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Oxygen sensor versus air fuel ratio sensor

Air fuel ratio sensor

What are the biggest difference between an Oxygen sensor and an air fuel ratio sensor?

1. Both measure oxygen in the exhaust stream, but a wide range air fuel ratio sensor (AFR) measures the actual amount of oxygen, as opposed to an ordinary oxygen sensor that just reports a rich or lean condition in the exhaust.

2. An AFR sensor can measure mixtures from extremely rich to extremely lean. They can even measure the oxygen content of air. This allows the ECM much more control over the air/fuel mixture, allowing the engine to run leaner and improve gas mileage.

3. While zirconia oxygen sensors have a typical lifespan of only 80,000 to 100,000 miles, AFR sensors are designed to last up to 150,000 miles.

4. In addition to the much greater accuracy, an AFR sensor reacts to changes in the exhaust stream much faster than ordinary O2 sensors. This allows the carmakers to use thinner coatings of precious metals in the catalytic converter because they won’t have to burn off as much fuel or store as much oxygen as engines outfitted with a conventional oxygen sensor.

How does an air fuel ratio sensor work?

A bit about conventional zirconia oxygen sensors first

A zirconia oxygen sensor acts like a battery, producing a signal (high or low) voltage that rises up to 0.9 volts when it detects a rich exhaust and as low as 0.3 volts in a lean exhaust stream. The change from rich to lean is fast, so the ECM has to devote lots of reading and analysis power to keep track and make sense of the rapid transitions.

Once the oxygen sensor detects a lean condition, the signal voltage drops low. But it can’t drop even lower if the oxygen level drops even further.

Wideband air fuel ratio sensor operation

An AFR works the opposite of a zirconia sensor; its output INCREASES as the exhaust gets leaner. It can even read an extremely lean 18:1 ratio.

Rather than working like a battery that generates voltage changes, an AFR sensor requires a power source. The amount of power varies with each carmaker. Toyota, for example, supplies 3.3 volts to the AFR electronics. The internal circuitry converts the power into a voltage that has a neutral bias. In other words, the voltage can flow in a positive or negative flow depending on what the sensor detects.

The bias (no voltage flow) happens when the sensor detects a stoichiometeric/lambda air/fuel mixture of 14.7:1. In other words, when the AFR sees lambda there’s no current flow to the ECM. As the mixture moves toward lean, the signal moves in the positive direction. When it detects a rich condition, voltage flow switches to a negative flow. The flow not only reverses but also changes in amplitude. So the signal the ECM sees is a liner progression instead of a switching high/low voltage signal.

Where are air fuel ratio sensors used?

Due to their higher costs, AFR sensors are used as upstream sensors (before the catalytic converter) so the ECM can have more data to improve performance and fuel economy. Since the downstream sensor only measures catalyst efficiency, carmakers can use the less expensive zirconia sensors for that application.

air fuel ratio sensor

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat

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