Inputs into the Sea from Recreational Marine Vessels
The 1985 NRC report Oil in the Sea did not discuss petroleum hydrocarbon inputs from operation of two-stroke engines used in outboard motors and personal watercraft (PWC) (also know as jet skis). In 1990, heightened awareness about the large number and the design inefficiencies of these engines led the US EPA to begin regulating the “non-road engine” population under the authority of the Clean Air Act. Engines that fell under this category include lawn mowers, grass trimmers, chain saws, etc., as well as outboard engines for boats. In the 1990 EPA regulations, there was only preliminary data on hydrocarbon inputs into surface water from two-stroke engines. Since 1990, studies have provided better quantification of the inputs of hydrocarbons and gas additives such as MBTE into the air and water from two-and four-stroke engines (Juttner et al., 1995; Barton and Fearn, 1997; M. S. Dale et al., 2000; Gabele and Pyle, 2000). For this report, oil and gasoline inputs to the sea are calculated for two-stroke engines ranging in size from 16-175 horsepower (20-230 kW) that are fueled by a mixture of oil and gasoline. Four-stroke engines discharge approximately 10 times less fuel that two-stroke engines, and were not included in the calculation because the population of four-stroke outboard engines is not known. Discharge rates of fuel for diesel outboard engine and from inboard engines are not well characterized and were also not included in the calculation.
DESIGN FEATURES OF TWO- AND FOUR-STROKE ENGINES
Both two- and four-stroke engines create mechanical energy (movement of a crankshaft) from the combustion of fuel in a confined space (cylinder). The names reveal the number of piston strokes required to complete one power or combustion cycle.
There are two strokes in the combustion cycle common to both engine types: a compression stroke and a power stroke. In two-stroke engines, the combustion cycle is completed in two-stroke pistons and a single revolution of the crankshaft. Power is generated with each revolution of the crankshaft. The following description starts with the piston at the bottom of the cylinder:
As the piston travels upwards to the top of the cylinder, it compresses the gases inside the cylinder as well as closes off the transfer and exhaust parts. This is considered the compression stroke. At the same time, the motion of the piston causes a vacuum in the crankcase and air is drawn into the engine for the next cycle.
Ignition occurs when the piston is near the top of its travel, causing the fuel and air to expand and force the piston downward. This is the power stroke. As the piston travels downward, the exhaust travels the exhaust port is opened and the hot expanding gasses leave the cylinder. At the same time, the fresh charge in the crankcase is pressurized. As the piston moves farther down, the transfer port is opened and the fresh charge enters the cylinder. Any remaining exhaust gasses are pushed out of the exhaust port.
There is no pump or oil circulation system in a 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 gasses 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, and 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.
Data Sources and Assumptions
Data on boating activity including number of two-stroke engines, average hours of use for each boating type (gasoline outboard and personal watercraft) and average horsepower was collected from the EPA Nonroad Emission Model (Jansen and Sklar, 1998). Several recent reports have measured discharge rates for fuel such as BTEX or fuel additives (e.g. MBTE) into surface water by recreational boating (Juttner et al., 1995; Barton and Fearn, 1997; Dale et al., 2000, Gabele and Pyle, 2000). In this calculation, BTEX was used as a surrogate for gasoline with aqueous discharge rates ranging from 0.20 to 0.70 g kW−1 hr−1 (Gabele and Pyle 2000). It is well established that comparable size four-stroke engines and direct injection two-stroke engines discharge approximately 5-10 times less fuel than standard two-stroke engines (Gabele and Pyle  and earlier references). To our knowledge there is no population data on the four-stroke engine population and the existing two-stroke population data does not differentiate between standard and DI engine types. Therefore we assume that all the two-stroke populations are standard models requiring fuel and gas mixtures (Tables 2-2 through 2-6).
The average hours of use nationwide for two-stroke PWC engines is 77.3 hours per year and for outboard engines is 34.8 hours per year and calculated from a model (US EPA (in preparation)). These values are lower than past values for average boating-use of 91 and approximately 150 hours/yr (US EPA 1991). (The former of these 1991 estimates was provided by the National Marine Manufacturers Association and based on boater surveys; the latter is from an earlier EPA model). The average hours of use in this study does not distinguish between seasonal differences between regions where boating use may vary considerably. For example, states in northern latitudes generally have a shorter boating season and it is limited to the summer season.
The EPA population model also does not distinguish between engines used in coastal waters and those used in inland lakes and rivers that may or may not connect to the coast. For these calculations, we assumed that between 20-80 percent (average 50 percent) of the petroleum hydrocarbon discharge from two-stroke engines was to fresh water such as lakes and rivers that either did not connect to the coastal water or was included in Section F on petroleum hydrocarbon inputs from nonpoint sources.
Engine is the two- stroke standard engine population is from the boater registrations for each coastal county from the EPA population model.
Horsepower for two-stroke personal watercraft followed the EPA population model and divided into 4 categories (≤ 18.5, 35.4, 44.4, 75.1, 111) and 10 categories for outboard engine population (≤ 2.4, 5.2, 8.7, 15, 21.6, 35.7, 48.5, 78.3, 139, 228).
Hours of engine use per year is 77.3 hour year−1 for PWC and 34.8 hours year−1 for outboard engines for the entire United States (US EPA in preparation)
Discharge rate for BTEX is 0.21 g kW−1 hour−1 (Benzene), 0.70 (Toluene), 0.2 (Ethylbenzene), 0.55 (Xylene) (Gambel and Pyle 2000).
Conversion factor I 0.75 kW/ horsepower−1.
Density of hydrocarbon (gasoline) is 739.966 g L−1
Conversion factor II 3.79 l/gallon−1
BTEX is 37.4% of gasoline (Saeed and Al-Mutairi 1999)
Assumption: the amount of fuel that enters the marine environment is estimated at 50% (range 20-80 %)
Oil mixture is 2 % of the fuel mixture in two-stroke engines.
Final fuel inputs were reduced by 45% to account for the decrease in fuel emission with increased engine size. (www.epa.gov/otaq/certdata.htm).
Overall, oil and gas inputs from two-stroke outboard motors are estimated to be between 0.6 to 2.5 million gallons per year (average 1.6 million gallons) or between 2,100 and 8,500 tonnes (average 5,300 tonnes) per year for coastal waters of the United States.