ME-SFI ignition system, function - GF15.12-P-0001M

ENGINE 275.950 in MODEL 215, 220 

ENGINE 275.951 in MODEL 230 up to Model Year 8 

ENGINE 275.953 in MODEL 216, 221 up to Model Year 8 

ENGINE 275.954 in MODEL 230 up to Model Year 8 

ENGINE 275.980 in MODEL 215, 220 

ENGINE 275.981 in MODEL 230 up to Model Year 8 

ENGINE 275.982 in MODEL 216, 221 up to Model Year 8 

ENGINE 285.950 in MODEL 240 

ENGINE 285.980 in MODEL 240 

Fig 1: ME-SFI Ignition System Diagram

Courtesy of MERCEDES-BENZ USA

When the crankshaft is rotating, an alternating voltage is generated in the crankshaft position sensor by the teeth of the drive plate.

During this operation, each tooth generates an alternating voltage signal. No voltage is generated as a result of the gap created by 2 missing teeth. The ME-SFI [ME] control unit detects the TDC position of cylinders 1 and 7 via the 2nd negative signal edge after the gap:

If the signal from the camshaft Hall sensor of the respective cylinder bank is at 0 V ("Low") at this moment, the ME-SFI [ME] control unit processes this to detect ignition TDC for cylinder 1 and, with the ME-SFI 2.7.1, additionally for cylinder 7. This is used to actuate the ignition modules and the fuel injection valves.

The high voltage distribution is performed statically and directly from the ECI ignition modules to the spark plugs.

1. Determining the ignition angle in line with the input signals and actuating the ECI ignition modules 

   For determining the ignition angle, the ME-SFI control unit essentially analyzes the signals of the following sensors:

   • Pressure sensor downstream of throttle valve actuator (engine load)
   • Crankshaft position sensor
   • Camshaft Hall sensor
   • Coolant temperature sensor
   • Charge air temperature sensor

   The ME-SFI [ME] control unit actuates the output stages in the ECI ignition modules. The output stages generate the high voltage from an ac voltage for each spark plug.

   The ignition angles can be checked using the DAS.

2. Ignition angle adaptations 
   1. 2.1 Catalytic converter heating up (warming-up phase) 

      The ignition angle is continuously retarded for about 20 seconds in order to warm up the catalytic converter more rapidly to its operating temperature, if:

      • coolant temperature at start > 15°C and < 40°C
      • Selector lever position P or N

      At the same time idle speed is increased by the idle speed control.

   2. 2.2 No-load speed 

      To assist idle speed control, the ignition angle can be retarded briefly by as much as 36° crank angle or advanced by as much as 20° crank angle.

      The control by altering the ignition angle reacts more rapidly than altering the position of the throttle valve (idle speed control).

   3. 2.3 Deceleration fuel shutoff 

      When combustion is restored (actuation of the fuel injectors), the ignition angle is briefly retarded in order to prevent a sudden increase in torque.

   4. 2.4 Overheating and pinging protection 

      The ignition angle is retarded under load, as a function of the charge air temperature and coolant temperature, in order to prevent any knocking tendency at high charge air and coolant temperatures.

   5. 2.5 Transmission overload protection 

      In order to protect the shift elements of the automatic transmission from excessive thermal stresses during powershifts (1-2-1, 2-3-2), the ignition angle is briefly retarded during the gearshift and the engine torque is thus reduced. The ME-SFI [ME] control unit is supplied with a signal from the ETC [EGS] control unit for this purpose via the CAN.

   6. 2.6 ESP control mode 

      In order to reduce the engine torque as rapidly as possible in the ESP/ASR control mode, the ignition angle is retarded by the throttle valve actuator (opening angle reduced) before the commencement of the control cycle. The information flows from the ESP/ASR control unit over the CAN data bus to the ME-SFI control unit.

   7. 2.7 Anti-knock control (AKC) 

      If uncontrolled combustion (knocking) occurs at one or several cylinders, the ignition angle at the relevant cylinder or cylinders is "retarded".

   8. 2.8 Smooth engine running analysis 

      To restrain the three way catalytic converter from thermal overload through combustion misfiring and in order to keep the exhaust emission values, the smooth operation of the engine is continuously monitored.

      If combustion misfiring is identified at one or several cylinders, the corresponding fuel injection valves are no longer actuated after a certain number of combustion misfires.

      Detection of combustion misfires is performed through a smooth running analysis and at high rpms, in addition through the evaluation of the ion current. The size of the ion current is detected by the ECI ignition module and is transmitted to the ME-SFI control unit.

   9. 2.9 Double ignition 

      Two spark plugs for each cylinder are beneficial on the basis of the valve arrangement for achieving optimal exhaust emission levels and smooth engine running.

      Each spark plug is actuated via its own ignition coil in the ECI ignition modules separately by the ME-SFI control unit.

      In the lower part load range up to approx. 2000 rpm both ignition sparks of a cylinder are triggered simultaneously. At medium and high loads, ignition is offset by up to 10° crank angle.

      The order of actuation is constantly varied in this case in order to ensure uniform wear of both spark plugs and to avoid deposits on one side of the combustion chamber. The signals for ignition circuit change and ignition offset are transmitted by the ME-SFI control unit to the respective ECI ignition module.