MX-6 L4-1991cc 2.0L DOHC (1993)
Three-Way Catalytic Converter: Description and Operation
Catalytic Converter / Exhaust System
A three-way catalytic converter is used downstream of the exhaust manifold to reduce oxides of nitrogen (NOx) and oxidize hydrocarbons (HC)
and carbon monoxide (CO). The converter contains the catalyst elements platinum, palladium, and rhodium, which promote oxidation and
reduction reactions at high temperatures. Emissions are converted into carbon dioxide, water vapor and nitrogen.
THE OXIDATION REACTION
To change HC and CO into harmless materials, catalytic elements (platinum and palladium) start an oxidation, or burning, reaction in the catalytic
converter. Oxidation is the addition of oxygen to an element or compound. During the process, oxygen (O2) mixes with excess HC and CO to
form H2O and CO2. Considerable heat (900°F to 1,600°F) is generated by oxidation reduction. Use caution while working on or around the
catalytic converter and other exhaust system components.
THE REDUCTION REACTION
Reduction is the opposite of oxidation. The reduction reaction changes NOx to harmless nitrogen and free oxygen, which is then consumed in the
oxidation reaction with other pollutants. Catalytic elements required for the reduction process are platinum and rhodium.
The Three-Way Catalytic Converter (TWC) is designed to react with all three major exhaust pollutants, but works best to reduce NOx when the
CO level of the exhaust is between 0.8 and 1.5 percent. As the CO level increases or decreases from that percentage, converter efficiency
decreases. Exhaust gas first passes through the catalyst, where oxides of nitrogen are reduced, then through the catalyst where HC and CO are
changed to carbon dioxide and water vapor.
