Three Way Catalytic (TWC) Converter Conversion Efficiency
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A TWC convertor requires a stoichiometric air fuel ratio of 14.7 pounds of air to 1 pound of gasoline, or 14.7:1, for high conversion efficiency. In order to achieve these high efficiencies, the air to fuel ratio must be tightly controlled with a narrow window of stoichiometry. Deviations outside of this window greatly decrease the conversion efficiency. For example a rich mixture decreases the HC and CO conversion efficiency while a lean mixture decreases the NO x conversion efficiency.
For vehicles using E85 the required air to fuel ratio is 9.8:1. Other gasoline/ethanol mixtures require a variable air to fuel ratio between 14.7:1 to 9.8:1 dependent on the percentage of ethanol content.