For 2010, the U.S. Environmental Protection Agency (EPA) will require a reduction in nitrogen oxide (NOx) from diesel engine exhaust. All but one engine supplier for medium- and heavy-duty trucks have announced they will be using a technique known as Selective Catalytic Reduction (SCR). While this is a proven technology, it will be the first time it is implemented on heavy-duty trucks in North America.
"The SCR system is in addition to the exhaust aftertreatment that is generally already in place - in our case, that would be the diesel particulate filter (DPF)," says David McKenna, director of powertrain marketing and sales, Mack Trucks. "The EGR valve and the EGR cooler remain the same, and the DPF remains essentially the same with some very minor updates and improvements."
Mack Trucks asserts SCR is a proven technology that delivers improved fuel economy and near-zero emissions. "It's already being successfully used around the world by hundreds of thousands of trucks every day, including those in extreme operating conditions," says Dennis Slagle, president and CEO. "And it clearly works here in North America, as our test trucks have been demonstrating."
David Siler, director of marketing, Detroit Diesel, adds, "SCR has absolutely proven to be reliable and cost efficient around the world. Nearly 100% of all vehicles meeting the latest Euro 5 standard are doing so with SCR. It has been around in the stationary power generation business since the 1980s, and has evolved into one of the most cost-effective emissions-reducing technologies that just so happens to deliver economic improvements."
Cummins also has significant experience with SCR. "In 2006, Cummins launched its midrange engines certified to the Euro 4 standard using SCR for commercial vehicle applications in Europe," notes Christy Nycz. "Cummins has built and shipped over 45,000 SCR engines to date. Cummins Emission Solutions has built and shipped over 200,000 SCR systems."
How SCR works
"SCR is an exhaust aftertreatment system that utilizes basic chemistry to reduce NOx to near-zero levels, while allowing the vehicle's engine to operate at optimal conditions to reduce stress and fuel consumption," Siler explains. "The chemical NOx reduction is the result of introducing very small doses of a non-toxic diesel exhaust fluid (DEF) into the vehicle's exhaust downstream, while not adversely affecting the engine's performance like has been generally accepted in emissions changes prior to 2010."
An SCR system consists of a few basic components: an SCR catalyst aftertreatment chamber, DEF tank, pump and lines, and control and monitoring system. DEF is injected into a catalytic converter. The DEF fluid is made up of water and urea. The urea turns to ammonia when heated. This reacts with the exhaust stream when passed over the catalyst, forming nitrogen and water vapor, both of which are harmless.
SCR may actually simplify the engine design. "SCR won't increase the need for complex and heavy air management and cooling systems as it appears extreme EGR systems will, nor will it degrade engine power density like extreme EGR systems have shown a tendency to do," says Siler. "However, SCR components will add between 100 and 400 lbs. to the chassis, depending upon application and the size of the DEF tank. Since most SCR systems have had years of development time to integrate with the leading chassis manufacturers, the impact on vocational packaging requirements are being minimized."
Exactly how much DEF will SCR-equipped trucks consume? According to estimates by McKenna regarding Mack Trucks, the system will dose at a rate of 3% of the fuel rate. "If you consume 100 gal. of fuel each day, you would consume approximately 3 gal. of DEF," he says.
Benefits of SCR
"By selectively eliminating the NOx downstream of the combustion chamber, the NOx produced by the engine can be much higher," says Ed Saxman, product manager - drivetrain, Volvo Trucks North America. "This allows the truck to be retuned for maximum fuel efficiency."