Lambda sensor, basic function

P07.04-2376-73

Overview

This document contains information on:

 General

 Function

General

The oxygen sensor upstream of the catalytic converter records the residual oxygen content in the exhaust for the following tasks:

 Lambda control

 Selfadaptation of mixture formation

 Function chain tests (for European On-Board Diagnostics)

The lambda sensor downstream of the catalytic converter (guide and diagnostic sensors) detect the residual oxygen content in the exhaust gas for the following tasks:

 Two-sensor control

 Monitoring catalytic converter efficiency

Function

Oxygen sensor upstream of catalytic converter

The oxygen sensor upstream of the catalytic converter is designed as a potential-free, insulated, planar (flat) wideband oxygen sensor. The sensor element consists of a combination of a Nernst concentration cell (sensor cell) and an oxygen pump cell. The exhaust in the measuring chamber for the concentration cell passes through the diffusion gap. By applying the supply voltage at the pump cell platinum electrodes, oxygen in pumped into or out of the measuring chamber. If λ = 1 is already present in the measuring chamber, oxygen transport is not required. In this case, the pump current = 0 mA.

The combustion engine control unit subsequently regulates the voltage so that the mixture composition in the measuring chamber is constantly set to the value λ = 1. In the case of lean exhaust gas, the pump cell pumps oxygen out of the measuring chamber. The combustion engine control unit detects this process by means of the negative pump current.

If the exhaust gas is in contrast rich, oxygen is pumped into the measuring chamber. The oxygen is generated by the catalytic decomposition of carbon dioxide (CO2) and water. The combustion engine control unit detects this process by means of the positive pump current. The respectively required pump current is the indicator for the air/fuel ratio lambda.

Oxygen sensor downstream of catalytic converter

The oxygen sensor downstream of the catalytic converter is designed as a potential-free, insulated, planar (flat) non-linear oxygen sensor. The sensor element consists of a ceramic probe body that becomes conductive for oxygen ions from approx. 300 °C. If the oxygen concentration on both sides of the sensor ceramic differs, a signal voltage is produced at boundary surfaces as a result of particular properties of the sensor ceramic (Nernst voltage). The electronics in the combustion engine control unit provides a so-called sensor countervoltage of approx. 450 mV to the lambda sensor. The signal voltage of the lambda sensor has a steep voltage jump during the transition from a lean to rich mixture. Due to this property, only the mixture condition can be evaluated. A quantitative evaluation of the mixture condition can only be realized via the lambda sensor upstream of the catalytic converter.

P07.61-0427-01

Oxygen sensor heater

To quickly bring the ceramic probe bodies to operating temperature, the lambda sensors are heated with a heating output of approx. 7 to 10 W. The combustion engine control unit actuates the heaters of the lambda sensors using a ground signal. For a very cold exhaust system and while condensation is present, the lambda sensor heaters are deactivated to prevent damage due to a thermal shock. The power supply of the lambda sensor heaters is realized via the engine management.