No silver bullet for emissions: fleet, equipment operators use variety of approaches to reduce diesel emissions

Diesel Progress North American Edition, Feb, 2004 by Edward P. Richards

Joe Bjerke breathed a little easier when the dispatch center passed along the phone message from a local motorist. "I was following Bus 65 this morning, like I do every day," the caller reported. "But today I noticed that the bus wasn't smoking anymore. I wanted to tell you that whatever you're doing, I appreciate it."

Just the day before, Bus 65--a 1989 Thomas with a 8.3 L Cummins 240 hp engine--had become the first bus in the Clovis, Calif., Unified School District's fleet to be retrofitted with a Lubrizol Engine Control Systems (ECS) Purinmuffler diesel oxidation catalyst and to begin operating on the PuriNOx technology, a low-emission diesel fuel system, both from The Lubrizol Corp., Cleveland.

"We're used to getting complaints about our buses smoking" said Bjerke, director of transportation for the sprawling district in Fresno County in California's Central Valley. "To have someone notice the difference, especially so quickly, sends a very positive message to our community."

That's particularly significant in the Fresno area, which the American Lung Association reports now ranks behind only Greater Los Angles as the metropolitan area with the worst ozone air pollution in the nation.

More than a decade ago, school district officials began an effort to apply technology to reduce pollution from school buses. Part of this effort involved an evaluation of alternative fuel options, which eventually led to the district's commitment to buy CNG buses.

Yet Bjerke, whose buses traveled a total of 1.1 million miles during the last school year, recognized that there is no "silver bullet" when it comes to reducing emissions. Three fourths of his fleet still runs on diesel and the economic realities of state and local funding for education mean that most of those buses will be in service for some time before replacement. So the challenge was to find a way to reduce emissions from the older buses still on the road.

"People would propose different aftertreatment solutions, but they all were designed for 1994 buses and newer," Bjerke said. "We were most concerned about our older buses, the ones that were really spewing out soot and emissions and drawing the most complaints from the community."

In the spring of 2003, Bjerke learned about the potential benefits of ECS aftertreatment hardware, along with the increased benefit of combining these aftertreatment devices with PuriNOx fuel technology. When used in conjunction with the ECS Purimuffier diesel oxidation catalyst, the PuriNOx system can reduce emission of nitrogen oxides by up to 30% and harmful diesel particulate matter up to 65%.

A $39,700 grant from Valley Clean Air Now (Valley CAN), a nonprofit group created to help support and promote real-world solutions to improve the region's air quality, provided the initial opportunity for the district to see for itself on six buses at virtually no cost. More recently, a $286,700 award from the U.S. EPA has 'allowed the balance of the diesel buses to be fitted with aftertreatment hardware and to be converted to PuriNOx.

Altogether, 51 buses from model years 1976 through 1997 will be retrofitted. Most are being equipped with either Lubrizol ECS Purifier or Purimuffler oxidation catalysts, a conversion process that usually takes less than an hour. Five newer buses received Purifilter diesel particulate filters and now run on ultra low-sulfur diesel furl (ULSD).

Bjerke conceded that he was initially hesitant to believe that the benefits would be all that had been promised. So the district conducted its own before-and-after opacity tests on tailpipe smoke. "Our 1990 buses with Detroit Diesel 6-71 engines were some of our worst smokers, coming in at an average of about 25% smoke," he said. After the switch to PuriNOx Technology, one bus showed a 79% reduction in smoke--from 25.1% to 5.2%. Bus 65 posted similar results after being fitted with a Purimuffler and switching to PuriNOx Technology, going from 20.1% to 5.32% for a 74% reduction.

There was even room for improvement with 1990 Detroit Diesel 6V-92 engines, which were already producing much lower smoke levels thanks to their electronic emissions controls. One such bus achieved a 44% smoke reduction--from 8.1% to 4.6%--while the conversion of another bus has appeared to eliminate visible smoke completely.

"We have seen some very impressive improvement in tailpipe smoke just from switching from CARB diesel to PuriNOx," Bjerke said. "We have no reason to doubt that there has been a reduction in nitrogen oxides (N[O.sub.x]) and particulate matter (PM) emissions as well, although we don't have the testing equipment that's needed to confirm our belief."

While there may be no silver bullet, diesel operators in every industry face similar pressures--from regulators, employees and communities--to reduce emissions.

Finding the right combination of technologies for any situation is best accomplished through an upfront fleet survey. Variables to consider include the composition of the fleet (diesel vs. alternative fuels, etc.), vehicle age, operating conditions, duty cycle and fueling infrastructure.