Ignition System: Description and Operation

Ignition System Layout:

PURPOSE
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 will produce reduced power and unacceptable tail pipe emissions.

Spark Plug Wire Location:

CONSTRUCTION
The "motionless" or direct ignition system consists of:
^ Digital Motor Electronic (DME) control unit
^ Four individual Ignition coils
^ Ignition wires with suppression connectors
^ Spark plugs

Electrical Diagram:

OPERATION
The primary circuits of the ignition coils are switched by the power output stage of the DME control unit. The uncorrected basic ignition time is used as a load signal for the ignition. Based upon these load conditions 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.

Ignition Timing Program Map:

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.

Knock Control
Knock control allows ignition advance right up to the knock limit without the need for a safety margin. Ignition is retarded only when the actual knock point is detected, and then only the cylinder(s) identified as knocking. The best ignition timing point can be selected for optimum fuel consumption and operating efficiency.

Signals produced by the knock sensors are compared to set values in the DME programming. When these predetermined values are reached, that combustion process is viewed as "knocking" and the timing is immediately retarded (e.g. 3°). This can occur for one or more cylinders as needed. The ignition retard occurs well before the knock poses a hazard for the engine. Following knock retardation, the ignition is advanced step by step until the optimum timing point is reached, or until knock is again detected.

Refer to Computers and Control Systems for more information.