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cylinder, with a result that a quicker firing charge is obtained and also better scavenging of the burnt gases. The carburetor may be placed far lower than the lowest level of the cylinder, thus eliminating any danger of flooding, while the straight thrust of the piston rod and cross-head bearing do away with all side wear on the piston and cylinder walls. The cold charge of gas inside the piston also helps materially to cool the same, while the heat from the piston aids in better vaporization of the gas before its transfer to the firing chamber. The only objection to this type of motor is the additional height made necessary by the cross-head and connecting rod below the cylinder level. In stationary or ma use this is no serious objection, but for vehicle use it would necessitate a very high engine hood or small clearance beneath the shaft.
Several makers of two-port motors have also resorted to placing a throttle valve in the by-pass as shown in Fig. 16 (T), and while the speed and power of a motor so equipped may be controlled to some extent by this arrangement, yet as a rule I have found them rather unsatisfactory. A throttle on the fuel-mixing apparatus is far more reliable and easier to adjust. Some motors are also manufactured with a fourth, or air, port, and wonderful results are claimed for this type of motor. Undoubtedly it possesses many good points, but as a rule the fewer ports there are the more reliable the engine. To obtain the best results each port must be accurately designed and finished, and in a small engine the variation of a small fraction of an inch in the size of one or more ports will make a wonderful difference in the
power and operation of the motor. Moreover, the first part of a motor that wears loose is generally the piston or the connecting rod between the piston and crank shaft. As a slight wear here causes the piston to have considerable play up and down in the cylinder, it will be easily seen that the variation in the amount of opening of each port during operation is considerable. With every port added this variation, due to looseness, increases the resulting variation of the amount of fuel admitted to the cylinder, and as the amount of play varies quite a little according to the load of the engine and its speed, it is very difficult to adjust a motor thus
In fact the results from a three- or four-port motor with a loose piston or connecting rod are almost as great as if the ports were rapidly but unevenly opened and closed by hand. Any one can thus readily understand why a strictly two-port motor will run more satisfactorily when old or badly worn than a three- or four-port engine.
Probably the most reliable and efficient two-cycle motor yet produced is the Elmore, which is used in the well-known Elmore automobiles. This motor, which is illustrated in Fig. 17, is entirely distinct from most other two-cycle motors and in its construction and operation it overcomes most of the objectionable features of the two-cycle engine. The figure is purely diagrammatic and represents a section of one of the four cylinders and a portion of the base cut away
The piston of the Elmore motor differs materially from that of other motors, inasmuch as the lower half or base is much greater in diameter than the piston proper (Fig.