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.

The first description reviews the operation of the 4 stroke power cycle. Each 4 stroke image depicts a piston in a cylinder, a spark plug and 2 valves; one intake, one exhaust. The valves are held closed by means of a spring and opened by a rotating eccentric called a camshaft. The camshaft is driven from the crankshaft by means of gears or a drive belt and timed to the up and down movement of the piston. To complete all 4 strokes the crankshaft makes 2 revolutions.

The piston now begins to move upward and starts to compress the air/fuel mixture in the cylinder. Both valves are closed. This continuing upward motion compresses the mixture to about 100-120 PSI, around 7 or 8 times atmospheric pressure (the compression "ratio"). As the piston reaches the top of the cylinder the spark plug fires and ignites the compressed mixture.

The fuel air mixture now BURNS very rapidly, increases in pressure generated by combustion force the piston downward in the cylinder. Both valves are still closed. This is the only stroke that creates power in the 4 stroke cycle.

Upon completion of the power stroke the piston starts to move upward again and now the exhaust valve starts to open. The continuing upward movement forces the hot burned gases out past the exhaust valve. When the piston reaches the top of the cylinder the exhaust valve closes. The piston starts to go back down and the cycle repeats itself.

The advantage to the 4 stroke is that the combustion process is very efficient at varying RPM ranges with almost no unburned fuel escaping into the atmosphere. 4 stroke engines also develop significant torque at low Rpm's. The big drawback is there is only one power stroke for every 2 revolutions of the crankshaft so the engine lacks the burst of power experienced with the 2 stroke engine. Four strokes are more complex as well as generally much heavier as a result of additional parts, e.g. camshaft, valve train, balance shafts, etc. required to complete the power cycle. This additional complexity does not reduce the engine's reliability. Four strokes have a proven track record in reliability and dependability.

Next, let's review the two stroke theory of operation and examine the various types of these engines and how they work.