31
C A T A L Y T I C C O N V E R T E R S
Figure 12
Typical Catalytic Converters
The first attempt at reducing emission levels in automobiles was to get the air-fuel ratios as close as possible to
stoichiometric. However, even engines designed for low emissions, and which are operating properly, may not have
HC and CO emission levels low enough to meet Clean Air Standards. Catalytic converters are installed to further
reduce emission levels.
A catalyst is a substance that increases the rate of a chemical reaction without being used up it. The catalytic converters
used in automobiles contain a combination of the noble metals platinum, palladium and rhodium. These metals are
applied to small beads or to ceramic baffle materials. These materials, called substrates, provide tremendous surface
area for exhaust gases to come in contact with the noble metal catalysts.
Figure 12 shows the construction of two typical catalytic converters.
In operation, exhaust gases pass into the catalytic converter from the engine, where the gases flow past the catalytic
metals. Contact with these metals causes reactions, known as catalytic oxidation, in the exhaust gases, adding O
2
to the
molecular structure of HC and CO. This turns the HC into H
2
O (water) and CO2 (carbon dioxide). The CO is
converted to CO
2
. Figure 13 shows the equations for this process.
Oxidation of any compound requires an abundance of O
2
.
In most cases, engines are equipped with an auxiliary air
inlet device, typically called an A.I.R. (Air Injection Reaction) pump or a pulse air system, to inject additional air into the
exhaust manifold or catalytic converter, providing the oxygen needed for the reaction (oxidation) to take place.
Converters of this type are called oxidation or two-way converters since they only treat two gases.
