Start, post-start phase and warm-up enrichment, function - GF07.10-P-1007MMW

ENGINE 276.9 in MODEL 212.095 as of model year 2014 

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

• Circuit 87M ON (engine timing ON)
• Engine running

The function requirements "internal combustion engine running" includes all function sequences of the fuel low pressure and high-pressure circuit, as described in the function "fuel supply".

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 (N310) enriches the fuel-air mixture.

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 

Function sequence for starting mixture enrichment 

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

For the starting mixture enrichment, the ME-SFI [ME] control unit computes 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) via "circuit 30"
• Electronic ignition lock control unit (N73), "circuit 50 signal" via chassis CAN 1 (CAN E1)
• Engine shutoff time

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

Starting mixture enrichment remains active until the engine speed dependent on the particular coolant temperature is exceeded. The higher the coolant temperature, the shorter 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.

Fig 1: Engine Enrichment Rate/Time Graph

Courtesy of MERCEDES-BENZ USA

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 computes 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
• Fully integrated transmission control unit (Y3/8n4), selector lever position via the drive train CAN (CAN C)
• Correction programming with Xentry Diagnostics

An additional quantity of fuel is added to the engine by the ME-SFI [ME] control unit in line with the performance map. To do this, the ME-SFI [ME] control unit extends the injection time of the fuel injectors.

Function sequence for warm-up enrichment 

After start and post-start enrichment, the fuel-air mixture is further enriched by the warm-up enrichment. The warm-up enrichment compensates for fuel which precipitates on the still cold cylinder barrel. The ME-SFI [ME] control unit computes the injection time for the warm-up enrichment using the following sensors and signals:

• Left and right intake camshaft Hall sensors (B6/4, B6/5), intake camshaft positions
• Left and right exhaust camshaft Hall sensors (B6/6, B6/7), exhaust camshaft positions
• Coolant temperature sensor
• Intake air temperature sensor (B17)
• Intake manifold intake air temperature sensor (B17/1)
• Pressure sensor downstream of throttle valve (B28/7), engine load
• Crankshaft Hall sensor, engine speed
• Correction programming with Xentry Diagnostics

An additional quantity of fuel is added to the engine by the ME-SFI [ME] control unit in line with 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 characteristics map. The left and right intake camshaft solenoids (Y49/4, Y49/5) and the left and right exhaust camshaft solenoids (Y496, Y497) determine the exhaust gas recirculation rate due to the valve overlap. 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 is mainly dependent on the coolant temperature at engine start. For partial load, the crucial factor is the engine load. Once enabled, the lambda control monitors the injection time.