Page 30 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

12 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

Types of catalytic converters

There are three main types of catalytic converters used in spark-ignition engines, which are
described in detail in [8].

A “one-way or oxidaiioti ca/a/ytk cotiverter” (fig. 6a) oxidizes the unburned CO and HC in
the exhaust gases and converts them in COz and HzO. In fuel injection engines the necessary
oxygen for the oxidation is simply received by increasing the quantity of air in the engine
air/hel mixture. In engines which use carburetors, a supply of secondary air in the exhaust pipe
upstream of the converter is necessary.

A ‘‘two-way catalytic corwer/er” (fig. 6b) comprises two consecutive catalytic bodies. The first
body is used to convert NO, to ammonia (NH3). The second body converts non-burnt and
partially burnt hydrocarbons (HC) to water and carbon dioxide (CO2) and carbon monoxide
(CO) to C0.r by oxidation. The regulation of the engine aidhe1 mixture becomes necessarily
rich (excess of hel) in order to convert the NO,. The supply of secondary air between the two
bodies may transform a part of NH, back to NO,. The system is not optimal, but it can be used
in the case of engines with carburetors without electronic control.

A “lhree-way ca/aly/ic converler” (fig. 6c) has the capability to eliminate efficiently the three
basic pollutants CO-NO,-HC. The main condition is that the engine aidfuel mixture should be
kept stoichiometric (14.7 gram air for 1 gram of petrol), which is the theoretical proportion for
a complete combustion of the mixture. The coefficient ?L (definition of h - see next chapter)
characterizes the importance of the difference between the real aidfuel ratio and the ratio
theoretically needed. This condition is satisfied by making use of a ?L (lambda) sensor or an
oxygen sensor upstream of the catalytic converter (fig. 4 and fig. 5c). These sensors detect the
composition of the exhaust gases. An electronic unit receives the output signal of the ?L or the
oxygen sensor and corrects the quantity of fuel injected to the engine. Three-way catalytic
converters are the most commonly used converters nowadays.
As shown in fig. 7, the three-way catalytic converter removes three pollutant in the exhaust
gases, i.e., NO,, HC, and CO at the same time by reducing NO, and oxidizing HC and CO,
when the air-fuel ratio of the exhaust gas is the stoichiometric air-he1 ratio. However, when
the air/fuel ratio of the exhaust gas becomes rich compared to the stoichiometric air-he1 ratio,
the ability of the three-way catalytic converter to oxidize HC and CO becomes low, and when
the air-he1 ratio of the exhaust gas becomes lean compared to the stoichiometric air-fuel ratio,
the ability of the three-way catalytic converter to reduce NO, becomes low.

The rate of decrease in the ability to reduce NO, when the air-fuel ratio is lean is more rapid
than the rate of decrease in the ability to oxidize HC and CO when the air-fuel ratio is rich.
Therefore, when the air-fuel ratio of the exhaust gas periodically swings between rich and lean
under the air-fuel ratio control, as explained in next chapter, the purification of the NO, by the