Ignition system

Ignition system

The ignition coil consists of two primary windings connected to control unit (A) via terminals 34 and 35 supplied by battery voltage, and two high tension secondary windings whose outputs are connected directly to the spark plugs of cylinders 1-4 and 3-2.

The two spark plug pairs are supplied with high voltage whenever current is cut off in the primary winding by the power module. The high tension solid state distribution exploits the different pressure conditions existing simultaneously in cylinder pairs 1-4 and 3-2.

In practice, if an air/fuel mixture is present, the compression phase of one cylinder corresponds to the exhaust phase of the other.

If, under these conditions, the voltages required to set off a spark between the electrodes of a spark plug are examined, it may be noted that the voltage is very high during the compression stage (10 - 15 kV), but extremely low during the exhaust stage (in the order of 500 V).

Two sparks therefore cross the secondary circuit of each coil. One of these generates about 500 V and is in essence a wasted spark because it is triggered in the cylinder in exhaust phase, the other generates 10 - 15 or more kV in the cylinder in compression-exhaust stage in order to burn the air fuel mixture in the cylinder.

ENGINE IDLE CONTROL

The main aim of this strategy is to maintain engine speed at around the mapped setting (warm engine: 850 rpm): the position assumed by the actuator is dependent on engine conditions/rpm and vehicle speed.

Start-up stage

When the key is inserted, the actuator takes up a position dependent on engine temperature and battery voltage (open-loop position).

Warming-up stage

The rpm is mainly corrected on the basis of coolant temperature.

With the engine warm, idle management is dependent upon the signal from the rpm sensor. The ECU introduces sustained idle when external loads are activated.

Over-run

When the accelerator is released when the car is not idling, the ECU governs the actuator (step motor) by measn of special delivery curve (dash-pot curve), i.e. delays the return of the plunger to its housing to bring about a reduction in engine braking effect.

FUEL VAPOUR RECIRCULATION MANAGEMENT

Fuel vapours (legally recognised pollutants) are directed to an active carbon filter and then on to the engine where they are burnt. This is achieved by means of a solenoid controlled by the ECU only when engine load conditions are appropriate for permitting effective combustion without operation being impaired. The ECU compensates for the extra fuel by reducing fuel flow to the injectors.

CONTROLLING KNOCK

This function detects the presence of knock by processing a signal from a knock sensor. The ECU continually compares signals from the sensor with threshold values, which are in turn continually updated to take into account background noise levels and engine age.

The ECU is therefore able to detect the presence of knock (or incipient knock) and reduce the ignition advance until the effect disappears. The advance is then gradually restored to baseline levels.