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Oil in the Sea III: Inputs, Fates, and Effects
ogy described in Appendix I, is 140,000 tonnes per year. The minimum estimate is 6,800 tonnes per year and the maximum estimate per year is 5,000,000 tonnes.
Recreational Marine Vessels
The 1985 NRC report Oil in the Sea did not discuss petroleum hydrocarbon inputs from operation of two- and four-stroke engines used in outboard motors and personal watercraft (PWC) (also know as jet skis) and small recreational vessels (<100 GT vessels). In both cases, there has been a significant increase in the use of recreational vessels, especially in marinas located in the coastal zone. In 1990, heightened awareness about the large number and the design inefficiencies of two-stroke engines led the U.S. EPA to begin regulating the “non-road engine” population under the authority of the Clean Air Act. Engines that fall under this category include lawn mowers, grass trimmers, chain saws, as well as outboard engines for boats. EPA has issued regulations to decrease the exhaust emissions from small marine engines by 75% by 2005 through new design features (see below). In the 1990 EPA regulations, there were only preliminary data on hydrocarbon inputs into surface water from two-stroke engines. Since then, many more studies have better quantified the inputs of hydrocarbons and gas additives such as MBTE into the air and water from two- and four-stroke engines (Jüttner et al. 1995; Barton and Fearn, 1997; M.S. Dale et al., 2000; Gabele and Pyle, 2000).
Design Features of Two- and Four-Stroke Engines
There is no pump or oil circulation system in a standard two-stroke engine, so oil is added to the gasoline to lubricate the moving parts in the engine. There is no extra valve mechanism to operate, as the piston acts as the valve, opening and closing the necessary ports. These features make these engines powerful and lightweight and therefore very popular as outboard engines on small boats.
Fuel and fuel additives that are not combusted, can enter the surface water directly with the exhaust gases through the exhaust port. Depending on how the fuel is introduced to the combustion chamber, two-strokes may emit unburned fuel and fuel additives. Before 1998, conventional two-stroke engines used either carburetors or injectors to mix fuel with air as it entered the crankcase. Since 1998, marine outboard manufacturers have been producing new, direct injected (DI) two-strokes, although the technology is still in its infancy. While there are various techniques used in DI, they all inject the fuel directly to the cylinder after or nearly after the exhaust ports close. Direct injected two-stroke engines generally have 80 percent less hydrocarbon emissions than their predecessors. In DI two-strokes, oil is introduced directly to the crankcase to lubricate the moving parts and not mixed with the fuel. As the name implies, four-stroke engines use four piston strokes for each combustion cycle including an intake stroke where fuel and air enter the combustion chamber a compression stroke combustion or power stroke and
PHOTO 11 Recreational vessels, especially those with older, two-stroke engines contribute about 6 percent of the total load of petroleum entering North American waters each year. Tri-level “boat racks” along Falmouth Harbor, in Falmouth, Massachusetts, emphasize how enjoyment of recreational marine boating has expanded, leading to a shortage of slips and vessel storage facilities. (Photo courtesy of Paul Dery.)