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Both NOx and unburned HC were reduced by changing the direction of the flame propagation. It is generally said that the optimum ignition position of spark ignition engine is in the center of combustion chamber. However by igniting arround the chamber and propagating the flame toward the center, a smooth heat release pattern due to the decrease in the flame area and a decrease in the unburned gas entering the ring crevise can be anticipated. These effects of this combustion process, which was named the surrounding combustion process (SCP), were experimntally confirmed using the constant volume combustion vessels and the spark ignition engine equipped with six spark plugs per cylinder. Next, the steps for decreasing the number of ignitions TCre considered, and additional three spark plugs for SCP were installed in the four valve pentroof combustion chamber. With this engine, the NOx reduction and the capability of SCP to further improve the lean burn engine fuel economy were confirmed.

The causes of the cyclic combustion variation in a lean operating SI engine have been identified using multivariate analysis on the pressure-time data. Principal component analysis on the combustion characteristics obtained from the pressure-time data was conducted in order to select an index of an optimal released heat pattern for analyzing the causes of the cyclic combustion variation. Using this index and the released heat quantity, the IMEP variation was subjected to multiple regression analysis to identify the causes of the cyclic combustion variation. Optimizing the fuel injection timing and swirl ratio made it possible to enrich the mixture near the spark plug. With the lean limit thus extended, a SI engine was operated in a lean range, and the resultant pressure-time data were analyzed. It was found that the main cause of the IMEP variation in the lean operating SI engine was the released heat quantity variation.