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 two-stroke engine completes its power cycle in only one crankshaft revolution with two strokes of the piston. There are no valves, camshafts, springs chains, etc. so the engine is much less complex and lighter. Instead of valves tThere are a series of strategically located transfer ports - intake and exhaust, cut into the sides of the cylinder wall. The ports are on opposite sides of the cylinder. The transfer ports are opened and closed by the up and down movement of the piston. To accomplish a complete power cycle both sides of the piston are used; consequently several events occur simultaneously during each stroke. They are:

On the down"power"stroke the piston is forced towards the crankcase reducing its volume and creating a positive pressure. As it continues downward travel it starts first to uncover the exhaust ports. Exhaust gas begins to rush out of the cylinder. Then the intake ports are uncovered. The fresh air/fuel charge in the crankcase is forced into the cylinder and continues to push the remaining exhaust gases out.

The 2 stroke process of purging exhaust gases from the cylinder and filling it with a fresh air/fuel charge is called scavenging. Two stroke engines use 2 different scavenging methods, cross-scavenging and loop scavenging. Both differing designs have particular advantages.

Two stroke cross-scavenged engines can be identified by the irregular shape of the top of the piston called a deflector. This deflector directs the incoming air/fuel up, towards the top of the cylinder. This creates a wall or column of fresh mix that sweeps across the cylinder towards the exhaust ports. As the column advances it pushes the spent exhaust gases out of the exhaust ports.

Pistons in loop scavenged engines are generally near flat. They do not rely on deflectors to aim the fuel/air mix, rather they have shaped intake ports and combustion chambers to control the scavenging of the cylinder. Several intake ports are aimed upwards and arranged such that their combined streams flow upward and then LOOP down toward the exhaust ports.

Cross-scavenged engines are better performing at idle and low speed. All older motors of any horsepower are of this design. Until the late 60's it was not economical to try to produce this design in quantity at a reasonable cost.

Loop charged engines, although having poorer idling characteristics are more fuel efficient and perform better at higher RPM's than crossflow as they have lighter pistons. This lowers the strain on the connecting rods, bearings and crankshaft. OMC created the first US production looper in 1968 with the 3 cylinder 55 HP.

Now that you have been thru the basics of the current marine and outboard engine technology, what is all the furor in current outboards? What outboard engine choice should you make? What brand should you purchase? is an "I-O" a better choice? How does the EPA figure in this equation?