EGR ESTIMATION METHOD FOR INTERNAL COMBUSTION ENGINE AND EGR ESTIMATION DEVICE FOR INTERNAL COMBUSTION ENGINE

An EGR estimation method for an internal combustion engine estimates an EGR rate in an intake and exhaust system , of an internal combustion engine, the intake and exhaust system , of an internal combustion engine including an intake system , an exhaust system , and an EGR device , the EGR device includes an EGR passage and an EGR valve . The EGR estimation method includes determining a gas replacement state by exhaust gas and fresh air in an upstream EGR passage, which is a portion of the EGR passage between the EGR valve and the exhaust passage, when fuel cut of the internal combustion engine is started and the EGR valve is fully closed; and estimating the EGR rate based on a result of the determination.