port motor may, however, be transformed to a two-port
engine even after its efficiency as a three-port has expired.
Many of the best so-called two-port motors are in reality a sort of combination of the two-and three-port
types. Such a motor is illustrated in Fig. II. In this motor the inlet, or fuel, port, instead of being in the base as illustrated in Chapter I, is situated in the by-pass B midway between the base and the inlet to cylinder C.
Fig. 10.-Gray Model “T” Motor
The operation of this type of motor is precisely the same as in the straight two-port, but it possesses many advantages over either the two- or three-port type.
The suction created by the piston is greater than in either of the other types, while no great pressure is required in the base
as in the three-port. The fuel-mixing device is elevated more than in the two-port, which is a distinct advantage in a boat, and the gas or vapor striking on the walls of the by-pass, instead of on the base of the engine, results in far better vaporization and consequently better combustion. The walls of the base are usually cold or nearly so and the mixture of gasolene and air tends to cool them off even more; whereas in the motor under consideration the walls of the by-pass are always warm while the motor is in operation and the mixture of gasolene and air striking this warm metal is immediately thoroughly vaporized. Many of the motors of this type have an opening in the base into which the fuel intake may be screwed. In such cases the operator may use his motor as a regular two-port or as a combination as desired. In many motors of this class very superior results are obtained by attaching an auxiliary air inlet in the base. This is closed when starting or running slowly, but when running at high speed is gradually opened until the best possible results are obtained. Such an auxiliary inlet is shown in Fig. 12 (A), and is known as an “Accelerator.”
Two-cycle motors are also made which possess some of the best features of both the two- and three-port types. An example of this style of engine is the Grasser motor illustrated in Fig. 13. This motor has a vaporizer A, as well as an automatic air valve B, a third port C, and a transfer port D, which acts as the second port on a two-port motor. In operation the piston on its upward stroke draws a charge through the vaporizer A into the crank case E, until it reaches a point where the third