=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.
As the admission and exhaust in the two stroke cycle engine each occur once per revolution, and are controlled directly by the piston position at opposite ends of the stroke, it is evident that the direction of rotation is not affected by gas control or valve timing, as in the case of the four stroke cycle engine. The factor that does determine the direction of rotation in the two stroke engine is the time at which ignition occurs in regard to the angular position of the crank. By changing the relation between the crank position at the end of the compression stroke and the time at which the spark occurs, it is possible to reverse the engine even when it is running.
Should the engine be standing still in the position shown by Fig. 6, with the crank on the dead center, when ignition occurred, there would be no more tendency to turn the crank to the right than to the left, providing of course, that there was no effect from the momentum of a revolving fly wheel. If ignition occurred with the crank inclined ever so little toward the right, the pressure of the piston would force the crank downwards in a right handed direction. If the crank were inclined to the left, the tendency would be for left handed rotation.
If the ignition system were arranged so that the spark occurred when the crank was inclined towards the right every time that the piston came up on the compression stroke, we should have continuous rotation in a right hand direction. By shifting the sequence of the spark so that it occurred with the crank on the left we would cause the engine to stop and reverse to left handed rotation. This is exactly the method used in reversing two stroke motors in practice, the change in the ignition being accomplished by advancing or retarding the mechanism that dispatches the spark (“Timer” or “Commutator”).