Page 129 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Toyota Motor Co. Ltd 111

2) estimating a balance between the oxygen absorbing power and the oxygen releasing power
of the catalytic converter. This estimation comprises the following sub-steps:
a) calculating a difference between a target amount of fuel required to
maintain the aidfuel ratio of the engine at the stoichiometric aidfuel ratio
and an actual amount of fuel calculated at the air/hel ratio feedback
control step 1
b) detecting the change of the downstream air/fuel sensor. This change
detecting step has a hysteresis characteristic between a threshold used for
a change from a rich state to a lean state and a threshold used for a
change from said lean state to said rich state.
c) integrating a fuel difference calculated at step 2a to estimate the oxygen
balance from the time when a first change of the output of the
downstream aidfuel sensor is detected at step 2b to the time when a
second change is detected at step 2b
3) detecting deterioration of the catalytic converter at least in accordance with the output of
the downstream aidfie1 sensor. The deterioration of the catalytic converter is detected in
accordance with:
a) the length of the response curve of the output of the upstream aidfuel
sensor and the length of the response curve of the downstream aidfuel
sensor, or
b) the frequency of deviations of the fuel difference integral integrated at
step 2c from a fixed upper limit or a fixed lower limit
4) inhibiting the detection of the deterioration degree of the catalytic converter (step 3) when
the balance estimated at step 2 deviates from at least one fixed threshold
5) restraining from inhibiting the detection until a fixed interval is elapsed after the change of
the output of the downstream aidfbel sensor is detected at step 2b though the fuel
difference integral integrated at step 2c deviates from a fixed upper limit or a fixed lower
limit

The problem of inhibiting misjudgment is explained in fig. 55. Fig. 55a denotes the output of
the downstream sensor and fig. 55b denotes the balance between the oxygen absorbing power
and the oxygen releasing power.

The downstream sensor detects the rich state before time t2, and its output changes fiom the
rich state to the lean state between time t2 and t3, It detects the lean state between time t3 and
ts, and its output changes from the lean state to rich state between time t5 and t6. It detects the
rich state between time tb and tn, and its output changes from the rich state to the lean state
between time tr and t9.

Therefore, the converter absorbs oxygen before time t3, releases it between time t? and t6,
absorbs it between t6 and t9, and releases it after time tg. The amount of oxygen stored in the
converter increases gradually before time t?, the amount of oxygen stored in the converter
decreases gradually between time t3 and t6, and the oxygen absorbing power increases
gradually after time ts