How Does an Outboard 2-Cycle Work?

Two-strokes are more powerful at low speeds.

Outboard motors refer to a propulsion system used in boats which enable steering control and propulsion. An outboard 2-cycle, or two-stroke motor, is a type of motor that utilizes more piston strokes.

Spark Plugs

Outboard two-cycle engines use spark plugs to ignite the combustion mixture, or air and fuel combination, used by the engine to generate power in the combustion stroke. When the spark plugs ignite, the piston moves down, compressing the mixture in the crankcase and opening the exhaust port. The cylinder pushes the exhaust gas from the cylinder, which then fills with a fresh supply of fuel.

Compression

The piston moves back toward the spark plug for the compression stroke, compressing the combustion mixture in the piston and generating a vacuum in the crankcase. This vacuum causes the reed valve to open, and the combustion mixture to be drawn out of the carburetor. The spark plug fires in the combustion stroke once the piston ends its compression stroke, and the two-stroke cycle is then repeated by the spark plugs.

Power Valve System

Many outboard 2-cycle motors use a power valve system. Power valve systems close the top of the exhaust port during the piston cycles, which alters port timing and results in enabling better low-speed engine power.

References

• The Yacht Market: Outboards - 2 Stroke or 4 Stroke?
• "Outboard Engines: Maintenance, Troubleshooting, and Repair"; Edwin R. Sherman; 2008

Resources

• "Understanding the Outboard Motor"; Eugene Stagner; 2002