Engine control system and control method thereof

摘要

An engine that re-circulates its exhaust gas suffers decreased accuracy in estimating an EGR rate real-time especially while the operating state of the engine 19 is in a transitional state, which often results in torque fluctuations and deteriorated exhaust gas. A sensor 12 for directly detecting an EGR flow rate is disposed in an EGR path 9. An EGR rate and in-cylinder oxygen concentration are calculated from the output value of that sensor 12. In addition, when this EGR rate calculation method is used, the calculation is properly switched between a steady operation state characterized by a low load and small rotational fluctuations and a transitional operating state including the acceleration and deceleration. This makes it possible to correctly estimate the EGR rate and the in-cylinder oxygen concentration under a wide range of engine operation conditions, and thereby to avoid the fluctuation of torque and the deterioration of exhaust gas.