Tim Jackson, senior vice president and Chief Technology Officer, Tenneco (www.tenneco.com; Lake Forrest, IL) is convinced that there will be an increase in the number of diesel engines in the U.S.-yes, even in passenger cars with engines 2.0 liters and above in displacement. That's because, he explains, there is technology that will meet the emissions regulations not just of today, but future regulations by the EPA. (As well as in Europe and China.) Tenneco, which has long been offering diesel aftertreatment technology for diesels, including particulate filters and oxidation catalysts, is now moving into the area of selective catalytic reduction (SCR). Briefly, in a system there is a diesel oxidation catalyst that handles the carbon monoxide and unburned hydrocarbons, then the particulate filter that takes out the particulate matter-on the order of 99.9% of the particulates. But now added to that is the company's SCR technology, which it acquired from Combustion Components Associates (CCA; www.cca-inc.net; Monroe, CT). The system is called "ELIM-NOx" and, as the name implies, it is designed to remove nitrogen oxide (NOx) emissions from diesel exhaust.
"The thing about this is that unlike other technologies that rely on heat"-which would be a system based on engine management, which utilizes heavy amounts of high-pressure piezo injectors and variable-vane superchargers-"or precious metals like platinum and palladium"-which would be a NOx absorber, which stores NOx on the face of a catalyst, which has to be regenerated by running the engine sufficiently rich to generate the heat to burn it off-"is that it requires the injection of a reductant, urea." The trade name for this liquid, which is held by the German Association of the Automobile Industry (VDA) is "AdBlue." Essentially, this liquid is vaporized to create ammonia; the ammonia reacts on the face of the SCR catalyst with NOx and the result is nitrogen and water vapor. Jackson points out that nitrogen is the primary component of the air we breathe. This approach, he says, "Has been around a long time in the stationary power industry to clean up stack emissions from coal-fired boilers." CCA is one of the companies that developed the technology for those stationary applications; Tenneco has obtained that company's mobile business.
The urea looks like water. It doesn't smell like water. It smells like ammonia. The key element of the ELIM-NOx system, Jackson says, is the injector system that transforms the urea into droplets on the order of 50 microns in diameter. He points out that unlike, say, a fuel injector, this injector is operating in the exhaust system, which is at least twice as hot as the conditions within a cylinder head. So there is a fluid recirculation system that cools the purpose-built injector so that it operates effectively. The amount of urea injected is predicated on a variety of engine operating parameters (e.g., RPM, load); the signals are pulled directly off of a CAN bus and used to determine the rate. (Jackson points out that one problem related to injecting too much: "ammonia slip," which results in a rather unpleasant smell emerging from the tailpipe. Think about the sort of smell you may encounter when driving by a farm field in the spring after it has been fertilized.) The system is scalable; it can be used on engines ranging in size from 22 hp to 3,000 hp with the change of a single part. Jackson says that they're also able to quickly setup a vehicle with the system: Tenneco installed it on an existing 2008 Ford Super Duty "and got it working to 85% NOx conversion efficiency in less than two-weeks."
The system requires a reservoir for the urea, generally a tank on the order of 9 to 16 gallons. According to Jackson, in Europe the engine calibrations are such that the amount of urea used is approximately 3 to 4% of fuel consumption. In North America the calibration is on the order of about 2%. So, for a 20-gallon tank of fuel, there is a half gallon of urea used. The urea tank is sized so that a vehicle can travel about 5,000 to 6,000 miles before there needs to be a fill-up; Jackson says that in Europe AdBlue is available at some 800 service stations and that it is also readily available in stores. He describes it as being somewhat like windshield washer fluid.
Given the size of the urea tank, he thinks vehicles with engines of two-liters or less will probably go with a NOx absorber system rather than an SCR. But above two, SCR makes sense.
Jackson explains one of the real advantages of the SCR system as regards clean air and better fuel economy. He says that high heat and high cylinder pressures lead to NOx. But they also lead to better fuel economy. So absent an effective aftertreatment system it is necessary to detune the engine so that it won't run as hot. But with the SCR system, the engine can be calibrated so that it runs hot, then the consequent NOx can be handled by the aftertreatment system. "You get better fuel economy, cleaner air, and, I think, ultimately, a higher residual value," Jackson concludes.