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Recreational Vehicles

Marine Engines

Marine engines serve a wide variety of applications.  The smallest marine engines, virtually all of which use gasoline, are used in recreational outboards and personal watercraft.  Small gasoline or diesel marine engines provide auxiliary power on many vessels.  Larger marine engines provide propulsion for both recreational and commercial applications.  Recreational sterndrive and inboard engines tend to be gasoline, though diesel engines are making inroads into that market.

Commercial engines, virtually all diesel, power vessels such as tugs, ferries, and crew/supply boats.  These engines also provide auxiliary power on larger vessels.   The largest marine diesel engines, sometimes exceeding 60,000 hp, propel ocean-going vessels.  EPA intends to have emission control requirements for all marine engines.  Engines are grouped under three control programs reflecting their application and, to some extent, the fuel they use.

Gasoline Outboards and Personal Watercraft marine Engines

Gasoline outboards and personal watercraft contribute about 5 percent of the national mobile source VOC inventory.  However, in areas with large boat populations, the contribution of these recreational marine engines may exceed 10 percent of the regional HC inventory.  These engines typically employ 2-stroke technology, which changed very little over the last 50 years.  Regulations to control exhaust emissions from new outboards and personal watercraft went into effect in July 1996.  The emission controls for these engines involve increasingly stringent standards over the course of a nine-year phase-in period beginning in model year 1998.  By the end of the phase-in, each manufacturer must meet an emission standard, on a corporate-average basis, that represents a 75 percent reduction in HC compared to unregulated levels.  The gradually decreasing emission standard allows manufacturers to determine the best approach to achieving the targeted reductions over time, manufacturers are able to phase in the types of control technologies in the most sensible way, while minimizing the cost impact to the consumer.

Commercial Diesel Marine Engines

Commercial diesel marine engines contribute about 8 percent of the national mobile source NOx inventory, and about 1 percent of the national mobile source PM inventory.   In areas with large commercial ports or near busy shipping lanes, the contribution of diesel marine engines to the local NOx and PM inventory may be much higher.

EPA proposed regulations for the control of exhaust emissions from new marine diesel engines in November 1998.  The proposed emissions limits, which vary depending on the size of the engine, are similar to emission limits for corresponding land-based nonroad or locomotive engines.  These limits would apply beginning with engines manufactured in 2004.  In addition, a more stringent set of emission limits based on the nonroad Tier 3 approach will be evaluated in 2003.  At that time, EPA will confirm the Tier 3 limits or adjust them to reflect the technologies manufacturers can apply at that time.   The more stringent Tier 3 emission limits would apply beginning with engines manufactured in 2008.

The proposed emission limits for very large commercial marine diesel engines are the same as those contained in Annex VI of the International Convention on the Prevention of Pollution from Ships (MARPOL).  Consistent with MARPOL Annex VI, these proposed limits will apply to engines installed on ships constructed on or after January 1, 2000.

Recreational Sterndrive and Inboard Engines

Recreational sterndrive and inboard engines can be either gasoline or diesel engines. While their contribution to national mobile VOC and NOx levels is smaller that the other two marine engine categories, their emissions are expected to increase due to the growing number of recreational vessels.  EPA did not finalize emission limits for gasoline sterndrive and inboard engines as part of the 1996 marine rule.  Likewise, EPA did not propose limits for recreational diesel engines in the commercial diesel engine rule.   Consequently, these recreational engines remain unregulated at this time.  EPA has started in a separate rulemaking to consider emission limits for these engines however.

For more information on Engine Certification, see the Office of Transportation and Air Quality's Engine Certification Information Center.


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