Functional description

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: Ignition Coil

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 main influential parameters are:

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