Page 77 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Chapter 1.2

Daimler-Benz AG

In DE3443649 (1986) workshop test method is proposed. At constant operating conditions,
a
a controlled slight oscillation of the engine air/fuel ratio around the stoichiometric one with a
certain frequency is imposed. First the upstream probe is used as the control probe and the
corresponding frequency indicati0n.h is determined. Then the downstream probe is used as a
control probe for the same operating conditions and the corresponding frequency of the
control .f2 is determined (two-point closed loop control). The quotient of these two controlled
frequencies is formed

.A
AZ=-
.fi
and compared with a given required value range of quotients. If the quotient falls out of this
range then the catalytic converter is considered as exhausted. This range is predefined once for
the actual type of vehicle or catalytic converter. It obviates the need for road testing.

In US4622809 (1986) at predefined operating conditions the engine airhe1 ratio forcibly
oscillates with a certain frequency. The corresponding output voltages of the upstream
(control) and downstream (test) probes are shown in fig. 27a.
B is the maximum permissible amplitude of the downstream probe signal. If this limit is

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