Snap-On Diagnostics DGA 1000, Руководство пользователя

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5-12
Fundamentals
Catalytic Converters
The first attempt at reducing emission levels in automobiles was to
get air-fuel ratios as close to stoichiometric as possible. However,
even engines designed for low emissions, and that are operating
properly, may not have
HC and CO emission levels low enough to
meet clean air standards. To assist in lowering these emissions,
catalytic converters were installed.
A catalyst is a substance that increases the rate of a chemical
reaction without being used up itself. Automobile catalytic
converters contain a combination of three noble metals - platinum
(Pt), palladium (Pd) and rhodium (Rh). These metals are applied to
small beads or ceramic baffle materials, called substrates, that
provide a tremendous surface area for exhaust gases to contact
the noble metal catalysts.
In operation, exhaust gases pass into the catalytic converter from
the engine, where the exhaust gases flow past the catalytic metals.
Contact of the exhaust gases with these metals causes reactions,
known as catalytic oxidation, adding
O
2
to the molecular structure
of
HC
and
CO
. This turns the
HC
into water (
H
2
O
) and the
CO
into
CO
2
. These converters are called oxidation or two-way converters
since they only treat two gases.
Figure 5-10 : Typical Catalytic Conver ters