Four-Stroke Engines
Four-stroke engine designs have proven practical for most outboard applications and several outboard manufacturers now offer a range of four-stroke engines in their product line-up. These engines are currently available in power ranging from 2-to 130-horsepower. Unlike conventional two-stroke engine designs, four-stroke technology eliminates exhaust-gas scavenging by the crankcase air/fuel/oil mixture, which significantly reduces HC emissions. Recent technological advancements, including fuel injection and innovations in engineering design, have enabled this technology to be expanded to include a wider range of power options.
Four-stroke technology has been proven in automotive applications, as well as off-road engine applications, such as stem drive and inboard marine engines and outdoor power equipment. Until recently, many of the marine industry's engine experts felt that four-stroke technology would be limited to applications under 100-horsepower due to the weight increases from the added components common to basic four-stroke engine designs. As engine technologies and designs have advanced, however, four-stroke technology has recently broken that perceived 100-horsepower barrier, which was no small feat for marine engine manufacturers.
Most four-stroke engines currently available on the market meet or exceed the EPA 2006 standards. Virtually every marine engine manufacturer offers a range of four-stroke power options for nearly any marine application. These manufacturers include Honda Marine (13 models), Mercury Marine (13 Mercury models), OMC (16 Evinrude models), Suzuki Marine (6 models) and Yamaha Marine (10 models).
Catalysts
A third technology has also been considered feasible for two-stroke outboard and personal watercraft applications catalysts. While catalyst technology is simple and has proven effective for the automotive industry, its adaptability for marine applications may offer manufacturers many challenges. Many marine engines use water to help cool the engine and quiet the exhaust to comply with U.S. Coast Guard regulations for engine surface temperature reduction and for noise reduction purposes. In many cases, the cooling water is saltwater, which becomes very corrosive when heated and may reduce the longevity of the catalyst. If catalysts are to be used, one of the primary challenges is to design the system such that the saltwater will not come into contact with the catalyst, which also may create packaging constraints for some confined engine designs and applications.
A second challenge is that marine engines often operate at higher temperatures for extended periods. This type of operation can potentially lead to a significant loss of conversion efficiency of the catalyst over time. Engines equipped with catalysts and closed-loop, electronic-fuel-injection systems, like automotive engines, often can achieve more than 90 percent HC conversion efficiency. These engines, however, do not operate at higher temperatures for extended period, which keeps the catalyst from reaching the high temperatures that can result in deactivation of the catalyst.
