Catalytic converter

In chemistry, a catalyst is a substance which affects the speed of a reaction without itself being consumed. Chemical reactions also occur in passenger vehicle catalytic converters. There are two types: computer-controlled and open-loop catalytic converters. The chemical processes which take place in both types are the same.

The interior of the catalytic converter contains the precious metals platinum, rhodium and palladium. As soon as the hazardous substances in the exhaust gas – carbon monoxide, hydrocarbons and nitrogen oxide – come into contact with the precious metals, they are instantly converted:

  • carbon monoxide and the hydrocarbons are converted to water and carbon dioxide (through oxidation).
  • Nitrogen oxides are converted to nitrogen, oxygen and carbon dioxide (through reduction).

Since three groups of hazardous substances are converted, the device is called a three-way catalytic converter. Modern, computer-controlled catalytic converters achieve a reduction rate of more than 95 percent.

The lambda sensor regulates the air-fuel mixture.
With a controlled catalytic converter the lambda sensor ensures that the optimal mixture is always combusted

Catalytic converters, computer-controlled

In order for the three-way catalytic converter to convert as many hazardous substances as possible, it requires a special air-fuel ratio in the engine: 14.7 kg of air to 1 kg of fuel. This is where the lambda sensor comes into play: Together with the engine control unit they form a control circuit, which ensures that only this ideal ratio is used for combustion. Therefore, the term catalytic converter with "lambda control” is also used - in other words a controlled catalytic converter.

With an open-loop catalytic converter there is not control circuit and/or no engine control unit which ensures a constant air-fuel mixture.
With an open-loop catalytic converter there is no control circuit and/or no engine control unit which ensures a constant air-fuel mixture

Catalytic converter, open-loop

At the beginning of the 1980s open-loop catalytic converters were still used. They do not work as effectively and constantly as computer-controlled catalytic converters, because the engine deviates from the ideal air-fuel ratio more frequently.

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