Ignition System: Description and Operation

Coil Pack:

Spark Plug Wire Location:

The high tension ignition system must be able to deliver sufficient energy to ignite the air/fuel mixture under all conditions. If sufficient ignition energy is not available ignition does not occur and the resultant combustion miss produces reduced power and unacceptable tail pipe emissions. For this reason BMW uses the "motionless" direct ignition system. It consists of the DME control unit, individual coils for each cylinder, ignition wires with interfence suppression and improved spark plugs. The system operates without moving parts thus secondary contact wear and tear is eliminated. Other advantages include a wider ignition timing adjustment range, elimination of secondary arcing and stray radio frequency interference with improved ignition coil performance during peak loads.

Ignition Timing Program Map:

The primary side of the coils are connected with the positive battery terminal via the ignition switch. During the primary current-flow time (coil saturation) the coils are connected to ground through the power output transistors of the DME control unit. Based upon load conditions received by the DME control unit in relation to various engine speeds a three dimensional ignition performance characteristic "Basic Ignition Map" is used to optimize the ignition to achieve the most favorable exhaust gas and fuel consumption performance for any given load or speed condition.
When the throttle valve is in the closed position, the bottom line of the basic ignition map is selected as the idle/coast characteristic. For engine speeds below the nominal values, the ignition timing is advanced to obtain idle stabilization. For coasting ignition timing is programmed to obtain minimum exhaust gas pollutants and optimum driveability performance. For the wide open throttle position, the top line of the basic ignition map is selected. This full-load characteristic is programmed with engine knock limits taken into consideration. For very high ambient temperatures the ignition is retarded as a function of engine and intake air temperature.