Auto Service World
Feature   April 1, 2008   by Auto Service World

Planar Oxygen Sensors Explained

Thought’s not the only maker of planar oxygen sensors– virtually every major supplier manufactures them for their OE customers–the announcement by Delphi that it would be offering planar oxygen sensors as part of its aftermarket offering highlights what has been a trend of continual advancement in the field of oxygen sensors.

In 1976 the unheated sensor was introduced. In 1982, the heated sensor arrived. The planar sensor arrived at the OE level in 1994, and 1998 saw the introduction of the planar wide band lambda sensor.

Veterans behind the counter will surely remember the earliest offerings, originally called lambda sensors in deference to their European lineage (“lambda” being the Greek letter used to define the air/fuel mixture in automotive circles). Those early sensors were pretty crude affairs in comparison with the modern equivalents.

The early oxygen sensors relied on engine heat only to reach proper operating temperature, about 325 to 350 degrees C (which could take a while to reach), and only then would be able to provide a voltage signal to the ECU to allow the fuel/air mixture to be varied. To add insult to injury, they also had a much shorter life than we are accustomed to today.

Technically speaking, even when the subsequent constructions added heating to shorten the time between engine startup and proper operation (which is to lessen emissions)–the operation was largely the same.

An O2 sensor relies on the fact that when the zirconium dioxide ceramic sensor reaches operating temperature, it registers a difference between the oxygen content of the 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 something 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.

To reach operating temperature faster, a unit that featured new “fast light off” sensor technology was developed, the first to use a planar sensor element. Tests conducted on Delphi sensors in 2000 revealed some significant improvements. In results published in an SAE paper, tests were conducted in open-loop and closed-loop mode under steady and transient conditions using a 1996 model year 2.4-litre DOHC in-line 4- cylinder engine with a close-coupled catalytic converter. Overall performance of the sensor showed relatively quick reaction time to reach the operating temperature. During the first 30-second cold-start period of the FTP 75 cycle, the closed-loop control and air-fuel ratio were adjusted to as lean as possible up to the stoichiometric air-fuel ratio without affecting driveability. Engine-out hydrocarbon emissions for the first cold cycle were reduced by 8.1% using the sensor and a shorter open-loop time. The operating characteristics of the sensor during a cold start made it possible to achieve closed-loop control in less than 10 seconds. Early closed-loop control can contribute directly to reducing hydrocarbon emissions, stated the engineers who published the paper.

But in the real world, even this style of sensor can only read good/no-good, and constantly cycles between the two, even if it gets there faster.

In this regard, the latest wideband planar oxygen sensor is a real game-changer.

This type of oxygen sensor is not an on/off switch; instead, it measures the actual air-fuel ratio. This provides much more precision, and also flexibility in fuel mapping with lean-burn strategies, for example.

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 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 and allowing the vehicle to enter closed loop operation sooner, resulting in reduced cold-start emissions.

These sensors can be identified as having five or more wires.

Wideband oxygen sensors are always used in the upstream, pre-catalyst position in the exhaust system.

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