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Aiming for improving fuel economy of vehicles, a new two-stroke gasoline engine utilizing stratified-charge auto-ignition has been designed and tested. The engine concept was to switch the combustion mode according to the engine operating load and speed, i.e., homogeneous-charge spark-ignition for the high-load range, homogeneous-charge auto-ignition for the mid- to low-load range, stratified-charge auto-ignition for the low-load low-speed range, and stratified-charge spark-ignition for idling. Using this engine concept, an experimental two-stroke engine was successfully operated free of irregular combustion throughout the range of operating loads. For small motorcycle applications, a great improvement in fuel economy and exhaust emissions was estimated from the experimental data.

The auto-ignition attracts researchers as an ultimate combustion method that could simultaneously reduce fuel consumption and NOx emissions. The authors have studied auto-ignition combustion in the two-stroke gasoline engines aiming at vehicle engine applications. However, our attempts were in an impasse with the onset of irregular combustion in the lower speed and the extremely lower load range. As a solution for this problem, this paper proposes a new auto-ignition concept, i.e. Stratified Charge Auto-Ignition (SCAI), which focuses on the thermal distribution and mixture formation in the combustion chamber. Visualization of the direct injection spray formation was conducted first, and then a combustion chamber design was determined by using a CFD simulation, so as to form a mixture at the hottest spot in the combustion chamber.