Jae-Soon Choi

Assessing Reductant Chemistry During In-Cylinder Regeneration of Diesel Lean NOx Traps

Lean NOx Trap (LNT) catalysts are capable of reducing NOx in lean exhaust from diesel engines. NOx is stored on the catalyst during lean operation; then, under rich exhaust conditions, the NOx is released from and reduced by the catalyst. The process of NOx release and reduction is called regeneration. One method of obtaining the rich conditions for regeneration is to inject additional fuel into the engine cylinders while throttling the engine intake air flow to effectively run the engine at rich air:fuel ratios; this method is called “in-cylinder” regeneration. In-cylinder regeneration of LNT catalysts has been demonstrated and is a candidate emission control technique for commercialization of light-duty diesel vehicles to meet future emission regulations. In the study presented here, a 1.7-liter diesel engine with a LNT catalyst system was used to evaluate in-cylinder regeneration techniques. Characterization of the exhaust reductant chemistry during in-cylinder regeneration was performed. The effects of various injection strategies and fuels and the resulting exhaust chemistry on the performance of the LNT catalyst were analyzed. In addition, exhaust species measurement of NOx and select reductants were performed inside of the catalyst cells with a capillary-based mass spectrometry technique. The effect of various injection parameters on exhaust chemistry species and LNT performance are discussed. Results indicate that fuel chemistry does affect engine-out hydrocarbon (HC) species, but not engine-out carbon monoxide (CO) or hydrogen (H2) generation. Higher engine-out CO and H2 correlate to improved NOx reduction, irrespective of HCs.

Effective Model for Prediction of N2O and NH3 Formation During the Regeneration of NOx Storage Catalyst

In this paper we propose an effective global kinetic model that allows prediction of N2O and NH3 formation during the reduction of stored NOx in dependence on the composition of the rich mixture (H2/CO/C3H6), actual operating temperature, and length of regeneration period. A bench flow reactor equipped with a high-speed FTIR was used to measure dynamic evolution of gas components during periodic lean/rich operation of a fully formulated NSRC catalyst (PtPdRh/Ba/Ce–Zr/Mg–Al/Al2O3).

Cold-start emissions control in hybrid vehicles equipped with a passive adsorber for hydrocarbons and nitrogen oxides

We present results from a computational study of the potential for using low-cost sorbent materials to trap the emissions of hydrocarbons and nitrogen oxides temporally during cold-start periods in hybrid electric vehicles and plug-in hybrid electric vehicles operating over transient driving cycles. The hydrocarbon adsorption behavior of a candidate sorbent composed of Ag-beta-zeolite was characterized in a laboratory flow reactor to estimate the kinetic parameters for a one-dimensional transient adsorber device model. This model was then implemented in the Powertrain Systems Analysis Toolkit to simulate a passive hydrocarbon adsorber device on a hybrid vehicle. The results indicate that such an adsorber can substantially reduce the hydrocarbon emissions by temporarily storing them until the three-way catalyst is sufficiently warm to remove them from the exhaust. A similar adsorber device model was simulated for nitrogen oxide control, using an initial set of conjectured kinetic parameters for transition metal oxides based on limited information in the literature. These latter simulations revealed the need to pursue additional experimental studies to characterize fully this class of sorbents. Such studies are especially relevant in the present context of rapidly evolving vehicle technology, because emission controls of this type do not involve any penalty in fuel consumption or require any change in engine operation.

Nitrogen Selectivity in Lean NOx Trap Catalysis with Diesel Engine In-Cylinder Regeneration

NOx emissions have traditionally been difficult to control from diesel engines; however, lean NOx trap catalysts have been shown to reduce NOx emissions from diesel engines by greater than 90% under some conditions. It is imperative that lean NOx traps be highly selective to N 2 to achieve the designed NOx emissions reduction. If selectivity for NOx reduction to NH 3 or N 2 O is significant then, ultimately, higher levels of pollution or greenhouse emissions will result. Here studies of the N 2 selectivity of lean NOx trap regeneration with in-cylinder techniques are presented. Engine dynamometer studies with a light-duty engine were performed, and a lean NOx trap in the exhaust system was regenerated by controlling in-cylinder fuel injection timing and amounts to achieve rich exhaust conditions. NH 3 and N 2 O emissions were analyzed with FTIR spectroscopy. Both engine and bench experiments show that excess reductant delivery during regeneration leads to high NH 3 emissions and poor N 2 selectivity. Specific design of in-cylinder regeneration techniques that minimize excess reductant or allow O 2 purge can optimize N 2 selectivity of the lean NOx trap catalyst.

Chapter 9:Reduction of Stored NOx with CO/H2 and Hydrocarbons: A Spatial Resolution Analysis

Lean NOx traps (LNT) are multi-function multi-component catalysts which operate in an integral and transient reactor mode to reduce NOx in the exhaust from diesel or lean-burn gasoline engines. This chapter addresses the regeneration phase of the LNT operation, discussing various types of reductants and reactions involved. We describe how reductants of different reactivity influence catalyst functions, reactions, and overall regeneration efficiency. The presented examples highlight the importance of understanding the spatial and temporal development of key reactions and their interplay to rationalize the global performance of practical LNTs which are generally honeycomb-shaped ceramic monoliths. The emphasis is on explaining global NOx removal performance (i.e., activity and selectivity) based on the insights gained through spatially resolved techniques, including initial distribution and redistribution of stored NOx; oxygen storage and reduction; transformation of feed reductants; and formation and utilization of reduction intermediates. Pathways leading to N2O and NH3 byproduct formation as well as mitigation strategies are also discussed.