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Exhaust Treatment Function - GF14.00-P-3000MRA

Engine 274.9 in model 205 (except 205.047/053/054/147/247/253) 

Engine 274.9 in model 253 (except 253.354/954) 

Function requirements for exhaust treatment, general points 

Exhaust treatment, general 

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

To do this, amongst other things, the near-engine mounted catalytic converter (three-way catalytic converter) rapidly brought up to operating temperature, in order to reduce the exhaust emission for a cold start.

IMPORTANT The ME-SFI [ME] control unit (N3/10) actuates the intake camshaft and exhaust camshaft solenoids (Y49/1, Y49/2) to determine the valve overlap. The exhaust gas recirculation rate is thereby controlled by internal exhaust gas recirculation.

Function sequence for exhaust treatment 

The following subsystems are involved in exhaust treatment:

Function sequence for catalytic converter 

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

Through oxidation, carbon monoxide is converted to carbon dioxide (CO2 ) and hydrocarbon to water (H2 O)+ carbon dioxide.

Through reduction the nitrogen oxides are converted into nitrogen (N2 )+ carbon dioxide.

Function sequence for NO x  storage catalytic converter (for code 920 (Gasoline direct injection with stratified charge)) 

In homogeneous mode with λ=1 the hydrocarbon, carbon monoxide and nitrogen oxide pollutants are converted in the firewall catalytic converter.

In stratified operation with λ>1 the portion of nitrogen oxides and the conversion of the nitrogen oxides is performed by the NOx storage catalytic converter. It stores the nitrogen oxides through chemical binding.

If the storage capability is exhausted, the stored nitrogen oxides are converted into nitrogen and carbon dioxide by purging the NOx storage catalytic converter.

Purging of NO x  storage catalytic converter 

The fuel-saving stratified operation can only be enabled when the increasing amounts of nitrogen oxides in the NOx storage catalytic converter have been converted.

The ME-SFI [ME] control unit reads in the following sensors and signals for purging of the NOx storage catalytic converter:

The ME-SFI [ME] control unit uses the NOx sensor to detect any abrupt increase in NOx which indicates that the NOx storage catalytic converter is full. Homogeneous operation is activated as a reaction to this and the mixture enriched, so that the NOx storage catalytic converter can regenerate. Stratified operation is then activated again.

For NOx conversion a temperature range of 250 to 500°C in the NOx storage catalytic converter is optimal. For stratified operation or wide open throttle it must not get hot than about 850°C.

The exhaust temperature is monitored by the temperature sensor upstream of NOx storage catalytic converter. The temperature sensor determines the current exhaust temperature and routes it as voltage signals to the ME-SFI [ME] control unit for evaluation.

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

IMPORTANT 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 converter, resulting in fewer nitrogen oxides being stored. Stratified operation is therefore shortened and purging must be performed more frequently.

Additional function requirements for upshift delay (for automatic transmission) 

Function sequence for upshift delay (for A/T) 

The upshift delay brings the CAT downstream of the engine start more rapidly up to operating temperature.

The ME-SFI [ME] control unit controls the upshift delay according to the following sensor and signal:

The upshift delay is active for a maximum of 60 s and exclusively occurs electronically.

The ME-SFI [ME] control unit makes a request to the fully integrated transmission control unit (Y3/8n4) via drive train CAN to move the shift characteristic curves.

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 

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 efficiency 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 signals to monitor the catalytic converter efficiency:

IMPORTANT The oxygen stored during the "lean operating phase" is reduced totally or partially during the "rich operating phase". Aging reduces the oxygen storage capacity and HC conversion capacity of the catalytic converter. Because the catalytic converter has a high ability to store oxygen, the alteration of the oxygen content in line with the catalytic converter is fully dampened. Consequently, the oxygen sensor signal downstream of the catalytic converter has a low amplitude and is virtually constant.

When the catalytic converter is at operating temperature and the lambda control enabled, the amplitude variables of the oxygen sensor signals downstream and upstream of the catalytic converter are compared. If the catalytic converter is no longer operable, then the oxygen sensor signal of the oxygen sensor, sensor element upstream of the CAT and the oxygen sensor signal of the oxygen sensor, sensor element downstream of the CAT are the same size.

A number of measurements take place in the lower partial-load range in the specified rpm range. The results are compared with a characteristics map in the ME-SFI [ME] control unit.

If a fault is detected, the ME-SFI [ME] control unit actuates the engine diagnosis symbol in the instrument cluster (A1) via the drive train CAN, powertrain control unit, suspension FlexRay, electronic ignition lock control unit (N73) and user interface CAN (CAN HMI).

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

  Electrical function schematic for exhaust treatment   PE14.00-P-2051-97FBA 
  Overview of system components for gasoline injection and ignition system with direct injection   GF07.70-P-9998MRA
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