Lambda control, basic function

Overview

This document contains information on:

 General

 Function requirements

 Function

General

The lambda control monitors exhaust gas conversion in the catalytic converters. As a result, the maximum volume of exhaust gases is converted into carbon dioxide (CO2), water (H2O) and nitrogen (N2).

Function requirements

 Engine management ON (circuit 87M)

 Engine in operation

 Coolant temperature (depending on operating status)

 Operating temperatures of oxygen sensors are reached

 Overrun mode fuel shutoff deactivated

Function

Lambda control

For optimum exhaust gas conversion, the fuel/air ratio must be made to approximate an air index of lambda (λ) = 1. The oxygen sensors upstream and downstream of the catalytic converter detect the oxygen and nitrogen oxide content in the exhaust gas. In the process, they compare the oxygen concentration in the exhaust gas with the oxygen concentration in the ambient air.

 In exhaust gas technology, the variable lambda (λ) stands for the mass ratio of air to fuel in the combustion process. A differentiation is made here between the following conditions:

 Low oxygen content (rich mixture): λ  1

 Stoichiometric ratio (ideal mixture): λ = 1

 High oxygen content (lean mixture): λ  1

P07.10-2038-75

Lambda control circuit

The oxygen sensor element upstream of the catalytic converter responds to the oxygen content in the exhaust gas and sends a linear voltage signal to the combustion engine control unit that corresponds to the lambda value.

If the oxygen content is too high (lean mixture), the combustion engine control unit initiates an increase in the injection quantity to the cylinders through the fuel injectors. As a result, the oxygen content in the exhaust gas decreases and the fuel/air ratio alters in the direction λ = 1.

If the oxygen content is too low (rich mixture), the combustion engine control unit initiates a reduction in the injection quantity to the cylinders through the fuel injectors. As a result, the oxygen content in the exhaust gas increases and the fuel/air ratio alters in the direction λ = 1.

Due to the permanent adjustment of the oxygen content in the fuel/air mixture, a closed-loop control circuit is created. In order to avoid noticeable problems in the handling characteristics, the combustion engine control unit carries out the adjustments slowly. If the mixture is leaned out, the lambda control factor alters in a positive direction. If the mixture is enriched, the lambda control factor alters in a negative direction.

Two-sensor control

The two-sensor control also has the following function requirements:

 Two-sensor control for the operating temperature of the catalytic converter achieved

 Lambda control active

 Oxygen sensor downstream of catalytic converter fault-free

The two-sensor control monitors the function of the oxygen sensor upstream of the catalytic converter as well as the effect of the catalytic converter, with an additional oxygen sensor downstream of the catalytic converter (guide and diagnostic sensor). The combustion engine control unit determines the mean lambda value from the signals of the oxygen sensor element downstream of the catalytic converter (guide and diagnostic sensor). The mean lambda value is compared with the signals of the oxygen sensor upstream of the catalytic converter and a stored specified value. If the variance is too large in several measurements, this leads to a self-adjustment of the mixture formation with the aid of a correction value. If the correction value exceeds the prescribed limit value and if the following sources of error are excluded for the mixture formation then the oxygen sensor upstream of the catalytic converter must be replaced.

The following faults can, for example, occur during mixture formation:

 Unmetered air due to leaks in the exhaust system upstream of the oxygen sensor

 Wear or carbon deposits on the fuel injectors,

 Defective sensors or actuators

 Engine wear (e. g. leaky valves)

If the prescribed limit value is exceeded, the instrument cluster displays a fault message. Exceeding of the limit value is stored in the fault memory by the combustion engine control unit.