The fuel economy regulations being mandated all over the world require development of engines with improved fuel efficiency. It is important to recognize that the energy content in the exhaust varies with fuel efficiency; that is, as we increase the fraction of fuel energy that does useful work, the energy content in the exhaust will decrease. For example, recent tests conducted at GM R&D with an advanced lean gasoline engine have shown that its exhaust gas temperatures (with the engine calibrated for maximum fuel efficiency) under most of engine operating conditions of practical interest are substantially lower than exhaust temperatures of > 550°C typically attained at a close-coupled converter location with current production gasoline engines. This, coupled with the more stringent future emission standards (e.g., LEV-III/Tier 3 regulations in U.S.) and recent adoption of green-house gas regulations including CH4, makes catalytic oxidation of exhaust hydrocarbons (HCs) at low temperatures critically important.
Hydrocarbons in engine exhaust generally consist of unburned fuel components and partial combustion products, which include light (C1-C3) saturated and unsaturated HCs such as methane, ethane, ethene and propene. For gasoline engines, it has been commonly observed that C5 and C8 alkanes as well as C7 and C8 aromatics (toluene and xylene) are among the most abundant unburned fuel components in the exhaust. Regarding the nature of C5 and C8alkanes, our detailed GC-based speciation studies of gasoline exhaust HCs show that iso-pentane and iso-octane are the predominant species, with the concentrations of the corresponding n-alkanes (i.e., n-pentane and n-octane) being much lower.
The direct consequence of lower exhaust gas temperatures is that more HC species in engine exhaust are likely to remain unconverted even after the converter is fully warmed-up, thus presenting considerable challenges in HC emission control for future fuel-efficient engines. Thus, it is of practical importance to identify the types of HC species that are likely to contribute significantly to tailpipe HC emissions in cooler exhaust from next-generation gasoline engines and quantify the sensitivity of HC conversions to type & loading of noble metals (Pd vs Rh), O2concentration level and catalyst aging. To that end, we conducted laboratory reactor experiments aimed at investigating the catalytic oxidation of various HCs in gasoline engine exhaust (alcohol, alkanes, alkane isomers, alkene and aromatics) over modern commercial TWCs with variable noble metal loadings before and after aging.
This presentation will first discuss proper choice of “representative” HCs in gasoline engine exhaust based on HC speciation data and then examine their oxidation activities over degreened and aged Pd- and Rh-based commercial three-way catalysts as a function of noble metal loading. The oxidation activities were measured using a feedstream which contains not only the HC of specific interest and O2 but the other usual exhaust constituents, such as CO, H2, NO, CO2 and H2O as well. This would allow us to characterize the reactivity of each HC species under realistic conditions, namely, in the presence of the inhibition effects of CO and NO, and the promotion effects of H2 and H2O. Also discussed in this talk are the implications of our laboratory reactor data to emission control for next-generation fuel-efficient vehicles.