Page 59 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Catalytic Converter Functionality Diagnosis by Means of Oxygen or Air/Fuel Ratio Sensors 41

air/fbel sensors. The oxygen storage capacity of the catalytic converters in general varies with a
change in the temperature, which causes a variation in the indication of the sensor placed
downstream of the catalytic converter.

During the deterioration diagnosis procedure, the engine air/f%el ratio is controlled in such a
way as to achieve certain operating conditions. Great attention should be paid in order not to
increase the emissions of the internal combustion engine during this deterioration diagnosis
procedure.

A typical engine layout for monitoring the efficiency of a catalytic converter is shown in fig. 12
(see e.g. DE4139560 (1993)). Fig. 12 shows a catalytic converter having an oxygen probe 1
arranged upstream of the converter and an oxygen probe 2 arranged downstream of the
converter, a value-determining unit, an evaluation unit and a characteristic field.
The value-determining unit includes a computation device (CPU) and an operating-state
detector and can work as a control unit to regulate the air/fuel mixture supplied to the engine.

The computation unit receives an upstream-probe signal 11 from the upstream probe 1 and a
downstream-probe signal 12 from the downstream probe 2. From these signals, the
computation unit evaluates the performance loss of the catalytic converter. The computation
unit performs this evaluation during e.g. a steady-state operation of the engine to which the
catalytic converter is connected. The operating state is announced by a detector when the
engine is operated in the steady state. The value determined by the computation unit is
compared to a characteristic-field value in the evaluation unit with the characteristic-field value
being supplied from the characteristic field. If the comparison shows that the catalytic
converter has a performance loss which is impermissibly high, then the evaluation unit emits a
fault signal to the driver warning means e.g. a Malfhction Indication Lamp (MIL,).

Values of variables are supplied to the operating-state detector and the characteristic field and
indicate the operating state of the internal combustion engine. Such variables are: engine speed,
vehicle speed, water coolant temperature, load, temperature of exhaust gases etc.(fig. 12). The
control unit processes all the input and regulates the aidfuel mixture supplied to the engine.

The control unit may comprise calculation means, integrators, comparators, storage means etc.
Signal filtering means, A/D converters and other necessary data equipment devices may be
included in the system.

Most of the methods presented in this book concern on-board diagnostic methods although a
few methods applied by skilled technicians in workshops are also presented.