(62) Variable Speed Two Stroke Motor.
A variable speed two stroke cycle motor is described by C. Francis Jenkins in the Scientific American that seems to solve many of the problems encountered in designing a two stroke cycle motor for automobile purposes. As is well known, the present design of the crank-case compression type is wasteful of fuel, and ignites irregularly at low speeds and light running, and as nearly all automobiles are well throttled for a greater portion of the time it means that this type of motor is working under the greatest disadvantage.
Fig. 66. Jenkins Two Stroke Cycle Motor.
Since the greater part of the trouble is due to the dilution of charge by the residual gases, and as the spark plug of the motor is situated in the most diluted portion of the gas, it would seem that a change of spark plug location, or a change in the circulation of the fresh mixture in the cylinder would be a great aid in remedying the difficulty. With the spark continually in contact with fresh undiluted mixture it would be possible to run it as low speeds as with the four stroke motor, with a corresponding increase in the efficiency, and opportunity to run with a constant advance of the point of ignition. This is accomplished by any or all of the following conditions:
(1.) By keeping good gas separate from bad.
(2.) By placing the spark near the intake port.
(3.) By leaving the plug in its present position and deflecting the fresh gas to meet it.
(4.) By changing the location of the inlet port.
Fig. 58-a. Two Cylinder Marine Engine, of the Two Stroke Type. Built by Fairbanks-Morse and Company.
In the motor invented and described by Mr. Jenkins, the method given by (4) is adopted as shown by Fig. 66, in which the spark plug is placed at the point of admission of the gas and in a confined passage. The operation of the motor is as follows:
Carbureted gas is drawn into crank-case from the carburetor (not shown) in the usual manner, i. e., by the upward movement of the piston; and by its downward movement is forced through the rectangular port in the wall of the piston into the combustion passage within the water-jacket when the port in the piston wall registers with the lower end of this combustion passage, and drives ahead of it the bad gas remaining after the previous explosion. If the throttle is wide open the combustion space above the piston will be completely filled, and on the ignition of the charge the maximum pressure will be exerted on the piston. If, however, the throttle is but slightly open, the combustion passage only may be filled and none overflow into the combustion space above the piston. This small charge will be just as efficient in proportion to its volume as was the large charge, for it was compressed to practically the same extent and none was mixed with the bad gas of the previous explosion. It will, therefore, be obvious that the spark-plug is always swept by the fresh charge, be it large or small, and the ignition will be just as certain in one case as in the other, although the charge and consequent impulse may be only just sufficient to keep the engine turning over, and without missing a single explosion.
Fig. 64-d. Roberts Cylinder Showing Cellular Screen in the Intake Port. This Screen Prevents Crank Case Fires by Chilling the Cylinder Flame Before it Enters the Crank Case.
In the motor built to test and demonstrate this design, provision was made for a second spark-plug to be located in the top of the cylinder for speed work, if this was found necessary. No opportunity has yet been had for making track tests, though without regret, as this two-cycle motor will run idle without missing or “stuttering,” which was the thing heretofore impossible.