Page 36 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

18 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

Control of catalytic converters during a cold engine start-up

The temperature of the exhaust gas in a warmed-up spark-ignition engine can vary from 300 to
400 "C during idle, to about 1000 "C in full load operation. The temperature of the exhaust
gas of a warmed-up compression-ignition (diesel engine) contains a substantial quantity of
oxygen and is at lower temperatures (1 00-700 "C). However, during a cold engine start up, the
temperature of the catalytic converter is very low and the converter is not activated. Till the
moment that the activation (light-off) temperature of the converter is attained (-200-300 "C),
the HC and CO produced by the engine are not converted and contribute to a high pollution of
the atmospheric air. About 6040% of existing HC emissions are produced from the time that
the converter takes to start operating after vehicle ignition (fig. 1) for both the FTP '75 cycle
and the New European Driving Cycle (NEDC). This problem is discussed in detail in [2, 7, 9-
121.

Two main approaches exist to face this problem:
a) ac/ive approach, which relies on the controlled supply of additional energy to raise
exhaust gas temperature during cold start-up and consequently to the accelerated
activation of the catalytic bodies. Systems like electric heaters, afterburners and fuel
burners belong to this category.
b) passive approach. which relies on the employment of exhaust system design
changes in order to reduce cold start-up emissions (e.g. positioning of the catalytic
converter closer to the engine, use of secondary converters, HC adsorbents, heat
storage systems and heat exchangers and insulated exhaust pipes to reduce the heat
transfer of the exhaust gas between engine and catalytic converter)

I:'iectric heaters

An electric heater is installed upstream or inside the converter, or the converter itself consists
of electrically conductive material that can be heated if supplied with electric current (fig. 9d).
The addition of secondary air assists the oxidation of HC and CO and consequently to the
warming of the converter. In many cases, an electrically heated secondary converter (EHC) is
installed upstream of the main converter (fig. 9e). This EHC converter is of a small volume and
can be heated up very fast. It oxidizes the HC and the CO of the exhaust gases and the heat
produced warms up the main catalytic converter.

The heater is provided with electric current from the battery or the alternator of the vehicle.
The typical car battery is not a practical power source to supply the electrical power needed