Clean air to breathe is essential both for humans and the environment. At the same time, one of the defining features of our modern standard of living is our capacity for mobility. However, vehicles with internal combustion engines leave harmful exhaust gases in their wake. Catalytic converters provide the technical response to this problem by converting the gases in a way that greatly reduces the harmful effect on humans and the environment. This conversion process is continuously optimised and monitored.
A catalytic converter converts exhaust gases. To do this, the catalytic converter has a specific structure and is coated with a complex substance. When the exhaust gas passes through it, the pollutants are converted.
A catalytic converter for exhaust gases consists of a catalyst carrier (monolith), the housing and associated connecting pipes, and mounts. The monoliths are coated with a complex substance consisting of various inorganic oxides – the so-called washcoat – and certain noble metals such as platinum, palladium and rhodium.
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).
Oxidation catalytic converters are used in diesel engines. These catalytic converters broadly have the same structure as those used in petrol engines. The only difference lies in the coating.
The oxidation catalytic converter efficiently oxidises carbon monoxide (CO) and hydrocarbons (HC). However, because the combustion method involves excess air and the lambda (λ) value is therefore greater than 1, the exhaust gas has a high residual oxygen content and it is consequently not possible to reduce the nitrogen oxides (NOx).
To reduce the nitrogen oxides, storage catalytic converters are used for engines that run with excess air (λ > 1), i.e. direct-injection petrol and diesel engines.
Storage catalytic converters function discontinuously. The catalytic converter stores the nitrogen oxides and has to be regularly regenerated. This regeneration is achieved by briefly modifying the operating conditions (λ < 1).
The SCR catalytic converter (Selective Catalytic Reduction) for diesel engines (?) operates continuously and selectively converts the nitrogen oxides into elementary nitrogen.
By adding ammonia in the form of an exhaust fluid (AdBlue®) to the exhaust system upstream of the SCR catalytic converter, the nitrogen oxides are selectively converted into elementary nitrogen. A conversion rate of up to 90% can be achieved.
The decisive factor determining the effectiveness of pollutant conversion is the quality of the catalytic converter. The decisive criteria here are the complex construction, the presence of an adequate volume and carefully selected materials. Premium quality catalytic converters from ERNST guarantee effective pollutant conversion.
Powerful monolith
Catalytic converters are subject to extreme mechanical and thermal stresses during the catalytic reaction. That is the main reason why the monolith in the ERNST catalytic converters is made from a material that can reliably withstand large fluctuations in temperature, gas pulses and vibrations.
Only a catalytic converter that has the right dimensions for the engine in question and has a suitable noble metal coating ensures a high conversion rate throughout the entire lifetime of the component. ERNST guarantees these premium characteristics for all its catalytic converters.
