Ignition System: Description and Operation

Ignition Control:

PURPOSE
The ignition system delivers a timed spark with sufficient energy to ignite the air/fuel mixture under all conditions.

DESIGN
Motronic 1.8 ignition uses a "motionless" direct ignition system. It consists of the Motronic control module (ECM), two power stage transistors, individual coils for each cylinder, and spark plugs.

The system operates without moving parts, eliminating secondary contact wear and tear. Other advantages include a wider ignition timing adjustment range, elimination of secondary arcing and stray radio frequency interference, and improved ignition coil performance (40 Kv) during peak loads.

Ignition Timing Program Map:

OPERATION
The primary side of each coil is connected with the positive battery terminal via the ignition coil relay. During the primary current-flow time (coil saturation) the coils are connected to ground through the power stage. Power stage operation is controlled by the ECM.

The ECM uses a "Basic Ignition Map" to optimize ignition for the most favorable exhaust gas and fuel consumption performance for any given load or speed condition. Map computations are based on:

^ engine speed and position,
^ piston position,
^ engine load,
^ throttle position,
^ engine knock signals,
^ engine temperature, and
^ transmission shift data.

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.

Components covered in this section are:
^ Crankshaft position sensor
^ Ignition coil
^ Knock sensor
^ Power stage
^ Spark plugs

For additional information, refer to COMPUTERS AND CONTROL SYSTEMS.