Exhaust treatment function

Function requirements, general

 "Circuit 61 ON"

Exhaust treatment, general

The task of the exhaust treatment is observance of the limits for exhaust emissions set by the lawmaker in which the following pollutants in the exhaust are reduced:

– nitrogen oxides (NOx)

- hydrocarbons

– carbon monoxide (CO)

– soot particle

Pollutant reduction is supported by the following subfunctions:

- diesel particulate filter (DPF) preheating

– exhaust gas recirculation (EGR)

The CDI control unit (N3/21) reads in the following components:

– oxygen sensor (B85/2)

- catalytic converter temperature sensor (B19)

- temperature sensor upstream of diesel particulate filter (B19/12)

– diesel particulate filter differential pressure sensor (B28/8)

The exhaust treatment function is divided into the following function sequences:

 Function sequence for oxidation catalytic converter

 Function sequence for DPF

Function sequence for oxidation catalytic converter

The oxicat ensures lowering of the hydrocarbon, CO and NOx and generates the required thermal energy for the DPF regeneration phase by means of afterburning.

Function sequence for DPF

The diesel particulate filter consists of a ceramic honeycomb filter body made out of silicon carbide, which is coated with rare metal platinum. The passages of the diesel particulate filter are opened alternately at the front and rear and are separated from each other through the porous filter walls of the honeycomb filter body.

The precleaned exhaust which has passed though the oxidation catalytic converter flows into the ducts of the DPF which are open to the front and passes through the porous filter walls of the honeycomb filter body into the ducts which are open to the rear. After this, the cleaned and filtered exhaust is dissipated through the exhaust system. The particulate matter (particles) (PM) are held back in the honeycomb filter body of the diesel particulate filter.

If the soot particle content exceeds a map-based value, the CDI control unit will start the regeneration phase provided the prerequisites for regeneration are given. The CDI control unit receives the soot particle content in the DPF via the DPF differential pressure sensor.

Regeneration takes place by means of a periodical increase of the exhaust temperature. For this purpose, the following functions are initiated by the CDI control unit:

- additional post injection

- DPF glow function

- the soot particles retained in the DPF are mostly burnt off to produce carbon dioxide (CO2) by increasing the exhaust temperature. The ash produced remains in the DPF. During regeneration, the exhaust temperature is monitored by the CAT temperature sensor and the temperature sensor upstream of the diesel particulate filter. The diesel particulate filter differerential pressure sensor determines the exhaust pressure lines upstream and downstream of the DPF between the exhaust gas pressure upstream and downstream of the DPF. The soot particle content in the DPF is determined using a characteristics map on the basis of the pressure differential and the exhaust mass calculated by the CDI control unit. If the diesel particulate filter requires maintenance, the EOBD indicator lamp (P15e17) in the instrument cluster (P15) will signal this.

 On short-distance trips, regeneration is interrupted and distributed over several driving cycles. This means that more heating-up phases up to the required regeneration temperature will occur. Regeneration is imperceptible to the customer.