Ignition Coil: Description and Operation
Ignition Coil
The engine has an inductive ignition system with a static ignition voltage distribution.
Each cylinder has a separate ignition coil that is plugged directly into the cylinder head cover.
The ignition circuit of this coil ignition consists of:
- Ignition coil with primary and secondary coil
- Ignition output stage in the engine control unit
- Spark plug, connected to the secondary coil
Each spark plug is activated with high voltage by a separate ignition coil (rod ignition coil) as well as by a separate ignition output stage in the engine control unit.
Functional description
Before the desired point of ignition, the ignition output stage switches current from the vehicle network through the primary coil. While the primary power circuit is closed (dwell time), a magnetic field is built up in the primary coil. In the point of ignition, the current is interrupted again by the primary coil. The energy of the magnetic field discharges via the magnetically coupled secondary coil (induction). This generates high voltage in the secondary coil which generates the ignition sparks at the spark plug.
The necessary ignition voltage at the spark plug (ignition voltage requirement) must always be below the maximum possible ignition voltage of the ignition system (ignition voltage supply). Once the ignition spark has broken through, the remaining energy is provided at the spark plug during spark combustion.
The point of ignition at which the ignition spark ignites the fuel-air mixture in the combustion chamber must be set very precisely. This ensures optimal torque as well as lower fuel consumption, simultaneously minimising pollutant emissions.
The major influencing variables are:
- Engine speed
- Engine torque
- Charge-air pressure
- Current oxygen sensor value
- Coolant temperature and intake-air temperature
- Fuel quality (octane number)
- Engine operation (engine start, idle speed, part-load, full load)
Structure and inner circuit
The ignition coil works according to the principle of a transformer. Two coils are applied on a common iron core. The primary coil consists of a thick wire with a few windings. One end of the coil is connected to the positive terminal (terminal 15) of the vehicle voltage via the overload-protection relay terminal 15.
The other end (terminal 1) is connected to the ignition output stage, which can then apply the primary current. The secondary coil consists of thin wire with a great many windings.
Signal path and target values
Calculation of the ignition signal also ensures that the ignition spark is provided in the right cylinder, at the optimal ignition timing angle as well as with the necessary energy. To achieve this, the speed signal of the crankshaft is detected. The engine control unit uses this to calculate the crankshaft angle and the current engine speed.
The ignition output stages are then switched on and off at each desired crankshaft angle (appropriate range for petrol engine: -70° crankshaft before OT to +30° crankshaft after OT). As ignition is only required after every second revolution on a four-stroke engine, the camshaft sensor is required for unambiguous assignment of a cylinder.
The basis of sequential spark ignition is repeatedly switching the ignition coil on and off. This expands the actual ignition spark into a band of sparks. The individual sparks are stopped by early recharging. This means that no more energy is transferred to the spark plug in the fuel-air mixture. Residual energy remains in the ignition coil. This enables minimization of the recharging time. Sequential spark ignition is only implemented in the low engine speed range as well as for the warm-up phase (flushing the spark plug).
Observe the following target values for the ignition coil:
Variable Value
Voltage range 6 to 16 V
Voltage of secondary coil in up to 29 kV
normal operating mode
Maximum current consumption in 8 to 10.5 A
normal operating mode
Primary resistance Less than 600 m Ohms
Transformation ratio primary coil 1:80
to secondary coil
Temperature range -40 to 140 °C
Diagnosis instructions
Failure of the component
If the ignition coil fails, the following behavior is to be expected:
- Fault code memory entry in the engine control unit
- Combustion misfiring
General information
NOTE: Observe the exact location of the spark plug.
In contrast to the naturally aspirated engines, there is a distinctive charge movement on high-precision direct injection engines. The spark plug must be located geometrically directly in each injected fuel jet. On the N63 and N74, the exact seating is ensured by a directional cut thread on the spark plug and in the cylinder head. Here, a precise tightening torque of 23 Nm ± 3 Nm must be adhered to.
The location of the spark plug electrodes is particularly deep in the combustion chamber. This also enables the ignition of lean mixture compositions. This has a positive effect in the cold starting phase and the catalytic converter warm-up phase. In this phase, sequential sparks are also applied, whereby, in contrast to the earlier engine management systems, all the diagnosis functions with regard to the ignition remains active.
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