Closed loop fuelling (Engine management system - V8)

Closed loop fuelling

The ECM uses a closed loop fuelling system as part of its fuelling strategy. The operation of the three-way catalytic converter relies on the ECM being able to optimise the air/ fuel mixture, switching between rich and lean either side of lambda one. Closed loop fuelling is not standard for all markets, vehicles that are not fitted with HO₂S do not have closed loop fuelling.

The ideal stoichiometric ratio is represented by λ =1. The ratio can be explained as 14.7 parts of air to every 1 part of fuel.

Conditions To achieve closed loop fuelling, the ECM interacts with the following components: l HO₂S. l Fuel injectors.

Closed loop fuelling is a rolling process controlled by the ECM. The ECM uses information gained from the CKP, ECT, MAF/ IAT and the TP sensors, to operate under the following conditions: l Part throttle. l Light engine load. l Cruising. l Idle.

Function When the engine is operating in the above conditions, the ECM implements the closed loop fuelling strategy. The air/ fuel mixture is ignited by the high tension (ht) spark in the combustion chambers and the resulting gas is expelled into the exhaust pipe. Upon entering the exhaust pipe the exhaust gas passes over the protruding tip of the HO₂S. The HO₂S measures the oxygen content of the gas compared to that of ambient air and converts it into a voltage, which is measured by the ECM.

The voltage signal read by the ECM is proportional to the oxygen content of the exhaust gas. This signal can then be compared to stored values in the ECM's memory and an adaptive strategy can be implemented.

If the HO₂S informs the ECM of an excess of oxygen (lean mixture), the ECM extends the opening time of the fuel injectors via the Injector Pulse Width (IPW) signal. Once this new air/ fuel ratio has been 'burnt' in the combustion chambers the HO₂S can again inform the ECM of the exhaust gas oxygen content, this time there will be a lack of oxygen or a rich mixture. The ECM reduces the opening time of the injectors via the IPW signal using the ECM's adaptive fuel strategy. During closed loop fuelling the HO₂S will constantly switch from rich to lean and back again, this indicates that the ECM and the HO₂S are operating correctly.

Open loop fuelling

Open loop fuelling does not rely on information from the HO₂S, but the air/ fuel ratio is set directly by the ECM, which uses information gained from the ECT, MAF/ IAT, the TP sensors and also the vehicle speed sensor (VSS). The ECM uses open loop fuelling under the following conditions: l Cold start. l Hot start. l Wide open throttle. l Acceleration.

The ECM uses open loop fuelling to control fuel quantity in all non adaptive strategy conditions. The ECM implements fuelling information carried in the form of specific mapped data contained within its memory.

Because there is no sensor information (e.g. HO₂S), provided back to the ECM, the process is called an 'open loop'.

The ECM will also go into open loop fuelling if a HO₂S fails.