The next round of Environmental Protection Agency emissions regulations go into effect in 2010, again requiring new technologies for heavy-duty trucks to achieve compliance. Diesel engine manufacturers that have revealed their solution currently are divided into two camps – those using selective catalytic reduction (SCR) and those using an enhanced version of cooled exhaust gas recirculation (EGR).
The 2010 standards allow two-tenths of a gram of nitrogen oxides (NOx) per horsepower hour – just a tenth of the NOx allowed in 2004, a percentage reduction comparable to the drop in soot enabled by the diesel particulate filter.
To meet this standard, at least four manufacturers – Detroit Diesel, Paccar, Volvo and Mack – will use SCR, a form of aftertreatment to reduce NOx outside the engine. At least one other manufacturer, Cummins, will use an enhanced version of cooled EGR. International, likewise, has said it won’t go the SCR route with its heavy-duty engines that debut this year. Caterpillar, which is introducing heavy-duty engines in 2009, has not announced a decision as of press time.
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,” Saxman 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? Urea will be consumed at a rate of roughly three percent of the diesel fuel burned, Saxman says. Urea is made using natural gas rather than crude oil, so urea costs 25 percent less than diesel fuel, Saxman says.
Moreover, “Diesel fuel [prices] will go up faster than urea,” he says.
Detroit Diesel’s 2010 DD15 and its sister 13-liter and 16-liter engines will consume urea at a rate of about two percent of the diesel fuel burned, says David Siler, Detroit’s marketing director. While SCR engines potentially have higher up-front 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 Diesel program manager for 2010 heavy-duty engines. He adds: “We are confident that our new DD series engines and 2010 aftertreatment will provide the customer with the lowest possible life-cycle costs.”
That’s because Detroit’s 2010 system is based on proven, mature technologies already in production, which “allows us to 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, preventive maintenance and fuel economy.” Plus, SCR reduces component stress and heat rejection and extends oil-drain intervals, he says.
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.”
Cummins’ heavy-duty emissions strategy for 2010 mainly consists of enhanced EGR. When exhaust is cooled to about 230
