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A failure detection apparatus for an internal combustion engine includes failure detecting means (S22-S30) for detecting abnormality of fresh air quantity detecting means (air flow sensor) based on the result of comparison between a fresh air quantity detected by the fresh air quantity detecting mea

A failure detection apparatus for an internal combustion engine includes failure detecting means (S22-S30) for detecting abnormality of fresh air quantity detecting means (air flow sensor) based on the result of comparison between a fresh air quantity detected by the fresh air quantity detecting means (air flow sensor) and a fresh air quantity reference value set by fresh air quantity reference value setting means (S20), and a supercharger equipped with variable nozzles/vanes. The fresh air quantity reference value setting means sets the reference value (S16) in accordance with not only the operating state (engine speed Ne, fuel injection quantity Qf, etc.) of the engine but also a target vane opening set for the variable nozzles/vanes of the supercharger by target vane opening setting means (S12, S14).

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What is claimed is: 1. A failure detection method implemented by a failure detection apparatus for an internal combustion engine, the failure detection apparatus including fresh air quantity detecting means for detecting a quantity of fresh air introduced into a combustion chamber of the engine, an

What is claimed is: 1. A failure detection method implemented by a failure detection apparatus for an internal combustion engine, the failure detection apparatus including fresh air quantity detecting means for detecting a quantity of fresh air introduced into a combustion chamber of the engine, and a supercharger equipped with variable nozzles/vanes having a turbine and a compressor, the turbine being operated by an exhaust stream whose flow rate has been adjusted by the variable nozzles/vanes, to drive the compressor and thereby supercharge the engine with intake air, said failure detection method comprising: a target vane opening setting step of setting a target vane opening for the variable nozzles/vanes in accordance with an operating state of the engine such that an air-fuel ratio or excess air ratio of an exhaust system of the engine becomes equal to a predetermined value corresponding to the operating state; a vane opening control step of controlling the variable nozzles/vanes in accordance with the target vane opening set in said target vane opening setting step; a fresh air quantity reference value setting step of setting a reference value for the fresh air quantity in accordance with the operating state of the engine and the target vane opening set for the variable nozzles/vanes in said target vane opening setting step; and a failure detecting step of detecting abnormality of the fresh air quantity detecting means, based on a result of comparison between the fresh air quantity detected by the fresh air quantity detecting means and the reference value set in said fresh air quantity reference value setting step. 2. The failure detection method according to claim 1, wherein said failure detection apparatus further includes exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, and said fresh air quantity reference value setting step comprises correcting the target vane opening for the variable nozzles/vanes based on a difference between the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, and the predetermined value, and setting the reference value based on the corrected target vane opening. 3. The failure detection method according to claim 1, wherein said failure detection apparatus further includes exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, and said vane opening control step comprises correcting the opening of the variable nozzles/vanes such that the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, coincides with the predetermined value. 4. The failure detection method according to claim 1, wherein said failure detection apparatus further includes exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, and if the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, is different from the predetermined value, said fresh air quantity reference value setting step suspends the control of the variable nozzles/vanes in said vane opening control step and sets the reference value based solely on the operating state of the engine. 5. A failure detection method implemented by a failure detection apparatus for an internal combustion engine, the failure detection apparatus including fresh air quantity detecting means for detecting a quantity of fresh air introduced into a combustion chamber of the engine, a supercharger equipped variable nozzles/vanes having a turbine and a compressor, the turbine being operated by an exhaust stream whose flow rate has been adjusted by the variable nozzles/vanes, to drive the compressor and thereby supercharge the engine with intake air, an EGR passage for allowing part of exhaust gas to be recirculated from an exhaust system of the engine to an intake system of same as EGR gas, and an EGR valve inserted in the EGR passage for controlling a quantity of the EGR gas by varying an opening thereof, said failure detection method comprising: a target vane opening setting step of setting a target vane opening for the variable nozzles/vanes in accordance with an operating state of the engine such that an air-fuel ratio or excess air ratio of the exhaust system becomes equal to a predetermined value corresponding to the operating state; a vane opening control step of controlling the variable nozzles/vanes in accordance with the target vane opening set in said target vane opening setting step; a target EGR valve opening setting step of setting a target EGR valve opening for the EGR valve in accordance with the operating state of the engine such that the air-fuel ratio or excess air ratio of the exhaust system becomes equal to the predetermined value; an EGR valve control step of controlling the EGR valve in accordance with-the target EGR valve opening set in said target EGR valve opening setting step; a fresh air quantity reference value setting step of setting a reference value for the fresh air quantity in accordance with the operating state of the engine, the target vane opening set for the variable nozzles/vanes in said target vane opening setting step, and the target EGR valve opening set in said target EGR valve opening setting step; and a failure detecting step of detecting abnormality of the fresh air quantity detecting means, based on a result of comparison between the fresh air quantity detected by the fresh air quantity detecting means and the reference value set in said fresh air quantity reference value setting step. 6. The failure detection method according to claim 5, wherein said failure detection apparatus further includes exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, and said fresh air quantity reference value setting step comprises correcting at least one of the target vane opening and the target EGR valve opening based on a difference between the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, and the predetermined value, and setting the reference value based on said at least one of the corrected target vane opening and the corrected target EGR valve opening. 7. The failure detection method according to claim 5, wherein said failure detection apparatus further includes exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, said vane opening control step comprises correcting the opening of the variable nozzles/vanes such that the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, coincides with the predetermined value, and said EGR valve control step comprises correcting the opening of the EGR valve such that the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, coincides with the predetermined value. 8. The failure detection method according to claim 5, wherein said failure detection apparatus further includes exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, and if the air-fuel ratio or excess air ratio of the exhaust system, detected by the exhaust concentration detecting means, is different from the predetermined value, said fresh air quantity reference value setting step suspends the control of the variable nozzles/vanes in said vane opening control step as well as the control of the EGR valve in said EGR valve control step and sets the reference value based solely on the operating state of the engine. 9. A failure detection apparatus for an internal combustion engine, comprising: fresh air quantity detecting means arranged in an intake system of the engine, for detecting a quantity of fresh air introduced into a combustion chamber of the engine; fresh air quantity reference value setting means for setting a reference value for the fresh air quantity in accordance with an operating state of the engine; failure detecting means for detecting abnormality of said fresh air quantity detecting means, based on a result of comparison between the fresh air quantity detected by said fresh air quantity detecting means and the reference value set by said fresh air quantity reference value setting means; a supercharger equipped with variable nozzles/vanes arranged across the intake and exhaust systems of the engine and having a turbine and a compressor, the turbine being operated by an exhaust stream whose flow rate has been adjusted by the variable nozzles/vanes, to drive the compressor and thereby supercharge the engine with intake air; target vane opening setting means for setting a target vane opening for the variable nozzles/vanes in accordance with the operating state of the engine such that an air-fuel ratio or excess air ratio of the exhaust system becomes equal to a predetermined value corresponding to the operating state; and vane opening control means for controlling the variable nozzles/vanes in accordance with the target vane opening set by said target vane opening setting means, wherein said fresh air quantity reference value setting means sets the reference value in accordance with not only the operating state of the engine but also the target vane opening set for the variable nozzles/vanes by said target vane opening setting means. 10. The failure detection apparatus according to claim 9, further comprising exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, wherein said fresh air quantity reference value setting means corrects the target vane opening for the variable nozzles/vanes, based on a difference between the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, and the predetermined value, and sets the reference value based on the corrected target vane opening. 11. The failure detection apparatus according to claim 9, further comprising exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, wherein said vane opening control means corrects the opening of the variable nozzles/vanes such that the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, coincides with the predetermined value. 12. The failure detection apparatus according to claim 9, further comprising exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, wherein, when the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, is different from the predetermined value, said fresh air quantity reference value setting means suspends the control of the variable nozzles/vanes by said vane opening control means and sets the reference value based solely on the operating state of the engine. 13. A failure detection apparatus for an internal combustion engine, comprising: fresh air quantity detecting means arranged in an intake system of the engine, for detecting a quantity of fresh air introduced into a combustion chamber of the engine; fresh air quantity reference value setting means for setting a reference value for the fresh air quantity in accordance with an operating state of the engine; failure detecting means for detecting abnormality of said fresh air quantity detecting means, based on a result of comparison between the fresh air quantity detected by said fresh air quantity detecting means and the reference value set by said fresh air quantity reference value setting means; a supercharger equipped with variable nozzles/vanes arranged across the intake and exhaust systems of the engine and having a turbine and a compressor, the turbine being operated by an exhaust stream whose flow rate has been adjusted by the variable nozzles/vanes, to drive the compressor and thereby supercharge the engine with intake air; target vane opening setting means for setting a target vane opening for the variable nozzles/vanes in accordance with the operating state of the engine such that an air-fuel ratio or excess air ratio of the exhaust system becomes equal to a predetermined value corresponding to the operating state; vane opening control means for controlling the variable nozzles/vanes in accordance with the target vane opening set by said target vane opening setting means; an EGR passage for allowing part of exhaust gas to be recirculated from the exhaust system of the engine to the intake system as EGR gas; an EGR valve inserted in said EGR passage, for adjusting a quantity of the EGR gas by varying an opening thereof; target EGR valve opening setting means for setting a target EGR valve opening for said EGR valve in accordance with the operating state of the engine such that the air-fuel ratio or excess air ratio of the exhaust system becomes equal to the predetermined value; and EGR valve control means for controlling said EGR valve in accordance with the target EGR valve opening set by said target EGR valve opening setting means, wherein said fresh air quantity reference value setting means sets the reference value in accordance with not only the operating state of the engine but also the target vane opening set for the variable nozzles/vanes by said target vane opening setting means and the target EGR valve opening set by said target EGR valve opening setting means. 14. The failure detection apparatus according to claim 13, further comprising exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, wherein said fresh air quantity reference value setting means corrects at least one of the target vane opening and the target EGR valve opening based on a difference between the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, and the predetermined value, and sets the reference value based on said at least one of the corrected target vane opening and the corrected target EGR valve opening. 15. The failure detection apparatus according to claim 13, further comprising exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, wherein said vane opening control means corrects the opening of the variable nozzles/vanes such that the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, coincides with the predetermined value, and said EGR valve control means corrects the opening of said EGR valve such that the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, coincides with the predetermined value. 16. The failure detection apparatus according to claim 13, further comprising exhaust concentration detecting means for detecting an exhaust concentration to thereby detect the air-fuel ratio or excess air ratio of the exhaust system, wherein, when the air-fuel ratio or excess air ratio of the exhaust system, detected by said exhaust concentration detecting means, is different from the predetermined value, said fresh air quantity reference value setting means suspends the control of the variable nozzles/vanes by said vane opening control means as well as the control of said EGR valve by said EGR valve control means and sets the reference value based solely on the operating state of the engine.