1-18
FEATURES
GEN
INFO
Functions of components
Catalyst
Because the conditions in which NOx is generated are directly opposed to those of CO and HC, there is
a limit to the extent to which the concentration levels of these harmful elements can be reduced in the
combustion stage. Hence, the function of the catalyst is to clean the exhaust gas at a high degree of
efficiency by removing CO, HC, and NOx in the exhaust stage.
This model has adopted a monolith type metallic catalyst with a honeycomb construction, which
achieves a low exhaust resistance through the large surface area of the catalyst body (with a high level
of cleaning efficiency).
Catalytic substances consisting of precious metals such as platinum and rhodium are adhered to the
wall surface of these honeycomb cells, which are enclosed in the exhaust pipe. As the exhaust gas
comes in contact with these catalytic substances, the chemical reactions of oxidation and reduction
advance in order to clean the exhaust gas.
S
The CO and HC oxidize with the oxidation function of platinum, and are converted into harmless
carbon dioxide (CO
2
) and water (H
2
O), resulting in cleaner exhaust gases.
CO + 1 / 2 O
2
CO
2
HC + O
2
CO
2
+ H
2
O
S
The NOx is reduced by the reduction function of rhodium, which converts NOx into harmless nitro-
gen (N
2
) and oxygen (O
2
), resulting in cleaner exhaust gases.
NOx
N
2
+ O
2
To clean the exhaust gases at a high rate of efficiency through the maximization of these catalytic capa-
cities, it is necessary to maintain and control the mixture in the vicinity of the stoichiometric airfuel ratio
of (14.7 : 1) at all times. As a means of maintaining the stoichiometric ratio, this system has adopted an
O
2
feedback compensation method that uses an O
2
sensor.
Large amounts of both CO and HC are generated when the mixture is rich (as indicated by insufficient
O
2
region
A
). Conversely, large amounts of NOx are generated when the mixture is lean (as indicated
by excessive O
2
region
B
). Under these conflicting characteristics, the system maintains the mixture
within an extremely narrow range
c
of stoichiometric ratio (14.7 : 1). As a result, the function of the
catalyst is maximized, making it possible to clean the exhaust gases at a high degree of efficiency.
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