Page 58 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

40 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

and Mg, with the consequence that the oxygen storing ability of the converter falls away (see
Table 3). Decreases in the ability of the catalytic converter to convert the reducing agents in
the exhaust stream results in free oxygen passing through the catalytic converter without
reacting during lean operation of the engine. Thus, the downstream sensor reflects the decrease
of conversion activity with more rich-to-lean fluctuation of oxygen, as a result of the increase
in untreated exhaust gases passing through the catalytic converter.

Summarizing, when the catalytic converter operates properly, it dampens the fluctuations in
oxygen content, thus the downstream sensor produces a distinguishably different sensor
response pattern from that obtained from the upstream sensor. The more advanced the
deterioration of the catalytic converter the more the voltage response pattern of the
downstream sensor tends to approach the voltage response pattern of the upstream sensor.
When the catalytic converter has completely deteriorated, the oxygen conversion efficiency is
completely lost. Therefore, the oxygen content of the exhaust stream downstream of the
catalytic converter resembles much more the oxygen content upstream of the catalytic
converter and therefore the response patterns of the two sensors resemble much more to each
other.
However, the system is very dynamic and non-linear as the catalytic converter deteriorates
with use over time. The ever changing oxygen storage capability of the catalytic converter
makes a simple moment by moment comparison of voltage of frequency sensor output non-
determinative of catalytic converter adequacy or failure. Additionally, other factors influencing
the determination of the efficiency of a catalytic converter like aging of the two sensors,
operating conditions of the engine. temperature of exhaust gases etc. complicate even further
this determination. For that reason sophisticated substantially real time phase difference,
crossing ratio, integration methods etc. applied by computer algorithms are necessary to result
in a successful means of substantially real time determination of catalytic converter adequacy
or failure.

As mentioned in the previous paragraph, an important factor that influences the precision of
assessment of the efficiency of the catalytic converter is the deterioration or aging of the
oxygen sensors installed upstream or downstream of the catalytic converter. Particularly the
upstream oxygen sensor, which is directly exposed to hot exhaust gases, undergoes faster
deterioration than the downstream oxygen sensor. The rate at which the deterioration proceeds
is thus different between the upstream and downstream oxygen sensors, which results in an
error in the result of deterioration determination (see US5325664 (1994)). For this reason,
modem methods to assess the functionality of catalytic converters comprise simultaneous
assessment of degradation of oxygen sensors and compensation of the errors due to this
degradation.

Additionally, the sensors installed upstream and downstream of the catalytic converter may
have different characteristics, due to manufacturing tolerances. These tolerances must be taken
into account during the assessment of the condition of the catalytic converter.

The temperature of the catalytic converter is also an important factor that can influence the
correct determination of the deterioration of a catalytic converter by means of oxygen or