Air/Fuel Ratio Feedback Compensation - Closed Loop Operation
In order to obtain the most efficient engine performance with a good balance of power and fuel economy, the air/fuel mixture must be kept as close to the theoretical air/fuel ratio of 14.7:1 as possible. Maintaining the 14.7:1 air/fuel ratio also provides efficient operation of the 3-way catalytic converter (TWC) and a high clarification rate of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) in the exhaust gas stream, In order to accomplish this, the powertrain control module (PCM) first compares the input voltage from the heated oxygen sensor 1 (HO2S 1) with a specified reference voltage.
The HO2S 1 sensor is a wide band or wide range design. As with all oxygen sensors, the wide band HO2S measures the amount of oxygen in the exhaust system, but provides more information than the typical 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 PCM 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 PCM controls the pump current through the oxygen pumping cell in order to maintain a constant voltage in the oxygen sensing cell. The PCM monitors the voltage variation in the sensing cell and attempts to keep the voltage constant by increasing or decreasing the amount of current flow, or oxygen ion flow, to the pumping cell. By measuring the amount of current required to maintain the voltage in the sensing cell, the PCM can determine the concentration of oxygen in the exhaust. The HO2S input signal is displayed as a voltage within the range of 1-5 volts, with a steady value of 3.2-3.3 volts equal to the air fuel ratio of 14.7:1. If the HO2S 1 input voltage is lower 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. If the input voltage from the HO2S 1 is higher than the specified reference voltage, the PCM determines that the air/fuel ratio is leaner 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 Voltage | HO2S 1 Lambda | Fuel System Response |
|---|---|---|---|---|
| Lean Mixture | High Oxygen | High Voltage | More Than 1.0 | Rich Command |
| Rich Mixture | Low Oxygen | Low Voltage | Less Than 1.0 | 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 throttle actuator control (TAC) system and the air intake system. Refer to Air Intake System Description .