Ignition coils

Each spark plug is activated by high voltage from a separate ignition coil (pencil ignition coil) and also a separate ignition output stage in the Digital Motor Electronics (DME).

The ignition output stage switches on a current from the on-board electrical system through the primary coil before the required ignition time. While the primary circuit is closed (closing time), a magnetic field is built up in the primary coil. When the ignition time is reached, the current is interrupted again by the primary coil. The energy of the magnetic field discharges via the magnetically coupled secondary coil (induction). This creates a high voltage in the secondary coil, which generates the ignition spark at the spark plug.

The ignition voltage that is needed at the spark plug (required ignition voltage) must always be greater than the maximum possible ignition voltage of the ignition system (available ignition voltage). Once the ignition spark has broken through, the remaining energy is converted at the spark plug during the spark duration.

Fig 1: Identifying Ignition Coils Components

Courtesy of BMW OF NORTH AMERICA, INC.

The ignition time when the ignition spark ignites the fuel-air mixture in the combustion chamber must be set with extreme accuracy. This provides an optimum torque and low consumption, together with minimal emissions.

The essential variables are:

• Engine speed
• Engine torque
• Charging pressure
• Current lambda value
• Coolant temperature and intake air temperature
• Fuel grade (octane number)
• Engine operation (engine start, idle, partial load, full load).

The ignition coil operates according to the principle of a transformer. 2 coils are placed onto a shared ferric core. The primary coil consists of a thick wire with just a few windings. One end of the coil is attached to the positive terminal (terminal 15) of the vehicle voltage via the load-shedding relay terminal 15. The other end (terminal 1) is connected to the ignition output stage, which can use it to switch the primary circuit. The secondary coil consists of thin wire with many windings.

The ignition signal calculation ensures that the ignition spark is produced in the correct cylinder, with optimum ignition timing and with the necessary energy. To do this, the speed signal from the crankshaft is recorded. used by the Digital Motor Electronics (DME) to calculate the crank angle and the engine speed.

The ignition output stages are switched on and off for any required crank angle (petrol engine: -70 crankshaft degrees before top dead center up to +30 crankshaft degrees after top dead center). In a 4-stroke engine, ignition is only required after every second revolution which means the camshaft sensor is necessary in order to clearly allocate a cylinder.

Fig 2: Charge Current Signal Waveform

Courtesy of BMW OF NORTH AMERICA, INC.