Among the many unknowns surrounding the upcoming changes in diesel emission regulations, there is one certainty: Buying and operating school bus engines built after Jan. 1, 2007, will cost more. How much more? Well, nobody’s sure just yet.
Every aspect of the transition to ‘07-compliant on-highway diesels comes with added expense. The new hardware — mainly exhaust-stream particulate traps and bigger cooling components — could bump bus prices by $1,900 (EPA estimate) or more (industry estimate). Whatever the figure, it will be fattened with princely sums charged for the reformulated fuels and oils needed to keep those engines running properly. If that weren’t enough, fuel economy is expected to take a slight hit — perhaps 1 to 2 percent — and oil drain intervals might be shortened somewhat, at least in the near term.
Pros outweigh cons
The upside is, of course, reduced exhaust emissions. The regulation — formally known as “the heavy-duty engine and vehicle standards and highway diesel fuel sulfur control requirement” — dramatically cuts two harmful components of spent fuel: particulate matter (PM) and oxides of nitrogen (NOx). Both pollutants will be reduced by about 90 percent. The new standards allow PM levels of just 0.01 g/bhp-hr and NOx levels of 0.20 g/bhp-hr. The NOx limit, however, is being phased in between 2007 and 2009, offering manufacturers a temporary reprieve before the next — and probably last — round of air-quality regs take effect in 2010.
Reaching the ‘07 mandated targets has been a sizable challenge for engineers across the heavy-automotive spectrum. Three or four years ago, trade publications were filled with dire predictions about the emissions-reducing methods being considered at the time — most of which seemed futuristic, overly complex and grandly unaffordable. Those worries have largely subsided now, though, because cooled exhaust gas recirculation, the primary NOx-cutting tool, has proven itself among North American fleet operators. Also, continuous refinements in combustion technology have enabled engine companies to meet the EPA’s next set of demands with a minimal amount of unfamiliar hardware, the sum of which is expected to add about 100 pounds to the weight of a medium-duty vehicle.
Trap will be key element
“A particulate trap accounts for about 90 percent of the new equipment we’ll be using,” says Steve Flammersfeld, a sales manager for Freightliner LLC. “The package also includes an intake throttle, hydrocarbon doser and some added software to control everything. There will be increased EGR, but it’s the same system we’ve had. The base engine [an MBE 900] is unchanged.”
Except for Caterpillar’s choice of ACERT instead of EGR, all manufacturers are taking roughly the same approach in this latest round of emissions compliance. They’re using ignition timing and EGR, or ACERT, to lower cylinder temperatures and suppress NOx formation, then routing the exhaust gasses through a filter, or “trap,” to strain off particulate matter.
When the accumulated PM reaches a certain volume, it’s converted to ash through a chemical process called “regeneration.” This is accomplished with either internal operating temperature or, lacking that, one of several mechanical maneuvers that will elevate heat to an appropriate level.
In developing the latest batch of diesel engine technology, designers considered two types of regenerating (self cleaning) particulate filters: passive and active. All chose the latter for heavy vehicles because it’s apparently more dependable across a wide range of environmental and operating conditions. Passive systems rely solely on an engine’s heat to perform the soot conversion, or oxidation, process; but active systems adjust internal temperature as needed to accomplish the task. Both types of devices, though, are sulfur intolerant.