Engine management (MEMS 3), fuel and emission systems (Operation): Description and operation

OPERATION

Acceleration Enrichment

When the throttle pedal is depressed, the ECM receives a rising voltage from the TP sensor and detects a rise in manifold pressure from the MAP sensor. The ECM provides additional fuel by increasing the normal injector pulse width and also provides a number of extra additional pulses on rapid throttle openings.

Over-Run Fuel Cut-Off

The ECM implements over-run fuel cut-off when the engine speed is above 1600 rpm with the engine at normal operating temperature and the TP sensor in the closed position, i.e. when ECM senses that the vehicle is ’coasting’ with the throttle pedal released. The ECM indexes the IAC valve open slightly to increase the air flow through the engine to maintain a constant manifold depression to keep emissions low.

Fuel is immediately reinstated if the throttle is opened. If the engine speed drops below 1600 rpm on over-run, fuel is progressively reinstated.

Over-Speed Fuel Cut-Off

To prevent damage at high engine speeds the ECM will implement fuel cut-off at engine speeds above approximately 7000 rpm. Fuel is reinstated as the engine speed falls.

Ignition Switch Off

In the first 10 seconds after the ignition is switched off, the ECM drives the IAC valve to its power down position (ready for the next engine start), and stores any required information.

The ECM then monitors the engine bay temperature using the ambient air temperature sensor. If the temperature is above a certain limit, the ECM will drive the engine bay fan for 8 minutes, and will then power down. If the engine bay temperature is below the limit, the ECM will power down after 10 seconds.

Fuel Quantity

The ECM controls fuel quantity by providing sequential injection to the cylinder head intake ports. Sequential injection allows each injector to deliver a precise amount of fuel to the cylinder intake ports, during the induction stroke, in cylinder firing order.

The CMP sensor and reluctor allows the ECM to synchronise injection at cranking speed for starting. The precise quantity of fuel delivered is controlled by adjusting the duration of the injector open time.

To achieve optimum performance the ECM is able to ’learn’ the individual characteristics of an engine and adapt the fuelling strategy to suit. This capability is known as adaptive fuel strategy.

Adaptive fuel strategy must be maintained under all throttle positions except:

• Cold start
• Hot start
• Wide open throttle.

All of the above throttle positions are deemed to be ’open loop’. Open loop fuelling does not rely on information from the HO₂S, but sets the air/fuel ratio (AFR) according to the stored data in the ECM.

During a cold start, the ECM references the ECT sensor to calculate the appropriate amount of fuel required to support combustion and adjusts the idle speed to the correct ’fast idle’ value. This strategy is maintained until the HO₂S is hot enough to provide an accurate feedback signal.

The specific nature of the other open loop conditions means that the HO₂S feedback is unsuitable as a control value for fuelling. Adaptive strategy also allows the ECM to compensate for wear in engine components and allow for production tolerances in mass produced components such as sensors.