Subaru 2002 Impreza Service Manual page 115

THREE-WAY CATALYST
Emission Control
(Aux.
Emission Control Devices)
4. Three-way Catalyst
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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 (carri-
er). To avoid damaging the catalyst, only unleaded gasoline should be used.
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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 concentra-
tions 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.
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Therefore, the air-fuel ratio needs to be controlled to around the stoichiometric ratio to purify the
exhaust gases most efficiently.
EC-7