ME-SFI ignition system - function (Engine 113) - GF15.12-P-0001BK

ENGINE 113.967 in MODEL 219 

ENGINE 113.964 in MODEL 164.1 

ENGINE 113.971 in MODEL 251 

Fig 1: ME-SFI Ignition System Function Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

When the crankshaft is rotating, an alternating voltage is generated in the crankshaft position sensor by the teeth of the driven 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 control unit detects the TDC position of the crankshaft with the 2nd negative slope after the gap.

If the signal from the camshaft Hall sensor is at 0 V ("low") at this moment, the ME control unit processes this in order to detect ignition TDC. This is used for actuating the ignition coils and fuel injection valves.

The ME control unit actuates the ignition coils. When the ignition coil primary circuit is turned off, high-voltage is generated via induction.

High-voltage is distributed statically and directly from ignition coils to spark plugs.

Advantages are

• Essentially lower electromagnetic base noise (no naked sparks)
• no rotating parts
• Noise reduction
• no high voltage connections.

The ME-SFI control unit essentially analyzes the following signals for determining the ignition angle:

• Engine load
• Engine speed
• Crankshaft position
• Camshaft position
• Coolant temperature sensor
• Knock signals
• Throttle valve position
• intake air temperature.

The ignition angle can only be tested with the STAR DIAGNOSIS.

The engine torque can be changed rapidly via the ignition angle.

The following functions require adaptation of ignition timing.

Catalytic converter heating-up 

To bring the catalytic convertor quickly to operating temperature, the ignition angle is constantly adjusted for approx. 20 seconds to "Late" when the coolant temperature at start is between 15°C and 40°C and when with automatic transmission, selector lever position P or N is selected.

Idling 

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

Inertia fuel shutoff 

The ignition angle is briefly retarded when combustion is resumed (fuel injection valves actuated) in order to prevent a sudden increase in torque.

AC system 

Shortly before the coolant compressor is switched on, the engine torque increases. In doing so, the ignition angle is adjusted for a short time to "Early" to support idle speed control.

Transmission overload protection 

In order to protect the shift elements of the automatic transmission from excessive thermal stresses during powershifts, the ignition angle is briefly retarded during the gearshift and the engine torque is thus reduced.

The ME control unit is supplied with a signal for this purpose by the transmission control unit over the CAN databus.

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 databus to the ME control unit.

Intake air temperature/coolant temperature 

To prevent the knock angle at an air intake temperature over approx. 35°C or a coolant temperature over approx. 105°C, the ignition angle is adjusted to "Late" under load.

Anti-knock control (AKC) 

During uncontrolled combustion (knocking) at one or several cylinders, the ignition angle is adjusted to "Late" at the knocking cylinders.