1. A catalyst degradation diagnosing device that is provided in an internal
combustion engine, which executes air-fuel ratio feedback control of the internal
5 combustion engine based on a detection value of an oxygen concentration sensor
configured to detect an oxygen concentration in an exhaust gas in an exhaust pipe on a
downstream side of a catalytic converter interposed between exhaust pipes coupled to an
exhaust port of the internal combustion engine, and that diagnoses a state of degradation
of a catalyst of the catalytic converter, the catalyst degradation diagnosing device
10 comprising:
a torque computation unit configured to calculate a torque value of the internal
combustion engine;
a fuel injection amount capture unit configured to capture a fuel injection amount
that is supplied for one combustion of the internal combustion engine;
15 a downstream air-fuel ratio level count unit configured to count the number of
times an output signal of the oxygen concentration sensor passes a theoretical air-fuel
ratio level based on the output signal;
an air-fuel ratio level setting unit configured to set an air-fuel ratio level in the
exhaust port in a pseudo manner based on the torque value and the fuel injection amount;
20 an upstream air-fuel ratio level count unit configured to count the number of
times the air-fuel ratio level passes the theoretical air-fuel ratio level; and
a catalyst degradation diagnosis unit configured to diagnose the state of
degradation of the catalyst based on count values of the upstream air-fuel ratio level count
unit and the downstream air-fuel ratio level count unit.
25
2. The catalyst degradation diagnosing device according to claim 1,
wherein the air-fuel ratio level setting unit is configured to set the air-fuel ratio
level to a value larger than the theoretical air-fuel ratio level in a case where the fuel
injection amount is equal to or less than a predetermined value.
30
- 26 -
3. The catalyst degradation diagnosing device according to claim 1 or 2, further
comprising:
a data sorting unit configured to sort the set air-fuel ratio level based on an
exhaust volume setting expression for sequentially setting an exhaust volume that enters
5 the exhaust pipe and an exhaust gas volume that is a variable of the exhaust volume
setting expression,
wherein the data sorting unit is configured to initialize the exhaust volume at a
point in time when the set air-fuel ratio level passes the theoretical air-fuel ratio level,
integrate the exhaust volume that enters the exhaust pipe after the initialization, and sort a
10 current value of the set air-fuel ratio level as a non-use value that is not used to determine
whether the air-fuel ratio level passes the theoretical air-fuel ratio level in a case where
the initialization is performed in a state where an integrated exhaust volume obtained by
the integration does not exceed a predetermined value.