Page 39 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Introduction 21

Disadvantages of the method are: increased fuel consumption, a significant number of
components, the necessity to monitor safe ignition procedures and to protect the converter
from overheating. Advantages and disadvantages of the above described methods are
extensively discussed in [ 121.

Close-coupled converters

Positioning the converter close to the exhaust port of the internal combustion engine is an
efficient way of increasing the inlet temperature of the converter during cold start-up of the
engine (fig. 9b). In high load condition of the engine the aging of the catalyst material can be
accelerated due to high temperatures of the exhaust gases (up to 1000 "C).

Palladium is a more suitable catalyst material for close-coupled converters than rhodium and
platinum because:
a) it has a lower operating temperature
b) it is extremely effective in removing hydrocarbons, particularly with rich aidfie1
mixtures
c) it is much more tolerant of high temperatures

Secondary converters

A secondary or starter or light-off or auxiliary or pre-catalytic converter or pre-catalyst
upstream can be also installed upstream of the main catalytic converter (see fig. 9c). This
converter is installed close to the exhaust manifold and has a small volume in order to be
activated very fast (small thermal inertia). The oxidation of the HC and CO on this converter
releases heat that activates fast the main converter. After the main converter is activated, the
secondary converter is usually bypassed to protect it from high temperatures developed close
to the exhaust manifold (see US5089236 (1992), EP0727567 (1996)). As already mentioned
above, an electric heater may also be installed in the secondary converter to activate even
faster the main converter (fig. 9e).
Secondary converters use palladium-rhodium or palladium only as catalysts with high precious
metal loading thus favoring exothermic oxidation reactions and consequently producing heat
utilized to heat-up the main converter.