Browse Publications Technical Papers 770298
1977-02-01

A Three-Way Catalytic Muffler Using Progressive Air Injection for Automotive Exhaust Gas Purification 770298

The present paper shows that the simultaneous removal with a single catalytic muffler of the three types of pollutants (NO, CO and HC) is possible without requiring a very precise adjustment of the engine equivalence ratio or permanent control of the supply of secondary air added ahead of the muffler.
The system studied consists of a muffler in which secondary air is supplied from a plurality of points located within the catalyst bed itself in order to have an oxygen concentration in the treated gas that increases progressively between the inlet and the outlet.
Experimental muffler designs are presented for monolithic or bead-type catalysts.
The numerous advantages of this technique as compared with the well-known dual bed system are described. It is in particular shown that the NO reduction rate is increased on precious metal catalysts when oxygen is added to some extent in a reducing exhaust gas, with maximum efficiency being reached just below the stoichiometric proportion. Ammonia formation due to the presence of oxygen is also reduced in the first part of the catalytic bed, and its reoxidation in the oxidizing section is directed toward N2 formation since oxygen is only in large excess near the outlet.
The results of several tests made on an engine bench, show that the ratio between the flow of exhaust gas and secondary air may vary widely with little change in the efficiency of the muffler. Some data obtained on a vehicle, using the CVS procedure, confirm these results.

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