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For fuel economy improvement by lean-burn gasoline engines, extension of their lean operation range to higher loads is desirable as more fuel is consumed during acceleration. Urgently needed therefore is development of emission control systems having as high NOx conversion efficiency as three-way catalysts (TWC) even with more frequent lean operation. The authors conducted a study using catalysts loaded with potassium (K) as the only NOx trapping agent in an emission control system of a lean-burn gasoline engine.

A new single-brick Ba + alkali metals NOx-Trap catalyst has been developed to replace a two-brick NOx-Trap system containing a downstream three-way catalyst. Major development efforts include: 1) platinum group metals selection for higher HC oxidation with potassium-containing washcoat, 2) alumina and ceria selection, and Rh architecture design for more efficient NOx reduction and 3) NiO to suppress H2S odor. Mitsubishi Motors' 1.8L GDI™ with this Delphi new NOx-Trap catalyst with H2S control achieves J-LEV standard with less cost and lower backpressure compared to the previous model. It is further discovered that incorporation of NiO into the NOx-Trap washcoat is effective for H2S control during sulfur purge but has a negative impact on thermal durability and sulfur resistance. Further study to improve this trade-off has been made and preliminary results of an advanced washcoat design are presented in this paper. Details will be reported in a future publication.

Two-brick (tandem) three-way underfloor catalyst systems provide greater emission reduction performance compared to comparable single brick TWC systems, which contain the same amount of platinum group metal (PGM) for the same catalyst volume. This superior emission reduction performance is speculated to be due to front catalyst activity promoted by heat transfer from reverse exhaust gas flow in the gap between the front and the rear catalyst of tandem TWC system (hereinafter, tandem gap). Furthermore, the following findings were obtained by conducting experiments with model catalysts. 1) During catalyst light-off phase, conversion efficiency strongly depends on activity of the front portion of catalyst where temperature rises rapidly.

The widening of the selectivity windows of a three-way catalyst under A/F ratio modulation was shown to result from retention of adsorbed species on catalyst surfaces and their subsequent reaction with gas-phase reactants. A reaction model was developed based on adsorption properties of CO, HC and NOx and by computer simulation, the conversion efficiencies of CO, HC and NOx under A/F ratio modulation were calculated. The calculated and experimental results were in good agreement.

This paper describes the results of studies on the behavior of air-fuel ratios under feedback control, the effect of air-fuel ratio modulation on three-way catalyst conversion efficiency and emission test results with and without feed back control. As a further measure for decreasing automobile exhaust emissions, the three-way catalyst activity for reduction of CO, HC and NOx emissions is most effectively utilized when the normal engine air-fuel ratio perturbations are controlled and limited. In order to attain such an objective, this report describes the governing characteristics of an air-fuel ratio control system using an EFl engine coupled to a ZrO2 type O2 sensor and feed back loop. The conversion efficiency characteristics of a conventional three-way catalyst, using systematically modulated air-fuel ratios, and the resultant reduction of exhaust emissions with these systematic fluctuations and limited perturbations are also defined.

A mean value turbocharged engine model is useful in terms of accuracy and convenience for fuel economy strategies or engine controller development. Turbocharging process is a feedback system with a positive gain, i.e. increasing exhaust work leads to increasing a cycle work. The gain of the feedback system is determined mainly by exhaust work ratio in a cycle and inertia of the turbine. The work ratio was investigated based on engine test with EGR. A turbocharging process model was obtained using the work ratio in a cycle and theoretical equations. The model is applied to investigate manifold absolute pressure response with EGR.