Page 147 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Nippon Denso Co. 129

Engines with multiple cylinder groups

The method of US5279114 (1994) is applied to the case of a multiple cylinder bank engine
with an aidfuel sensor installed in each cylinder bank exhaust pipe upstream of the catalytic
converter and an aidfuel sensor installed in the common exhaust pipe section downstream of
the catalytic converter. The general layout is this of fig. 56. The method comprises the steps:
1) executing a feedback control of the aidhel ratio of each cylinder bank in accordance with
the results of detected output signals performed by the plurality of upstream airhe1 ratio
sensors and the downstream aidfuel ratio sensor
2) discriminating the deterioration state of the catalytic converter in accordance with results of
detected output signals performed by the upstream aidfuel ratio sensor corresponding to a
predetermined cylinder bank and the downstream aidfie1 ratio sensor
3) adjusting the air/hel ratio control quantity at the time of feedback-controlling the aidfuel
ratio so as to eliminate an influence of exhaust gas emitted from cylinder banks except for
said predetermined cylinder bank of a plurality of said cylinder banks. This elimination of
influence can be achieved by one of the following methods:
a) reducing air/hel correction coefficients of all cylinder banks except
for the predetermined cylinder bank at the time of feedback-
controlling the aidfuel ratio, or
b) using an air/fuel ratio correction coefficient of the predetermined
cylinder bank as the aidfie1 ratio correction coefficients for the other
cylinder banks at the time of feedback-controlling said air/fuel ratio
so as to cause the phases of the upstream aidfuel ratio sensors to be
synchronized with one another, or
c) subjecting all cylinder banks except for the predetermined cylinder
bank to a dither control in which the air/fbel ratio is changed relative
to a target aidfuel ratio

Fig. 61 illustrates changes of the wave forms of the air fuel ratio for the case of left and right
bank upstream sensors with coinciding phase (fig. 61a), with non-coinciding phase (fig. 61b)
and with a reduced aidfbel correction coefficient for the right bank sensor (fig. 61c). The left
cylinder bank is considered to be the predetermined cylinder bank mentioned above. The case
of fig. 61a is an ideal one where the output signal of the downstream sensor results in a high
amplitude oscillation, but in reality the phases of the two upstream sensors never coincide
because they are independently feedback-controlled. The case of fig. 61b results in an almost
flat signal output of the downstream sensor and no real detection of the catalytic converter can
take place. In the case of fig. 61c the amplitude of the right cylinder bank is considerably
decreased, so it does not influence the total variation of the output signal of the downstream
sensor and the deterioration of the catalytic converter can take place by comparing the output
signals of the lee cylinder bank airhiel sensor and of the downstream air/hel sensor.

The case of fig. 61 d corresponds to a dither control of the aidfie1 ratio of the upstream sensor
placed in the right cylinder bank. The airhe1 correction coefficient for this sensor swings