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Aftertreatment systems have become an integral component of engines today due to the implementation of increasingly stringent emissions regulations. "In the last decade, aftertreatment systems used by our global OEM customers have become increasingly complex and more densely packaged," says Dr. Korneel De Rudder, Development Manager, Europe at Donaldson Co. Inc.
This increased complexity is necessary to meet the ever-tightening emissions standards being implemented in the U.S., Europe and other parts of the world he says. "As a result, there are more catalysts and sensors incorporated into a given package size," De Rudder explains. "For many off-road machines, because under-hood space is limited, we need to be creative to fit these new components onto the machines – and the components need to be more densely packaged."
To address this, De Rudder says Donaldson has focused on developing components and systems which allow for less 'dead' volume inside the afterteatment system. "With earlier generation systems, 'dead' volume was often used to increase the residence time of the DEF (diesel exhaust fluid; also called AdBlue), which enhanced its decomposition and mixing in the exhaust gas,” he says. "Modern systems have less space for 'dead' volume, which results in less residence time and drives the need for improved mixer performance. At Donaldson, we've developed a comprehensive range of compact and advanced DEF/AdBlue mixing systems that are specifically designed to be more efficient at achieving effective mixing in smaller spaces."
Combining the SCR and DPF functions into a singular SCR-on-Filter Substrate (SCRoF) is becoming more common in the industry as a means of meeting system packaging challenges says Gary Simons, Engineering Director, North America at Donaldson Co. Inc. "Because SCRoF substrates have higher porosity than DPFs and SCRs, they are more fragile. Canning them requires careful management of the installation forces and the substrate mat holding pressures."
As aftertreatment system packages get larger and more complex, De Rudder also notes they tend to have more surface area. This larger external surface can lead to heat rejection issues. Proper insulation will become more critical and those designs which have already been implemented will need to be further refined.
DEF/AdBlue mixing is an important aspect of aftertreatment systems, as well, says De Rudder. "It is commonly expected the ammonia (NH3) uniformity requirements need to exceed 0.990 to meet emission standards. This level of performance is expected with little or no DEF/AdBlue crystallization."
This crystallization is a recurring challenge with many aftertreatment systems he says. While some crystallization is possible, minimizing its occurrence and impact is the goal for all equipment manufacturers. De Rudder says the industry currently struggles with defining a specification for consistent testing of crystallization. However, Donaldson has developed its own test cycle to measure crystallization so it can apply appropriate mixer solutions to minimize its occurrence.
"Because it is more likely to generate crystallization/deposits during challenging duty cycles, at Donaldson we often recommend that mixers are designed to allow for effective function even during demanding duty cycles – rather than an unoptimized system that will result in a greater risk of deposits," says De Rudder.
Although there are no new emissions regulations for off-road equipment currently on the books in the U.S. and Europe, the California Air Resources Board (CARB) has proposed a potential Tier 5 regulation. And with many countries looking to reduce emissions from all industries, stricter regulations are possible in the future.
Although the proposed CARB Tier 5 emissions regulation has not yet been formally accepted, Donaldson’s De Rudder and Simons say changes that could be included in the final rule will likely impact aftertreatment system designs. Reductions in NOx, particulate matter (PM), carbon dioxide (CO2) as well as increases in the useful life/warranty limits and colder duty-cycle tests are expected to be part of the regulation. "The reduction in criteria pollutants and CO2 will drive a change in emissions strategy," they say. "Dual DEF/AdBlue injection systems are possible, as well as closer coupling of the aftertreatment system to the engine."
In addition, De Rudder and Simons say there is growing consensus in the industry that use of EGR will be necessary to meet the next generation of emissions limits. "This means that engines that are calibrated for very high raw NOx and low PM will become less prevalent," they explain. "This will lead to reduced burden on DEF/Adblue mixers – which will bring other trade-offs/challenges such as the need for greater regeneration of DPFs."
"Today’s off-road engines tend to rely on passive DPF regeneration with an off-line active regeneration when needed, while many on-highway engines can have active regenerations during normal operating with minimal impact to the user," adds De Rudder and Simons.
They go on to say the higher raw PM which results from the reintroduction of EGR will increase the need for more frequent active regeneration. This will likely require OEMs to develop similar regeneration strategies used for on-highway engines.
Dr. Korneel De Rudder, Development Manager, Europe
"During the last 15 years, global emission regulations have evolved rapidly," says De Rudder. "Because the industry rushed from one legislation to the next, this may have led to some over-engineering, with little time devoted to cost and product optimization."
However, now there is more time being given between emissions regulations which is better enabling the industry to optimize aftertreatment systems and harmonize between different platforms he says. OEMs too have been better able to focus their design efforts on optimized products and functionality instead of meeting tight emissions regulation deadlines which has also benefited the industry and likely will continue to be the case going forward.
Additionally, many in the industry see passenger car and heavy-duty on-highway standards merging at the EURO 7/VII level. "If, and when, this happens, there will be questions regarding how the next Stage/Tier off-road standards can be harmonized with the on-highway EURO VII limits," says De Rudder and Simons.
In general, further technological developments will be necessary for aftertreatment system designs whether that be to meet customer requirements or future emission regulations.
"We also expect to see significant changes on the low-power side of the market," concludes De Rudder. "Specifically, electrification will play a significant role by replacing some diesel engines and hybridization of engines and machines will make high-power electricity available for use during the heat-up phase of the exhaust," which will aid aftertreatment system performance.