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SCR is Coming
by Rolf Lockwood
There are two approaches for meeting the 2010 medium- and heavy-duty diesel emissions regulations, and sooner or later you'll have to decide: cooled-exhaust-gas recirculation or selective catalytic reduction, EGR or SCR. Unless you're tied to a specific truck brand that makes the choice for you, it might be a tough decision. We won't do a full-blown engineering investigation of the two technologies here - we don't know enough about all the '010 variations yet - but let's have a look at the rough welcome that SCR is getting in some quarters. Is it deserved? ‘Probably not' appears to be the answer, so let's explore. First off, a reminder about the basics of EGR, which we've known since 2002. It circulates cooled exhaust gas back into the engine's air intake, lowering combustion temperature and reducing the formation of nitrous oxides, or NOx. All but Caterpillar used this, but in '07 even Cat added EGR to its ACERT technology. All engine makers needed diesel particulate filters (DPF) to meet the stringent '07 limits on particulate matter (PM), but 2010 is only about NOx, which must be dialed way down. Enter SCR, used by most engine makers in Europe, but not all, for a couple of years now (MAN and Scania make both but favor EGR). In fact, SCR's been used to reduce stationary-engine emissions since the 1980s. Its main benefit is that it's said to increase fuel efficiency by three percent or so compared to an '07 EGR motor. European experience seems to bear this out. In all SCR engines a fluid called urea (also called DEF, or diesel exhaust fluid) is injected into the exhaust stream. It combines with exhaust heat and a catalyst to convert NOx into harmless pure nitrogen and water vapor, and it costs a little less than half the price of diesel fuel, though that's a moving target since the cost of both has been rising sharply. An SCR truck engine also uses a relatively mild form of EGR, ironically, as well as a DPF.
So who's using what for 2010? In fact, there's a split down the middle of the market. On the SCR side we have Mack/Volvo and Detroit Diesel, plus the new engines coming from Paccar in 2010, based on Europe's DAF motor. On the other side we have Cummins and International, its heavy-duty MaxxForce engines co-developed with Germany's MAN. They'll continue to use a slightly more ‘aggressive' form of EGR with DPF for on-highway trucks, saying EPA standards can be achieved by in-cylinder means, especially high-pressure fuel injection. They won't need any NOx aftertreatment. Caterpillar won't be making a 2010 pronouncement. As you'll read in Channel 19 (p. 8), the company is getting out of the on-highway engine business at the end of 2009. Daimler Trucks North America president and CEO Chris Patterson, on the other hand, makes his company's 2010 plans clear, though they've been known since 2006. Same with Volvo/Mack. "We will be utilizing Daimler's BlueTec technology for our Detroit Diesel engines beginning in 2010," Patterson says. "The technology is... the only means of meeting the stringent nitrous-oxides standard for heavy-duty diesel engines in 2010 while actually reducing diesel fuel consumption in comparison with the technology used in 2007 engines." "BlueTec" is the Daimler name for the SCR system that's already being used in thousands of Mercedes trucks in Europe. Cummins vice president and heavy-duty engine chief Steve Charlton says not much will actually change in his company's '010 motors. Next-generation EGR doesn't add complexity, he says, and power, torque, fuel economy, and maintenance intervals will remain the same as today. The 2010 Cummins ISX is getting "refinements" to combustion, air handling, and EGR integration. It will use the XPI high-pressure common-rail fuel system designed and built by the company's joint venture with Scania, said to maintain high injection pressures regardless of engine speed. It will also continue to use the variable-geometry turbocharger and DPF designed in-house. Cummins is unique here, because its medium-duty engines will actually use SCR for 2010. The addition of a NOx-reduction system will be straightforward because in 2006 it launched this very technology in Europe to meet the Euro 4 standard. Asked why it will use both EGR and SCR, the company just says it will provide "the right solution" for the application. OK, so what's the controversy? It began when International Truck & Engine president Dee Kapur made some very negative comments about SCR earlier this year. DTNA and its Detroit Diesel subsidiary have been working ever since to "dispel the SCR misinformation being tossed about." When we asked about SCR a couple of months back, Kapur said, "We don't like it." He pointed to the infrastructure required to replenish urea tanks. He also said urea is known to gel in cold climates. And then he said urea was toxic. DTNA's senior VP Mike Delaney counters this way: "Given our driving conditions characterized by steady cruising speeds on the highways and enormous overland distances, the new BlueTec system will show its advantages early on," he says. "The BlueTec system with SCR technology is projected to increase fuel efficiency by about three to five percent." The key challenge is indeed urea distribution. It's a common chemical, but it will have to be routinely available at truckstops and the like. That's because a 20-gal tank probably won't last much more than 5,000-6,000 miles, meaning you'll have to fill it between normal service intervals. We don't yet have a distribution system in place, and SCR critics are quick to pounce on this, but Delaney isn't worried. "With the massive effort currently underway to establish the DEF distribution infrastructure in North America, we're not concerned in the least about availability," he says. "With just the 800 or so outlets created by the participants already committed, we have more than enough to get started, but there will be a lot more than that. Let's put it this way - with a 5,000-mile range, you'd have to try pretty hard to miss them." In many cases, says Detroit Diesel marketing director Dave Siler, urea will first be sold in 1,000-gal ‘mini bulk' dispensers, with proper pumps being added as demand dictates. As for the poor cold-weather performance of urea, Detroit Diesel's EPA 2010 program leader Rakesh Aneja says it's not an issue. Yes, it does gel at about 12 degrees F or -11 degrees C, but it warms up and flows in little time. In fact, Aneja says, you can start a cold engine and run it right away with no ill effects. Is urea toxic, a form of ammonia? It's neither. Urea is actually two thirds water, Aneja explains, and while a small amount of ammonia is produced during the chemical reaction in the aftertreatment system, it stays there. So in the end, what will we have? The need to worry about and pay for one more fluid will keep some truck operators in the EGR camp, especially in the long-haul world. On the other hand, if SCR really does produce significant fuel savings, the sharply rising cost of diesel will be a very compelling factor in its favor. It really does come down to those two factors.
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