Studies confirm clean-diesel, ULSD can fix old bus problem

Diesel Fuel News, Oct 27, 2003 by Jack Peckham

California Air Resources Board (CARB) this month unveiled a real-world emissions study confirming that kids riding in some old diesel buses can be exposed to relatively high amounts of particulate matter (PM) emissions.

But either fitting a bus with a diesel particulate filter (DPF) and ultra-low sulfur diesel (ULSD) fuel, or replacing an old bus with compressed natural gas (CNG) can dramatically slash such PM emissions, the study found.

CNG however has higher toxic formaldehyde emissions than an DPF-equipped clean diesel, the study shows. Until recently, CNG school buses with oxidation catalysts weren't available, thus causing higher "toxic" emissions than DPF-equipped clean diesels.

"Bus-to-bus variability was relatively high and therefore with the small number of buses studied [five diesels of 1975-1998 vintages, one DPF-diesel (1998 vintage) and one CNG (2002), and the limited number of routes covered, the findings of this study should not be viewed as inherently typical for all buses under all conditions," CARB found.

In response to the CARB study, California Association of School Transportation Officials spokesman Doug Snyder told Diesel Fuel News that his association doesn't recommend either DPF/clean-diesel or CNG buses.

But far more diesel buses could be cleaned up with DPFs than with the money spent on costlier CNG, Snyder confirms. While the required ULSD can cost about 10 cents/gallon more than ordinary diesel fuel in California, building new CNG refueling infrastructure is far more expensive, he said.

So, for the same dollars spent, "clean-diesel would let you fix more buses, but in some areas there is more grant money available for CNG," Snyder said.

"We're in agreement" with the CARB study conclusions, adds International Truck & Bus spokesman Roy Wiley. "Our Green Diesel school bus technology is as good as anything out there," he said, pointing to new information on International's emissions web site (www.geendieseltechnology.com).

Coincidentally, International just released its own new study on California school-bus pollution. This study shows that an eight-year-old, properly maintained school bus adds almost no PM emission to the bus interior, contradicting another study sponsored by various California "green" groups (see Diesel Fuel News 2/18/02, p5).

The new study by "California EnSight" consulting group employed a fuel-borne iridium tracing scheme, in tests on a 1995 International bus using ordinary diesel fuel (not ULSD) and lacking any exhaust after treatment.

This iridium tracing scheme allows measurement of the portion of diesel exhaust from the bus's own engine, in contrast to the aethelometer measurement used in the earlier "green" group study that only measures black carbon, from all sources.

Remarkably, the EnSight study found that "less than 0.5% of the P[M.sub.2.5] measured inside the bus is from its own exhaust," and PM levels were "small relative to the typical estimated levels of diesel particulate matter based on surrogates, such as black carbon measurements, reported in California ambient air."

What's more, the estimated PM levels in the school bus were 150 times less than the average black carbon levels in metro Los Angeles. Hence, the study found that "exposure to diesel particulate matter while riding inside a school bus is likely to be no greater than the exposure from standing near or driving on any roadway in the Los Angeles area."