Lean Gasoline Engine Reductant Chemistry During Lean NOx Trap Regeneration

Lean NOx Trap (LNT) catalysts can effectively reduce NOx from lean engine exhaust. Significant research for LNTs in diesel engine applications has been performed and has led to commercialization of the technology. For lean gasoline engine applications, advanced direct injection engines have led to a renewed interest in the potential for lean gasoline vehicles and, thereby, a renewed demand for lean NOx control. To understand the gasoline-based reductant chemistry during regeneration, a BMW lean gasoline vehicle has been studied on a chassis dynamometer. Exhaust samples were collected and analyzed for key reductant species such as H2, CO, NH3, and hydrocarbons during transient drive cycles. The relation of the reductant species to LNT performance will be discussed. Furthermore, the challenges of NOx storage in the lean gasoline application are reviewed. INTRODUCTION Engines that operate at lean air-to-fuel ratios ("lean engines") offer the potential to reduce fuel consumption and, thereby, lower greenhouse gas emissions.[1] A difficult problem to address when commercializing vehicles with lean engines is the control of oxides of nitrogen (NOx) emissions. For stoichiometric combustion, the three-way catalyst technology commonly found on gasoline-engine vehicles controls emissions of NOx as well as carbon monoxide (CO) and hydrocarbon (HC) emissions; however, the three-way function is only effective in the low-oxygen exhaust conditions associated with stoichiometric combustion. Lean combustion results in oxygen-rich exhaust where the reduction of NOx is difficult. Recently, diesel engine vehicles have been commercialized for the U.S. market that meet NOx emissions regulations by employing catalysts for lean NOx control.[2] Two catalytic approaches have been used: ureabased selective catalytic reduction (urea-SCR) and lean NOx trap (LNT) catalysts. The urea-SCR approach requires the on-board storage of urea which is introduced into the exhaust upstream of the SCR catalyst, converted to NH3 via hydrolysis reactions, and interacts with NOx on the SCR catalyst. In contrast, the LNT technology utilizes fuel from the vehicle and advanced engine controls to enable periodic operation of the engine at rich air-to-fuel ratios which produces oxygen-depleted exhaust suitable for reducing NOx stored on the LNT catalyst surface. Both technical approaches have been proven effective in controlling NOx from diesel engines. Despite the commercialization of diesel vehicles in the U.S. with lean NOx control, the U.S. passenger market is dominated by gasoline-engine vehicles. Gasoline engines can be operated lean, and direct injection gasoline engines show promise for significant fuel economy gains from lean operation. However, gasoline engines do Page 2 of 9 not typically operate as lean as diesel engines, and portions of the load and speed map would likely be handled with stoichiometric operation. Thus, NOx control for lean gasoline engines would be required for both lean and stoichiometric conditions. The ability to operate gasoline engines with a wide range of air-to-fuel ratios offers advantages for the LNT catalyst approach for controlling NOx emissions since LNTs require rich air-to-fuel operation for reducing the NOx stored during lean operation. In this study, a lean gasoline engine in a BMW vehicle has been studied on a chassis dynamometer to evaluate the exhaust conditions during lean and rich airto-fuel operation to better understand the chemistry of lean gasoline engine exhaust as related to the LNT technology. EXPERIMENTAL METHODS Experiments were conducted on a MY2008 BMW 1-series 120i lean gasoline engine vehicle. The engine was a model N43B20 2.0-liter, 4-cylinder engine with direct injection. The vehicle was operated on a chassis dynamometer with ultra-low sulfur certification gasoline fuel. The vehicle had previously accumulated sufficient mileage during use to degreen the catalysts. The vehicle's exhaust system is composed of two three-way catalysts (TWCs) which were located in two exhaust manifolds (one for each pair of cylinders: 1-4 and 2-3) close coupled to the engine. Downstream of the merge point for the two exhaust manifolds, the LNT was located in an underfloor position. The muffler was located downstream of the LNT, and the tailpipe was connected to the dilution system of the chassis dynamometer emissions measurement system. Several transient drive cycles were driven with the vehicle; the drive cycles included: US Federal Test Procedure-75 (FTP), US06 Supplemental Federal Test Procedure (US06), and Highway Fuel Economy Test (HFET). In addition to the transient drive cycle operation, the vehicle was operated at various speed and load conditions at near steady-state conditions for the purpose of collecting emissions data. During the steady-state operation, the vehicle speed remained constant and the vehicle load was changed in a stepwise fashion. Emissions from the engine and the subsequent catalytic-induced emissions changes were measured with a variety of analytical techniques. Two sets of emission analyzers were used to measure engine out emissions upstream of the TWCs from exhaust pulled from each manifold. The emission analyzers included: heated chemiluminescence for NOx (California Analytical Instruments Model 400), non-dispersive infrared for CO and CO2 and paramagnetic for O2 (all three gases covered by California Analytical Instruments Model 300) on chilled exhaust, and heated flame ionization detector for hydrocarbons (California Analytical Instruments Model HFID). Another set of the same instruments (with different scale ranges) were used to characterize tailpipe emissions diluted by the dilution tunnel. Detailed measurements relative to the LNT regeneration were made with a FTIR (MKS Multi Gas 2030HS) and a capillary-based magnetic-sector mass spectrometry technique called SpaciMS.[3] The FTIR and SpaciMS measurements were primarily made in exhaust downstream of the TWCs and upstream of the LNT; however, some measurements were also made at engine out and tailpipe positions. The FTIR allowed measurement of NO, NO2, NH3, N2O, CO, CO2, and hydrocarbons. To complement the FTIR data, the SpaciMS was configured to measure H2 and O2. In addition to the exhaust analyzers, thermocouples and universal exhaust gas oxygen (UEGO) sensors were mounted at various positions in the exhaust as well. The average of the two UEGO sensors mounted in the engine out positions of both exhaust manifolds served as the master timing signal for aligning the data from the analyzers. Since various analyzers have different delay times in exhaust transfer and measurement, a common signal was needed to time align measurements of exhaust species from different analyzers. The UEGO air-to-fuel signal was converted to exhaust concentrations for O2,