Three Way Catalytic Converter (TWC) Conversion Efficiency
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A TWC requires a stoichiometric air/fuel ratio, 14.7 pounds of air to 1 pound of gasoline (14.7:1), for high conversion efficiency. In order to achieve these high efficiencies, the air/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 NOx conversion efficiency.
For vehicles using E85 the required air/fuel ratio is 9.8:1. Other gasoline/ethanol mixtures require a variable air/fuel ratio between 14.7:1 to 9.8:1 dependent on the percentage of ethanol content.