In petrol engines, three-way catalytic converters are used to convert the pollutants. The name refers to the three chemical processes that take place almost simultaneously as soon as the operating temperature is reached.
In a catalytic converter, three chemical conversion processes take place practically simultaneously:
• Unburned hydrocarbons (HC) are converted into carbon dioxide (CO2) and water vapour (H2O)
• Carbon monoxide (CO) is converted into carbon dioxide (CO2)
• Nitrogen oxides (NOx, NO2) are converted into nitrogen (N2) and oxygen (O2)
In order for a three-way catalytic converter to operate effectively, it requires a certain amount of oxygen in the exhaust gas. A lambda sensor at the inlet to the catalytic converter monitors the control of the preparation of the mixture.
The chemical processes in the catalytic converter operate most effectively at a lambda (λ) value of 1. A second lambda sensor, located downstream of the catalytic converter, checks all the control systems that have an influence on the exhaust gas (OBD - On-Board Diagnostics).