A catalyst deterioration diagnostic device for a flexible fuel engine which is configured to combust flexible fuel and includes:
- a flexible fuel supplier which is configured to supply the flexible fuel having different mixture ratios, in which gasoline and alcohol are mixed at different ratios;
- a catalyst which is configured to purify exhaust gas generated by combustion of the supplied flexible fuel;
- a front sensor which is configured to detect a state of the exhaust gas before passing through the catalyst;
- and
- a rear sensor which is configured to detect the state of the exhaust gas after passing through the catalyst,
- the catalyst deterioration diagnostic device comprising:
- a processor which is configured to perform a diagnosis process of diagnosing a state of deterioration of the catalyst based on at least a signal of the rear sensor,
- the front sensor being an upstream oxygen sensor which is configured to output a signal which is reversed between a rich state and a lean state indicating that oxygen concentration is higher than the oxygen concentration in the rich state, in accordance with increase and decrease of the oxygen concentration in the exhaust gas before passing through the catalyst,
- the rear sensor being a downstream oxygen sensor which is configured to output a signal which is reversed between a first state and a second state in which the oxygen concentration is higher than the oxygen concentration in the first state, when the oxygen concentration in the exhaust gas after passing through the catalyst increases and decreases in such a way that a state in which the oxygen concentration is lower than a predetermined range and a state in which the oxygen concentration is higher than the predetermined range are repeatedly alternated, and
- the catalyst deterioration diagnostic device being configured to
- (A) control the flexible fuel supplier by setting a cycle and amplitude of increase and decrease of a flexible fuel amount in a diagnosis period, either
- (I) without being based on the signal of the downstream oxygen sensor but based on at least the signal of the upstream oxygen sensor and a mixture ratio of the flexible fuel, or
- (II) based on at least the signal of the upstream oxygen sensor and the signal of the downstream oxygen sensor such that,
- in a period that is a combination of multiple diagnosis periods,
- (a) the signal of the downstream oxygen sensor when the catalyst is deteriorated is reversed multiple times irrespective of
- (a1) a difference in the mixture ratio of the flexible fuel, and
- (a2) a difference in magnitude of at least one of a cycle or an amplitude of the signal of the downstream oxygen sensor due to the difference in the mixture ratio of the flexible fuel, and
- (b) the signal of the downstream oxygen sensor when the catalyst is normal (b1) is maintained in the first state without being reversed to the second state or (b2) is reversed at least once with at least one of a cycle that is large or an amplitude that is small as compared to the signal of the downstream oxygen sensor when the catalyst is deteriorated, irrespective of the mixture ratio of the flexible fuel,
- the diagnosis period having a length equal to or longer than one cycle of the signal of the upstream oxygen sensor during execution of the diagnosis process and having a length such that the length of one driving cycle is longer than an integer multiple of the length of the diagnosis period,
- (B) generate a single set of diagnostic data based on the signal of the downstream oxygen sensor in the diagnosis period,
- (B1) even when at least one of the cycle or amplitude of the signal of the downstream oxygen sensor in the diagnosis period is different due to the difference in the mixture ratio of the flexible fuel,
- (B2) even when the number of reversals of the signal of the downstream oxygen sensor in the diagnosis period is smaller than the number of reversals of the signal of the upstream oxygen sensor in the diagnosis period, when the catalyst is deteriorated, and
- (B3) even when the signal of the downstream oxygen sensor is reversed in the diagnosis period when the catalyst is normal, and
- (C) diagnose the state of deterioration of the catalyst based on at least one set of the diagnostic data generated based on the signal of the downstream oxygen sensor in a total diagnosis period which includes at least one diagnosis period, which has a length equal to or longer than any two cycles of the signal of the upstream oxygen sensor, and to which features of the respective signals of the downstream oxygen sensor when the catalyst is normal and when the catalyst is deteriorated are reflected,
- (C1) even when at least one of the cycle or amplitude of the signal of the downstream oxygen sensor in the diagnosis period is different due to the difference in the mixture ratio of the flexible fuel,
- (C2) even when the number of reversals of the signal of the downstream oxygen sensor in the diagnosis period is smaller than the number of reversals of the signal of the upstream oxygen sensor in the diagnosis period, when the catalyst is deteriorated, and
- (C3) even when the signal of the downstream oxygen sensor is reversed in the diagnosis period when the catalyst is normal.