The most common internal combustion engines of today can be defined as either four-stroke or two-stroke cycle. Two-stroke or four-stroke refers to the number of strokes the piston makes in the cylinder to complete one power cycle. A stroke is the movement of the piston in one direction, moving the piston from the top to the bottom of the cylinder is one stroke. A running internal combustion engine continually repeats a power cycle called: intake, compression, power and exhaust. Your automobile or stern drive engine is most likely a four stroke design. The majority of existing outboard motors use two stroke technology. However the current movement in emissions regulations is pushing the design of current outboards towards the 4 stroke and direct injection two stroke design. Efforts to build a 4 stroke outboard in the past have been many and varied, mostly unsuccessful as the design technology and precision production that can be achieved today were impossible to achieve then. Resulting motors were bulky and unreliable. Those motors that were viable were for the most part rejected by the boating public.

 

On the up stroke the top side of the piston is compressing only air in the cylinder. At the same time the bottom side of the piston is pulls more fresh air into the crankcase past a one way reed valve. Near the top of the stroke, the direct injector sprays a plume of fuel directly into the cylinder after the intake and exhaust ports are closed. This is accomplished in several ways depending on the engine manufacturer, all serve the same purpose and have more or less the same components. That fuel/air mixture is ignited by the spark plug. The rapidly burning fuel expands and begins forcing the piston down.

On the down "power" stroke the piston is forced toward the crankcase. As it continues its downward travel it starts first to uncover the exhaust ports. Exhaust gases begin to rush out of the cylinder. The intake ports then uncover; fresh air compressed on the downstroke is then forced into the cylinder and continues to push the remaining exhaust out of the cylinder. Since there was only air in the crankcase the cylinder is cleaned very effectively and readied for the next injection of fuel.