Functioning method of a Lambda sensor

There are two types of Lambda sensors. They differ in respect of the ceramic elements with which the composition of the exhaust gas is measured.

Zirconium dioxide sensors:

The outer side of the zirconium dioxide element is in direct contact with the exhaust gas. The inner side is in contact with air. Both sides of the element are coated with a thin platinum layer. Oxygen-ions pass the element and leave an electric charge on the platinum layer. The platinum layer functions as an electrode; the special signal is transmitted from the element to the connecting wire of the sensor.

The zirconium dioxide element becomes conductive for oxygen ions from a temperature of approx. 300° C. When the concentration of oxygen on both sides of the zirconium dioxide element is different a voltage arises because of the particular properties of the element. With a lean ratio fuel : air a low voltage is produced. If the ratio is rich, the voltage is high.

The typical voltage leap takes place at a fuel-air ratio of approx. 1:14.5. This ratio can also be characterised by the air number (λ = 1 corresponds to fuel-air ratio of 1:14.5, i.e. there is complete combustion.

Therefore the name Lambda sensor. The mixture controller of the engine control system controls the fuel-air ratio. Engine control obtains the information required for this by the Lambda sensor. The sensor produces a voltage only when the operating temperature of over 300° C is reached. Therefore, the element takes a certain time after the engine was started until it is heated by the exhaust gases.

Today, most sensors have an internal ceramics heater which reduces the time until the sensor can work.

Titanium dioxide sensors:

The titanium dioxide element does not produce a voltage as does the zirconium dioxide element. Instead, the electric resistance of the titanium dioxide element changes in relation to the oxygen concentration in the exhaust gas. At Lambda 1.0 there is a significant change of the resistance. When a voltage is applied to the element the output voltage changes in relation to the oxygen concentration in the exhaust gas. In contrast to the zirconium dioxide sensor, the titanium dioxide sensor does not need reference air as its method of functioning is different.

Therefore, also the dimensions of the titanium dioxide sensor are smaller. The sensor element, its platinum electrode, and the heating element are being built, using the thick-film multi-layer technology of NGK, on a ceramic substrate. Sensors, based on titanium dioxide and on zirconium dioxide are not interchangeable because of the different dimensions and control strategies, which are used for evaluating the sensor signal.