Idle speed control

Idle speed control

The ECU controls idle speed in three ways:

• - by means of a stepper motor that moves a plunger to act on the throttle by-pass;
• - by changes in ignition advance;
• - via asynchronous fuel injections.

Idle speed control is implemented to compensate for power taken up by the various appliances in order to ensure speed is maintained as constant as possible.

The ECU is able to take vehicle speed into account in order to control restoration of idle speed in different ways according to whether vehicle is stationary or moving.

Restriction of maximum rpm

Injection is cut off when 6600 rpm is exceeded.

Control of combusted gas - Lambda probe

The ECU ensures air-fuel ratio is maintained close to stoichiometric levels for as long as possible in order to permit efficient, long-lasting operation of the catalytic converter.

Fuel vapour recovery

Fuel vapours from the tank and subsequently collected in the active carbon filter are taken up again so that they can be burnt.

Vapour take-up and intake activation are governed by the ECU by means of a solenoid controlled with Duty Cycle proportional to rpm, engine load and base pulse constant.

Air conditioner compressor connection

The ECU is interfaced with the air conditioning system (where fitted) in order to stabilise idle speed fluctuations due to power absorption caused by compressor activation.

By controlling information such as engine load, coolant temperature and throttle angle, the ECU is able to de-activate the compressor if necessary.

Connection with anti-theft system

The anti-theft device can alter the logical status of one ECU pin to de-activate the engine.

Controlling ignition advance

The ECU also processes a control strategy for the solid-state electronic ignition.

The optimal advance angle is computed as follows:

1. A baseline advance angle is computed using a memory map whose input parameters are engine angular speed of rotation and absolute pressure measured in the inlet manifold. Two different one-dimensional tables are used when idling and under full load on the basis of engine speed.
2. The resulting values are added to the correction for coolant temperature during the engine warm-up period.
3. The idle table value is further corrected in the case of sudden drops in idle speed (e.g. due to activation of an appliance).
4. Speed is used in the case of over-run and subsequent cut-off.
5. The advance angle setting is also corrected under the following conditions:
   • – transitory throttle positions;
   • – return from cut-off due to changed throttle position;
   • – return from cut-off due to minium rpm threshold.