Two-step Finger Sensor

Function

If the task is the exact adherence of a certain Lambda value and not to produce measurement values, the two-step finger sensor is in use. The vehicle area was the first working area of this device, in the beginning it was still unheated. Although it can in principle also control another Lambda value, here it is responsible only for the observance of Lambda = 1.

How it works

Primarily the lambda oxygen sensor consists of a gas-impervious ceramic body from zircon dioxide (brown - left part) on which from the inside and outside an electrically conducting, gas-permeable platinum layer (red) is pressed. One is connected via the outer skin with ground and the other via a heat-resistant pipe with the control unit. The outside of the ceramic body is exposed to the exhaust gas (grey) and is circum washed by it, while the inside is exposed to the air (light blue). Protecting the sensor against sudden load, quick temperature changes and combustion residues there is a metal cover in the exhaust gas pipe with particularly formed openings for the exhaust gas.

In operation, starting at around 280 - 300°C, a difference in oxygen concentration between the internal and external platinum layer develops, depending on the exhaust gas and the outside air. Though the ceramics are permeable for oxygen, they are not for their electrons. Thus the lambda oxygen sensor tension is formed. It becomes the bigger, the fatter the mixture, the less oxygen there is in the exhaust gas.

To reach the working temperature as quickly as possible, taking also large distances to the engine into account, most lambda oxygen sensors are heated (dark red) and have, therefore, two additional connections. It is well-working if the ohmmeter shows about 2 - 15 ohms for the heating in the disconnected state with 20°C.