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A new air-cooled SI two-stroke gasoline engine has been developed for an arborist-chainsaw. This engine has a displacement of 25 cm3 and generates about 1.1 kW and complies with the latest CARB and EPA exhaust regulations by optimizing scavenging flows and the use of a catalytic converter. Characteristics of lightweight, compact and high power-to-weight ratio are required for handheld chainsaws, especially for arborist chainsaws. As a matter of course, these characteristics are needed for engine itself in order to satisfy such market requirements for hand-held power equipment. To realize lightweight and compact design, the placement of catalytic converter is optimized for the engine and a two-piece crankshaft is adopted.

This paper presents some simulative and experimental results of a low emission small two-stroke cycle engine. A 23.6cc air-head stratified scavenging engine complied with CARB Tire II exhaust emission regulation was used as a base engine. A mechanism of air/fuel mixture short-circuiting cause of THC emission was investigated by numerical simulation by means of Computational Fluid Dynamics (CFD) for the base engine. To reduce the air/fuel mixture short-circuiting, port design was improved and evaluated by means of CFD. New engine performance adopted the CFD results was compared with that of the base engine. As the results of the investigation, the new engine performance at a rated speed was achieved 40% reduction of THC emission having almost same power compared with the base engine. The new 23.6cc air-head stratified scavenging two-stroke cycle engine can meet to EPA Phase2 and CARB Tire III regulations without catalyst.