Emission Control Systems: Description and Operation

GF14.00-P-3000MMC Exhaust Treatment Function
ENGINES 157.9 in MODEL 216.3, 221.0 /1
ENGINES 278.9 in MODEL 216.3, 221.0 /1

Function requirements for exhaust treatment, general points
^ Circuit 87M ON (engine control ON)
^ Engine running

Exhaust treatment, general
The task of exhaust treatment is to reduce the exhaust emissions:

- Nitrogen oxides (NOX)
- Hydrocarbon (HC)
- Carbon monoxide (CO)

To do this, amongst other things, the firewall catalytic converter must be rapidly brought up to operating temperature in order to reduce the exhaust emissions for a cold start.

Function sequence for exhaust treatment
The following subsystems are involved in exhaust treatment:

^ Function sequence for firewall catalytic converter
^ Function sequence for transmission shift delay
^ Function sequence for monitoring the catalytic converter efficiency

Function sequence for firewall catalytic converter
The pollutant in the exhaust emitted by the engine are converted chemically by the near-engine mounted firewall catalytic converters (three-way catalytic converters) for Lambda =1.
Through oxidation, carbon monoxide is converted to carbon dioxide (CO2) and hydrocarbon to water (H2O) and carbon dioxide.
Through reduction the nitrogen oxides are converted into nitrogen (N2)+ carbon dioxide.

Additional function requirements for transmission shift delay
^ Coolant temperature at start < 35°C
^ Vehicle speed < 40 km/h

Function sequence for transmission shift delay
Transmission shift delay brings the firewall catalytic converter more quickly up to operating temperature after engine start. The ME-SFI [ME] control unit controls the transmission shift delay according to the following sensor and signal:

- Coolant temperature sensor (B11/4)
- ESP control unit (N47-5), vehicle speed over the chassis CAN (CAN E)

Transmission shift delay is active for a maximum of 60 s and is entirely electronic.

The ME-SFI [ME] control unit makes a request to the electric controller unit (VGS) (Y3/8) via the drive train CAN (CAN C) for adjustment of the shift characteristics.
Partial load gear shifts (1-2-1, 2-3-2) thus take place at higher engine speeds or at higher vehicle speeds.

Additional function requirements for monitoring the catalytic converter efficiency
^ Firewall catalytic converters at operating temperature
^ Lambda control enabled

Function sequence for monitoring the catalytic converter efficiency
Hydrocarbon (HC) emissions must not exceed the limit specified by the legal requirements.

The tasks of monitoring a firewall catalytic converter is to obtain from the oxygen storage capacity of firewall catalytic converters a statement about its aging and thus about the HC conversion.

The ME-SFI [ME] control unit reads in the following sensors to monitor the catalytic converter efficiency:

- LH and RH O2 sensors upstream of CAT (G3/3, G3/4)
- LH and RH O2 sensors downstream of CAT (G3/5, G3/6)
- Crankshaft Hall sensor (B70), engine speed

The oxygen stored during the "lean operating phase" is then reduced totally or partially during the "rich operating phase". With aging, the oxygen storage capacity of firewall catalytic converter is reduced, and so therefore is HC conversion.
Changes in the oxygen content downstream of the firewall catalytic converters are almost completely dampened by the high oxygen storage capacity of the firewall catalytic converters.
Consequently, the signals from the oxygen sensors downstream of firewall catalytic converters have low amplitude and are virtually constant.

When firewall catalytic converters are at operating temperature and the lambda control is enabled, the signal amplitudes of the oxygen sensors upstream of the firewall catalytic converters are compared with those downstream.
If the firewall catalytic converters are no longer operational, the oxygen sensor signals from the O2 sensors upstream of catalytic converter and from the O2 sensors downstream of catalytic converter are the same size.

A number of measurements take place in the lower partial-load range in the specified engine rpm range.
The results are compared with a characteristic map in the ME-SFI [ME] control unit.
If there is fault detection the ME-SFI [ME] control unit actuates the engine diagnosis indicator lamp (A1e58) in the IC (A1) via the chassis CAN and backbone CAN (CAN F).

Any faults detected are stored in the fault memory of the ME-SFI [ME] control unit. These can be read out and deleted with the Diagnosis Assistance System (DAS).