As regulations drive criteria pollutant and fuel economy standards, researchers in Southwest Research Institute’s Advanced Combustion Catalyst and Aftertreatment Technologies (AC2AT) consortium have made great strides in diesel emission control systems development and research in advanced engine technology and catalyst applications.

The AC2AT consortium takes a pre-competitive, collaborative approach to understanding the complex nature of emissions from today’s high-performance, high-efficiency gasoline and diesel engines. To improve aftertreatment and fuel efficiency strategies, the consortium focuses on four areas: characterizing emissions from advanced combustion regimes, developing low-temperature catalysts and emission control solutions, decoupling interactions of soot and ash on advanced wall-flow filter systems, and developing 3-D emission control system simulation tools.

Characterizing emissions from advanced combustion regimes

New combustion strategies are being developed in the truck and off-highway industry to meet increasingly strict fuel efficiency and criteria pollutant standards. Researchers are evaluating high temperature, low exhaust gas recirculation (EGR) diesel combustion, and low-temperature, dual-fuel reactivity controlled compression ignition (RCCI) strategies to gauge their effects on overall fuel efficiency. Historically, higher temperature combustion strategies have been used to increase fuel efficiency. The AC2AT group, however, uses a combination of both hot and cold combustion strategies to meet current and future fuel economy and greenhouse gas emission (GHG) standards.

Reducing pumping and friction losses in hot combustion regimes helps increase fuel economy and reduces hydrocarbon and particulate emissions. The challenge, however, is increased NOx emissions levels.

Fuel economy improvements in cold combustion regimes are due primarily to reduced heat transfer losses, reduced energy rejected to the exhaust stream, and significantly reduced NOx emissions. The challenges here, however, are not only increased hydrocarbon emissions and more complex particulate emissions, but significantly lower exhaust gas temperatures making catalytic emission control more difficult.

While traditional emission control strategies may work for the hot combustion regime, the changing nature of particulate and hydrocarbon emissions and lower exhaust gas temperatures require new strategies for cold combustion regimes. The AC2AT consortium is evaluating strategies that include hydrocarbon selective catalytic reduction (HC-SCR), partial oxidation catalysts for particulate control, and passive hydrocarbon storage catalysts. These all provide processes for mitigating the challenging hydrocarbon emissions from low-temperature combustion strategies.

The AC2AT team evaluates these challenges by determining the nature of emissions from advanced combustion concepts and comparing them to conventional strategies. As combustion strategies change to meet fuel economy standards, the researchers also observed a change in the amount and composition of criteria pollutant emissions coming from the engine. These changes can alter the conditions under which the catalyst must operate. AC2AT researchers are developing optimized powertrain solutions that can achieve the most stringent heavy-duty NOx emissions of 0.02 g/hp-hr, while also achieving the GHG standards for 2025 and beyond.

Development of low-temp catalysts and emission control solutions

It is becoming increasingly important in the truck and off-road industry for researchers to seek innovative emission control solutions. As engines become more efficient, exhaust gases get colder. New criteria pollutant emissions regulations require technologies to mitigate emissions under both cold-start and sustained low-temperature operation.

To meet these challenges, the AC2AT team is reinventing ways to use existing technologies as well as experimenting with new catalysts. Passive NOx adsorber (PNA) catalysts, hydrocarbon storage traps, and in-exhaust burners are some of the existing technologies being evaluated. Newer technologies to improve low-temperature performance include catalytic approaches to improve diesel exhaust fluid (DEF) decomposition and utilization, mixed metal oxide SCR on filter, and passive HC-SCR.

These efforts are multi-faceted and include reducing low-temperature emissions via trapping and subsequent reduction (passive NOx adsorbers and HC storage traps) as the aftertreatment system reaches operating temperature. New catalyst technologies effective for emissions reductions at temperatures below 150°C (302°F) include tailor-made oxidation catalysts, novel use of catalysts to improve DEF injection quality and reduce deposit formation, and new zeolite SCR formulations.

The AC2AT team is working to better understand the behavior of DEF decomposition to develop improved injection mixing systems that reduce deposit formation and growth. New physical and chemical strategies that reduce the formation of deposits are also being developed. Part of these efforts are focused on using DEF at lower temperatures to improve the NOx reduction potential of SCR systems. Reducing the propensity for DEF deposits results in direct benefits of improved NOx conversion at low-temperature operating conditions as well as improved fuel economy from less frequent high-temperature regenerations.

Decoupling interactions of soot and ash on advanced wall-flow filter systems

One of the most innovative areas of this research is investigating the viability of advanced wall-flow catalysts, such as SCR on filters. Rather than coating filters with precious metals to promote soot oxidation, AC2AT is using existing technologies that coat the filter substrate with an SCR catalyst, not only to reduce NOx emissions, but to reduce the packaging volume of the entire system.

Although SCR on filter has been used in light-duty applications since 2015, the process has yielded unforeseen challenges relating to the impact of soot and long-term ash interactions on catalyst performance. AC2AT researchers are working to understand how soot and ash can interact with the washcoat, substrate and diesel exhaust fluid or urea—and whether this interaction impacts NOx conversion efficiency over time.

In addition to traditional SCR on filter technologies, which utilize copper zeolite SCR catalyst technology, the AC2AT members are also evaluating new strategies that make use of mixed metal oxide SCR technology. While traditional copper zeolite catalysts are highly effective for NOx reduction, they offer no benefit for soot oxidation. Using copper zeolite SCR on filter typically would require increased filter regeneration events to remove stored soot, resulting in additional fuel consumption.

Mixed metal oxide SCR catalysts, on the other hand, present an opportunity for simultaneous NOx reduction and soot oxidation under the right operating conditions. These mixed metal oxide SCR catalysts typically use metals such as manganese and cerium instead of vanadium or copper/iron exchanged zeolites. While mixed metal oxides have been used in some production applications, their market share is relatively small. The consortium is evaluating this new strategy to quantify the benefits on NOx conversion and fuel economy and to identify any potential challenges which may exist.

Evaluating these mixed metal oxides for SCR on filter is still in the research phase, and if successful, it will still be a number of years before we can expect to see any full-scale production.

Development of 3-D emission control system simulation tools

To streamline the development and integration of an aftertreatment system, the AC2AT team is developing innovative tools such as 3-D emission control system simulation packages. Zero-D and 1-D simulation tools have long been used to improve system performance via analysis-led design. While these strategies may be acceptable for certain emission control systems, the non-uniformities experienced with the injection of DEF require 3-D simulation to more accurately and precisely predict NOx conversion performance.

AC2AT is working to combine 3-D computational fluid dynamics (CFD) with custom chemical kinetics for DEF decomposition and catalytic NOx conversion. Developing these unique tools will enable the industry to predict DEF deposit formation and growth as well as overall NOx conversion efficiency for different system configurations and DEF delivery and mixing systems.

All of the kinetic tools and simulation processes AC2AT is developing are compatible with major CFD software packages, and are provided to all consortium members to use for their system development. These new tools are capable of accurately predicting DEF deposit mass and location. Further improvements and validation exercises are under way to improve the accuracy and to add 3-D performance simulation capability. These capabilities will enable OEMs and suppliers not only to reduce cost of emission control system development, but also improve overall NOx conversion efficiency.

About AC2AT

The four-year AC2AT consortium was formed in 2014 with members representing engine manufacturers and affiliated businesses in the automotive industry. The consortium has 11 members from various specialties and markets. Participants include suppliers such as Tenneco, Eberspaecher and Bosch, to major truck and off-highway OEMs such as John Deere, Cummins and Volvo.

Through its collaborative approach with industry leaders, the AC2AT consortium maximizes its efforts to develop the latest processes and technologies for truck and off-highway developments. In its first two years, the consortium filed two patent applications with several more in progress. Consortium members receive a royalty-free license for patents earned by SwRI engineers through the consortium during the program.

By combining membership fees, members share substantially more pre-competitive research dollars than would be possible with funding from a single client. New members can enroll for $95,000 per year. The next Program Advisory Committee (PAC) meeting for the consortium is planned for the week of October 23, at SwRI headquarters in San Antonio, TX.

Dr. Cary Henry, program manager for the AC2AT consortium and manager of catalyst and aftertreatment R&D activities at the Southwest Research Institute, and Crystal Smith Henry, senior publicist for R Public Relations, wrote this article for Truck & Off-Highway Engineering.