Mack affiliate Volvo AB already uses SCR in Europe, so its use in Mack’s 2010 platform is hardly a surprise.
Nowhere are the fruits of globalization more apparent in the truck manufacturing industry than in the preparations for the next round of Environmental Protection Agency emissions standards. The U.S. heavy-duty diesel engine industry is dividing into two camps to meet the upcoming standard of only two-tenths of a gram of nitrogen oxides per horsepower hour. That’s just a tenth of the NOx allowed in 2004, a percentage reduction comparable to the drop in soot enabled by the diesel particulate filter.
At least four manufacturers – Detroit Diesel, Paccar, Volvo and Mack – will use selective catalytic reduction (SCR), a form of aftertreatment to reduce NOx outside the engine. Two other manufacturers – Cummins and International – said they will rely on high-pressure fuel injection systems to reduce NOx through enhanced cooled exhaust gas recirculation (EGR) without an aftertreatment in heavy-duty applications, although Cummins plans to use SCR in medium-duty applications. At press time, Caterpillar was the only heavy-duty diesel engine supplier that had not yet announced its technology choice.
The technology choices in many ways mirror what these engine makers’ European sister companies and strategic partners are doing to meet current and upcoming emissions reductions on that continent. Mercedes Benz and Volvo AB – affiliates of Detroit Diesel and Volvo/Mack, respectively – already use SCR in Europe. The same goes for Paccar, which uses SCR in diesel engines built for its DAF trucks and likewise will use SCR for Paccar engines in Kenworth and Peterbilt heavy-duty trucks beginning in 2010.
Cummins and International, meanwhile, have forged engine development alliances or joint ventures with Scania and MAN, respectively. Both of those European engine makers so far have pursued emissions reductions without using SCR.
So which basic approach is preferred? That’s a tougher question, and it may come down to how you operate – and what happens between now and 2010.
Eureka for urea?
SCR feeds a small amount of ammonia-containing urea solution into a catalytic exhaust chamber, where the ammonia combines with NOx to form harmless byproducts. Urea occurs naturally in the urine of humans and other mammals, which is one reason some in the industry prefer to call it “diesel exhaust fluid” or DEF, but it’s also manufactured as an industrial chemical.
SCR will be similar to turning the clock back to the days before EGR engines, says Ed Saxman, Volvo’s drivetrain product manager. “Urea and NOx become nitrogen and water,” Saxman says. “That’s how SCR aftertreatment works. Why are we going to use it to kill NOx? Because everything else the engine makes can be reduced by making combustion more efficient.
“When you don’t have to reduce the engine-out NOx, you can do amazing things with today’s technology, which includes 35,000-PSI injectors in our case. That includes greater power density, greater thermal efficiency and reduced heat rejection when compared with higher levels of EGR.”
Volvo’s 2010 engines will reverse the trend of worsening fuel economy ushered in by the 2002 emissions standards, Saxman says. “You can turn the knobs back,” he says. “For somebody who uses a lot of fuel, it puts dollars into his pocket.”
What about the cost of adding a new chemical to the mix? Relative to the cost of diesel, it’s far less. Urea will be consumed at a rate of roughly 3 percent of the diesel fuel burned, Saxman says.
Detroit Diesel’s 2010 DD15 and its sister 13-liter and 16-liter engines will consume urea at a rate of about 2 percent of the diesel fuel burned, says David Siler, Detroit’s marketing director. While SCR engines potentially have higher upfront costs, the long-term benefits more than make up for that, Siler says. “The financial implications of our choice were in the center of our discussion on which path to take in 2010.”
“It’s premature to talk about front-end costs quantitatively because several influencing factors are still in rapid flux,” says Rakesh Aneja, Detroit’s program manager for 2010 heavy-duty engines. He adds, however, that because Detroit’s 2010 system is based on proven, mature technologies already in production, the engine maker can “effectively break away from the three-year product release cycle that we have witnessed in this decade,” Aneja says. “SCR provides us the opportunity to break the classic trade-offs associated with NOx, PM and fuel economy.” Plus, SCR reduces component stress and heat rejection and extends oil-drain intervals, he says.
SCR’s proponents argue that urea should be readily available to the diesel engine industry because other sectors already demand it in large amounts: Agriculture uses it in fertilizer, and utilities use it to reduce NOx emissions at power plants. The amount needed by engine manufacturers will be “a very small percentage of present production,” Saxman says. Siler agrees, saying urea producers “are investing in their capacity to meet the demand in plenty of time for the 2010 engines.”
In the Paccar platform, SCR will be used in concert with EGR. “The combination of SCR and EGR will provide Paccar customers a highly efficient solution to meet the rigorous 2010 emission requirements,” says Craig Brewster, Paccar assistant vice president.
Brewster says the company’s vehicles have operated SCR emissions systems in Europe successfully for years. “Paccar is working with SCR distributors to ensure a nationwide infrastructure is in place to serve our customers,” he says.
Current work on Paccar’s $400 million engine production facility in Columbus, Miss., is expected to wrap up in late 2009. The facility will produce the 12.9-liter and 9.2-liter MX and PR engines, respectively, for Kenworth and Peterbilt tractors, launching Paccar heavy-duty engines in the North American market.
Cummins’ heavy-duty emissions strategy for 2010 mainly consists of enhanced EGR. When exhaust is cooled to about 230 degrees Fahrenheit and fed back into the engine with the intake air, it becomes a potent way to kill NOx. But meeting the 2010 standard with more EGR is not as simple as it might appear.
You can’t just increase the size of the EGR system components. Your engine would have to endure excessive cylinder and turbocharger boost pressures, and the total volume of air and exhaust the turbo would have to jam in would be much higher than now. Increasing the size of the engine block would get these stresses under control, but such a beast would be difficult to fit under an aerodynamic hood; it also would be too heavy and costly to be practical.
There are other more subtle ways to put in a lot more EGR, says Steve Charlton, Cummins vice president of heavy-duty engineering. What matters, Charlton says, is not the amount of EGR going in, but the total volume of gases: air plus exhaust.
Since diesels need more air than combustion requires, one possibility is to improve “air utilization,” or how effectively the air and fuel mix in the combustion chamber. A better mixing process can bring down the amount of needed air. In Cummins’ case, half the equation was the XPI High-Pressure Common Rail injection system.
Present injectors trap a tiny volume of fuel under an injector plunger that is driven down by a rocker lever and the camshaft. The camshaft and rockers are mounted on the cylinder head and driven off the crankshaft by a geartrain.
Common rail, on the other hand, uses a crankshaft-driven pump and routes the fuel from the pump to all the injectors via small high-pressure passages, running at about 31,900 PSI. Common rail has one big advantage: The pressure always is there.
In the unit injector system, the speed at which the injector plunger moves downward drops with engine RPM because the crankshaft and geartrain slow down. The slower motion means less pressure produced.
The unit injector system also must build up pressure every time the engine fires because the fuel in the injector is unpressurized once injection ends. A short time lag ensues at the beginning and end of each injection cycle, as low-pressure fuel enters the combustion chamber.
Common rail guarantees maximum injection pressure even at 1,100 RPM and throughout every injection cycle. The higher pressure causes the fuel to mix better with the air.
Cummins also redesigned the combustion bowl. Helped by computer simulations, the designers have increased the force of the air motion and the turbulence so that the air moves more rapidly through the sprays and does a better job of supplying needed oxygen. The engine needs less air to burn the same amount of fuel.
Because of these improvements, less air goes in, and the total exhaust and air going in is only slightly higher than what was needed in 2007, Charlton says. The exhaust coming out contains less oxygen, which minimizes the increase in exhaust volume needed to get the NOx down, he says.
Thanks to this improved combustion system, Cummins says, its 2010 ISX will use the current bore and stroke for the lower power ratings, and the engine will increase in size only from 15 liters to 16 liters for the maximum power ratings – all while keeping stresses in line with today’s engines.
Cummins also will add a close-coupled catalyst to increase the effectiveness of active DPF regeneration, which has to occur more frequently when the exhaust contains less NOx.
A different means of dealing with EGR is demonstrated in the SCR camp, in the new Detroit Diesel DD15: The fuel will be pumped in slowly at first, then much faster later, during each injection cycle. This means the engine needs less air while achieving the same smoke limit, says Edward Crawford, Detroit director of product engineering.
International, which won’t use SCR in 2010, plans to address emissions requirements through an advanced fuel system, air management, combustion and controls.
“I have publicly been an advocate of customer-friendly emissions control solutions which do not add additional costs to our truck and bus customers,” says Daniel Ustian, chairman and chief executive of International parent Navistar. “While SCR is a means to achieve the NOx reduction requirement for 2010, it comes with a steep cost to our customers. Our ability to achieve our goals without adding customer cost and inconvenience is a competitive advantage for International.”
Caterpillar officials say they will build on their proprietary ACERT technology for 2010, but are not ready to announce the final details.
The bottom line
Proponents of SCR believe that their technology choice will benefit truck owners by restoring some lost fuel economy – even taking into account the cost of urea. Those pushing enhanced EGR focus on the freedom from both the costs and the management burden of using urea, as well as the comfort of relying on familiar technology. As fleets continue to test the planned solutions in greater numbers, the choice – and air – may become clearer.
Where’s the infrastructure?
Urea poses challenges for truckstop operators
Yes, truckstops plan to carry urea in time for the 2010 engines, but that doesn’t mean they’re wild about doing so.
“So far, we are looking at this as a cost and a condition of being able to carry diesel fuel,” says Sarah Dodge, NATSO vice president of government affairs. “It may prove to be a profit burden for us, rather than a profit center.”
Urea, which has been used in Europe for years, is sold in much smaller volumes than diesel
fuel at a much lower price. Yet it requires its own infrastructure, including heated or underground tanks and dispensing and metering equipment.
U.S. truckstops are “anticipating more headaches than anything,” Dodge says.
The fluid will be needed not just for big rigs but for a variety of vehicles, including
large pickups. Some distribution will begin by summer because of diesel-powered cars and SUVs.
Most truckstops will keep urea in a storage tank because customers will need more than a washer-fluid-sized package.
“We can’t figure how much we will need to store,” Dodge says, which makes it challenging to develop a business plan.
Urea also has a short shelf life that varies with the weather; it deteriorates rapidly when too hot and freezes at about 12 degrees Fahrenheit.
Some of the truckstop problems posed by urea, of course, are the same ones posed by diesel fuel and gasoline: how to ship it in, how to clean it up if it spills, how to deal with liability issues if it runs out or if bad urea is dispensed into customers’ tanks – and how to do all of the above affordably.
Along with the American Petroleum Institute and the U.S. Department of Energy, NATSO plans to develop an online service to help customers find truckstops, dealerships, service centers and retailers that sell urea. The listing for each location will include whether urea is sold at the pump or by the bottle, what types of vehicles the facility services, and whether the dealer participates in a urea quality-control program.