Vehicle applicability: This diagnostic supplement covers multiple Ford vehicles and engines. Follow the application named in the source below. Some references apply to vehicles other than Mustang.
Description and Operation 1-61
Catalyst And Exhaust Systems
Typical Inline Engine
Item | Part Number | Description |
|---|---|---|
1 | — | Engine |
2 | — | HO2S12 |
3 | — | Catalytic Converter |
4 | — | Universal HO2S11 |
5 | — | Exhaust Manifold |
Catalytic Converter
A catalyst is a material that remains unchanged when it initiates and increases the speed of a chemical reaction. A catalyst also enables a chemical reaction to occur at a lower temperature. The catalytic converter assists in controlling the concentration of exhaust gas products released to the atmosphere. It contains a catalyst in the form of a specially treated ceramic honeycomb structure saturated with catalytically active precious metals. As the exhaust gases come in contact with the catalyst, they are changed into mostly harmless products. The catalyst initiates and speeds up heat producing chemical reactions of the exhaust gas components so they are used up as much as possible.
Light Off Catalyst
As the catalyst heats up, converter efficiency rises rapidly. The point at which conversion efficiency exceeds 50% is called catalyst light off. For most catalysts this point occurs between 246°C to 302°C (475°F to 575°F). A light off catalyst is a three way catalytic (TWC) converter that is located as close to the exhaust manifold as possible. Because the light off catalyst is located close to the exhaust manifold it achieves the required temperature faster and reduces emissions more quickly than the catalyst located under the body. Once the catalyst lights off, it quickly reaches the maximum conversion efficiency for that catalyst.
Three Way Catalytic (TWC) Converter Conversion Efficiency
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 NO conversion efficiency.
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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.
2016 Powertrain Control/Emissions Diagnosis, Gasoline Engines, 3/2015
2016 Mustang workshop publication supplied to Mustang Atlas. Original technical text and illustrations; HTML formatting by Mustang Atlas. Source page numbering is retained for traceability. Follow the configuration, revision, warnings, and cross-references in the material. About this edition.