In Aftertreatment, less Is More

August 2, 2016

Terry Costlow

Simulations can help Perkins develop ways to combine particulate filters and SCR to shrink aftertreatment systems.

For design teams focused on aftertreatment, reducing package size has become almost as important as removing emissions. Combining catalysts, improving filters and integrating sensors are a few of the techniques being used to minimize package sizes.

Now that suppliers have figured out how to meet current regulatory requirements, they’re striving to shrink packages while also gearing up for future cuts in emission levels. A range of strategies are being employed.

“We have optimized diesel oxidation catalyst (DOC) technology to improve emissions performance during active regeneration conditions while at the same time reducing the precious metal loading of the catalyst,” said Jason Schneider, manager of product engineering at John Deere Power Systems. “We have also adapted new diesel particulate filter (DPF) substrate designs offering lower backpressure. These two advances allowed size reductions on the order of 25% more than our production launch of Final Tier 4 compliant systems with no negative performance impact to the engine.”

There’s also an effort to combine catalysts to shrink space requirements. While the benefits can be significant, developers note that it’s often difficult to design systems that can save space and meet reliability requirements.

“Catalyst multi-functionality has the potential to reduce the number of substrates required in the system, thus shrinking the overall package space,” said Dr. Ben Patel, Vice President, Clean Air, Global Research & Development at Tenneco. “However, complexity increases because now the DPF’s soot loading mechanisms compete with those for NOx reduction, and both must be carefully managed to avoid catalyst and filter failures. Oxidation catalysts are becoming integrated with NOx adsorbers onto a single catalyst substrate upstream of other NOx reduction catalyst components. The NOx adsorber stores NOx under low exhaust temperature conditions, for example start up and idle, and releases it above the SCR catalyst activation temperature. That improves overall NOx conversion efficiencies.”

However, some combination strategies simplify system production. Reducing component counts saves space while simplifying procurement and manufacturing functions.

“The combination of NOx sensor and O2 sensor actually reduces the complexity of the system and provides cost reduction benefits,” said Alan Chewter, Senior Manager, Powertrain Systems, at IAV Automotive Engineering. “Improvements are being made in particulate matter sensors to increase durability, repeatability and sensitivity. In combination with advanced signal processing techniques, it is becoming possible to effectively estimate the sooting rate as well as the total accumulated soot mass over time.”

Much of the focus is on reducing size within aftertreatment systems. But it’s possible to alter engine designs to help reduce the complexity of cleansing systems. When engines emit fewer pollutants, smaller systems can be used to meet environmental requirements.

“People are trying to reduce soot from the engine so the particulate filter acts more like a sponge,” said Dave Rodgers, Engine Business Unit Director at Ricardo. “You don’t want to be cleaning the particulate filter too often. There’s also a push to put the particulate filter and SCR in the same box to reduce costs.”

A range of techniques have been employed to reduce the size of mechanical components like pipes and valves. That can also help reduce noise, which is an important factor in many operating areas.

“Mixers have advanced and significantly reduced the mixing lengths needed for diesel exhaust fluid dosing, offering light-off improvements to the catalyst,” Patel said. “The integration of exhaust valves has offered significant improvements in sound quality, engine power and reduced muffler volumes, becoming more important as space becomes even more premium.”