Performance Of Three-way Catalysts Research Articles

An efficient product design tool for aftertreatment system

A numerical simulation technique based on the conservation of mass and energy in the gas phase has been developed to optimize the aftertreatment system with the lowest costs. Both oxygen storage capacity model and catalyst deterioration model have been integrated into the three-way catalyst performance model. Applications have been discussed, including XY-L 1.5L I3 gasoline turbocharged direct injection (GTDI) production hybrid vehicle, BY-L 1.5L I4 GTDI hybrid vehicle, XY-L (overseas version) 1.5L I4 GTDI hybrid vehicle, and G-L7 1.5L I4 GTDI hybrid vehicle. Vehicle tests in support of the proposed model have been described. The developed model covers the complete range of the cold start, high temperature and volume flow conditions. To optimize a three-way catalyst performance, this work simulated the effect of air fuel ratio, space velocity, temperature, biasing adjustment on the catalyst efficiency. The simulations presented the technique’s capability of well predicting emissions on fresh and full useful life aged aftertreatment systems, respectively, and carried out under transient conditions. The investigation indicated that excess fueling was used upon engine start to heat the catalyst up to its full operating temperature of greater than 350°C. The model results prompted a redesign of the I3 and I4 1.5L GTDI China 6b, Euro VId and Tier 3/SULEV30 exhaust systems over the world light vehicle test procedure (WLTP) and federal test procedure (FTP) cycles, respectively, for example, the model suggested that the latest design for the SULEV30 aftertreatment system on XY-L (overseas version) 1.5L I4 GTDI hybrid vehicle with the revised calibrations in the areas of cold-start and closed-loop fuel treated emissions of NMOG, NOx, and CO to the 70% of the SULEV30 standards with a $64 cost reduction relative to the baseline.

Efficient control of automotive emission by Pt-based and Rh-based three-way catalysts: The critical role of phase structure of ceria-zirconia support

Rationally design efficient supports material is essential for heterogeneous catalysts with enhanced activity in many industrial catalytic processes, accordingly a clear understanding of the support effect is necessary. Herein, two ceria-zirconia materials without/with partial κ-Ce2Zr2O8(CZ/κCZ) structure were achieved and then employed as supports for Rh-based and Pt-based three-way catalysts (TWCs), respectively. According to the results of systematic characterizations, it has been manifested that the surface conditions of supports significantly affected the activity and stability of Rh-based and Pt-based catalysts. As κCZ exhibits an intrinsic Zr-rich surface with more oxygen vacancies with respect to CZ, Rh species supported on κCZ mainly occupy the Zr sites over the surface, thus exhibit a more electronic-rich and active state due to the decreased Rh-Ce interfaces, which weakened the undesirable electron transfer from Rh to Ce. As a result, κCZ delivered as an ideal support for improving the performance of Rh-based TWCs effectively. On contrary, comparing with Pt/CZ, more serious sintering of Pt nanoparticles and inferior activity as well as stability have been detected in Pt/κCZ due to the decreased Pt-Ce interfaces, which weakened the anchor effects of the support through Pt-O-Ce bonds. This finding enhances the understanding of support effects for the development of advanced TWCs.

Influence mechanism of Pd, Pt, Rh/γ-Al2O3 on NH3 formation for NO reduction by CO

The byproducts of NO reduction by CO have attracted great interest in the three-way catalysts, but the influence of active components (Pd, Pt, and Rh) on the formation mechanism is still elusive. Herein, Pd/, Pt/, and Rh/Al2O3 were prepared for investigating the NH3 formation during NO reduction by CO under wet conditions. The strongest metal-support interaction in Rh/Al2O3 is attributed to the highest interfacial Rh–O–Al bond from the results of XAFS and NMR. The smallest particle size and highest redox property on Rh/Al2O3 led to the highest NH3 yield below 300 °C. Pt/Al2O3 produced more NH3 than Pd/Al2O3 above 350 °C because of robust response to lattice oxygen reacted with NO and reduction by CO. The optimal strategy for minimizing byproducts on the premise of preserving three-way catalyst performance was the PtRh/γ-Al2O3 where the molar ratio of Pt: Rh was 9:1.

Impact of Cylinder-to-Cylinder Dispersion of Exhaust Gas Recirculation on the Three-Way Catalyst Performance and Tailpipe Emissions of Spark-Ignition Engines

Abstract New generations of spark-ignition engines include exhaust gas recirculation (EGR) to improve the engine efficiency. Depending on the design of the EGR routing, some differences in the total amount of recirculated gases that reach each cylinder can be induced. This affects the air-to-fuel ratio on each cylinder due to the combination of the different temperature and composition of the gases at the intake valve closure. As a consequence, significant deviations in the combustion process and the subsequent composition upstream the three-way catalyst can be reached. This paper explores these effects on catalyst performance and tailpipe emissions, individualizing the behavior for each regulated species. The study was performed in a four-cylinder naturally aspirated engine with Atkinson cycle and a close-coupled three-way catalyst. The most significant deterioration in conversion efficiency appeared for the nitrogen oxides, directly linked to the EGR dispersion level. In the case of carbon monoxide (CO) emissions, no significant impact was observed except at high average EGR rates, where one or more of the cylinders exceeded the EGR tolerance for that speed and load. Based on these results, a strategy where the fuel injector command is adapted to correct the air-to-fuel ratio deviations induced by the EGR was developed and implemented.

Assessment of exhaust raw emissions and aftertreatment performance in a retrofitted heavy duty-spark ignition engine operating with liquefied petroleum gas

This study investigates the effects of varying Liquefied Petroleum Gas compositions on pollutant emissions and three-way catalyst performance in retrofitted heavy-duty spark-ignition engines. It focuses on fuels with varying propane and butane ratios, providing new insights into the influence of the composition of Liquefied Petroleum Gas on catalyst activity and tailpipe emissions. Representative conditions from the world Harmonized Transient Cycle were selected to comprehensively assess engine emissions and catalyst performance. Experiments under constant and variable λ conditions tested the catalyst's response concerning raw emissions, exhaust temperature, and space velocity, all variables affected by engine operation. Key findings indicate that catalyst performance is not significantly affected by Liquefied Petroleum Gas composition. Under constant λ conditions, the catalyst achieved high conversion efficiency of carbon monoxide, unburned hydrocarbons, and nitrogen oxides, especially under rich conditions (λ = 0.990). However, rich conditions can lead to elevated ammonia levels, although nitrous oxide generation remains low in all circumstances. Implementing λ pulse strategies reduced ammonia formation to around 25 mg/kWh, ensuring high catalyst conversion efficiency for all pollutant species after optimizing the amplitude and frequency of the pulses to establish the balance of reducing-oxidizing species and oxygen storage for each operation mode. Crucially, Liquefied Petroleum Gas composition was determined to be non-critical for the catalyst in these engines, promoting the use of this fuel in retrofitted engines and promoting for a shift towards cleaner transportation.

Exploring the Impact of Palladium Loading on Pd-Based Three-Way Catalyst Performance and Propane Reactivity for Emission Reduction in Liquefied Petroleum Gas Engines

To reduce air pollution worldwide, regulations on exhaust gas emissions from ships are becoming increasingly stringent. One fuel that is being considered as an alternative to replace the heavy fuel oil used in existing ship engines and thereby reduce harmful emissions, such as NOx, SOx, and greenhouse gases, is sulfur-free liquefied petroleum gas (LPG). To assess the viability of this alternative, it is necessary to understand propane reactivity, the main component of LPG, and develop after-treatment devices applicable to LPG engines. This research evaluated the performance of three prototype Pd-based three-way catalysts (TWCs) with varying Pd loadings (6.5, 4.1, and 1.4 g/L), focusing on their effectiveness concerning propane reactivity in LPG engines. For the fresh samples, catalysts with 4.1 g/L Pd demonstrated performance that was comparable to, or even surpassed, those containing 6.5 g/L Pd. Notably, the temperature of 50% conversion (T50) for NO and C3H8 in the fresh Pd-4.1 was lower by 14 °C and 10 °C, respectively, compared to the fresh Pd-6.5 sample, despite having 37% less precious-metal loading. However, after hydrothermal aging at 900 °C for 100 h, the performance of the 4.1 g/L Pd catalyst significantly deteriorated, exhibiting lower efficiency than the 6.5 g/L Pd catalyst. The study also delved into various probe reactions, including the water–gas shift and propane steam reforming. Advanced analytical techniques, such as N2 physisorption and scanning transmission electron microscopy, were employed to elucidate the texture and structural characteristics of the catalyst, providing a comprehensive understanding of its behavior and potential applications. Through this research, within the efforts of the maritime sector to address challenges posed by emission regulations and rising costs associated with precious metals, this study has the potential to contribute to the development of cost-effective emission control solutions.

Key design and layout factors influencing performance of three-way catalytic converters: Experimental and semidecoupled numerical study under real-life driving conditions

The optimization of three-way catalyst (TWC) design and layout is important for reducing mobile source emissions and requires numerical modeling. To date, the high computational cost of accurate models has inspired the development of numerous tradeoff approaches; however, none of them is fully satisfactory. To address this gap, we herein theoretically and experimentally explore the key design and layout factors influencing TWC performance under transient driving conditions, starting with the optimization and verification of a simplified reaction mechanism. This mechanism is used to construct a semidecoupled posttreatment GT-Power model featuring boundary conditions derived from experimental and upstream engine modeling results, and experiments on a real engine are carried out to evaluate the model accuracy. The performance of three posttreatment systems is analyzed to identify the key performance-affecting parameters, and the following conclusions are drawn. (1) For a single reactor, cell density is the most important factor influencing pressure loss.(2) The exhaust temperature rapidly decreases in the longitudinal direction of the reactor in the initial start stage, although the reaction rate always decreases in this direction during the whole cycle.(3) One-dimensional plug-flow reactor simulations do not account for inhomogeneity and local turbulence, thus overestimating the performance of large reactors.(4) Under the employed conditions, the exhaust temperature is sufficiently high for pollutant conversion, although the efficiency of this conversion often decreases in the high-speed stage. Moreover, we show that high-speed adaptation can be enhanced by increasing the crosswise size of the reactor and propose a new reactor structure. Compared with our model, this structure is characterized by a lower catalyst loading and NOx conversion efficiency but achieves higher hydrocarbon and CO conversion efficiencies. Thus, our study provides a useful and effective method for evaluating the design/layout of posttreatment devices and facilitates the development of more efficient TWCs, contributing to the establishment of a more ecofriendly society.

Intra-catalyst CH4 oxidation pathways on a Pd/Al2O3/CeZrOx-based commercial catalyst and implications on NOx conversion profiles for a natural gas vehicle exhaust under lambda modulation

The performance of a three-way catalyst (TWC) in natural gas-powered vehicles is enhanced by periodic changes in air-to-fuel ratio (λ-modulation). The reaction networks and sequences inside the catalyst that facilitate such enhanced performance have not been extensively investigated. This work applied intra-catalyst measurements (SpaciMS) to analyze the transient spatiotemporal gas concentrations inside a Pd-based TWC to establish relationships between CH4, NOx, CO and H2 conversion pathways. Steam reforming and partial oxidation were revealed to be the main CH4 conversion routes. The cyclic rich-lean conditions combined with the oxygen storage capacity (OSC) of the TWC generate reduced and oxidized zones that are constantly moving within the catalyst, changing the dominant chemical reactions occurring on the surface. In the reduced zones, OSC is depleted while CH4 is converted through steam reforming and NOx is converted through reactions with H2, CO, and other surface-bound reducing fragments formed by CH4 conversion. In the oxidized zones, OSC is replenished, CH4 is converted by partial oxidation, and H2, CO, and NH3 are oxidized. The length of lean-rich phases impacts the catalyst performance significantly; too short or too long of a rich or lean phase can lower the overall conversion of reactive species. An inhibition of CH4 conversion was observed during the rich phase possibly due to CO-poisoning of active sites. The intra-catalyst measurements revealed that the catalyst consists of three distinct reaction zones and their lengths vary with modulation conditions. Various modulation frequencies, amplitudes, λ-centers, and temperatures were investigated which allowed an understanding of how these parameters affect the reaction zones and catalyst utilization. Understandings from this work can enable adaptive λ-control strategies to optimize the overall TWC performance over a range of vehicle operating conditions.

Impact of Cylinder Deactivation Strategies on Three-way Catalyst Performance in High Efficiency Low Emissions Engines

Reduction of CO2 emissions is a prevalent subject in the transportation sector. Cylinder deactivation (CDA) provides a method to reduce CO2 emissions at part load. However, there is little consideration of how these strategies affect catalyst performance.Effects of two CDA strategies on catalyst performance were studied. The first strategy of open-loop lambda control CDA improved catalyst CO conversion to 100 % and prevented NH3 formation over the catalyst. There was a NOX penalty if the duration exceeded 5 seconds. The closed-loop CDA strategy increased catalyst temperature by 300°C for durations of 60 seconds.Open-loop CDA can prevent NH3 formation and improve CO conversion. Closed-loop CDA strategy has potential to increase catalyst temperature and be effective at reducing light-off during cold starts without additional hardware. This gives the strategy an advantage over more complex heating options. Both strategies provide opportunities to help meet current and future emission regulations.

Non-carbon greenhouse gas emissions for hybrid electric vehicles: three-way catalyst nitrous oxide and ammonia trade-off

Transport sector decarbonisation is leading to increased demand for electrified powertrains including hybrid vehicles. The presence of an internal combustion engine and electric motor offer multiple performance and efficiency advantages. However, changes in the conditions that catalytic aftertreatment systems are subjected to can present challenges in meeting forthcoming emissions standards. This work investigated the three-way catalyst performance to abate regulated and unregulated emissions from a gasoline direct injection engine working under conditions related to hybrid vehicle operation. The focus on unregulated emissions of NH3 and N2O is of interest due to limited literature on their formation in conventional, and particularly hybrid, vehicle aftertreatment systems. Furthermore, the likelihood of their regulation when the EURO 7 emissions standards are introduced increases the pertinence of this work. For this particular engine and aftertreatment setup, it was found that starting the engine whilst the three-way catalyst temperature was below 150 ℃ led to an increase in tailpipe regulated emissions and N2O. Whilst, starting the engine when three-way catalyst temperatures were above 350 ℃ lead to tailpipe NH3 emissions. This was due to the selectivity of NO to form N2O at lower temperatures and NH3 at higher temperatures. For the case of the studied catalyst, a vehicle energy management strategy opting to start the engine with the three-way catalyst within a targeted temperature range allowed for a trade-off between regulated emissions, N2O and NH3. These findings are significant since it can be used to optimise hybrid vehicle control strategies minimising both regulated and unregulated emissions.

Direct Observation of Rhodium Aluminate (RhAlOx) and Its Role in Deactivation and Regeneration of Rh/Al2O3 under Three-Way Catalyst Conditions

To improve the performance of three-way catalysts (TWC), it is essential to further develop our understanding of the microstructure–performance relationships in next-generation TWC. Light-off measurement combined with comprehensive structural imaging using advanced characterization techniques reveals the correlation between catalyst activity and atomic-scale morphology of deactivated and regenerated Rh/Al2O3 catalysts. Using scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS), and extended X-ray absorption fine structure spectroscopy (EXAFS), we show direct evidence of rhodium aluminate (RhAlOx); its formation and disappearance can be attributed to the deactivation and regeneration of Rh/γ-Al2O3. Specifically, regeneration relies on the ability to restore active nanometer-sized Rh nanoparticles from inactive RhAlOx, in which the preferential interstitial sites of inactive Rh are found to be the octahedral sites of the cubic spinel γ-Al2O3. A deep understanding of the long-debated deactivation mechanism in Rh/Al2O3 will enable more efficient usage of Rh in TWC applications.

Improved Pd/CeO2 Catalysts for Low-Temperature NO Reduction: Activation of CeO2 Lattice Oxygen by Fe Doping.

Developing better three-way catalysts with improved low-temperature performance is essential for cold start emission control. Density functional theory in combination with microkinetics simulations is used to predict reactivity of CO/NO/H2 mixtures on a small Pd cluster on CeO2(111). At low temperatures, N2O formation occurs via a N2O2 dimer over metallic Pd3. Part of the N2O intermediate product re-oxidizes Pd, limiting NO conversion and requiring rich conditions to obtain high N2 selectivity. High N2 selectivity at elevated temperatures is due to N2O decomposition on oxygen vacancies. Doping CeO2 by Fe is predicted to lead to more oxygen vacancies and a higher N2 selectivity, which is validated by the lower onset of N2 formation for a Pd catalyst supported on Fe-doped CeO2 prepared by flame spray pyrolysis. Activating ceria surface oxygen by transition metal doping is a promising strategy to improve the performance of three-way catalysts.

Highly Stable and Active Solid‐Solution‐Alloy Three‐Way Catalyst by Utilizing Configurational‐Entropy Effect

Since 1970, people have been making every endeavor to reduce toxic emissions from automobiles. After the development of a three-way catalyst (TWC) that concurrently converts three harmful gases, carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NOx ), Rh became an essential element in automobile technology because only Rh works efficiently for catalytic NOx reduction. However, due to the sharp price spike in 2007, numerous efforts have been made to replace Rh in TWCs. Nevertheless, Rh remains irreplaceable, and now, the price of Rh is increasing significantly again. Here, it is demonstrated that PdRuM ternary solid-solution alloy nanoparticles (NPs) exhibit highly durable and active TWC performance, which will result in a significant reduction in catalyst cost compared to Rh. This work provides insights into the design of highly durable and efficient functional alloy NPs, guiding how to best take advantage of the configurational entropy in addition to the mixing enthalpy.

O2 dosage as a descriptor of TWC performance under lean/rich dithering in stoichiometric natural gas engines

The effects of oscillations in gas composition, known as lean/rich dithering, on the performance of a commercial Pd-based three-way catalyst (TWC) for stoichiometric natural gas (NG) engines were evaluated using synthetic exhaust flow reactor experiments. Under simulated NG exhaust conditions, NO conversion was intimately correlated to CH4 conversion at slightly fuel-rich operating conditions. CH4 conversion significantly varied with lambda (or O2 concentration) and depended on the direction of the lambda change. The dynamic CH4 conversion is likely related to the change of catalyst oxidation state and structures of Pd active sites. The CH4-NO cross-over point was found at a rich-biased lambda rather than stoichiometry. Compared to static operation, catalyst performance was much higher under a realistic dithering condition. The impacts of dithering parameters including amplitude and frequency on CH4 and NO conversions were explored. O2 dosage is proposed as an effective descriptor of the TWC dithering performance. In the O2-dosage space, the performance of distinct dithering parameters collapse, helping to elucidate the fundamental influence of dithering parameters on TWC performance. With the help of O2 dosage, an optimal operating window leading to high CH4 and NO conversions was identified. A higher dithering amplitude is required to keep the catalyst in a reduced state for higher CH4 and NO conversions, while an appropriate O2 dosage that does not exceed the breakthrough OSC is necessary to avoid excess O2 that results in NO slip. The dithering amplitude and O2-dosage-metric maps provide a new method for studying dynamic TWC performance and identifying optimum operation strategies.

Effect of La Addition on the Performance of Three-Way Catalysts Containing Palladium and Rhodium

Three-way catalysts containing palladium and/or rhodium were prepared using γ­Al2O3 doped with lanthanum oxide as a support. All the samples were obtained by an incipient wetness impregnation of the support with an aqueous solution of nitrates. In order to investigate the metal–support interaction, the support was additionally calcined at 800 °C before the impregnation procedure. Characterization of the support thermally treated within a range of 600–1000 °C by low-temperature nitrogen absorption, X-ray diffraction analysis, and electron paramagnetic resonance spectroscopy has revealed that the treatment conditions strongly affect the textural properties, the phase composition and the concentration of electron-donor sites on the surface of the support. Deposition of metals by the impregnation of initial support with a joint solution of Pd and Rh nitrates has led to formation of small Pd–Rh alloyed nanoparticles with strong metal–metal interaction, which was confirmed by a testing reaction of ethane hydrogenolysis. No alloy formation was observed in the case of mechanical mixing of the separately prepared Pd-only and Rh-only catalysts as well as in the case of preliminary calcined support impregnated with a joint solution of Pd and Rh nitrates. Bimetallic Pd–Rh catalyst of alloyed type was shown to be the most promising in terms of catalytic performance and thermal stability.

Designed synthesis of highly active CeO2-ZrO2-Al2O3 support materials with optimized surface property for Pd-only three-way catalysts

In this work, a series of CeO2-ZrO2-Al2O3 (CZA) support materials were prepared by co-precipitation method combing with hydrothermal treatment at different temperatures. Then their supported Pd-only three-way catalysts (TWCs) were prepared, characterized and compared with commercial Pd/CZAS catalyst. It is found that the catalytic performance of Pd-only TWCs shows a strong dependence on the surface properties rather than the bulk structures of CZA supports. Higher hydrothermal temperatures promote the self-assemble of Al2O3 into nanorods, and the formation of CeO2-ZrO2 (CZ) solid solution with larger crystal size, which widens the pore size distribution of CZA and improves the thermal stability of CZ nanoparticles with lower growth rate, respectively. However, the specific surface area, surface oxygen vacancies and surface Ce content of CZA decrease with hydrothermal temperature rising, which in return decreases the activity of the Pd/CZA catalysts. While it should also be noted that all the as prepared Pd/CZA exhibit better activity than the commercial Pd/CZAS, and the T50 (temperature for 50% conversion) for CO, NO, C3H6, C3H8 over Pd/CZA100 are 48 °C, 30 °C, 26 °C and 31 °C lower than those for Pd/CZAS, respectively. It further indicates that highly active Pd-only TWCs could be constructed by optimizing the surface properties of CZA support via hydrothermal treatment at modern temperature.

Recent advances in gasoline three-way catalyst formulation: A review

Development of three-way catalyst technology has been critical in maintaining air quality regulations for gasoline engines via the conversion of pollutants from the internal combustion engine exhaust. The development of improved three-way catalyst formulations is an important challenge for automotive industry. Indeed, in order to meet increasingly stringent environmental regulations around the world, the development of more efficient catalysts depends on a complete understanding of the many parameters related to three-way catalyst design. In this review paper, some of these parameters are examined in relation to three-way catalyst performance, and especially low-temperature activation performance, with a focus on more recently published work. In particular, washcoat composition, platinum group metal ratios and loading, and substrate design are considered. The effect of these parameters with regard to the conversion efficiency of carbon monoxide, unburned hydrocarbons, and nitrogen oxides pollutants is summarized.

Transition metal doping of Pd(1 1 1) for the NO + CO reaction

The replacement of platinum group metals by non-noble metals has attracted significant attention in the field of three-way catalysis. Here, we use DFT calculations to comprehensively study NO reduction by CO and CO oxidation on Pd(111) and transition metal doped Pd(111). Whilst direct NO dissociation is very difficult on metallic Pd(111), doping with transition metals can substantially lower the reaction barrier for NO dissociation. The lowest barrier is predicted for Ti-doped Pd(111). An electronic structure analysis shows that the low barrier is due to the strong adsorption of N and O on surface sites involving Ti atoms. It relates to strong hybridization of the N and O orbitals with the half-filled d-band of the metallic surface. At the same time, the anti-bonding states are shifted above the Fermi level, which further strengthens the adsorption of N and O. A Brønsted-Evans-Polanyi relation for NO dissociation on TM-doped Pd(111) surfaces is identified. The complete reaction pathway for N2, N2O and CO2 formation on Pd(111) and Ti-doped Pd(111) was considered. Besides more facile NO dissociation, the energy barrier for CO oxidation is decreased for the Ti-doped surface. Microkinetics simulations confirm that the activity and selectivity for NO reduction and CO oxidation are drastically improved after Ti doping. Our findings indicate that doping of Pd with non-noble metal can further improve the performance of three-way catalysts.

운전조건 변화가 HCNG 엔진용 삼원촉매 전환효율에 미치는 영향

Abstract - Stoichiometric combustion engine with Three-way catalyst had an advantage that can reduce the harmful emissions effectively. Fuel equivalence ratio controlled from engine is very important because Fuel equivalence ratio with high conversion efficiency was narrow. This study analyzed the conversion efficiency under whole range of operating area for to evaluate the performance of three-way catalyst. In order to identify the Optimum conversion efficiency, the conversion efficiency due to change the control value of fuel equiv-alence ratio was investigated. The result show that conversion efficiency of emissions(more than 95%) has dis-covered by means of fuel equivalence ratio control at each test condition. As engine power increases, optimal fuel equivalence ratio tended to increase linearly under operating conditions of similar exhaust gas temperature. Key words : HCNG(Hydrogen- Compressed Natural gas Blends), stoichiometri c, TWC(Three-way Catalyst), conversion efficiency, equivalence ratio

Lean HCCI/Rich SACI Gasoline Combustion Cycling and Three-Way Catalyst for Fuel Efficiency and NOx Reduction

Multimode combustion (MMC) concepts using homogeneous charge compression ignition (HCCI) gasoline combustion at low loads and spark assisted compression ignition (SACI) gasoline combustion at medium loads have the potential for improved fuel efficiency relative to spark ignition (SI) gasoline combustion. Two MMC concepts are compared in this paper with respect to fuel efficiency and tailpipe NOx emissions. The first concept uses stoichiometric HCCI and SACI to allow standard three-way catalyst (TWC) operation. The second concept also uses HCCI and SACI, but cycles between lean and rich combustion and uses a TWC with increased oxygen storage capacity (OSC) for potentially even greater fuel efficiency improvement. This paper performs a preliminary comparison of the two MMC concepts by analyzing two scenarios: (1) cycling between stoichiometric HCCI at 2 bar BMEP (brake mean effective pressure) and stoichiometric SACI at 3 bar BMEP, and (2) cycling between lean HCCI at 2 bar BMEP and rich SACI at 3 bar BMEP. The effects of excess oxygen ratio during HCCI operation and the frequency of oxygen depletion events on TWC performance and fuel efficiency are investigated. Results show that MMC lean/rich cycling can achieve better fuel efficiency than stoichiometric HCCI/SACI cycling. NOx emissions are moderately higher, but may still be low enough to meet current and future emission regulations.