Monitoring Catalyst Efficiency

Monitoring the Efficiency of the Catalytic Converter







In this process the oxygen storage capacity of the TWC (three-way catalytic converter) Is determined from the signal characteristic of the oxygen sensor after the TWC. If the oxygen storage capacity of the TWC is greatly reduced due to aging, voltage amplitudes also occur at the oxygen sensors after the TWC.
The appearance of amplitudes after the TWC is dependent on the momentary load of the TWC's storage capacity. This load can vary with the engine load and rpm, which is why a load and rpm-dependent (r1/n) evaluation is performed.







The signal amplitude of the sensor after the TWC indicates its oxygen storage capacity

Diagnosis of Secondary Air Injection System
Through the secondary air system, additional oxygen is introduced behind the exhaust valves in order to reduce harmful emissions. This occurs after the engine is started, within a defined engine temperature range and for a predetermined time.

While the secondary air injection is active, the diagnosis of the system is performed. The additional air behind the exhaust valves leads to a surplus of oxygen in the oxygen sensors and therefore to a voltage reduction in oxygen sensors ahead of the TWC.


NOTE: The running time of the pump depends on the air flow rate

Oxygen Sensor Voltage Without Secondary Air:

Oxygen Sensor Voltage With Secondary Air:

Diagnosis is interrupted when the vehicle leaves a given load/rpm range. No further secondary air diagnosis takes place during this driving cycle.

Secondary air diagnosis is performed in conjunction with oxygen sensing. To this aim, during secondary air injection the oxygen sensing system is operated by the input of a lean nominal oxygen exhaust gas value at which thinning of secondary air is not adjusted. If a rich mixture is measured nonetheless, the system regulates this to = 1. The value required for this purpose is then converted to the actual secondary air mass.

In order to compensate the effect of a transient mixture adaptation on secondary air mass measurement, a so-called offset measurement of the secondary air mass is performed directly after secondary air injection when = 1 without secondary air.