Three-way catalytic converter and lambda sensor
Function, regulation and exhaust gas reduction
The modern engine control system in petrol engines ensures efficient exhaust gas after-treatment with less fuel consumption and environmentally harmful exhaust gases. The regulated three-way exhaust gas catalytic converter converts the exhaust gases / raw emissions such as NOx, CO and hydrocarbons into CO₂, water and nitrogen at temperatures above a certain value. The ratio of air to fuel is crucial for this. The precise lambda control with jump probe or wide range lambda probe upstream of the catalytic converter determines the residual oxygen in the exhaust gas.
This value allows the engine management system to calculate the amount of fuel to be injected so that the air-fuel mixture combusts in a stoichiometric ratio and therefore combusts optimally. The monitor probe downstream of the catalytic converter monitors the operation of the catalytic converter via the residual oxygen content and detects defects at an early stage. In this article, you will learn more about thermal behaviour, engine control and the role of the lambda sensors in modern vehicles. How do three-way catalytic converters work and why do you need them? You will find further explanations about control sensors, monitor probes and lambda sensors in this article.
Exhaust gas after-treatment with an exhaust gas catalytic converter
During combustion, raw emissions such as carbon monoxide (CO), nitrogen oxides (NOx), sulphur dioxide and hydrocarbons are generated in the cylinders in the petrol engine and in diesel engines. The exhaust gas after-treatment converts these raw emissions into less harmful exhaust gases. Ideally, only nitrogen, carbon dioxide (CO2) and water remain.
The conventional exhaust gas after-treatment method in a petrol engine is the "regulated three-way catalytic converter", also known as the exhaust gas catalytic converter. Three chemical reactions of reduction and oxidation occur simultaneously here. The lambda is determined using the lambda sensor upstream of the catalytic converter – also known as a control sensor. This value indicates the ratio of air to fuel in the mixture.
Using this value as an input variable, the vehicle control unit controls the addition of fuel to achieve efficient combustion and complete oxidation. This improves engine performance while reducing pollutant emissions.
The function of the lambda sensor
The lambda sensor is a central component in the exhaust system of vehicles with a petrol engine. The main functions of the lambda sensor are the measurement of the oxygen content in the exhaust gas and the regulation of the air / fuel ratio. The oxygen content in the exhaust gas indicates whether the air-fuel mixture is too rich or too lean. The engine control unit uses the information from the lambda sensor to regulate the mixture so that it is as stoichiometric as possible (air / fuel ratio of lambda = 1). Only then can the three-way catalytic converter efficiently convert all pollutants.
The lambda sensor is therefore also an important component for monitoring the general function of the catalytic converter and the exhaust gas after-treatment in the vehicle. If the values measured by the lambda sensor are incorrect or if the signals from the control sensor and the monitor probe are too similar, this indicates that hardly any exhaust gas purification is taking place in the catalytic converter – the catalytic converter is defective and the OBD (on-board diagnostics) reports a fault.
Video – How do the three-way catalytic converter and lambda sensor work
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