Conference report: SAE 2015 Congress
30 April 2015
The SAE 2015 World Congress & Exhibition was held on April 21-23 in Detroit, MI. The Congress followed the formula introduced in 2010, with a three-day schedule of technical sessions, but the duration of presentations was increased from 20 to 30 minutes. The conference program included 1405 technical papers covering all areas of automotive technology. There was some increase in the number of exhibitors from the last year, but the exhibition remains significantly smaller compared to the pre-recession years. The overall number of Congress participants exceeded 10,000 and was similar to the 2014 attendance.
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Sessions on vehicle emissions opened with an overview of global vehicular emission trends by Tim Johnson of Corning [Paper #2015-01-0993]. New regulations that set the emission requirements for light-duty vehicles in North America include the California LEV III program and US EPA Tier 3 standards—both emission rules are now final, while the new LEV III OBD requirements have been proposed by the California ARB. The LEV III/Tier 3 standards require significant cold start emission reductions in both gasoline and diesel engines. In diesel cars, this will likely require close-coupled SCR systems in the SCR-on-filter (SCRF) configuration (i.e., particulate filters coated with an SCR catalyst). High porosity SCR substrates can further enhance low temperature SCR performance due to the higher catalyst loadings compared to conventional flow-through substrates. Improved cold start performance can be also provided by passive NOx adsorber catalysts. In the European Union, RDE emission limits—designed to address the issue of increased in-use emissions compared to regulatory testing—are still under development. In diesel cars, the RDE limits will require wider use of SCR catalysts with focus on emission performance outside of the NEDC test cycle. In parallel with cleaner emissions, light-duty engines in the USA and in the EU must meet increasingly more challenging fuel economy and CO2 emission targets. These future CO2 targets, however, may be threatened by the increasing costs of vehicle efficiency technologies. As the CO2 targets become tighter, the predicted cost of the required technology increases steeply, while the customer savings on the cost of fuel decrease and the payback period becomes longer.
Important upcoming regulations for heavy-duty engines include the US EPA Phase 2 GHG emission standards that are scheduled to be finalized in 2016. After the adoption of the Phase 2 GHG standards, the clean air agencies are expected to commence the development process of new federal regulations to reduce NOx emissions from heavy-duty onroad engines to levels as low as 0.05-0.02 g/bhp/hr, reflecting the Optional Low NOx Standards adopted last year in California. In the nonroad engine sector, Stage V emission standards have been proposed in the EU, which include particle number (PN) emission limits for several engine categorizes. If the Stage V proposal is adopted, the PN limits will force the use of particulate filters on the affected engine categories. The proposed PN limit of 1×1012 1/kWh is particularly demanding because of the hot regulatory test cycle, which causes particle burn-off in the filter and lowers the filtration efficiency.
Engine Developments. Gasoline SI engine efficiency continues to improve with some development engines showing peak brake thermal efficiency (BTE) of 40% or higher. This is a considerable improvement over the efficiency of many of the latest production engines such as Toyota’s turbocharged 1.2 L ESTEC (36.2% BTE) [2015-01-1268] and GM’s range extender for the 2016 Volt (36.5% BTE) [2015-01-1272]. Honda presented an approach that resulted in a maximum BTE of 45%. Key to this high efficiency were: a geometric compression ratio of 17, late intake valve closing for 12.4 effective compression ratio, a stoke/bore ratio of 1.5 and 35% EGR. High turbulence intensity, a high energy ignition system and an optimized combustion chamber design were key to achieving the high EGR rates. A low pressure loop EGR system ensured sufficient exhaust flow for turbocharging. The boosting system required parallel sequential turbochargers to achieve the required range of boosted airflow. The engine was designed for 91 octane fuel [2015-01-1263].
A design for a more modest efficiency target of 40% was presented by Toyota for hybrid applications. This development was centered around a modified Prius engine. The latest production Prius engine achieves 38.5% BTE. To gain the additional efficiency, cooled EGR (28%), strong turbulence, high energy ignition, high tumble ratio and a piston bowl redesign was employed. Compression ratio is 13 and 91 octane fuel is required [2015-01-1254].
Diesel Aftertreatment. Most papers in diesel aftertreatment sessions focused on NOx emission control for both light- and heavy-duty (onroad and nonroad) applications. Due to the cold start emission challenges in meeting the LEV III/Tier 3 standards, as well as the anticipated future heavy-duty engine standards, NOx adsorber catalysts (also called lean NOx traps, LNT) have been receiving increasingly more attention. In several proposed aftertreatment configurations, a passive NOx adsorber was positioned upstream of the SCR catalyst, to store NOx during engine start-up, when temperatures are still below the SCR catalyst light-off. Many discussed systems also incorporated SCR catalysts coated on the diesel particulate filter (SCRF). The SCRF allows to position the SCR catalyst closer to the engine—exposing it to higher temperatures—and provides a more compact aftertreatment package.
FEV discussed diesel aftertreatment strategies for light-duty LEV III applications and their implications on OBD [2015-01-1040]. Single NOx aftertreatment devices (LNT or SCR) offer limited potential for LEV III diesels. Active LNTs would require increased regeneration frequency with subsequent increases in HC emissions. Also desulfurization of LNTs makes it very challenging to maintain high NOx conversion over the required lifetime. SCR catalysts already perform at high levels of conversion but would require additional means to speed up light-off; this would typically have a negative impact on HC emissions and fuel consumption. Another challenge with high conversion efficiency in single brick NOx catalysts is that the allowable deterioration in NOx conversion performance before emissions exceed the OBD threshold is relatively small; making reliable diagnostics very difficult. Aftertreatment systems that use two NOx aftertreatment devices (e.g., LNT + SCR, LNT + SCRF or SCRF + SCR) can address some of these challenges. High NOx conversions are possible for such combined approaches even though each device on its own has only a modest conversion efficiency. Also, a certain degree of redundancy is available since deterioration in NOx conversion of one device can be accommodated by an increase in conversion by the second. Diagnostics can also be greatly simplified since a relatively large decrease in conversion in one of the devices is required before the OBD threshold limit is exceeded.
SCR Technology. The main goals of the ongoing development of SCR technology include increased NOx conversion efficiency (overall and at low temperature), catalyst durability, and N2O emission control. Cummins presented a study on hydrothermal aging of Cu/SSZ-13 (Cu-chabazite) SCR catalyst [2015-01-1022]. A sample of Cu-SSZ-13 was hydrothermally aged between 550 to 900°C. The changes of performance in NH3 storage, oxidation functionality and NOx conversion of the catalyst were evaluated, and the catalyst samples were characterized for structural changes. Progressive hydrothermal aging up to 750°C decreased NOx conversion to a small degree, as well as NH3 storage and oxidation functions. Above 750°C, zeolite sites were destroyed due to dealumination.
US EPA emission standards include limits for N2O emissions (for both light- and heavy-duty engines) that became effective this year. These limits put some additional demands on the performance of SCR systems, as N2O can be produced over each of the SCR system components: a Pt-based DOC can catalyze N2O formation via HC-SCR processes, the SCR catalyst (especially copper-based formulations) can reduce NOx to N2O rather than N2, and the ammonia slip catalyst can oxidize NH3 to N2O. All of these processes, as well as formation of N2O over the regulatory test cycles, were discussed by Cummins [2015-01-1030]. Since most of the N2O formation occurs in the front part of the SCR catalyst, the authors suggested that N2O could be controlled by replacing the front part of the SCR catalyst with a low N2O formulation such as a vanadia or iron zeolite catalyst. The Cu catalyst would be used in the rear part to enhance NOx performance under non-optimal NO2/NOx ratios.
Gasoline Engine Aftertreatment. In the sessions on light-duty gasoline engine aftertreatment, presentations could be broadly categorized as those dealing with catalyst optimization (i.e., PGM reduction) and those dealing with performance improvements for upcoming emission limits such as LEV III/Euro 6/RDE/WLTP. Most papers focused on the NOx/NMOG performance of three-way catalysts (TWCs) but there were also a couple papers by Ford on lean burn aftertreatment systems. In other sessions, while there was considerable discussion on particulate emissions from GDI engines, there were only a small number of papers that dealt specifically with gasoline particulate filters (GPFs); two by filter manufacturers NGK [2015-01-1073] and Ibiden [2015-01-1011] discussed GPF integrated TWCs.
While the cost implications of reducing PGM usage are obvious, there is also some political pressure to reduce the use of some PGM metals such as rhodium. In the EU, initiatives such as the Raw Materials Initiative have attempted to reduce the risks of relying heavily on sources of such critical materials from unstable regions of the world.
Honda presented a Pd-only TWC that does not use rhodium [2015-01-1003]. Pd-only TWCs have traditionally performed poorly due to low NOx conversion, Pd sintering and phosphorus poisoning leading to significant increases in NMOG emissions with aging. In this Pd-only catalyst, (1) NOx purification performance was improved using an enhanced basicity support material that promoted the dissociation of NOx; (2) sintering of Pd was controlled using the anchoring effect of praseodymium-zirconia (PrZrOx); (3) poisoning inhibition of Pd and the support material was enabled by the phosphorus trapping effect of praseodymium.
Conference website: sae.org/congress