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This paper reviews the emissions reduction program that was pursued during the development of the small 26.2 cc, OHV, 4-cycle engine recently introduced by the Ryobi Group of Companies. This new engine's packaging and power density is designed to compete directly with pervasive, hand held, two-stroke power plants. The 4-cycle OHV design produces substantially less HC and CO emissions relative to the existing two stroke state-of-the-art hand held power plant. The challenge of minimizing NOx emission however is complicated by the practical limitations of low cost, diaphragm carburetor fuel metering systems. The levels of NOx emitted by the production engine are controlled by a careful manipulation of combustion chamber design, compression ratio, spark ignition timing, and internal and external exhaust gas recirculation. The results of experimental evaluations used to achieve a 1999 CARB standard capable design are discussed.

This paper outlines the design and construction of a small 26.2 cc overhead valve, 4-cycle engine recently developed by the Ryobi Group of Companies for hand held power equipment applications. Four cycle engines produce fewer hydrocarbon emissions and typically generate better low end torque than the commonly used two cycle. In order to displace the high power density two-stroke technology however, a four cycle design must be cost and performance competitive. Additionally, the engine must be durable and functional at operating speeds in excess of 8,500 r.p.m. The Design for Manufacturability and Design for Assembly methodology used to create the individual powertrain components and subsystems for the new, lightweight power plant to meet the desired cost, performance, and packaging objectives is reviewed.