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A new lean combustion process for internal combustion engines has been developed. This newly devised combustion system, designated as “Active Thermo-Atmosphere Combustion” (ATAC), differs from conventional gasoline and diesel engine combustion processes. ATAC can be applied most easily to two-stroke cycle gasoline engines. Stable combustion can be achieved with lean mixtures at part-throttle operation. With ATAC the fuel consumption and exhaust emissions of two-stroke cycle spark-ignition engines are remarkably improved, and noise and vibration are reduced.

A new scavenging method for two-stroke cycle engines - Multi-Layer Stratified Scavenging (MULS) - has been developed. The MULS method is achieved by separating the mixture generated by the carburetor into a rich mixture and a lean mixture between the inlet manifold and the scavenging ports, and by finely controlling the scavenging flows. With the MULS method the thermal efficiency and HC emissions of two-stroke cycle gasoline engines are considerably improved without sacrificing the brake specific power output and mechanical simplicity.