Air/Fuel Ratio Feedback Compensation (Closed Loop Operation)
In order to obtain efficient performance of the three-way catalytic converter (TWC) and a high clarification rate of CO, HC and NOx in the exhaust gas stream, the air/fuel mixture must be kept as close to the theoretical air/fuel ratio (14.7:1) as possible. In order to accomplish this, the powertrain control module (PCM) first compares the input voltage from fuel control heated oxygen sensors (HO2S) with a specified reference voltage. If the fuel control HO2S input voltage is higher than the specified reference voltage, the PCM determines that the air/fuel ratio is richer than the theoretical air/fuel ratio and reduces the fuel (lean command). If the input voltage from the fuel control HO2S is lower than the specified reference voltage, the PCM determines that the air/fuel ratio is leaner and increases the fuel (rich command). By repeating these operations, the PCM can adjust the air/fuel ratio in order to be closer to the theoretical air/fuel ratio. Control of the fuel delivery system as just described is known as closed loop operation.
| Air Fuel Mixture | Exhaust O2 Content | HO2S1 Output | Fuel System Response |
|---|---|---|---|
| Lean Mixture | High Oxygen | Low Voltage | Rich Command |
| Rich Mixture | Low Oxygen | High Voltage | Lean Command |
The closed loop fuel control operation will not take place under any of the following conditions:
- At engine start up.
- When the fuel injection is increased just after engine start up.
- When the engine coolant temperature (ECT) sensor is indicating a low coolant temperature.
- When the engine is operating under a high demand, such as wide open throttle (WOT).
- At fuel cutoff.
- When the HO2S 1 is cold, open loop operation.
Control of the air supply that is mixed with the metered fuel is detailed in the description of the air intake system. Refer to Air Intake System Description .