=STROKE 1.= We will consider the piston to be moving up on the compression stroke as shown in view (A), compressing the mixture in the combustion chamber D. While moving upwards in the direction of the arrow, the piston creates a vacuum in the crank case C drawing fresh mixture into the crank case. The piston at this time is covering the opening of the transfer port I and the exhaust port E so that the compressed mixture in the cylinder cannot escape. On reaching the end of the compression stroke, a spark occurs at S which drives the piston down and turns the crank towards the right as shown by the arrow. =STROKE 2.= When the piston uncovers the exhaust port E on its downward working stroke as shown by view B, the exhaust gases being under pressure rush out into the atmosphere as shown by the arrows, and relieve the pressure in the cylinder. Some of the burnt gas remains in the cylinder at atmospheric pressure as there is no scavenging action up to this point. While the piston has moved down on the working stroke it has compressed the mixture in the crank case ready for admission to the cylinder. The valve V prevents the escape of the gas during the compression. On reaching the end of the stroke the piston uncovers the transfer port which allows the compressed mixture in the crank case to rush into the cylinder through I, as shown by view C. Owing to the shape of the deflector plate Z on the piston head, the stream of mixture issuing from I is thrown up toward the top of the cylinder, as shown by the arrows, and consequently sweeps the remainder of the burnt gas before it through the exhaust port E. In this way the fresh mixture from the crank case scavenges the cylinder and fills it in one operation. Being filled with gas, the piston now moves up on the compression stroke for the next explosion as shown by view A. Unfortunately the scavenging action of the incoming gas is not complete for the whirling motion of the charge causes it to mix with the residual gas to a certain extent which, of course, reduces the heating effect of the fuel and reduces the power output. Another factor that reduces the output of this type of engine is the loss of explosive mixture through the exhaust port at low engine speeds with an open throttle. In this case, the piston speed being low, part of the mixture has time to pass over the deflector plate and through the exhaust opening before the piston closes the exhaust port. At very high speeds the charge is diluted by a considerable quantity of burnt gas which has not had time to escape through the port causing a further loss of power. With the throttle nearly closed on a light load, the impact of the incoming mixture is so slight that the percentage of exhaust gas left in the cylinder is very high. This dilution is so great that with moderately low speeds (easily within the capacity of the four stroke cycle engine) it is either impossible to ignite the charge or it is impossible to ignite two in succession.