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 [2000] 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.


Based on the discussion above, estimates for load of petroleum hydrocarbons to the ten U.S. coastal zones (see Tables 2-2 through 2-6) were calculated as follows.

Sample Calculation


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. (

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.

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