Exhaust treatment function - GF14.00-P-3000MM

ENGINE 276.9 in MODEL 204 

ENGINE 276.9 in MODEL 207, 212 (except 212.095), 218 up to model year 2014 

Function requirements for exhaust treatment, general points 

• Circuit 87M (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 for a cold start.

Function sequence for exhaust treatment 

The following subsystems are involved in exhaust treatment:

• Function sequence for firewall catalytic converters 
• Function sequence for NOx storage catalytic converters (for code (920) Gasoline direct injection with stratified charge) 
• Function sequence for transmission shift delay 
• Function sequence for monitoring the catalytic converter efficiency 

Function sequence for firewall catalytic converters 

The pollutants in the exhaust emitted by the engine are converted chemically by the near-engine mounted firewall catalytic converters (three-way catalytic converters) for λ=1.

Through oxidation, carbon monoxide is converted to carbon dioxide (CO₂ ) and hydrocarbon to water (H ₂ O)+ carbon dioxide.

Through reduction the nitrogen oxides are converted into nitrogen (N ₂ )+ carbon dioxide.

Function sequence for NOx storage catalytic converters (for code (920) Gasoline direct injection with stratified charge) 

In homogeneous operation with λ=1 the pollutants hydrocarbon, carbon monoxide and nitrogen oxide are also converted in addition to the firewall catalytic converters.

In homogeneous operation the limits of the nitrogen oxides are maintained by the firewall catalytic converters and the lambda control.

In stratified operation with λ>1 the nitrogen oxides are stored by a chemical reaction in the NOx storage catalytic converters. If the storage capability is exhausted, the stored nitrogen oxides must be converted into nitrogen and carbon dioxide by purging the NOx storage catalytic converters.

Purging of the NOx storage catalytic converters 

The fuel-saving stratified charge mode can only be activated if the increasing quantities of nitrogen oxides are converted in the NOx storage catalytic converters.

The ME-SFI [ME] control unit (N3/10) reads in the following sensors and signals for purging of the NOx storage catalytic converters:

• Temperature sensors upstream of NOx storage catalytic converter at right and left (B16, B16/1), exhaust temperatures
• right and left NOx sensors (N37/6b1, N37/5b1), nitrogen oxide and oxygen components in the exhaust via the right and left NOx sensor control units (N37/6, N37/5) and the drive train sensor CAN (CAN I)

The ME-SFI [ME] control unit recognizes from the RH and LH NOx sensors, based on a leap in the NOx concentration, that the NOx storage catalytic converters are full. Homogeneous operation is activated as a reaction to this and the mixture enriched, so that the NOx storage catalytic converters can regenerate. Stratified charge operation is then activated again.

For NOx conversion a temperature range of 250 to 500°C in the NOx storage catalytic converters is optimal. For stratified charge operation or full load it must not get hot than about 800 °C.

The exhaust gas temperature is monitored by the temperature sensor upstream of every NOx storage catalytic converter. The temperature sensors determine the current exhaust temperatures and lead these in the form of voltage signals for evaluation to the ME-SFI [ME] control unit.

Mixture formation is used to control the temperature of the NOx storage catalytic converters. The mixture formation is adapted on the basis of the stored temperature models in order to protect the NOx storage catalytic converters against excessively high temperatures.

In order to convert all pollutants in the exhaust, sulfur-free fuel is required. Sulfur blocks the spaces for nitrogen oxides in the NOx storage catalytic converters, so that less nitrogen oxides can be stored. Stratified charge operation is therefore shortened and purging must be performed more frequently.

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 converters up to operating temperature more quickly 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) (model 204, 207, 212), coolant temperature sensor (B11/4) (model 218)
• Electronic Stability Program control unit (N30/4) (except code (233) DISTRONIC PLUS) or Electronic Stability Program Premium control unit (N30/7) (for code (233) DISTRONIC PLUS), wheel 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 request via the drive train CAN (CAN C) to the fully integrated transmission control controller unit (Y3/8) to 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 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 task of the catalytic converter monitoring function is to determine the aging of the firewall catalytic converters and thus their HC conversion based on their oxygen storage capability.

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

• Left and right oxygen sensors upstream of catalytic converter (G3/3, G3/4)
• Left and right oxygen sensors downstream of catalytic converter (G3/5, G3/6)
• Crankshaft Hall sensor (B70), engine speed

The oxygen stored during the "lean operating phase" is reduced totally or partially during the "rich operating phase". With aging, the oxygen storage capacity of the firewall catalytic converters 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 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.

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.