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Urea-selective catalytic reduction (SCR) catalysts are the leading aftertreatment technology for diesel engines, but there are major challenges associated with meeting future NOx emission standards, especially under transient drive cycle conditions that include large swings in exhaust temperatures. Here we present a simplified, transient, one-dimensional integral model of NOx reduction by NH₃ on a commercial small-pore Cu-zeolite urea-SCR catalyst for which detailed kinetic parameters have not been published. The model was developed and validated using data acquired from bench reactor experiments on a monolith core, following a transient SCR reactor protocol. The protocol incorporates NH₃ storage, NH₃ oxidation, NO oxidation and three global SCR reactions under isothermal conditions, at three space velocities and at three NH₃/NOx ratios.

In order to further characterize and optimize the performance of Selective Catalytic Reduction (SCR) aftertreatment systems used on heavy-duty diesel engines, an accurately calibrated high-fidelity multi-step global kinetic SCR model and a reduced order estimator for on-board diagnostic (OBD) and control are desirable. In this study, a Cu-zeolite SCR catalyst from a 2010 Cummins ISB engine was experimentally studied in a flow reactor using carefully designed protocols. A 2-site SCR model describing mass transfer and the SCR chemical reaction mechanisms is described in the paper. The model was calibrated to the reactor test data sets collected under temperatures from 200 to 425 °C and SCR space velocities of 60000, 90000, and 120000 hr-1. The model parameters were calibrated using an optimization code to minimize the error between measured and simulated NO, NO₂, N₂O, and NH₃ gas concentration time histories.

Lean gasoline engines offer greater fuel economy than the common stoichiometric gasoline engine, but the current three way catalyst (TWC) on stoichiometric engines is unable to control nitrogen oxide (NOX) emissions in oxidizing exhaust. For these lean gasoline engines, lean NOX emission control is required to meet existing Tier 2 and upcoming Tier 3 emission regulations set by the U.S. Environmental Protection Agency (EPA). While urea-based selective catalytic reduction (SCR) has proven effective in controlling NOX from diesel engines, the urea storage and delivery components can add significant size and cost. As such, onboard NH3 production via a passive SCR approach is of interest. In a passive SCR system, NH3 is generated over a close-coupled TWC during periodic slightly rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean operation, NOX passes through the TWC and is reduced by the stored NH3 on the SCR catalyst.

A commercial three-way catalyst (TWC) was evaluated for ammonia (NH3) generation on a 2.0-liter BMW lean burn gasoline direct injection engine as a component in a passive ammonia selective catalytic reduction (SCR) system. The passive NH3 SCR system is a potential low cost approach for controlling nitrogen oxides (NOX) emissions from lean burn gasoline engines. In this system, NH3 is generated over a close-coupled TWC during periodic slightly rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean, NOX passes through the TWC and is reduced by the stored NH3 on the SCR catalyst. NH3 generation was evaluated at different air-fuel equivalence ratios at multiple engine speed and load conditions. Near complete conversion of NOX to NH3 was achieved at λ=0.96 for nearly all conditions studied. At the λ=0.96 condition, HC emissions were relatively minimal, but CO emissions were significant.

A hybrid LNT+SCR system is used to control NOx from a light-duty diesel engine with in-cylinder regeneration controls. A diesel oxidation catalyst and diesel particulate filter are upstream of the LNT and SCR catalysts. Ultraviolet (UV) adsorption spectroscopy performed directly in the exhaust path downstream of the LNT and SCR catalysts is used to characterize NH3 production and utilization in the system. Extractive exhaust samples are analyzed with FTIR and magnetic sector mass spectrometry (H2) as well. Furthermore, standard gas analyzers are used to complete the characterization of exhaust chemistry. NH3 formation increases strongly with extended regeneration (or “over regeneration”) of the LNT, but the portion of NOx reduction occurring over the SCR catalyst is limited by the amount of NH3 produced as well as the amount of NOx available downstream of the LNT. Control of lean-rich cycling parameters enables control of the ratio of NOx reduction between the LNT and SCR catalysts.

Two hybrid powertrain configurations, including parallel and series hybrids, were simulated for fuel economy, component energy loss, and emissions control in Class 8 trucks over both city and highway driving conditions. A comprehensive set of component models describing engine fuel consumption, emissions control, battery energy, and accessory power demand interactions was developed and integrated with the simulated hybrid trucks to identify heavy-duty (HD) hybrid technology barriers. The results show that series hybrid is absolutely negative for fuel-economy improvement of long-haul trucks due to an efficiency penalty associated with the dual-step conversions of energy (i.e. mechanical to electric to mechanical).

A prototype three-way catalyst (TWC) with NOX storage component was evaluated for ammonia (NH3) generation on a 2.0-liter BMW lean burn gasoline direct injection engine as a component in a passive ammonia selective catalytic reduction (SCR) system. The passive NH3 SCR system is a potential approach for controlling nitrogen oxides (NOX) emissions from lean burn gasoline engines. In this system, NH3 is generated over a close-coupled TWC during periodic slightly-rich engine operation and subsequently stored on an underfloor SCR catalyst. Upon switching to lean, NOX passes through the TWC and is reduced by the stored NH3 on the SCR catalyst. Adding a NOX storage component to a TWC provides two benefits in the context of a passive SCR system: (1) enabling longer lean operation by storing NOX upstream and preserving NH3 inventory on the downstream SCR catalyst; and (2) increasing the quantity and rate of NH3 production during rich operation.

An engine-aftertreatment computational model was developed to support in-loop performance simulations of tailpipe emissions and fuel consumption associated with a range of heavy-duty (HD) truck drive cycles. For purposes of this study, the engine-out exhaust dynamics were simulated with a combination of steady-state engine maps and dynamic correction factors that accounted for recent engine operating history. The engine correction factors were approximated as dynamic first-order lags associated with the thermal inertia of the major engine components and the rate at which engine-out exhaust temperature and composition vary as combustion heat is absorbed or lost to the surroundings. The aftertreatment model included catalytic monolith components for diesel exhaust oxidation, particulate filtration, and selective catalytic reduction of nitrogen oxides (NOx) with urea.

For renewable fuels to displace petroleum, they must be compatible with emissions control devices. Pure biodiesel contains up to 5 ppm Na + K and 5 ppm Ca + Mg metals, which have the potential to degrade diesel emissions control systems. This study aims to address these concerns, identify deactivation mechanisms, and determine if a lower limit is needed. Accelerated aging of a production exhaust system was conducted on an engine test stand over 1001 h using 20% biodiesel blended into ultra-low sulfur diesel (B20) doped with 14 ppm Na. This Na level is equivalent to exposure to Na at the uppermost expected B100 value in a B20 blend for the system full-useful life. During the study, NOx emissions exceeded the engine certification limit of 0.33 g/bhp-hr before the 435,000-mile requirement.

The leading approach for reduction of NOx from diesel engines is selective catalytic reduction employing urea as a reductant (NH₃- or urea-SCR). For passenger vehicles, the best known NH₃-SCR catalysts are Cu-ZSM-5 and Fe-ZSM-5 and have been shown to function very well in a narrow temperature range. This technology is not directly transferable to off-road diesel engines which operate under a different duty cycle resulting in exhaust with different fractions of components than are present in passenger vehicle emissions. Our results show that Cu-ZSM-5 exhibits 90% NOx reduction efficiency in 250-450°C range while Fe-ZSM-5 is highly effective in 350-550°C range for off-road engines. However, a combination of these catalysts cannot efficiently reduce NOx in 150-650°C which is the desirable range for deployment in off-road diesel engines. In our efforts to increase the effective range of these catalysts, we initiated efforts to modify these catalysts by catalyst promoters.

The combination of advanced combustion with advanced selective catalytic reduction (SCR) catalyst formulations was studied in the work presented here to determine the impact of the unique hydrocarbon (HC) emissions from premixed charge compression ignition (PCCI) combustion on SCR performance. Catalyst core samples cut from full size commercial Fe- and Cu-zeolite SCR catalysts were exposed to a slipstream of raw engine exhaust from a 1.9-liter 4-cylinder diesel engine operating in conventional and PCCI combustion modes. The zeolites which form the basis of these catalysts are different with the Cu-based catalyst made on a chabazite zeolite which las smaller pore structures relative to the Fe-based catalyst. Subsequent to exposure, bench flow reactor characterization of performance and hydrocarbon release and oxidation enabled evaluation of overall impacts from the engine exhaust.

The combination of advanced combustion with advanced selective catalytic reduction (SCR) catalyst formulations was studied in the work presented here to determine the impact of the unique hydrocarbon (HC) emissions from premixed charge compression ignition (PCCI) combustion on SCR performance. Catalyst core samples cut from full size commercial Fe- and Cu-zeolite SCR catalysts were exposed to a slipstream of raw engine exhaust from a 1.9-liter 4-cylinder diesel engine operating in conventional and PCCI combustion modes. The zeolites which form the basis of these catalysts are different with the Cu-based catalyst made on a chabazite zeolite which has smaller pore structures relative to the Fe-based catalyst. Subsequent to exposure, bench flow reactor characterization of performance and hydrocarbon release and oxidation enabled evaluation of overall impacts from the engine exhaust.

Previously reported work with a full-scale ethanol-SCR system featuring a Ag-Al2O3 catalyst demonstrated that this particular system has potential to reduce NOx emissions 80-90% for engine operating conditions that allow catalyst temperatures above 340°C. A concept explored was utilization of a fuel-borne reductant, in this case ethanol “stripped” from an ethanol-diesel micro-emulsion fuel. Increased tailpipe-out emissions of hydrocarbons, acetaldehyde and ammonia were measured, but very little N2O was detected. In the current increment of work, a number of light alcohols and other hydrocarbons were used in experiments to map their performance with the same Ag-Al2O3 catalyst. These exploratory tests are aimed at identification of compounds or organic functional groups that could be candidates for fuel-borne reductants in a compression ignition fuel, or could be produced by some workable method of fuel reforming.

In this contribution, nuanced changes of a commercial Cu-SSZ-13 catalyst with hydrothermal aging, which have not been previously reported, as well as their corresponding impact on SCR functions, are described. In particular, a sample of Cu-SSZ-13 was progressively aged between 550 to 900°C and the changes of performance in NH3 storage, oxidation functionality and NOx conversion of the catalyst were measured after hydrothermal exposure at each temperature. The catalysts thus aged were further characterized by NH3-TPD, XRD and DRIFTS techniques for structural changes. Based on the corresponding performance and structural characteristics, three different regimes of hydrothermal aging were identified, and tentatively as assigned to “mild”, “severe” and “extreme” aging. Progressive hydrothermal aging up to 750°C decreased NOx conversion to a small degree, as well as NH3 storage and oxidation functions.

Alkali and alkaline earth metal impurities found in diesel fuels are potential poisons for diesel exhaust catalysts. Using an accelerated aging procedure, a set of production exhaust systems from a 2011 Ford F250 equipped with a 6.7L diesel engine have been aged to an equivalent of 150,000 miles of thermal aging and metal exposure. These exhaust systems included a diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst, and diesel particulate filter (DPF). Four separate exhaust systems were aged, each with a different fuel: ULSD containing no measureable metals, B20 containing sodium, B20 containing potassium and B20 containing calcium. Metals levels were selected to simulate the maximum allowable levels in B100 according to the ASTM D6751 standard. Analysis of the aged catalysts included Federal Test Procedure emissions testing with the systems installed on a Ford F250 pickup, bench flow reactor testing of catalyst cores, and electron probe microanalysis (EPMA).

Hydrogen (H2) is an excellent reductant, and has been shown to be highly effective when introduced into a variety of catalysts such as three-way catalysts, lean NOx traps (LNTs), and hydrocarbon lean NOx catalysts (also termed hydrocarbon selective catalytic reduction (SCR) catalysts). Furthermore, since lean-burn engines offer improved fuel efficiency yet difficult NOx emission control, H2 production during lean operation for the purpose of NOx reduction could be beneficial. On-board generation of hydrogen is being explored via catalytic or plasma-based reformers. A possible alternative to these add-on systems is generation of the H2 in-cylinder with standard fuel injection hardware. This paper details experiments relating to the production and measurement of H2 under net-lean operation in a common-rail diesel engine. In-cylinder fuel control is used to tailor the combustion process such that H2 is generated while maintaining a lean Air:Fuel ratio in the bulk exhaust gas.

The influence of fuel rail pressure (FRP) and urea-selective catalytic reduction (SCR) on particulate matter (PM) formation is investigated in this paper along with notes regarding the NOx and other emissions. Increasing FRP was shown to reduce the overall soot and total PM mass for four operating conditions. These conditions included two high speed conditions (2400 rpm at 540 and 270 Nm of torque) and two moderated speed conditions (1400 rpm at 488 and 325 Nm). The concentrations of CO2 and NOx increased with fuel rail pressure and this is attributed to improved fuel-air mixing. Interestingly, the level of unburned hydrocarbons remained constant (or increased slightly) with increased FRP. PM concentration was measured using an AVL smoke meter and scanning mobility particle sizer (SMPS); and total PM was collected using standard gravimetric techniques. These results showed that the smoke number and particulate concentrations decrease with increasing FRP.

Urea-SCR systems are potentially a highly-effective means of NOX reduction for light-duty diesel vehicles. However, use of urea-SCR technologies at low temperatures presents unique technical challenges. This study was undertaken to provide more knowledge about low temperature urea decomposition and the resulting effects on SCR performance. Data are presented for experiments using two SCR catalysts of differing size with a light-duty diesel engine. Analyses of the NOX reduction efficiency, NH3 storage phenomena, and unregulated emissions are shown. Over production of NO2 by the oxidation catalyst is demonstrated to be problematic at 25,000 hr-1 space velocity for a range of temperatures. This leads to production of N2O by both SCR catalysts that is higher when urea is injected than when NH3 is injected.

An operational challenge associated with SCR catalysts is the NH3 slip control, particularly for commercial small pore Cu-zeolite formulations as a consequence of their significant ammonia storage capacity. The desorption of NH3 during increasing temperature transients is one example of this challenge. Ammonia slipping from SCR catalyst typically passes through a platinum based ammonia oxidation catalyst (AMOx), leading to the formation of the undesired byproducts NOx and N2O. We have discovered a distinctive characteristic, an overlapping NH3 desorption and oxidation, in a state-of-the-art Cu-zeolite SCR catalyst that can minimize NH3 slip during temperature transients encountered in real-world operation of a vehicle.

NOx emissions from a heavy-duty diesel engine were reduced by more than 90% and 80% utilizing a full-scale ethanol-SCR system for space velocities of 21000/h and 57000/h respectively. These results were achieved for catalyst temperatures between 360 and 400°C and for C1:NOx ratios of 4-6. The SCR process appears to rapidly convert ethanol to acetaldehyde, which subsequently slipped past the catalyst at appreciable levels at a space velocity of 57000/h. Ammonia and N2O were produced during conversion; the concentrations of each were higher for the low space velocity condition. However, the concentration of N2O did not exceed 10 ppm. In contrast to other catalyst technologies, NOx reduction appeared to be enhanced by initial catalyst aging, with the presumed mechanism being sulfate accumulation within the catalyst. A concept for utilizing ethanol (distilled from an E-diesel fuel) as the SCR reductant was demonstrated.