A production heavy-duty (truck) gasoline engine was equipped with both TWC single-bed and TWC dual-bed catalysts as well as a closed-loop control, stoichiometric air/fuel ratio carburetor system. The exhaust HC, CO and NOx emissions were measured on the EPA heavy-duty transient test cycle (cold start) applicable for 1984 engines. Catalyst conversion efficiencies for HC, CO and NOx of 76%, 94% and 77% were achieved with the dual-bed system. Corresponding catalyst conversion efficiencies for the TWC single-bed catalyst system were 64%, 56% and 79%. At low mileage, HC/CO/NOx emissions with the dual-bed system and EGR were 0.68/3.60/0.74 gm/BHP-hr which were well below the 1984-85 Federal limits of 1.3/15.5/1.7 gm/BHP-hr. No attempt was made to optimize the various components of the total system in this program. A loss of approximately 3% in maximum horsepower was observed relative to the standard production engine. Fuel economy was unaffected. Several undesirable design characteristics of the air/fuel mixture preparation and particularly mixture distribution for this engine were identified and suggestions are made to modify the air/fuel mixing and manifold to achieve the degree of control now attained in current light-duty (passenger car) engines.