Exhaust gas cleaning, function

S07.10-0041-79

S49.20-A104-79

Function requirements for exhaust treatment, general points

 Circuit 87M ON (Engine management ON)

 Engine running.

Exhaust treatment, general

The task of exhaust treatment is to reduce the exhaust emissions:

 Nitrogen oxides (NOx)

 Hydrocarbons

 Carbon monoxide (CO)

 Soot particles

Pollutant reduction is supported by the following subfunctions:

 Diesel particulate filter (DPF)

 Exhaust gas recirculation (EGR)

 Injection of the AdBlue® reduction agent

Vehicles without code MT2 (emissions class Euro 6d-TEMP N1 GR.III/N2)

The CDI control unit (N3/33) reads in the following signals:

 Temperature sensor upstream of catalytic converter (B19/7)

 Temperature sensor upstream of diesel particulate filter (B19/17)

 Diesel particulate filter differential pressure sensor (B28/12)

 Oxygen sensor upstream of catalytic converter (G3/2)

 Control unit of NOx sensor unit upstream of SCR catalytic converter (A97/1), NOx sensor 1 upstream of SCR catalytic converter (B29)

 Control unit of NOx sensor downstream of SCR catalytic converter (A97/2), NOx sensor 2 downstream of SCR catalytic converter (B29/1)

 AdBlue® (SCR) control unit (N141)

Vehicles with code MT2 (emissions class Euro 6d-TEMP N1 GR.III/N2)

The CDI control unit (N3/33) reads in the following signals:

 Temperature sensor upstream of SCR catalytic converter (B16/12)

 Temperature sensor upstream of oxidation catalytic converter (B16/19)

 Temperature sensor downstream of SCR catalytic converter (B16/27)

 Temperature sensor downstream of oxidation catalytic converter (B19/8)

 Control unit of NOx sensor unit upstream of SCR catalytic converter (A97/1), NOx sensor 1 upstream of SCR catalytic converter (B29)

 Control unit of NOx sensor downstream of SCR catalytic converter (A97/2), NOx sensor 2 downstream of SCR catalytic converter (B29/1)

 AdBlue® (SCR) control unit (N141)

Function sequence for exhaust treatment

The following subsystems are involved in exhaust treatment:

 Function sequence for diesel particulate filter (DPF)

 Function sequence for SCR catalytic converter

Function sequence for diesel particulate filter (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 soot particles are retained in the honeycomb filter body of the DPF.

If the soot content exceeds a characteristics map-based value, the CDI control unit (N3/33) will start the regeneration phase provided the prerequisites for regeneration are given. The CDI control unit (N3/33) receives information on the soot content in the DPF via the diesel particulate filter differential pressure sensor (B28/12).

Regeneration takes place by means of a periodical increase of the exhaust temperature. To do this the following functions are initiated by the CDI control unit (N3/33):

 One additional post injection via the fuel injectors (Y76)

 DPF glow function via the glow output stage (N14/8) to the glow plugs (R9)

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.

The diesel particulate filter differential pressure sensor determines the pressure differential between the exhaust gas pressure upstream and downstream of the DPF via the exhaust gas pressure lines upstream of the DPF and downstream of the DPF. The soot particle content in the DPF is determined using a characteristics map on the basis of the pressure difference and the exhaust mass calculated by the CDI control unit (N3/33). Necessary maintenance of the DPF is signaled by the engine diagnosis indicator lamp (A1/1e58) in the instrument cluster (IC) control unit (A1/1).

 On short-distance trips, regeneration is interrupted and distributed over several driving cycles. This means that preheating phases up to the required regeneration temperature will occur more frequently. Regeneration occurs unnoticeably by the driver.

The conversion rate of the NOx percentage in the exhaust is temperature dependent and is up to 80 %. The reduction in the soot present is about 99 %.

The CDI control unit (N3/33) determines the load condition of the DPF via the diesel particulate filter differential pressure sensor (B28/12). If the soot content exceeds a characteristics map-based value, the CDI control unit (N3/33) will start the regeneration phase provided the prerequisites for regeneration are given.

Regeneration is performed by periodically raising the exhaust temperature with another post injection. Raising the exhaust temperature causes most of the soot particles stored in the DPF to be burnt off to CO2. The noncombustible ash remains in the DPF.

Function sequence for SCR catalytic converter

The AdBlue® reduction agent is injected upstream of the SCR catalytic converter and is converted to ammonia (NH3) through thermal decomposition (heat-induced chemical reaction ) and hydrolysis (water-induced chemical reaction).

There is a mixing element between the AdBlue® injection valve (Y130) and the SCR catalytic converter. This improves the hydrolysis of the AdBlue® reduction agent and ensures more uniform distribution of the AdBlue® upstream of the SCR catalytic converter.

In the SCR catalytic converter, the NOx contained in the exhaust is converted in the SCR catalytic converter together with the NH3 into molecular nitrogen (N2) and H2O.

The CDI control unit (N3/33) calculates the quantity of reduction agent required based on a characteristics map and sends it via the drive train sensor CAN (CAN I) to the AdBlue® control unit (SCR) (N141). This then initiates characteristic map-dependent injection of the calculated quantity of AdBlue® reduction agent through the AdBlue® injection valve (Y130).

NOx sensor 1 upstream of SCR catalytic converter and NOx sensor 2 downstream of SCR catalytic converter receive the ''Heat'' request from the CDI control unit (N3/33). The NOx sensors thus quickly reach their operating temperature.

If the "Reserve" fill level in the AdBlue® tank is reached, the driver is informed by means of an acoustic signal and at the same time over the instrument cluster control unit that he/she should find a workshop and have the required maintenance work carried out.

If the "Empty" fill level is reached in the AdBlue® tank, the plausibility of the "Empty" fill level condition is checked using a computer model. If the plausibility test also indicates an "Empty" fill level, an acoustic signal is issued and at the same time an entry is made in the fault memory of the CDI control unit and the engine diagnosis indicator lamp (A1/1e58) is actuated in the instrument cluster (IC) control unit. The driver then has up to 20 engine starts available, with an assumed trip distance of 32 km in each case.

The number of standing starts is displayed in the IC control unit (IC). The vehicle can no longer be started after the last remaining "Start".