Diesel particulate filters (DPFs), which remove unburned fuel particles, and diesel oxidation catalysts (DOCs), which remove oxides of nitrogen from exhaust have seen field use in buses and on-road trucks for 10 years or more. Demonstrations in off-road equipment since the late 1990s have taught the industry some valuable lessons.
"I'm working with some fleet managers who are changing out early DPFs," says Wade Wessells, project engineer in Donaldson's emissions group. Pioneers sometimes labored with exhaust filters that plugged early, requiring more maintenance than expected. "It was a fledgling market when these came out, though, and nobody really knew that much. Sometimes those engines didn't generate enough exhaust temperature to regenerate the filters properly, and they had to service them more frequently.
"We have thousands of products in the market now," Wessells adds. "We do the temperature data logging."
And so the industry has learned the best ways to adapt DPFs to the varied duty cycles of construction applications.
The typical DPF handles a huge volume of material, and its greatest practical challenge is that it must clean itself — or regenerate — to avoid restricting the exhaust flow and causing horsepower-robbing backpressure. Passive DPFs — the most-affordable style — rely on a minimum exhaust temperature to routinely burn captured particulate off their ceramic filter surfaces. Active DPFs add a system to the filter that fires up periodically to burn the particulate off the media.
Nevertheless, both types of DPF have to be removed periodically for thorough cleaning in special ovens. Vendors of the filters typically size them for 1,000 hours of operation between services, but a great deal depends on how clean the engine runs.
"You want to do a full service on any engine before installing a DPF," says Jeff Silver, with E Global Solutions, the New York-area Clearaire and Engine Control Systems dealer. "An engine with a bad injector can plug a DPF in a week, and the filter will need to be serviced. At $300 to $600 each for the service, you want your engines running as clean as possible."
Silver says burning something other than ultra-low-sulfur fuel can plug the DPF in a couple of days. He does, however, claim that the filter can be cleaned with servicing, but sometimes it requires multiple cleanings.
Silver supplies Yonkers Contracting, among other New York area contractors, with DPFs. Yonkers' initiation to diesel emissions reduction came in 2002 while the company was restoring the bathtub retainage around the World Trade Center property for the New York Port Authority. Before Yonkers was finished in 2004, they were involved in demonstrations of DPFs on a couple of 966 loaders.
Yonkers recently landed a subcontract to build foundations for towers 3 and 4 of the new Trade Center development, and New York's Local Law 77 will require ultra-low-sulfur diesel and DPFs on all of Yonkers' diesel equipment (probably 15 pieces) headed to Lower Manhattan.
"They (the New York City Department of Environmental Protection, enforcing Local Law 77) want 'best available technology,'" says Lou Marino, vice president of equipment operations for Yonkers. "We submit a list of the equipment we plan to use — make, model, engine serial numbers — to our retrofit supplier, and they come back and tell us what is required. It's typically either a DPF or DOC."
This will be Yonkers' first time back to Ground Zero since 2004, but not their first exposure to Local Law 77. Projects for the Department of Environmental Protection in Westchester and Rockland Counties also apply Local Law 77. Yonkers experience with 15 DPF-equipped machines, and three with DOCs, on those jobs keeps Marino from worrying about exhaust aftertreatment.
He says maintenance doesn't change. Machines perform like they normally do, and the company has had no problems with DPFs or a couple of DOCs they've used. Operators and the equipment staff keep an eye on the exhaust-backpressure warning lights that come with the DPF installation. And they have filters cleaned during seasonal downtime to avoid any problems when the machines are working.
"The longest life we've seen with a DPF is about 2,000 hours before we have them cooked and cleaned out," Marino says.
"We all want clean air and you have to do the right thing, but this is not pennies we're talking about," he says. The passive DPFs that Yonkers has bought so far have cost $17,000 to $25,000 each. "With all of our projects requiring retrofits, including the World Trade Center work this spring, we will have allocated about $200,000 to retrofits."
Costs of diesel emissions retrofits are related to the scarcity of options currently available, and a function of the volume of DPFs and DOCs produced today. Certainly that will change, as CARB completes its in-use-diesel emissions reduction rulings (there is also a ruling in the works that will require on-road retrofits), and other states start applying the ruling in their own jurisdictions.
The diesel-emissions-reduction business is an evolving market made expensive by the fact that the regulators chose to verify technologies rather than test machine emissions. It's a practical move, eliminating the virtually limitless effort of field enforcement. But it makes government bureaucracy responsible to evaluate emissions-reducing technologies. Inevitably the list of approved technologies is going to be short and grow slowly (see the ARB Verified Technologies List at www.arb.ca.gov/diesel/verdev/vt/cvt.htm).
Just as engine manufacturers have turned to exhaust aftertreatment to attain the greatest levels of emissions reduction, the regulators have focused on aftertreatment. It appears that California's in-use diesel rule is going to require Tier 3 engines or better, or verified retrofits.
Efforts to verify technologies continue, and as the regulation-born emissions-technology market matures, lower-cost options are knocking at verification's door. Diesel catalysts, like that from Emissions Technology or American Clean Energy Systems are working to get verified.
Benefits of a system that burns diesel fuel more thoroughly include reduced fuel consumption. Emissions benefits, while significant, are incidental enough that several contractors are buying the expensive catalysts today with little other motivation than slashing fuel bills.
Fred Weber Inc. had been testing the DC-100 catalyst from Emissions Technology (www.emissionstech.com) on a few high-horsepower quarry machines for a couple of years. After the systems consistently reduced fuel consumption 18 percent and 13,000-hour engine teardowns confirmed that it cleaned up components, the company installed the systems on about 60 machines — most of the Cat 3408 and larger engines in the fleet.
"They've made a big difference in underground mining facilities," says Ed Moss, maintenance manager with the St. Louis, Mo.-based integrated paving contractor. "The guys in our underground aggregate mines know when they're not working. They can see when the engines start smoking and smell it. They're motivated to keep the systems working, because it means they have to spend less on ventilation."
Emissions Technology claims as much as 43 percent reduction in particulate matter with more complete combustion of the diesel fuel and as much as 14-percent reduction in oxides of nitrogen. The less-than-$2,000 systems installed on each engine atomize the catalyst and administer it through the intake air.
At about 50 cents per hour, though, the catalyst is no small investment. The 18-percent fuel savings Weber has measured more than generously offsets that cost, and its value will continue to increase as petroleum costs escalate.
Diesel/electric hybrid drive trains represent another approach to emissions reduction that, for the moment, remains outside the range of verified technologies.