Engines with more than four cylinders have “overlapping” impulses, that is some cylinder on the engine is always delivering power, for before one cylinder reaches the end of the stroke, another has fired its charge and has started to deliver power. Thus the impulses “overlap” one another, and the result is an even and smooth application of power and a minimum of strain is imposed on the engine. Aeronautical and speed boat engine builders have carried the multiple cylinder idea to an extreme because of the nature of their work. Eight cylinder aeronautical engines are very common and there are several built having sixteen cylinders. The latter type of engine gives eight impulses per revolution. To avoid a great multiplicity of cylinders, and to save on floor space, the great majority of heavy duty stationary engines are built double acting, that is an explosion occurs alternately in either end of the cylinder. In effect, a double acting cylinder is the same thing as a two cylinder single acting engine, as it gives twice the number of impulses obtained with a single acting cylinder. The order in which the events occur in a four stroke cycle engine is as follows: =STROKE 1.= First outward stroke of the piston causes a partial vacuum in the combustion chamber thus drawing a charge of combustible gas into the cylinder through the open inlet valve. The exhaust valve is closed. See diagram A in Fig. 4. (Suction Stroke.) =STROKE 2.= Inlet valve closes at the end of the suction stroke and the piston starts on the inward stroke compressing the charge in the combustion chamber. See diagram B. (Compression Stroke.) At the end of the compression stroke, or a little before, the spark “S” occurs causing the ignition of the charge. See diagram C. =STROKE 3.= Working Stroke. As the pressure is now established in the cylinder, the piston moves down on the working stroke forcing the crank around against the load and supplying sufficient energy to the fly wheel to carry the engine through the three idle strokes. See diagram D. When the piston reaches the end of the working stroke, or a little before, the exhaust valve opens to reduce the pressure and to allow the greater part of the burnt gas to escape. See diagram E. =STROKE 4.= Scavenging Stroke. The exhaust valve remains open and the inwardly moving piston expels the remainder of the burnt gas through the exhaust valve, clearing the cylinder for the next fresh charge of mixture. See diagram F. The next stroke is the suction stroke explained under “Stroke 1.”