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This paper focused experimental and theoretical studies on the effects of air-head stratified scavenging and leaner combustion on reducing hydrocarbon emission and improving thermal efficiency of two-stroke cycle spark-ignition engine. The result showed that the amount of short-circuiting mixture with a conventional Schnurle scavenging two-stroke cycle engine was reduced from 30% to 9% with the air-head stratified scavenging two-stroke cycle engine which was developed in this study. Thermal efficiency was improved from 13.6% to 20.6%. As the result, the engine with air-head stratified scavenging and leaner combustion in this study can conform to the California Air Resources Board (CARB) 1999 or tier 2 emission standards regarding THC, CO and NOx emission.

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 experimental and theoretical results of a low emission two-stroke cycle spark ignition engine for chainsaws, applied a stratified scavenging method to reduce Total Hydrocarbon (THC) emission caused by air/fuel mixture short-circuiting during the scavenging process. Stratified scavenging methods are applied reed valves to control air sucked into transfer ports for stratified scavenging during sucking process. The new chainsaw engine, however, is applied piston valves instead of the reed valves to keep the same size and to be simple structure as conventional Schnurle scavenging engine chainsaws. The piston has grooves built with both sides. The grooves connect an air inlet to transfer ports to suck air into the transfer ports during the sucking process. The new chainsaw, having the same power as conventional chainsaws, is reduced THC emission to about 1/3 of conventional one and can meet to EPA Phase II regulation on and after 2005 without catalyst.

The object of this study is to evaluate the effects of air-head stratified scavenging with lean combustion using a 34cc two-stroke spark-ignition engine on reducing THC and CO emission and improving thermal efficiency. The result showed that the amount of short-circuiting mixture with a conventional Schnurle scavenging two-stroke engine was reduced from 30% to 9% with the air-head stratified scavenging two-stroke engine. Thermal efficiency was improved from 13.6% to 20.6%. As the result, the engine with air-head stratified scavenging in this study can conform to CARB 1999 or tier 2 emission standards regarding THC, CO and NOx emission.