Counter Talk: Knowledge Building: Wideband Oxygen Sensors

While oxygen sensors have been an indelible part of automotive technology for decades, the technology continues to evolve.

In general, an O2 sensor relies on the fact that when the zirconium dioxide ceramic sensor reaches the high temperatures generated by the car’s engine, typically 325 to 350 degrees C, it registers a difference between the oxygen content of outside air (the known constant) and the exhaust gas oxygen content. This difference is emitted as a voltage signal. If there is a shortage of free oxygen, as when unburned fuel is present in the exhaust, the sensor generates a voltage of somewhat over 0.8 volts; if there is free oxygen present, the sensor generates zero voltage. The signal is sent to the ECU, which measures the electrical switching points of the oxygen sensor voltage as the exhaust gas oxygen content changes. The engine computer reads this signal and adjusts the fuel mixture accordingly in order to maintain the ideal air-fuel ratio, referred to as stoichiometric, of 14.7:1.

If you see a trace of an oxygen sensor on a scope, you’ll see the telltale cycling of the voltages. In the real world, this style of sensor can only read good/no-good, and constantly cycles between the two. That is a good strategy, but not the best.

Hence the development of the Wideband Oxygen Sensor.

These sensors measure the actual air-fuel ratio, not just if it’s right or not. This provides much more precision, and also flexibility in fuel mapping with lean burn strategies, for example, and just the opposite for performance applications where leaning out at high rpm can be a serious problem.

To get this added precision, this type of sensor uses a very different construction and adds an oxygen pump. The oxygen pump uses a heated cathode and anode to pull some oxygen from the exhaust into a “diffusion” gap between the two components. The sensing element and oxygen pump are wired together in such a way that it takes a certain amount of current to maintain a balanced oxygen level in the diffusion gap. The amount of current required to maintain this balance is directly proportional to the oxygen level in the exhaust. This gives the engine computer the precise air/fuel measurements it needs to meet the new emission requirements.

The wideband oxygen sensor receives a reference voltage from the engine computer and generates a signal current that varies according to the fuel mixture.

When the air/fuel mixture is perfectly balanced at 14.7:1 (the stoichiometric ratio and lambda equals 2), the sensor produces no output current. When the air/fuel mixture is rich, the sensor produces a “negative” current that goes from zero to about 2.0 milliamps when lambda is 0.7 and the air/fuel ratio is near 11:1.

When the air/fuel mixture is lean, the sensor produces a “positive” current that goes from zero up to 1.5 milliamps as the mixture becomes almost air.

As noted, these sensors use a planar zirconia ceramic element, so that they heat up much faster than other types of sensors, reaching their operating temperature of 700 to 800 degrees C (about twice that of a conventional sensor) more quickly allowing the vehicle to enter closed loop operation sooner, resulting in reduced cold-start emissions.

These sensors can be identified with having five or more wires. In addition, these sensors are used with the newly developed gasoline direct injection engines. Direct injection engines can use stratified charges, which produce a very lean mixture in the combustion chamber, and these sensors must be used because of their ability to measure from very lean to very rich accurately.

Wideband oxygen sensors are always used in the upstream, pre-catalyst position in the exhaust system. There several terms used by the vehicle manufacturers to differentiate wideband oxygen sensors from switching type oxygen sensors that are synonymous with the term wideband oxygen sensor. These terms include Air/Fuel ratio sensor and A/F sensor. When a request is made for an Air/Fuel or A/F sensor, know that these parts are also known as wideband oxygen sensors.

Special thanks to Robert Bosch for providing information for this article.