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Regulations are being considered or have already been enacted to limit the exhaust emissions of hydrocarbons, CO and NOx from small engines, such as those used in the lawn and garden industry. One of the most promising ways for engine manufacturers to comply with current and future emission standards is through the use of catalysts. However, these small engines provide an environment with a number of challenges for emission catalyst activity and durability which are not found with automotive exhaust, which is traditionally where catalysts of this type have been used. Problems unique to the small engine can include extremely short catalyst residence times, high hydrocarbon and carbon monoxide to oxygen ratios, overall high levels of emissions leading to high reaction exotherms, and pertubated flow due to single cylinder operation. A number of catalyst variables were tested using 4-stroke engines.

An innovative design of ceramic filter for diesel engine exhaust particulate matter is described. It is a cellular ceramic honeycomb with porous walls which act as the filter medium. Test results relating the effects of wall thickness, cell density, porosity, and filter volume on collection efficiency, pressure drop and filter capacity are described. Regeneration of the filter by ignition of the accumulated soot has been accomplished with conclusions drawn about the influence of exhaust temperature, oxygen content, and the effect of the filter design.