ENGINE | 274.9 in MODEL 212 |
with CODE 924 (Bivalent natural gas drive) |
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 |
P07.61-2691-01
Function requirements for starting, the post-start phase and warm-up enrichment, general points
Circuit 87M (engine timing 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 (N310) enriches the fuel-air mixture.
At engine start under -15°C, gas operation is selected briefly, although gasoline operation may have been selected. Gasoline operation is maintained until various temperature-dependent parameter values have been reached.
The ME-SFI [ME] control unit extends the injection time of the fuel injectors (Y76) to achieve the parameter values. Only then can gas operation be selected. When changing into gas operation the ME-SFI [ME] control unit actuates the gaseous fuel gas injection valves (Y106)via 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
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 chassis CAN 1, the ME-SFI [ME] control unit extends the injection time of the fuel injectors.
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 a battery voltage of approx. 7 V.
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
The ME-SFI [ME] control unit allocates an additional fuel quantity to the engine dependent on the characteristics 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:
– intake camshaft Hall sensor (B615), intake camshaft setting
– exhaust camshaft Hall sensor (B616), exhaust camshaft setting
- coolant temperature sensor
– charge air temperature sensor upstream of throttle valve (B17/7)
– charge air temperature sensor downstream of throttle valve (B17/9)
– pressure sensor downstream of throttle valve (B28/7), engine load
– crankshaft Hall sensor, engine speed
– correction programming with Xentry Diagnostics
The ME-SFI [ME] control unit allocates an additional fuel quantity to the engine dependent on the characteristics 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 intake camshaft solenoid (Y49/1) and the exhaust camshaft solenoid (Y49/2) determine the exhaust gas recirculation rate by means of 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.
Referenced documents
- gf0770p9998mrc Overview of system components for gasoline injection and ignition system with direct injection