NATURAL GAS SCHOOL BUS EMISSIONS EXCEED LOW-EMITTING DIESEL BUSES IN RESEARCH REPORTED TO SOCIETY OF AUTOMOTIVE ENGINEERS CONFERENCE
DETROIT, Mich. (November 18, 2002) – Exhaust emissions from natural gas
school buses contain higher levels of air pollutants and toxic air
contaminants than those in school buses powered by advanced-technology,
low-emitting diesel engines.
That is the chief finding by an independent research laboratory under
contract to International Truck and Engine Corporation, presented to the
Society of Automotive Engineers conference. The research compares emissions
from a popular model natural gas bus with emissions from diesel school
“We now have a reliable basis for comparing the current relative toxicity of
natural gas and diesel engine exhaust,” said Dr. Charles A. Lapin, a
toxicologist and co-author of a forthcoming SAE paper on the research. “The
study shows that low-emitting diesel technology clearly has clean-air
advantages over natural gas when it comes to school buses.”
International Truck and Engine Corporation, which has begun selling a
low-emitting diesel engine certified to U.S. Environmental Protection Agency
(EPA) and California Air Resources Board (ARB) 2007 particulate and
hydrocarbon emission standards, sponsored the research along with
ConocoPhillips, a producer of the ultra-low-sulfur fuel that enables the use
of the new diesel technology.
The study raises questions about the basis for diesel exhaust regulation in
California, the nation’s leading state air pollution regulator, said Lapin.
Of the 41 toxic air contaminants (TACs) listed as present in diesel exhaust
by the California ARB, tests did not find 21 of them in the exhaust of any
of three tested power system configurations – conventional diesel,
low-emitting diesel or natural gas.
“Special sampling provisions were used specifically to detect low levels of
these contaminants,” Lapin said. “The fact that the contaminants were
missing casts doubt on previous statements about diesel toxicity.”
The natural gas bus exhaust had higher levels of six of California’s listed
TACs than the exhaust from the low-emitting diesel bus.
In the three tested bus configurations, the natural gas bus had the highest
emissions of nitrogen oxides (NOx), nitrogen oxide (NO), total hydrocarbons,
non-methane hydrocarbons, methane and carbon monoxide (CO), according to
The low-emitting diesel bus was found to be higher than both natural gas and
conventional diesel in two other emissions – nitrogen dioxide and carbon
dioxide – but the low-emitting diesel had the lowest emissions of the four
engine exhaust “criteria pollutants” regulated by EPA and the ARB: NOx, CO,
particulate matter, and hydrocarbons.
The natural gas bus had lower emissions of carbon dioxide than the two
diesel bus configurations, and lower emissions of nitrogen dioxide than the
low-emitting diesel bus. “The findings provide a serious challenge to
assertions that natural gas buses are inherently cleaner than diesel,” said
Dr. William Bunn, chief medical officer and vice president of International.
“Low-emitting diesel technology has the lowest emissions of most criteria
pollutants and toxic air contaminants – both in California and nationally,
under EPA’s listing. Diesel is clearly part of the clean-air future in
transportation, and we are glad to have this research available to decision
makers and customers who rely on diesel power for performance as well as
environmental compliance,” Dr. Bunn said.
Ahead of other manufacturers in the heavy-duty, low-emitting diesel market,
International has supplied more than 100 Green Diesel Technology® school
buses to California schools in the last year. Using ultra-low-sulfur fuel,
International’s low-emitting diesel bus engine, with a low-NOx engine
calibration and a catalyzed particulate filter, was certified by the U. S.
Environmental Protection Agency as well as the California ARB as reducing
particulates and hydrocarbons to the 2007 levels. In California, this engine
and reduced sulfur fuel enabled Green Diesel Technology® school buses to
qualify for state funds made available for the purchase of “lower emission”
The research is the first to provide comparable detail in testing the
emissions profile of low-emitting diesel school bus technology alongside
those of conventional diesel engines and natural gas engines used in school
The same diesel bus, an American Transportation Corporation rear-engine
school bus powered by a 2001 model year International® DT 530 engine, was
tested in both the conventional and the Green Diesel Technology®
configurations. The use of the same bus minimized the effects of
vehicle-to-vehicle variation. The low-emitting Green Diesel Technology® bus
used a catalyzed particulate filter and a low-NOx engine calibration, and
was fueled with less than 15 parts per million sulfur content diesel fuel
provided by ConocoPhillips. An 8.1-Liter John Deere natural gas engine
powered the second bus, a 2000 model year Blue Bird All-American, typical of
natural gas school buses sold prior to 2002 and now in service. More
recent-model natural gas buses may be purchased with an oxidation catalyst;
testing of natural gas transit buses with oxidation catalysts is being
conducted by the California ARB. Both the diesel and the natural gas buses
were in the same size category with approximately the same engine power
rating, and were well within their warranted service lives.
The research team ran three consecutive runs of a chassis dynamometer
driving cycle representing school bus operation. Each test run covered about
21 miles and took about 85 minutes. An “emission value” was calculated using
the average of the three test runs.