Farewell to active regeneration

  • 24-May-2010 10:30 EDT
Grafik_EPA_10.jpg

For Detroit Diesel, Eberspächer manufactures the new SCR system in a single box that contains catalytic converter, DPF, and SCR complete with sensors and cabling.

The production ramp-up of selective catalytic reduction (SCR) systems for Volvo Trucks and Detroit Diesel is an industry first in the U.S. truck market. Since both companies announced that they would go for SCR to achieve EPA 2010 nitrogen oxide (NOx) limits, exhaust technology supplier Eberspächer has installed a dedicated welding and assembly area with some 12,000 m2 (129,170 ft2) at its Brighton site in Michigan. An additional 9000-m2 (96,875-ft2) factory for canning and subassembly was built in Wixom, MI.

“Now that EPA '10 is in effect, SCR system production is getting up to speed as the demand for new trucks increases,” said Dr. Rainer J. Lehnen, Head of Global Product Development for the supplier’s commercial vehicles business unit. He looks back at a three-year international development phase as work on one program began in December of 2006.

Highly integrated exhaust gas aftertreatment

At the two production sites near Detroit, the supplier makes combined diesel particulate filter (DPF) and SCR systems. For Detroit Diesel, both are mainly integrated in a single box while the Volvo system comprises two boxes for DPF and SCR, based on customer requirements. The Tier 1’s role is that of system supplier: Eberspächer cooperates with other international suppliers for delivery of the dosage system, substrates and washcoats, and controls.

“Our job is to manufacture a system to last up to 1 million miles,” said Lehnen. “This is despite the fact that the systems have to withstand temperatures of up to 650°C.”

The Detroit Diesel single-box DPF and SCR system weighs around 180 kg (397 lb). During assembly, 43 m (141 ft) of welds are done per system. The finished product has to be air-tight despite the ferritic stainless steel’s tendency to warp and bend under the heat introduced by welding. Welds that require a high level of precision, such as the canning modules, are done by laser—the rest is MAG welding.

“It takes a lot of industrialization know-how and experience to translate all this into the current highly automated production,” Lehnen shared.

The exhaust gas flow through the one-box system is quite complex. First the exhaust’s carbon monoxide and hydrocarbon content is reduced in two oxi cat flutes. After that the gas is filtered by two wall-flow DPF units. Then the gas flow is redirected within a chamber that leads on to the mixture preparation chamber where the urea solution is injected.

“Individual OEM requirements define whether we use an additional mixer for this or not,” said Lehnen. “It mostly depends on the required quality of mixture preparation while the engine runs in the low part load area of the map.”

Finally, the exhaust gas passes through the SCR bricks. Despite all this, the exhaust back pressure is kept low, Lehnen explained.

The sheer dimensions of truck DPF and SCR and the high-temperature application make canning of catalytic substrates a challenge because the steel wall expands strongly with temperature increase while the ceramic substrate hardly expands at all. To ensure tightness and secure mounting under all conditions, the canning tubes are tailored in diameter. For that purpose, the supplier has chosen a process that starts with an oversized tube. During canning, the inner diameter of each tube section is optimized for the individual brick.

SCR—pride and prejudice

This transition to SCR is no small step, and it was a stony road toward making it. One of the hindrances to the success of SCR in the U.S. was that truck drivers are not inclined to heed more dashboard lights coming on, telling them to adapt their driving or break routine to the needs of active regeneration of the DPF. Having to refill an additional tank with diesel exhaust fluid (DEF) and to pay for the DEF did not really help to make SCR popular either. So why should this change now?

To understand this, a quick look back is helpful: EPA 2007 had resulted in the industry decision to use DPF technology to meet the soot limit. Under EPA '07, low NOx levels could be achieved by heavy EGR. Volvo’s EPA '07 engines, for instance, use heavy EGR between 20 and 35% to reduce NOx.

Now EPA '10 has changed the landscape: New trucks must not emit more than 0.2 g of NOx per hp and hour. This is an 83% reduction on EPA '07. Further increasing EGR to a massive level between 35 and 50% offers the chance to bring down the combustion temperature further and thus to reduce NOx formation. Yet such high EGR rates impact the power density of the engine.

Also, massive EGR necessitates even more active DPF regeneration, as neither the temperature level nor the NOx levels of the exhaust gas are sufficient to ensure passive regeneration. Instead, cyclical active regeneration is needed with massive EGR. This requires fuel injection and driver attention.

To make matters worse, an engine that is NOx-optimized does not deliver optimum efficiency. SCR gives engine designers the freedom to optimize the engine toward efficiency and low soot levels. A slight increase in NOx formation is the price, but SCR ensures EPA '10 compliance despite this.

Fuel efficiency benefits from passive regeneration of the DPF: The higher NOx level helps to regenerate the soot so that regeneration at low temperatures will suffice under on-highway conditions. Therefore, Detroit Diesel is convinced that its EPA '10 engines will offer up to 5% better fuel efficiency over comparable EPA '07 engines.

In mid-April, Volvo Trucks North America made the same claim for its new D11, D13, and D16 engines.

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