Ignition System: Description and Operation

High Tension Ignition System Components:

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 a simplified system consisting of the DME control unit, ignition coil, high-tension distributor cap, camshaft driven resistor rotor, ignition wires with suppression connectors, and spark plugs.

Ignition Timing Program Map:

The primary side of the coil is connected with the positive battery terminal via the ignition switch. During the primary current-flow time (coil saturation) the coil is connected to ground through the power output transistor 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.