GF14.00-P-3000MAW Exhaust Treatment, Function

GF14.00-P-3000MAW Exhaust Treatment, Function
ENGINES 156.9 in MODEL 204.0 /2 /3, 212.0 /2

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 air injection
^ 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 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
^ Coolant temperature >4°C and less than 60°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:
- LH and RH hot film MAF sensor (B2/6, B2/7), 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, the air pump switchover valve and the electric air pump are actuated simultaneously by the ME-SFI [ME] control unit for up to 40 s following engine start.

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 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 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:
- 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 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 control unit actuates the engine diagnosis indicator lamp (A1e58) on the instrument cluster (A1) through 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.