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, for example, 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 timing unit 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.
At engine start, gasoline operation is selected briefly, although gas operation may have been selected. Gasoline operation is maintained until various temperature-dependent parameter values have been reached. Only then can gas operation be selected.

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To do this the ME-SFI [ME] control unit extends the injection time of the injection valves (Y62) or the gas injection time of the gas gas injection valves (Y106) over the CNG control unit (N118/2).

Function sequence for start phase, post-start phase and warm-up enrichment

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.

Computation for extension of the injection time for starting mixture enrichment is undertaken by the ME-SFI [ME] control unit on the basis of the following sensors and signals:

- Coolant temperature sensor (B11/4)

- Crankshaft position sensor (L5), motor 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

To do this the ME-SFI [ME] control unit extends the injection time of the injection valves (N118/2) over the CNG control unit.Y62

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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 over the CNG control unit.

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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 for 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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- Electronic transmission control control unit (N15/3), selector lever position via the drive train CAN (CAN C)

- Correction programming using Xentry diagnostics

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 injection valves over the CNG control unit.

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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

- Coolant temperature sensor

- Charge air temperature sensor (B17/8)

- Crankshaft position sensor, engine speed

- Correction programming using Xentry diagnostics

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 injection valves over the CNG control unit.

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

On partial load, the crucial factor is the engine load.

Once enabled, the lambda control monitors the injection time.

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