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The introduction of a large amount of EGR gas into cylinders is effective for reducing exhaust emissions in diesel engines. As combustion characteristics under the condition of higher EGR rate are highly influenced by the amount of intake charge gas and its composition. It requires a precise control method for preventing the increase of NOx and particulate matter (PM) in spite of the transient conditions of supercharging and tolerance of turbocharger. In this study, a new control method was developed based on the observed oxygen concentration of charge gas and excess air ratio (stands for the oxygen concentration of burnt gas) which are the main physical factors governing NOx and PM formation. In a conventional method, the intake airflow rate is controlled to meet a target value determined in advance in the stationary engine operating condition.

A new method that optimizes the control map of hydrocarbon addition to diesel exhaust gas for hydrocarbon selective catalyst reduction (HC-SCR) has been developed. This method is comprised of a numerical HC-SCR model and a new optimization technique using Evolutionary Programming based on the evolution of living things. As a result of this evaluation, the number of calculations to obtain the optimized control map with this method was one third that using the conventional method. By using the obtained optimized control map, the NOx conversion was found to be greater by 18% than that with the constant addition of hydrocarbon.