Clean Marine Engines

Types of Marine Engines (Gasoline)

Traditional Engine

Carbureted 2-Stroke Marine Engines
Traditional carbureted 2-stroke marine engines are currently the most commonly used engines on motorboats and personal watercraft (PWCs). In these traditional engines, the intake and exhaust ports are both open during the piston's down-stroke, which allows some fuel to pass straight through without being burned. Because of this design, these engines lose approximately 20 to 30 percent of their fuel as it passes through the combustion chamber unburned or partially burned, thereby being released directly into the water and air as pollution.

Low-pollution Engines

4-Stroke Marine Engines
Four-stroke marine engines are the cleanest outboard engines and are currently available in horsepower ratings up to 225 hp. In the 4-stroke engine, the intake and exhaust valves are never open at the same time, preventing unburned fuel from escaping the combustion chamber and entering into the environment. As the most fuel efficient marine engines available, 4-stroke engines run on straight gasoline and do not require consumers to mix oil with the gas.

Direct Fuel Injection 2-Stroke Marine Engines
Two-stroke marine engines with direct fuel injection (DFI) spray fuel into the cylinder after the piston covers the exhaust port on the up-stroke. The fuel is injected into the top of the cylinder next to the spark plug. While this technology is not as efficient as a 4-stroke marine engine, it is much cleaner than the carbureted 2-stroke marine engine and also reduces oil use by 50 percent over the traditional engines. DFI engines are currently available in horsepower ratings from 90 hp to 225 hp.

*Reference: March 2000 brochure, "New, Economical and Environmentally Friendly Outboard Motors", Vermont Department of Environmental Conservation.