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The exhaust emissions of two stroke vehicles like motorbikes and scooters contribute to the pollution in urban areas of developing countries in South East Asia and India to a major extent. But also in Japan and selected European countries exhaust gas limitations become effective from 10/1998 and 06/1999 for these vehicles. To control this emissions catalytic aftertreatment by Hot Tubes® and/or monolith type catalysts are applied. Due to the constant rich operation of the two-stroke engines, common design criteria for three-way catalysts fail. Extremely high exhaust gas hydrocarbon concentrations lead to high exotherms during oxidation which increases the exhaust gas temperature to a range between 800 and 900 °C. Furthermore the lack of oxygen limits the CO and HC oxidation under certain engine operation conditions. Therefore, water-gas shift and steam reforming reactions play an important part in catalytic aftertreatment of two-stroke exhausts.

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.

Extensive research and development has been performed to develop the NOx-adsorber catalytic system, which would make Mitsubishi vehicles powered by the gasoline direct-injection (GDI™) engines comply with European Stage 4 emissions regulations. This NOx-adsorber catalytic system is a three-brick configuration, consisting of a three-way catalyst in the front (the front catalyst) and the rear catalytic converter, composed of a new NOx-adsorber catalyst and a conventional three-way catalyst (TWC). In the present research work, a special effort has been made to define the required performance of the front catalyst, particularly with HC reduction efficiency at the cold start, the steady-state leaner A/F and the transient phase of the A/F from leaner to stoichiometric. For HC reduction, it has been found that platinum (Pt) had the highest HC efficiency.