road emissions nationwide. The most recent California emissions standards regulate evaporative emissions on such engines for the first time and are expected to require catalyst after-treatment on more types of engines than in previous regulations. These California standards drew increased attention because of economic and safety issues related to controlling emissions from small SI engine equipment.

  • Voluntary programs. A number of incentive-based nonregulatory programs have provided for significant cost-effective emissions reductions in nonattainment areas.


The 1990 Clean Air Act (CAA) amendments focused regulatory attention on reducing emissions from nonroad vehicles and engines. In the 1991 “Nonroad Vehicle study” to Congress, EPA found that recreational marine engines contributed approximately 30% of the total hydrocarbon (HC) emissions from nonroad sources, second only to small SI lawn and garden engines (EPA 1991). Data presented in the figures in Chapter 2 confirm that percentage in 2002. Most recreational marine emissions come from SI engines, which can be divided into two groups: outboard and inboard engines. Outboard engines typically hang on the hull of a boat and are traditionally light two-stroke engines. Personal watercraft (PWC) commonly have two-stroke jet drives and have been regulated with marine outboard engines as discussed below. Inboard engines are within the hull of the boat and are mostly derivations of four-stroke automobile engines.

Separate standards apply to SI outboard and PWC engines, and this case study focuses on those sources. Early rule-makings note that EPA was most concerned about outboard and PWC engines since they use two-stroke engine technology with much higher rates of HC emissions then inboard or stern-drive engines (61 Fed. Reg. 52087 [1996]).

EPA Standards for Outboard and Personal Watercraft Engines


The U.S. Environmental Protection Agency (EPA) finalized first-time HC and nitrogen-oxide (NOx) emissions standards in 1996 for out-

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