Air-fuel ratio detection device for internal combustion engine

1. 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.

2. The air-fuel ratio detection device according to claim 1 ,

wherein the operational condition parameter used for the correction of the polytropic index in the expansion stroke is at least one of an engine speed, a temperature of engine cooling water, an ignition timing, an EGR rate and an engine load factor.

3. The air-fuel ratio detection device according to claim 2 ,

wherein the ECU is programmed to correct the polytropic index in the expansion stroke so as to be smaller as the engine speed is higher, as the temperature of the engine cooling water is higher, as the ignition timing is retarded, as the EGR rate is higher, or as the engine load factor is higher.

4. The air-fuel ratio detection device according to claim 1 ,

wherein the ECU is programmed to calculate an amount of heat release or a burning velocity from the in-cylinder pressure detected by the in-cylinder pressure-detector and determines that the cylinder for which the index value of the in-cylinder air-fuel ratio is to be calculated is a rich cylinder when the calculated amount of heat release or burning velocity is greater than a predetermined value.

5. The air-fuel ratio detection device according to claim 1 ,

wherein the ECU is programmed to calculate the polytropic index in the expansion stroke from in-cylinder pressures and in-cylinder volumes at two or more points in the expansion stroke at or after a combustion end point and before an opening timing of an exhaust valve.

6. The air-fuel ratio detection device according to claim 1 ,

wherein the internal combustion engine includes a plurality of cylinders,

wherein the in-cylinder pressure detector is configured to detect an in-cylinder pressure of each of the plurality of cylinders, and

wherein the ECU is programmed to determine that the air-fuel ratio is imbalance between cylinders when a difference in air-fuel ratio between the cylinders based on the calculated index value of the in-cylinder air-fuel ratio is greater than a predetermined criterion value.

7. The air-fuel ratio detection device according to claim 1 ,

wherein the reference operational condition is an operational condition where the operational condition parameter equals an arbitrary reference value.

8. The air-fuel ratio detection device according to claim 1 ,

wherein the relationship information is a curve that prescribes a relationship between the polytropic index in the expansion stroke and the index value of the in-cylinder air-fuel ratio in an X-Y plane with an X axis indicating the index value of the in-cylinder air-fuel ratio and a Y axis indicating the polytropic index in the expansion stroke.