Start, post-start phase and warm-up enrichment, function

P07.61-2691-01

Enrichment rate/time diagram

a Start

b Start of cutoff of post-start enrichment

c Start of warm-up enrichment

t Time

t1 Starting mixture enrichment (duration e.g. 2 s)

t2 Post-start enrichment (duration e.g. 15 seconds at coolant temperature of 15°C)

t3 Warm-up enrichment (duration dependent on coolant temperature)

A Enrichment rate

Function requirements for starting, the post-start phase and warm-up enrichment

 Circuit 87M ON (engine control ON)

 Engine running

Start phase, post-start phase and warm-up enrichment, general

To ensure stable and smooth engine running during a cold start, the ME-SFI [ME] control unit (N3/10) enriches the fuel-air mixture.

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Start phase, post-start phase and warm-up enrichment, function sequence

The function sequence is divided into the following subfunctions:

 Function sequence for starting mixture enrichment

 Function sequence for post-start enrichment

 Function sequence for warm-up enrichment

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Function sequence for starting mixture enrichment

To allow easier starting when the engine is cold, more fuel has to be injected during the start procedure.

For the starting mixture enrichment, the ME-SFI [ME] control unit calculates the injection time using the following sensors and signals:

- Coolant temperature sensor (B11/4)

- Crankshaft Hall sensor (B70), engine speed

- On-board electrical system battery voltage (G1) over "circuit 30"

- Electronic ignition lock control unit (N73), "circuit 50 signal" via the chassis CAN (CAN E)

- Engine off time

When it receives the "circuit 50 signal" from the electronic ignition lock control unit over the chassis CAN, the ME-SFI [ME] control unit extends the injection time of the fuel injectors (Y76).

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Starting mixture enrichment remains active until the engine speed corresponding to the particular coolant temperature is exceeded.

The higher the coolant temperature, the shorter is the injection time calculated by the ME-SFI [ME] control unit.

The injection time is also dependent on the battery voltage. In order to guarantee the start quantity for a low battery voltage, the ME-SFI [ME] control unit extends the injection time.



No further engine start is possible below an engine start of approx. 7 V.

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Function sequence for post-start enrichment

After a cold start the fuel-air mixture is enriched for a short time to ensure that the engine runs smoothly.

Fuel condensing in the cold cylinder barrels is compensated by the post-start enrichment.

The ME-SFI [ME] control unit calculates the injection time for the post-start enrichment according to the following sensors and signals:

- Coolant temperature sensor, coolant temperature at engine start

- Combustion cycles after start

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- Fully integrated transmission controller unit (Y3/8), selector lever position over the drive CAN (CAN C)

- Correction programming with the Diagnosis Assistance System (DAS)

The ME-SFI [ME] control unit assigns an additional quantity of fuel to the engine depending on the performance map.

To do this, the ME-SFI [ME] control unit extends the injection time of the fuel injectors.

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Function sequence for warm-up enrichment

After the start and post-start enrichment, the fuel-air mixture is further enriched by the warm-up enrichment.

The warm-up control compensates for fuel which precipitates on the still cold engine parts.

The ME-SFI [ME] control unit calculates the injection time for the warm-up enrichment using the following sensors and signals:

- Hot film MAF sensor (B2/5), engine load

- Intake air temperature sensor (B2/5b1)

- LH and RH intake camshaft Hall sensors (B6/4, B6/5), intake camshaft positions

- LH and RH exhaust camshaft Hall sensors (B6/6, B6/7), exhaust camshaft positions

- Coolant temperature sensor

- Crankshaft Hall sensor , engine speed

- Correction programming using DAS

The ME-SFI [ME] control unit assigns an additional quantity of fuel to the engine depending on the performance map.

To do this, the ME-SFI [ME] control unit extends the injection time of the fuel injectors.

The ignition angles of the ignition coils (T1) during warm-up are determined according to a specific performance map.

The LH and RH intake camshaft solenoids (Y49/4, Y49/5) and the LH and RH exhaust camshaft solenoids (Y49/6, Y49/7) determine the valve overlap via the exhaust gas recirculation rate. As the hot exhaust gases heat up the combustion chambers, fuel evaporation is increased and fuel enrichment can be reduced.

In the idle speed range, injection time mainly depends on the coolant temperature at engine start.

On partial load, the crucial factor is engine load.

Once enabled, the lambda control monitors the injection time.