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Conference report: SAE 2017 Congress

29 April 2017

The SAE 2017 World Congress was held on April 4-6 in Detroit, MI. The Congress followed the formula from the previous years, with a three-day schedule of technical sessions and a scaled-down exhibition, but the presentations were extended to 30 minutes. The technical program included 1,385 technical papers covering all areas of automotive technology, including a number of papers on engine and emission control topics.

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Technical sessions on engine and vehicle emissions were opened with a review of recent regulatory and technical developments in vehicle efficiency and emissions, presented by Tim Johnson of Corning [Paper #2017-01-0907]. Some of the important regulatory developments include the implementation of Real Driving Emissions (RDE) testing requirements in the EU, with a NOx conformity factor of 2.1 for light-duty diesels, and a PN conformity factor of 1.5 for GDI vehicles. China has finalized their stage 6 emission regulations for light duty vehicles, which represent some of the most stringent emission standards in the world—effective from 2023, the final China 6 limits will be 40% lower than the respective Euro 6 standards.

The presentation also covered a number of engine concepts for meeting future criteria pollutant and GHG emission standards. The gasoline direct injection compression ignition (GDCI) engine is targeting US Tier 3 Bin 30 standards while showing high fuel economy. Nissan commercialized their multilink variable compression ratio system, which can provide a ~10% improvement in fuel economy and lower cold start emissions. In heavy-duty engines, several manufacturers outlined strategies to reach 55% BTE. Among the approaches, Volvo proposed a novel six-stroke engine concept with two compression stages and two expansion stages, producing an effective compression ratio of 55:1 and 300 bar peak cylinder pressure.

An interesting development in SCR technology was the introduction of a new catalyst, utilizing a copper exchanged, high silica and small pore LTA zeolite. The Cu/LTA catalyst was shown to be hydrothermally durable up to 900°C. It has been also found that diesel EGR could be utilized to increase the NO2/NOx ratio, a finding that might be used for the management of future SCR systems.

Low NOx Heavy-Duty Engines. The California ARB low NOx heavy-duty vehicle program, which targets NOx emission levels of 0.02 g/bhp-hr, has been an important driver in the development of emission technologies for heavy-duty engines. Under this program, the Southwest Research Institute (SwRI) has completed a demonstration of a low NOx heavy-duty diesel engine [2017-01-0954][2017-01-0956][2017-01-0958]. The target was to achieve FTP NOx emissions of 0.02 g/bhp-hr, representing a 90% reduction from the current standard of 0.2 g/bhp-hr, in a manner compatible with California GHG reduction targets. The starting point for the development was a 2014 MD13TC Euro VI Volvo engine, equipped with cooled EGR, DPF, SCR and turbocompounding. Emission performance was evaluated over a number of tests, including the US HD FTP, WHTC, RMC-SET for GHG, CARB idle, as well as several vocational cycles.

A major challenge in the development was the control of cold start NOx emissions, making thermal management strategies a key component of the project. The mechanical turbocompounding made these challenges even greater, contributing to exhaust temperatures during early cold start that were as much as 50°C lower compared to a non-turbocompounded engine. The engine initial calibration for the cold start portion has been modified, including EGR modifications, multiple injections, intake throttling, and elevated idle speed. The engine reverted to the original, high fuel economy calibration when hot. A multitude of aftertreatment configurations were analyzed and screened. The final configuration included a passive NOx adsorber (PNA), a diesel fuel mini burner (MB) for SCR thermal management, an air-assisted urea (DEF) injection system, SCR on DPF (SCRF), SCR, and ammonia slip catalyst (ASC). A model based SCR control strategy was developed with mid-bed NH3 feedback.

Final aftertreatment configuration

With a fresh catalyst system, the engine achieved composite FTP NOx of 0.008 g/bhp-hr (0.027 g cold FTP, 0.005 hot FTP). In the final testing with an aged catalyst system, the engine achieved a composite FTP NOx of 0.034 g/bhp-hr (0.114 g cold FTP, 0.021 hot FTP), which was above the target value. The final aged NOx conversions were 95.8% over cold start FTP, 99.3% hot start, and 98.8% composite. The cold start performance loss was primarily the due to a loss of NOx storage capacity of the PNA. This was due in part to a canning failure that occurred during the development process and exposure to rich conditions during the engine calibration work (Pd-zeolite PNA becomes prone to sintering under rich exhaust conditions). The loss of the hot start performance was due to a certain loss of SCR activity. The final aged system also showed a 2.5% GHG emission increase over the FTP, including all effects such as regenerations and the burner operation.

SCR Technology. While NOx emission standards for heavy-duty engines have not changed since 2010, the real world SCR performance has been continuously improving since the launch of SCR equipped engines. In particular, newer SCR engines achieve higher NOx conversions at low temperature conditions—an important issue identified with many early SCR implementations. University of Minnesota [A. Kotz, oral only] compared real-world NOx emissions from urban buses powered by a model year 2013 Cummins ISL 8.9 L engine and by a 2015 ISL 8.9 L engine. The measurements were conducted over a 10-day period, with the buses accumulating of approximately 160 operating hours each. The 2013 and the 2015 bus achieved real-world NOx conversion efficiencies of 75% and 95%, respectively, with a 25% increase in overall urea dosing in the 2015 engine. Additionally, control strategy changes for the 2015 bus have improved system response and tightened the range of dosing to achieve closer to stoichiometric NOx:NH3 conditions. These changes have resulted in a relatively constant tailpipe NOx level, and real-world emissions similar to the FTP NOx standard (0.2 g/bhp-hr = 0.267 g/kWh) for SCR catalyst temperatures of 200°C - 350°C and engine out NOx ranging from 0 to 9 g/kWh. In addition to the improved control and increased dosing, additional design considerations such as improved thermal insulation, lowered dosing onset temperature, improved catalysts, and optimized fluid dynamics during urea injection may have contributed to the increased NOx conversion.

With future SCR systems targeting low operating temperatures below 200°C, there is more attention to the possible catalyst contamination by ammonium salts, such as ammonium nitrate (AN) and ammonium (bi)sulfate, that are formed at low temperatures. Cummins studied the impact of ammonium nitrate on a Cu/CHA SCR catalyst [2017-01-0953] and developed a kinetic model to describe AN accumulation and the impact on performance. In another talk, Cummins Emission Solutions examined the effect on hydrothermal aging on the formation and decomposition of ammonium nitrate on a different Cu/zeolite SCR formulation [2017-01-0946]. It was found that AN readily forms both as a result of the co-presence of NH3 and NO2, and also in the case where NH3 is pre-stored before NO2 is introduced. AN does not form if NO and NH3 are co-fed without NO2. AN loading correlates with the BET surface area of the Cu/zeolite SCR catalyst upon hydrothermal aging. AN thermally decomposes through both N2O and NH3/HNO3→NO2 routes. It should be noted that the SCR catalyst tested in the study—presumably a Cu/LTA zeolite—showed a remarkable hydrothermal durability, with a significant loss of surface area occurring only above 900°C.

BET surface area of degreened and aged Cu/zeolite catalysts [2017-01-0946]

An elegant explanation of the differences in the performance characteristics of copper- and iron-based zeolite SCR catalysts was presented by Ashok Kumar of Cummins [2017-01-0939]. The performance differences are explained by the relative difference in the acidic-basic nature of the two transition metals. The experiments conducted in the study showed that Fe-zeolite has relatively acidic nature as compared to Cu-zeolite that causes NH3 inhibition, and hence, explains low NOx conversion on Fe-zeolite at low temperature under standard SCR conditions. Similarly, the relatively basic nature of Cu-zeolite resulted in more prominent SOx storage and performance deactivation in the presence of acidic contaminant like SOx and required higher temperatures for desulfation as compared to Fe-zeolite.

SCR technology has been increasingly used in marine applications, often using low quality fuels. A team from CERTH/CPERI and Aristotle University studied the effects of soot and sulfur on SCR performance [2017-01-0947]. Extruded vanadia catalysts were tested on a single cylinder 4.6 kW four stroke DI engine using LSD and MGO fuels. An SCR performance assessment and a soot blow-off study were conducted. A range of fuel qualities and exhaust conditions were surveyed, such as the effect of soot accumulation on NOx performance, NH3 storage, and the evolution of soot and sulphate deposition at different exhaust temperatures.

Gasoline Aftertreatment. A team from the University of Tennessee and Oak Ridge studied a dual SCR system for the reduction of NOx in lean gasoline exhaust [D.W. Brookshear, oral only]. NH3 produced over an upstream Ag/Al2O3 HC-SCR catalyst was used to enable further NOx reduction over a downstream Cu/CHA NH3-SCR catalyst. Experimental work was conducted in a laboratory gas reactor (500 ppm NO, 5% H2O, 10% O2, N2 balance) using E85 vapors (1500-4500 ppm E85 on C1 basis) as the HC-SCR reductant. NOx reductions above 98% were achieved between 325-500°C. The performance curve was optimized by changing the C1:N ratio (E85 dosing) depending on temperature. The dual SCR strategy showed clear benefit in NOx conversion and fuel penalty compared to the Ag lean NOx catalyst alone. However, the NH3 and THC slip remained a problem. Cycling of the E85 reductant flow allowed to eliminate the NH3 slip and to reduce the THC slip. The project was part of the US DOE funded Co-Optima initiative.

Oxygen storage capacity (OSC) is one of the most critical characteristics of a TWC. OSC is closely related to the catalyst aging and performance, and is the basis for OBD monitoring of three-way catalysts. A study by Cummins looked into the lean breakthrough phenomena for TWC OBD on a natural gas engine [2017-01-0962]. OSC measurements were conducted in a micro reactor. Dual O2 storage model was proposed, including fast O2 storage sites located at the surface of ceria sites, and slow storage sites in the bulk ceria. Large differences were found in O2 breakthrough depending on space velocity. At high SV, most of the OSC storage was the fast storage (kinetically controlled). At low SV, the slow OSC provided a stronger contribution (diffusion controlled). Correlations of breakthrough time and breakthrough OSC as a function of oxygen concentration and space velocity were established, which were used to formulate an alternative methodology for TWC OBD.

Catalyst zone flow, which could cause potential TWC OBD issues, was discussed by Ford [2017-01-0961]. Under the US OBD requirements, air-to-fuel ratio imbalance must not cause tailpipe emissions to exceed 1.5 times the standards. However, in some engines, zone flow occurs in the TWC catalyst due to the impact of particular cylinders with different gas composition. In the presence of zone flow, different portions of the catalyst substrate may be operating with different air-to-fuel ratios, despite good overall control of the mean air-to-fuel ratio. This condition can result in a simultaneous high NOx (from the lean zone) and high HC/CO (from the rich zone). A metric called the zone flow index (ZFI) has been defined to quantify the level of velocity overlap that occurs within the catalyst substrate for each cylinder. A highly close-coupled catalyst test case was examined, with measurements confirming catalyst zone flow. A hardware flow redistribution device was incorporated at the converter inlet to reduce the zone flow. This solution has been implemented in a commercial vehicle model.

The recently adopted European RDE testing requirements will affect the emission system design in all types of light-duty engines. Umicore investigated the impact of RDE regulations on exhaust aftertreatment in gasoline direct injection vehicles [2017-01-0924]. Two Euro 6b vehicles were tested: a D-segment station wagon (0.075 kW/kg) and a C-segment hatchback (0.1 kW/kg). Emissions from both cars were compared over a number of test cycles: NEDC, WLTC, RTS 95, FTP+US06, as well as two RDE modes, a moderate and a dynamic RDE route. Among the drive cycles, the FTP+US06 provided the best approximation of the RDE results. The WLTP results were also close to RDE for PN emissions, but not for NOx and other gases. The test were also conducted with a system that included a TWC and a catalyzed gasoline particulate filters (cGPF), demonstrating that the TWC+cGPF configuration could provide a robust aftertreatment solution for both NOx and PN emissions under RDE requirements.

Conference website: wcx17.org