Ignition Discharge Module
PRINCIPLE OF OPERATION, IGNITION DISCHARGE MODULE
Ignition Discharge Module
Ignition Discharge Module is mounted on the camshaft cover over the spark plugs. The ignition discharge module contains four ignition coils, the secondary windings of which are connected directly to the spark plugs.
The ignition discharge module is supplied with Batt+ from the main relay and grounded at grounding point G7P.
When the main relay operates, battery positive is supplied to the ignition discharge module which converts 12 V to 400 V DC. This voltage is stored in a capacitor which is connected to one pole of the primary winding of each of the 4 ignition coils.
Four trigger leads from pins 9 (cyl. 1), 10 (cyl. 2), 11 (cyl. 3) and 12 (cyl. 4) of the Trionic control module are connected to the ignition discharge module.
When the control module grounds pin 9, the other pole of the primary winding of the ignition coil for cylinder 1 will be grounded (via the ignition discharge module's battery positive input) and 400 V will be transformed up to max. 40 kV in the ignition coil for cylinder 1. Ignition takes place in the same way for cylinders 2, 3 and 4.
If there is no continuity in the power supply or the ignition discharge module is grounded, the engine will not start.
If there is an open circuit in any of the trigger leads, the cylinder concerned will not fire.
Ignition Control
When the engine is started, the ignition timing is 10° BTDC. To facilitate starting when the engine coolant temperature is below 0°C, the control module will ground the relevant trigger lead 210 times per second from 10°BTDC to 20° ATDC, producing a "multispark" sequence. This function ceases at engine speeds above 850 rpm.
At idling speed a special ignition control curve is used. Normal ignition advance is 0° - 8°, depending on the engine version.
If engine speed drops, e.g. when the radiator fan cuts in, the ignition is advanced up to a maximum of 20° BTDC to increase engine torque and restore engine speed. Similarly, the ignition is retarded if engine speed increases.
Ignition control when the engine is idling compensates for rapid changes in engine idling speed.
When the throttle leaves the idling position, ignition control reverts to normal and is again dependent on load and engine rpm.
Combustion Signals
The Trionic system does not include a camshaft position sensor. A camshaft position sensor is normally required for sequential knock control and fuel injection.
The Trionic system has to decide whether cylinder 1 or 4 is firing when the crankshaft position sensor indicates that cylinders 1 and 4 are at top dead center.
This is done as follows: One pole of the secondary winding of each of the 4 ignition coils is connected in the usual way to the appropriate spark plug. The other pole is not grounded directly but connected to an 80 V source. As a result, there is a current of 80 V across the spark plug gap at all times except at the moment when a spark is produced.
When combustion takes place, the temperature in the combustion chamber is extremely high. The gases are ionized and begin to conduct current. This causes a current to pass across the spark plug gap (without producing a spark).
The ionization current is measured in pairs, cylinders 1+2 and cylinders 3+4. If combustion takes place in cylinder 1 or 2, the ignition discharge module sends a battery positive pulse to pin 17 of the Trionic control module. Similarly, the ignition discharge module sends a battery positive pulse to pin 18 of the control module if combustion takes place in cylinder 3 or 4.
If the crankshaft position sensor indicates that cylinders 1 and 4 are at top dead center and a battery positive pulse is applied to pin 17 of the control module at the same time, the control module knows that cylinder 1 is the cylinder in which combustion has occurred.
When the engine is started, the control module does not know which of cylinders 1 and 4 is in the compression position. Consequently, ignition takes place in cylinders 1 and 4 simultaneously. Ignition takes place in cylinders 2 and 3 in a similar manner. As soon as combustion signals are applied to pins 17 and 18 of the control module, ignition timing and fuel injection are both synchronized with the engine's firing order.
If there is an open circuit on both the leads connected to pins 17 and 18 of the control module, no synchronization of ignition timing or fuel injection will occur and knock control will take place in parallel on cylinders 1+4 and 2+3. This does not have any noticeable effect on the operation of the system.
Knock Control
The Saab Trionic system does not have a conventional knock sensor. Instead, the ignition discharge module analyses the ionization currents for all the cylinders and sends signals to pin 44 of the Trionic control module. This function is adaptive in regard to inimical fuel additives.
Through the combustion signals the control module knows which cylinder has fired and if the signal on the knock lead is above a certain level at the same time, the control module will register knocking on this cylinder. The ignition will then be retarded 1.5° for this cylinder.
If the knocking persists, the ignition will be retarded additionally but not by more than a maximum of 12°. If ignition is retarded by more than a certain amount on all cylinders, the quantity of fuel injected will be slightly increased.
If knocking occurs when the pressure in the intake manifold is more than about 140 kPa, knock control takes place somewhat differently: first of all both the fuel injection and ignition matrices are changed and if this does not help, the boost pressure is lowered.
The aim is to achieve good performance even when knocking occurs.
If there is an open circuit on the lead to pin 44 of the control module, basic charging pressure is obtained and the ignition will be retarded by 12° when the load on the engine is so great that knocking could occur.
Burn-off
When the ignition has been turned off and the engine has stopped, the main relay remains operated for a further 6 seconds. The Trionic control module then grounds all the trigger leads 210 times a second for 5 seconds. Each electrode gap burns off impurities with more than 1000 sparks.