Page 104 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

86 Methods for Monitoring and Diagnosing the Eficiency of Catalytic Converters

Interruptions occur at the CPU when the A/D converter completes an A/D conversion and
generates an interrupt signal; when the second crank angle sensor generates a pulse signal; and
when the clock generator generates a special clock signal.
The intake air amount data from the airflow meter and the coolant temperature data from the
coolant sensor are fetched by A/D conversion routine(s) executed at predetermined intervals,
and then stored in the RAM ; i.e., the intake air amount data and the coolant temperature data
in RAM are renewed at predetermined intervals. The engine speed is calculated by an intempt
routine executed at 30 DEG CA, Le.. at every pulse signal of the crank angle sensor, and is
stored in the RAM.

In JP2091440 (1990), at certain conditions of air flow inducted to the engine and of engine
load, the aidfuel ratio of the engine oscillates from lean to rich and vice versa. The aidfirel ratio
is adjusted in accordance with the outputs of the upstream and downstream oxygen sensors.
When the number of oscillations of the downstream sensor exceeds a certain value, the
catalytic converter is considered as degraded.

In JP2030915 (1990), JP2033408 (1990) and JP2207159 (1990) the air/firel ratio of the
engine changes from a lean to a rich state. The time that is needed for the downstream sensor
to also change from lean to rich state is measured (lime lug). When this time is less than a
predetermined value then the catalytic converter is considered as degraded.

In JP3057862 (1991) the ratio of the reversals of the output signals of the two oxygen sensors
during feedback-control is formed and compared to a predetermined threshold. If the threshold
is exceeded then the catalytic converter is considered as deteriorated.

In US5088281 (1992) the main criterion used for the evaluation of the efficiency of the
catalytic converter is also the lime lug (phase shift) between the time point of a signal switch
indicated by the upstream oxygen sensor and the corresponding time point that the
downstream oxygen sensor senses this signal switch. The signal switch can be a forced change
of the engine aidfuel ratio from stoichiometry to a rich state or from stoichiometry to a lean
state or a combination of the two. The catalytic converter is considered as degraded when the
measured time lag is lower than a predetermined value.
One embodiment of the method is shown in fig. 41. The engine aidhel ratio starts at time
point T2 switching forcibly from lean to rich and vice versa according to fig. 41a. Figs. 41b,c
and d show the signal of the downstream sensor for the cases of a small, a medium and a large
deterioration of the catalytic converter respectively. The time (T1- T2) is long enough to expel
all oxygen from the catalytic converter (rich state). The time (T5 - T3) corresponds to a lean
state and at Ts oxygen is filly stored in the catalytic converter. T4 (T4’, Ti’) is the time point at
which the downstream sensor detects a switch from a rich side to a lean side. T6 (T6’. Tg).) is
the time point at which the downstream sensor detects a switch from a lean side to a rich side.