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.

Direct Ignition Component Locations:

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

Electrical Diagram:

OPERATION
The primary side of the eight Direct Ignition System (DIS) 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.

Knock Sensor Location:

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.

Ignition Circuit Monitoring:

Secondary Ignition Monitoring
The DME control module continuously checks secondary ignition system integrity.

Monitoring detects misfires on a cylinder-selective basis via a shunt (240 ohm resistor), and monitoring circuit in the DME control module. If the primary ignition circuit monitoring is recognized as OK, but secondary ignition voltage is not reached, the coresponding fuel injector output stage is switched off.

Refer to Computers and Control Systems for more information.