Three-Way Catalyst - Subaru LEGACY 2002 Quick Reference Index

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Emission Control (Aux. Emission Control Devices)

4. Three-way Catalyst

The basic material of three-way catalyst is platinum (Pt), rhodium (Rh) and palladium (Pd), and

a thin coat of their mixture is applied onto honeycomb or porous ceramics of an oval shape (car-

rier). To avoid damaging the catalyst, only unleaded gasoline should be used.

The catalyst reduces HC, CO and NOx in exhaust gases through chemical reactions (oxidation

and reduction). These harmful components are reduced most efficiently when their concentrations

are in a certain balance. These concentrations vary with the air-fuel ratio. The ideal air-fuel ratio

for reduction of these components is the stoichiometric ratio.

Therefore, the air-fuel ratio needs to be controlled to around the stoichiometric ratio to purify the

exhaust gases most efficiently.

THREE-WAY CATALYST

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