The Lambda sensor

What are the functions of the Lambda sensor?

To improve the exhaust gas characteristics of Otto engines three-way catalytic converters were developed in combination with Lambda control.

How does this technique work?

We all know the principle: Engines burn organic matter, i.e. petrol, and because of this emit gases which harm the environment. In order to alleviate this unpleasant effect the catalytic converter is employed, which converts the emissions into gas less harmful to the environment.

In order to enable this, a Lambda sensor is installed between the engine and the catalytic converter. which measures the residual oxygen in the exhaust gas at the electrodes and passes the information to the engine control unit. This processes the signal and adjusts the optimum fuel-air mixture of the mixture controller.

The Lambda sensor

This is an oxygen sensor made of zircon dioxide (ZrO2), working according to the principle of W. Nernst (a German scientist, 1864-1941). He discovered that the normal potential of an electrode can be described in dependence of the acting oxygen concentration.

Projected to the Lambda sensor the residual oxygen contained in the exhaust gas can be measured by means of the electrodes of the Lambda sensor.

The sensor is so positioned in the exhaust gas stream that the outer electrode (outer platinum layer) is immersed in the exhaust gas, while the inner electrode is in contact with the ambient air.

Because the zircon dioxide ceramics material becomes conductive from about 300° C for oxygen ions, an electric voltage develops with differing oxygen content between outer and inner electrode of maximum 1 V, which is picked off via the cable terminals.

The oxygen concentration developing at the outer electrode depends to a high degree on the fuel-air ratio and the chemical reaction at the electrode surface and, thereby, produces in the Lambda (l) range the leap of the sensor voltage curve.