Page 40 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

22 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

Secondary converters create a high back pressure which reduces the overall engine efficiency
and robs the engine of power output.

Hydrocarbon adsorhenls

Another approach to control the hydrocarbon emission during the cold-start of the engine is by
installing a HC adsorbent upstream of the catalytic converter. This adsorbs HC during cold
operation of the engine and releases HC during a hot operation of the engine, when the
catalytic converter is activated. The catalytic converter then converts the released HC.
Crystalline molecular sieves such as zeolites have HC adsorbing properties. Zeolites are
aluminum silicates with controlled porosity used as cracking catalysts in petrochemical
refineries and retain hydrocarbons on their surface via the attraction of electric charge. As the
exhaust temperature increases, the heat desorbs the trapped hydrocarbons by breaking the
electric bonds, permitting them to flow into the main catalytic converter for oxidation.
The HC adsorbent can be also bypassed during a hot operation of the engine to spare the HC
adsorbent from harmfid thermal excursions (see EP0727567 (1996), EP0602963 (1994)).

Secondary air

During the cold start phase, most engines must operate with richer than stoichiometric fbel
mixtures in order to ensure smooth operation without stalling. The lack of oxygen in the
exhaust allows non-oxidized CO and HC to be emitted to the environment. This problem is
solved by injecting secondary air in the exhaust gas downstream of the engine exhaust ports.
The injected air oxidizes the CO and HC of the exhaust gas and the heat produced lights-off
the catalytic converter.

The light-off time achieved in this way in FTP '75 test cycle is less than 40 sec (compared to
100 sec of the conventional catalytic system). The respective reduction of cumulative HC
emissions in the FTP '75 test cycle can reach even 50% (depending on the engine and exhaust
piping characteristics).

Secondary air injection requires a separate piping system, as well as I an electrically driven
(~9, 171).