Page 85 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Ford Motor Co. - Ford France SA - Ford Werke AG - Ford Motor Co. Ltd. 67

For linear decrease in time of the A/F modulation frequency, the oxygen storage can be
determined by measuring the time that it takes for the downstream sensor to start switching
after the test cycle has begun.

The downstream sensor might be an EGO or a hydrocarbon sensor

In US5099647 (1992) an UEGO sensor and a HEGO sensor are installed respectively
upstream and downstream of the catalytic converter. The outputs of the two sensors are
coupled to a complementary filter set characterized by a crossover frequency.

The output of the upstream sensor is applied to a high pass filter, while the output of the
downstream sensor is applied to a low pass filter. A summer receives inputs from each of the
high and low pass filter sections and provides an output to a feedback controller which in turn
controls a fuel metering system applying fuel to the engine. The method comprises three
embodiments:

1) A low crossover frequency is established. The catalytic converter is then considered as
exhausted when the measured amplitude of the downstream sensor is less than a
predetermined amplitude, or
2) the crossover frequency can be increased until the catalyst monitoring signal reaches a
predetermined amplitude. The crossover frequency that achieves this predetermined
amplitude is then determined. The catalytic converter is considered as aged when this
crossover frequency is greater than a predetermined one, or
3) the crossover frequency increases until the engine control means reaches airlfuel ratio limit
cycle operation. The crossover frequency that achieves this limit cycle operation is then
determined. The catalytic converter is considered as exhausted when this frequency of
airhel limit operation is greater than a stored frequency.

A lrmif cycle is defined as the cycle of variation in aidfuel ratio control signal from a rich limit
to a lean limit and back to the rich limit again.

In US5157919 (1992) the control system operates in a limit cycle mode, and frequencies of
the limit cycles are changed by changing parameters of a system controller. Parameters of the
catalytic converter are defined based on such limit cycle frequencies. These parameters are
matched with experimentally developed functions to estimate catalytic converter efficiency.
The method comprises the following steps:
1) initiation of a closed loop air/fUel ratio control system using the downstream EGO probe
2) measuring the frequency of the limit cycle
3) changing one or more times, operating or structural parameters of a system controller to
generate different limit cycle frequencies
4) measuring the frequency of each limit cycle
5) solving a system of equations which relate the limit cycle frequencies to the catalytic
converter parameters and