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 to 1, for high conversion efficiency. 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 NOx conversion efficiency.

For vehicles using E85 the required air to fuel ratio is 9.8 to 1. For vehicles using E100 the required air to fuel ratio is 9 to 1. Other gasoline/ethanol mixtures require a variable air to fuel ratio between 14.7 to 1 to 9.8 to 1 dependent on the percentage of ethanol content.

Fig 1: TWC Converter Conversion Efficiency Chart

Courtesy of FORD MOTOR CO.