Transportation Science

Why do cars need catalytic converters?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. Cars pollute the air, and clean-air laws have made catalytic converters a legal requirement because they convert harmful pollutants from an engine's exhaust into cleaner emissions. The devices work thanks to chemistry and the notion of a catalyst - - something that causes or speeds up a chemical reaction without itself being changed. The chemical reaction strips the exhaust of some of its polluting potency. Catalytic converters contain two types of catalyst: reduction and oxidation. They consist of a metal catalyst coating - - usually palladium, platinum and/or rhodium - - on a ceramic form built to expose the maximum surface area to the exhaust stream.

    The first stage of catalytic conversion, the reduction catalyst, uses a catalyst made of platinum and rhodium to diminish nitrogen oxide emissions. When a nitrogen oxide molecule touches the catalyst, the catalyst tears the nitrogen atom from the molecule and traps it, thereby freeing the oxygen. The second stage of catalytic conversion is the oxidation catalyst, which reduces unburned hydrocarbons and carbon monoxide by oxidizing or burning them with a catalyst composed of palladium and platinum. The final stage of catalytic conversion uses a control system to monitor the exhaust stream and regulate the fuel injection system. An oxygen sensor detects the oxygen level in the exhaust and transmits this data to the engine's computer. The computer then regulates the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio, ensuring that the engine is running at close to the ratio that pollutes the least.

    Although catalytic converters are great pollution reducers, there is always room for improvement. One flaw is that they require a relatively high temperature: When your car is cold, the efficacy of the catalytic converter is almost nil. One solution is to place the converter closer to the engine, where hot gases can heat it up quicker, but this may shorten the converter's lifespan. Another solution is to preheat the catalytic converter so it can start doing its job sooner. The drawback here is the electrical systems in most cars can't provide enough power to heat the converter fast enough; most people won't want to wait for the converter to warm up before they can start their cars.

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