Emission Control Systems: Description and Operation

GF14.00-P-3000MG Exhaust Treatment Function
ENGINES 271.8 in MODEL 204.0 /2 /3, 207.3 /4, 212.0 /2

Function requirements for exhaust treatment, general points
^ Circuit 87 M ON (engine timing 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 during cold starts.

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

^ Function sequence for firewall catalytic converter
^ Function sequence for air injection up to 12/2009 (except code (498) Japan version)
^ Function sequence for transmission shift delay (vehicles with automatic transmission)
^ Function sequence for monitoring the catalytic converter efficiency

Function sequence for firewall catalytic converter
The pollutants in the exhaust emitted by the engine are converted chemically by the near-engine mounted firewall catalytic converter (three-way catalytic converter) 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 air injection
^ Coolant temperature > 10°C and less than 60°C
^ Engine speed < 3000 rpm
^ Engine at idle or partial load operation

Function sequence for air injection up to 12/2009 (except code (498) Japan version)
After engine start, air injection brings the firewall catalytic converter more quickly up to operating temperature and thus improves the exhaust emission values during warm-up.
The ME-SFI [ME] control unit (N3/10) manages the air injection according to the information from the following sensors:
- Coolant temperature sensor (B11/4)
- Pressure sensor downstream of throttle valve (B28/7), engine load
- Crankshaft Hall sensor (B70), engine speed

The injected air is provided by the electric air pump (M33), which is actuated by the ME-SFI [ME] control unit through the secondary air injection relay (N10/1kP).

If the air pump switchover valve (Y32) is actuated by the ME-SFI [ME] control unit at the ground end it switches through the vacuum from the intake manifold to the secondary air injection air shutoff valve. This opens and the injected air is blown from the electric air pump into the exhaust ports of the cylinder head.

The injected air reacts with the hot gases in the exhaust ports and in the firewall catalytic converter. Oxidation of carbon monoxide (CO) and hydrocarbons (HC) takes place (afterburning).
The afterburning causes an increase in the exhaust temperature (exothermic reaction), which also heats up the firewall catalytic converter.

To provide the air injection, the air pump switchover valve and the electric air pump are actuated simultaneously by the ME-SFI [ME] control unit for up to 150 s following engine start.

After actuation of air injection, it is prevented from reoccurring until the coolant temperature has risen to > 60°C and then drops again to below < 40°C.
The electrical air pump also has sufficient time to cool down again.

Diagnosis
For diagnosis with Xentry Diagnostics, air injection must be enabled for up to 120 s (less than this if the engine is warm).
Prior to the next actuation, it is essential to maintain a cooling time of 30 minutes, otherwise the electric air pump may be damaged (overheated).

* Additional function requirements for transmission shift delay (vehicles with automatic transmission)
^ Coolant temperature at start <50°C
^ Vehicle speed < 53 km/h

Function sequence for transmission shift delay (vehicles with automatic transmission)
The transmission shift delay warms up the firewall catalytic converter more rapidly to its 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
- Electronic Stability Program control unit (N30/4) or Electronic
Stability Program Premium control unit (N30/7) (vehicles with DISTRONIC PLUS), wheel speed via the chassis CAN (CAN E)

Transmission shift delay is active for a maximum of 160 s and is entirely electronic.

The ME-SFI [ME] control unit makes the request via the drive train CAN (CAN C) to the electronic transmission control unit (N15/3) (for code (423) 5-speed automatic transmission (NAG)) or the fully integrated transmission control controller unit (Y3/8) (for code (427) 7-speed automatic transmission) move 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 converter 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 purpose of monitoring the catalytic converter effectiveness is to use the oxygen storage capacity of the firewall catalytic converter to determine the degree of aging, and therefore the degree of HC conversion.

The ME-SFI [ME] control unit reads in the following sensors to monitor the catalytic converter efficiency:
- Crankshaft Hall sensor, engine speed
- Oxygen sensor downstream of catalytic converter (G3/1)
- Oxygen sensor upstream of catalytic converter (G3/2)

The oxygen stored during the "lean operating phase" is then reduced totally or partially during the "rich operating phase". Aging reduces the oxygen storage capacity and HC conversion capacity of the firewall catalytic converter.
Due to the firewall catalytic converter's high oxygen storage capacity, the change in oxygen content is nearly fully attenuated downstream of the firewall catalytic converter.
Consequently, the oxygen sensor signal downstream of the firewall catalytic converter has a low amplitude and is virtually constant.

When the firewall catalytic converter is at operating temperature and lambda control is enabled, the amplitudes of the oxygen sensor signals downstream and upstream of the firewall catalytic converter are compared.
If the firewall catalytic converter is no longer operable, the oxygen sensor signal of the oxygen sensor upstream of CAT and the oxygen sensor signal of the oxygen sensor downstream of CAT are the same.
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 a fault is detected, the ME-SFI [ME] control unit actuates the engine diagnosis indicator lamp (A1e58) on the instrument cluster (A1) via the chassis CAN (CAN E).

Any faults detected are stored in the fault memory of the ME-SFI [ME] control unit. These can be read out and deleted with Xentry Diagnostics.