Air/Fuel Ratio Feedback Compensation - Closed Loop Operation
The wide band heated oxygen sensor (HO2S) measures the amount of oxygen in the exhaust system and provides more information than the switching style HO2S. The wide band sensor consists of an oxygen sensing cell, an oxygen pumping cell, and a heater. The exhaust gas sample passes through a diffusion gap between the sensing cell and the pumping cell. The engine control module (ECM) supplies a voltage to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. An electronic circuit within the ECM controls the pump current through the oxygen pumping cell in order to maintain a constant voltage in the oxygen sensing cell. The ECM monitors the voltage variation in the sensing cell and attempts to keep the voltage constant by increasing or decreasing the amount of current flow to the pumping cell. By measuring the amount of current required to maintain the voltage in the sensing cell, the ECM can determine the concentration of oxygen in the exhaust. The HO2S voltage is displayed as a lambda value. A lambda value of 1 is equal to a stoichiometric air fuel ratio of 14.7:1. Under normal operating conditions, the lambda value will remain around 1. When the system is lean, the oxygen level will be high and the lambda signal will be high or more than 1. When the oxygen level is low, the lambda signal will be low or less than 1. The ECM uses this information to maintain the proper air/fuel ratio.
In order to obtain efficient performance of the 3-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 of 14.7:1 as possible. In order to accomplish this the PCM first compares the actual lambda value of the heated oxygen sensor 1 (HO2S 1) with the reference value of 1. If the HO2S 1 lambda value is less than the 1, the PCM determines that the air/fuel ratio is richer than the theoretical air/fuel ratio and reduces the fuel. If the lambda value of the HO2S 1 is more than 1, the PCM determines that the air/fuel ratio is lean and increases the fuel. 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 | HO2S 1 Output | Fuel System Response |
|---|---|---|---|
| Lean Mixture | High Oxygen | More than One | Rich Command |
| Rich Mixture | Low Oxygen | Less than One | 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 at wide open throttle (WOT).
- During 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 .