Ignition discharge module

Ignition discharge module

Ignition

The ignition discharge module is mounted on the camshaft cover over the spark plugs. There are four ignition coils in the ignition discharge module, the secondary windings of which are directly connected 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, Batt+ is supplied to the ignition discharge module, which converts 12 V to 400 V DC voltage which is stored in a capacitor. The 400 V voltage is connected to one pole of the primary windings of the ignition coils.

Four trigger wires are connected to the ignition discharge module from Trionic ECM pin 9 (cyl. 1), pin 10 (cyl. 2), pin 11 (cyl. 3) and pin 12 (cyl. 4).

When the ECM grounds pin 9, the other pole of the primary winding for cyl. 1 will be grounded and 400 V transformed up to max. 40 kV in the ignition coil for cyl. 1. Ignition takes place in the same way for cyl. 2, 3 and 4.

If there is an open circuit in the power supply or ground to the ignition discharge module, the engine will not start.

If there is an open circuit on 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 assist starting at an engine coolant temperature below 0° C, the ECM will ground the trigger lead concerned 210 times per second from 10° BTDC to 20° ATDC, producing a "multispark". This function ceases at engine speeds above 850 rpm.

During idling, a special ignition control curve is used. Normal pre-ignition is approx. 6- 8°.

If the engine speed drops, e.g. when the radiator fan is switched on, the pre-ignition increases up to 20° BTDC in order to increase engine torque and restore engine speed. In the same way, ignition is retarded and engine speed is increased.

Ignition control during idling compensates for rapid changes in engine idling speed.

When the throttle position moves away from idling, ignition control changes over to normal control depending on load and engine speed.

Combustion signals

The Trionic system does not have a camshaft sensor. A camshaft sensor is normally required for sequential knock control and fuel injection.

Saab Trionic must decide whether cyl. 1 or cyl. 4 is firing when the crankshaft position sensor indicates that cyl. 1 and cyl. 4 are at top dead centre.

This is done as follows: one pole of the secondary windings of the ignition coils is connected in the usual way to the appropriate spark plug. The other pole is not directly grounded but is connected to a voltage of 80 V. This means that there is constantly a voltage of 80 V across the spark plug gap except at the precise moment when the spark is produced.

When combustion takes place, the temperature in the combustion chamber is very high. The gases are ionized and start to conduct current. This means that a current passes across the spark plug gap (without creating a spark)

The ionization current is measured in pairs, cylinders 1+2 and cylinders 3+4. If combustion occurs in cylinder 1 or 2, the ignition discharge module sends a Batt+ pulse to Trionic ECM pin 17. In the same way, the ignition discharge module sends a Batt+ pulse to the ECM pin 18 if combustion occurs in cylinders 3 or 4.

If the crankshaft position sensor indicates that cylinder 1 and cylinder 4 are at top dead centre and a Batt+ pulse arrives on ECM pin 17 at the same time, the ECM knows that it is cylinder 1 that has fired.

When the engine is started, the ECM does not know which of cylinders 1 and 4 is in the compression position, and ignition consequently takes place on both cylinders 1 and 4 simultaneously. In the same way ignition takes place on cylinders 2 and 3. As soon as combustion signals arrive on pins 17 and 18 of the ECM, both ignition and fuel injection are synchronized to the engine firing order.

If there is an open circuit on leads wires to pins 17 and 18 of the ECM, no synchronization of ignition or fuel injection occurs, 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 regulation

Saab Trionic does not have a conventional knock sensor. Instead the ignition discharge module analyzes the ionization currents for all the cylinders and sends signals to pin 44 on the Trionic ECM. This function is adaptive with respect to interfering fuel additives.

The ECM knows through the combustion signals which cylinder has fired and if the signal on the knock lead at the same time is above a particular value, the ECM registers knocking on this cylinder. Ignition is then retarded 1.5° on this cylinder.

If knocking is repeated, ignition is further retarded, but to a maximum of 12°. If the ignition reduction is above a particular value on all the cylinders, the amount injected is slightly increased.

If knocking occurs when the pressure in the intake manifold is above approx. 140 kPa, knock control takes place in a different way: first both the fuel injection and ignition matrices are changed, and if this does not help the boost pressure is reduced.

The aim is to achieve good performance even in the event of knocking.

If there is an open circuit on the lead to ECM pin 44, basic charging pressure is obtained and ignition is retarded by 12° when the engine load is so high that there is a risk of knocking.

Burn-off

After the ignition switch has been turned off and the engine has stopped, the main relay remains activated for a further 6 seconds. The Trionic ECM then grounds all the trigger leads 210 times per second for 5 seconds. Each electrode gap burns off impurities with more than 1000 sparks.