Emission Control Systems: Description and Operation

GF14.00-P-3000MI Emission Control Function
ENGINE 272.9 (except 272.98) in MODEL 204.0 /2 as of model year 2009 /AEJ 08
ENGINE 272.9 (except 272.98) in MODEL 204.9, 207.3 /4, 212.0 /2
ENGINE 273.9 in MODEL 207.3 /4, 212.0 /2

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

Exhaust gas cleaning, general
The task of the 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 the exhaust treatment
The following subsystems are involved in the exhaust treatment:

^ Function sequence for firewall catalytic converter
^ Function sequence for air injection
^ 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 converter) for Lambda =1 (converted).
Through oxidation the carbon monoxide in the carbon dioxide (CO2) and hydrocarbon are converted into water (H2O)+ carbon dioxide.
Through reduction the nitrogen oxides is converted into nitrogen (N2)+ carbon dioxide.

Additional function requirements for air injection
^ Coolant temperature >-10°C and less than 35°C
^ Coolant temperature >7°C and less than 35°C (with code (494) USA version)
^ Engine speed < 2500 rpm
^ Engine at idle or partial load operation

Function sequence for air injection
The air injection more rapidly warms up the firewall catalytic converters to operating temperature after starting and thus improves the exhaust emission values in the warming-up phase.
The ME-SFI [ME] control unit (N3/10) manages the air injection according to the information from the following sensors:

- Hot film MAF sensor (B2/5), engine load
- Coolant temperature sensor (B11/4)
- 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 the vacuum from the variable intake manifold to the air shutoff valves. This opens and the injected air is blown from the electric air pump into the exhaust ducts of the cylinder heads.

The injected air reacts with the hot exhaust gases in the outlet ducts and firewall catalytic converters. Oxidation of carbon monoxide (CO) and hydrocarbons (HC) takes place (afterburning).
This afterburning leads to a further increase in the exhaust gas temperature (exothermic reaction), which additionally heats the firewall catalytic converters.

To provide the air injection, both the air pump switchover valve and the electric air pump are actuated by the ME-SFI [ME] control unit for up to 40 s.

Following actuation, air injection remains locked. Renewed enable of air injection will only take place if the firewall catalytic convertors cool down again due to a sufficiently long stoppage time of the engine. The electrical air pump also has sufficient time to cool down again.

Diagnosis
For diagnosis with the 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
^ 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
- Electronic Stability Program control unit (N30/4) or Electronic Stability Program Premium control unit (N30/7) (for DISTRONIC PLUS), vehicle speed via 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 the requests to the fully integrated transmission control controller unit (Y3/8) to move the shift characteristics via the drive train CAN (CAN C).
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 should 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.

For camshaft adjustment the ME-SFI [ME] control unit reads in the following sensors for monitoring the catalytic converter efficiency:
- Crankshaft Hall sensor, engine speed
- LH and RH oxygen sensors upstream of catalytic converter (G3/3, G3/4)
- LH and RH oxygen sensors downstream of catalytic converter (G3/5, G3/6)

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 working effectively, the oxygen sensors signals upstream have the same amplitude as those downstream.

A number of measurements take place in the lower partial-load range in the specified engine rpm range.
The results are compared with a performance 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 the Xentry diagnostic system.