Page 91 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

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This value is then compared to a pre-determined, stored value of the catalytic converter
efficiency in order to determine if the converter is within an acceptable efficiency range.

In US5289678 (1994) a dual closed loop airhe1 ratio control system is used with two EGO
sensors installed upstream and downstream of the catalytic converter. The number of times the
downstream EGO sensor switches from rich to lean and vice versa is compared to the
corresponding number of times the upstream EGO sensor switches during normal system
operation over the course of a test period.

The ratio of the switching frequencies is calculated and compared to a pre-determined, stored
value in order to draw conclusions about the efficiency of the catalytic converter. Typically, for
a well hnctioning catalytic converter, the upstream sensor may switch ten times for each
downstream sensor switch. As the catalytic converter degrades the ratio moves closer to one-
to-one. A normalization of the frequency ratio calculation allows the reduction of dependence
of engine speed and load on the downstream sensor.

In US5319921 (1994) the test of the catalytic converter takes place at steady operating
conditions. During test mode, a test signal is produced, which has a total variation of zero
mean value. The test signal is a periodic function having a frequency higher than the natural
frequency of the limit-cycle of the downstream EGO sensor. The control of the fuel in the
engine takes place by means of a control signal produced from this test signal and the signal of
the downstream EGO sensor (e.g. by summing the two signals). After a certain number of
cycles the controlling system locks up the test signal i.e. the output signal from downstream
EGO sensor and the controller of the system oscillate with the same frequency as the test
signal. Then a value of the downstream output signal is compared to a stored value
corresponding to a minimum acceptable efficiency of the catalytic converter. This output can
be the integral, over a test interval, of an absolute value of the downstream output signal
deviation from stoichiometry (h=l), divided by the integral over the test interval of the fuel
flow control signal deviation from stoichiometry. A failure signal is produced when the
measured efficiency is lower than an acceptable value.

In US5351484 (1994), the efficiency of a light-off catalytic converter installed upstream of a
main catalytic converter is monitored. Two EGO sensors are installed upstream and
downstream of the light-off catalytic converter. The monitoring is activated when the mass of
the air flow into the engine is such that the space velocity is sufficiently low and the catalytic
converter monitoring is not saturated. The ratio of the frequencies of the downstream EGO
switching to the upstream EGO switching is used then as criterion of degradation of the
catalytic converter.

In US5353592 (1994) an attempt is made to provide a catalytic converter monitoring method
while concurrently maintaining airhe1 control. Two EGO sensors upstream and downstream