Exhaust treatment, function

Function requirements for exhaust treatment, general points

 Circuit 87M (engine control 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 catalytic converter must be rapidly brought up to operating temperature in order to reduce the exhaust emissions during cold starts.

Exhaust gas recirculation rate

The ME-SFI [ME] control unit determines the valve overlap through actuation of the intake camshaft and exhaust camshaft solenoids (Y49/1, Y49/2). In this way the exhaust gas recirculation rate is controlled over an inner ARF.

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Function sequence for exhaust treatment

The following subsystems are involved in exhaust treatment:

 Function sequence for catalytic converter

 Function sequence for transmission shift delay

 Function sequence for monitoring the catalytic converter efficiency

Function sequence for catalytic converter

The pollutants in the exhaust emitted by the engine are converted chemically by the catalytic converter (three-way catalytic converter) for λ=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 transmission shift delay

 Coolant temperature at start <50°C

 Vehicle speed < 53 km/h

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Function sequence for transmission shift delay

The transmission shift delay warms up the 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 (B11/4)

- Electronic Stability Program control unit (N30/4) wheel speed via the chassis CAN (CAN E)

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

The ME-SFI [ME] control unit makes a request to the dual clutch transmission fully integrated transmission control unit (Y3/14n4) via the drive train CAN (CAN C) to shift 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.

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Additional function requirements for monitoring the catalytic converter efficiency

 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 catalytic converter to determine the degree of aging, and therefore the degree of HC conversion.

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

- Crankshaft Hall sensor (B70) (with direction of rotation recognition), motor rpm

- Sensor element for oxygen sensor downstream of catalytic converter (G3/1b1)

- Sensor element for oxygen sensor upstream of catalytic converter (G3/2b1)

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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.

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When the catalytic converter is at operating temperature and lambda control is enabled, the amplitudes of the oxygen sensor signals downstream and upstream of the catalytic converter are compared.

If the 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 the catalytic converter 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.

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.