Air-fuel ratio detection device for internal combustion engine

An air-fuel ratio detection device for an internal combustion engine including one or more cylinders and each cylinder including a fuel injection valve to directly inject a fuel into the cylinder, the air-fuel ratio detection device comprising:

• an in-cylinder pressure detector configured to detect an in-cylinder pressure;
• and
• an ECU, the ECU programmed to:
• (a) determine whether a cylinder for which an index value of an in-cylinder air-fuel ratio is to be calculated is a rich cylinder, for which the in-cylinder air-fuel ratio is richer than a stoichiometric air-fuel ratio, or a lean cylinder, for which the in-cylinder air-fuel ratio is leaner than the stoichiometric air-fuel ratio;
• (b) calculate a polytropic index in an expansion stroke from the in-cylinder pressure detected by the in-cylinder pressure detector;
• (c) correct the calculated polytropic index based on an operational condition parameter of the internal combustion engine;
• and
• (d) calculate the index value of the in-cylinder air-fuel ratio based on the corrected polytropic index in the expansion stroke, a result of the determination of whether the cylinder for which the index value of the in-cylinder air-fuel ratio is to be calculated is a rich cylinder or a lean cylinder, and relationship information that prescribes a relationship between the polytropic index in the expansion stroke and the index value of the in-cylinder air-fuel ratio under a reference operational condition;
• wherein said ECU is further programmed to control the fuel injection valve in each cylinder, based on the index value of the in-cylinder air-fuel ratio, to adjust a fuel injection amount for each cylinder so as to set an air-fuel ratio of exhaust gas from said one or more cylinders to a stoichiometric air-fuel ratio.