| 摘要 |
The air-fuel ratio of a lean-burn engine is controlled so as to be adjusted to a lean-burn air-fuel ratio with a low NOx concentration. As this is done, an actual combustion fluctuating rate is detected from the cylinder pressure, and the air-fuel ratio is adjusted to the rich side with respect to the lean-side limit, whereby a satisfactory driving performance can be secured. The state of NOx exhaust is determined by an actual NOx concentration, and the air-fuel ratio is adjusted to the lean side with respect to the allowable limit of the NOx exhaust. On the other hand, the combustion fluctuating rate and an allowable limit value of the combustion changing rate under the driving conditions concerned are compared, while the NOx exhaust rate and an allowable limit value of the NOx exhaust rate under the same driving conditions are compared. The EGR amount is decreased when the combustion fluctuating rate is higher than the allowable limit value, and is increased when the NOx exhaust rate is higher than the allowable limit value of the NOx exhaust rate. Thus, the NOx exhaust can be decreased.
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