Abstract: A catalystic converter includes an annular catalyst bed containing catalyst beads and defined by inner and outer coaxial perforated tubes. An outer shell coaxial with the catalyst bed defines a passage between the outer shell and the outer perforated tube. The passage is connected to an outlet port. Exhaust gas flows radially outward through the catalyst bed and then through the passage to the outlet port. Air is injected into the passage at a selected axial location and is drawn into the catalyst bed by the reciprocating action of the engine. As a result, the single catalyst bed has an inner zone with a scarcity of oxygen which reduces oxides of nitrogen and an outer zone with oxygen present. Unburned hydrocarbons and carbon monoxide are oxidized in the outer zone.