HFM Ignition System Design and Function - RA0705HFM0035X (07.5-0035)
- Ignition system engine 104 NOTE: The information which follows describes the differences compared to the ignition system of engine 111 HFM.
Location of ignition coils and distribution of the high voltage
The ignition coils are located at the cylinder head cover. Two spark plugs are supplied simultaneously with high voltage from each ignition coil.
Ignition coil Cylinders T1/1 2 and 5 T1/2 3 and 4 T1/3 1 and 6 Ignition coils
The three twin-spark ignition coils are actuated by the HFM control unit through a power output stage each, switching alternately to ground. The high voltage is thus distributed by an ignition coil so that one spark plug fires in the power stroke of the cylinder while the other spark plug fires in the exhaust stroke of the cylinder offset by 360°. One crankshaft revolution later, the corresponding cylinders have advanced two strokes and the spark plugs again fire, with the roles now being switched, however.
The second and third twin-spark ignition coils also generate two sparks each, offset, however, by a crank angle of 120° in each case.
The ignition coils (T1/1, T1/2, T1/3) are mounted directly on a spark plug with a spark plug connector via the secondary output (4a). The secondary output (4b) runs via an ignition cable to the other cylinder. The guide sleeve (W) is at the same time the ground connection for the ignition coil.
The primary current is limited by the HFM control unit to about 7 A. The maximum ignition voltage is about 32 kV.
The resistance of the secondary winding must be tested between both high voltage terminals; specification 5.2 -8.5 kΩ.
The resistance of the primary winding between terminals 1 and 15; specification 0.3 -0.4 Ω.
Primary terminals conduct voltage up to 400 V. The guide sleeve (W) must always be connected to the vehicle ground.
Persons with heart pacemakers should not work on this ignition system .
- Ignition system engine 111
Ignition coils
The ignition coils (T1/1, T1/2) have two high voltage terminals (4a, 4b). Each high voltage terminal supplies a high voltage to a spark plug each.
The primary current is limited by the HFM control unit to about 7 A. The maximum ignition voltage is about 32 kV.
Primary terminals conduct voltage up to 400 V. Core stack/holder must always be connected to vehicle ground.
Persons with heart pacemaker should not work on this ignition system .
The ignition coil is similar in design to a transformer, the primary and secondary windings being isolated from each other. The two ends of the secondary winding are each provided with a high voltage terminal and together with two spark plugs form a self-contained circuit. During ignition, a high voltage is produced in the secondary winding. The ignition sparks jump across simultaneously at both spark plugs, one ignition spark jumping across from the central to the ground electrode and one ignition spark from the ground to the central electrode. This produces a positive and a negative high voltage to ground at the secondary terminals. The ignition spark in the uncompressed exhaust stroke requires only a low level of energy so that practically the entire energy stored in the ignition coil is available for the power stroke.
As the secondary winding is connected in series with both spark plugs, an open circuit (eg. at the spark plug connector) affects both ignition sparks. The resistance of the secondary winding must be tested between both high voltage terminals.
Specification 5.2 -8.5 kΩ.
The resistance of the primary winding between terminals 1 and 15, specification 0.3 -0.4 Ω.
Benefits of the distributorless high voltage distribution:
- Significantly lower electromagnetic interference level (no open sparks)
- No rotating parts
- Reduction in noise
- Fewer high voltage connections
- Fewer high voltage connections
Location of ignition coils and distribution of the high voltage
The ignition coils are positioned between the intake pipes. Each ignition coil supplies two spark plugs simultaneously with high voltage.
Ignition coil Cylinders T1/1 1 and 4 T1/2 2 and 3 Courtesy of MERCEDES-BENZ OF NORTH AMERICA.
Note regarding testing ignition system with oscilloscope
If no image appears on the oscilloscope when testing with the diagnostic tester, the secondary test clamps at the high voltage cables must be switched over at ignition coil T1/1 from ignition cable 1 to ignition cable 4, and at T1/2 from ignition cable 2 to ignition cable 3.
The three twin-spark ignition coils are actuated by the HFM control unit through a power output stage each, switching alternately to ground. The high voltage is thus distributed by an ignition coil so that one spark plug fires in the power stroke of the cylinder while the other spark plug fires in the exhaust stroke of the cylinder offset by 360°. One crankshaft revolution later, the corresponding cylinders have advanced two strokes and the spark plugs again fire, with the roles now being switched, however.
The second and third twin-spark ignition coils also generate two sparks each, offset, however, by a crank angle of 180° in each case.
Multi-spark ignition
Multiple sparks are produced in succession in the ignition timing point in order to increase smooth engine starting at coolant temperatures of <0 °C, but only up to a maximum 10° after TDC and an engine speed up to 600 rpm.
Synchronization of ignition sequence
The ignition sequence is synchronized during the first engine revolutions in order to achieve correct assignment of the ignition timing point to the respective cylinder. The HFM control unit requires the signal of the camshaft position sensor (L5/1) for recognizing TDC of cylinder 1, and the signal of the crankshaft position sensor (L5) (magnet segment) for detecting ignition circuit 2 (cylinders 2 and 3).
Ignition during starting
When the engine is started and up to about 600 rpm, the ignition timing point is controlled only via the rear edges of the segments at the flywheel/driven plate by the crankshaft position sensor (L5). The transition from the fixed ignition timing point to the ignition timing point calculated in accordance with the operating state of the engine at that moment is not made until engine speed has increased to about 800 rpm.
The following information is processed in the HFM control unit for this purpose:
- Coolant temperature
- Engine speed/crankshaft position/ignition circuit recognition
- Camshaft position TDC cylinder 1
Ignition during warming-up
The ignition timing point is corrected in the warming-up phase in line with coolant temperature and engine load.
The following information is processed in the HFM control unit for this purpose:
- Coolant temperature
- Engine speed/crankshaft position/ignition circuit recognition
- Air mass
Ignition during full load
The HFM control unit recognizes "full load" above a fixed throttle valve angle. The HFM control unit takes into account the "advanced" correction angle from the warming-up and heating function.
The following information is processed in the HFM control unit for this purpose:
- Coolant temperature
- Engine speed
- Throttle valve position
- Air mass
Ignition during idling
When the engine is idling, idle speed contact closed, 4 fixed ignition maps are set as a function of coolant temperature and type of transmission (manual or automatic). These are not influenced by the HFM resistance trimming plug.
The following information is processed in the HFM control unit for this purpose:
- Coolant temperature
- Engine speed/crankshaft position/ignition circuit recognition
- Idle speed recognition
Ignition during decel fuel shut-off
When the injection valves are re-actuated (see Function of gasoline injection system under deceleration fuel shut-off), the ignition timing point is retarded by up to 15° CA for up to 15 ignitions in order to prevent a sudden increase in torque when fuel injection is resumed after deceleration fuel shut-off.
The following information is processed in the HFM control unit for this purpose:
- Coolant temperature
- Engine speed/crankshaft position/ignition circuit recognition
- Engine operating state