Lambda Sensor

The so called lambda sensor is an exhaust gas oxygen sensor which is utilized in motor vehicles with three-way catalysts for the reduction of pollutants in the exhaust gas. It monitors the concentration of oxygen in the exhaust gas in order to keep the air-fuel ratio within tight limits, close to the optimum catalyst efficiency which is at the stochiometric ratio for complete combustion of about 14.7 kg air per 1 kg gasoline.

The lambda sensor utilizes quite the same solid state electrolyte material that was used in the Nernst lamp (yttrium stabilized zirconia). It is characterized by its high conductivity for O2- ions at elevated temperatures (about 700 K).

Usually, at the heart of the sensor is a hollow cylinder of solid electrolyte. The inner and outer walls are covered with microporous platinum electrodes. The outer wall is immersed in the stream of exhaust gas whereas the inner wall is exposed to ambient air whose oxygen content of about 21 % by volume serves as a reference.

For the reduction reaction of oxygen

O2 + 4 e- = 2 O2-

according to the Nernst equation the sensor voltage is approximately

Us = (R T / 4 F) ln [p(O2, reference) / p(O2, exhaust)]

(temperature T, partial pressure p, gas constant R, Faraday constant F)

The largest manufacturers of lambda sensors are Bosch, Germany and NTK, Japan.

bosch0258001051.jpg (28 KB)

Two used lambda sensors (Bosch 0258001051)
Part of the protective slotted metal cap has been cut off from the bottom one in order to expose the ceramic sensor element.

Walther Nernst homepage

Revised 2005-06-28