Durability of Low-Emissions Small Off-Road Engines

The goal of the project was to reduce tailpipe-out hydrocarbon (HC) plus oxides of nitrogen (NO_x) emissions to 50 percent or less of the current California Air Resources Board (CARB) useful life standard of 12 g/hp-hr for Class I engines, or 9 g/hp-hr for Class II engines. Low-emission engines were developed using three-way catalytic converters, passive secondary-air induction (SAI) systems, and in two cases, enleanment. Catalysts were integrated into the engine's mufflers, where feasible, to maintain a compact package. Due to the thermal sensitivity of these engines, carburetor calibrations were left unchanged in four of the six engines, at the stock rich settings. To enable HC oxidation under such rich conditions, a simple passive supplemental air injection system was developed. This system was then tuned to achieve the desired HC+NO_x reduction. Engines were then aged for either 250 hours (Class 1), or 500 hours (Class 2), with emission testing at intervals to track muffler-out emissions and catalyst performance. Results demonstrated that emissions from these engines could be significantly reduced.