Scientific Study Underscores the Clean-Air Advantages of Low-Emitting Diesel Technology

What is the foreseeable future for school buses?

For many years to come, engines that run on fossil fuels, including low-emitting diesel engines, will be the dominant power source for school buses. A federal rule that begins to take effect in 2006 will assure widespread availability of ultra-low sulfur diesel fuel and mandatory production of diesel school buses with near-zero emissions.

What is the latest scientific information comparing diesel vehicles to natural gas vehicles?

A study conducted in 2002 by the Southwest Research Institute of San Antonio, Texas, under funding provided by International Truck and Engine Corporation and ConocoPhillips, showed that low-emitting diesel technology, which is now used in many California school buses, produces lower emissions — including particulates (or “soot”) and nitrogen oxides — than current natural gas school buses now offered for sale.

Exactly what did this new study reveal?

This research found that a low-emitting diesel school bus, typical of those now being sold, had lower emissions of nitrogen oxides (NOx), particulate matter (PM), soluble organic fraction of PM (SOF), total hydrocarbons (THC), non-methane hydrocarbons (NMHC), methane and carbon monoxide (CO) than a bus fueled with natural gas.

How do natural gas school buses now offered for sale compare with school buses now offered for sale with low-emitting diesel technology with regard to the potential toxicity of the emissions?

This is a critical question — and the answer is that diesel wins in six critical toxic air contaminant categories. Low-emitting diesel school buses, as tested in this study, were cleaner than the tested natural gas bus with regard to acetaldehyde, acrolein, benzene, formaldehyde, methyl ethyl ketone and propionaldehyde.

Did the natural gas bus in this scientific study rate lower than the diesel school bus in any emission of toxic concern?

No. The natural gas school bus did not have lower emissions of any toxic air contaminant in this study.

How many buses were tested in the Southwest Research Institute study? Are they representative of the buses actually in use today?

Two buses were used, in three configurations. A diesel school bus, typical of buses currently being sold in California, was configured as a conventional diesel bus using regular diesel fuel, and also as a low-emitting diesel bus using ultra-low-sulfur diesel fuel. The second bus, using natural gas, is typical of buses currently in service. Both buses were in the same size category and had about the same engine power rating. This enabled the science team to characterize the exhaust emissions from school buses now transporting students.

Can you provide specifics on the school buses and the configurations?

The diesel bus was a 1998 American Transportation Corporation rear-engine school bus equipped with a 2001 model year DT530 diesel engine Model C275.

The low-emitting diesel configuration was that of an International Truck and Engine Corporation Green Diesel Technology® school bus, over 100 of which are currently in service in California. The major components of International’s Green Diesel Technology® are the addition of an Engelhard DPX catalyzed diesel particulate filter (CDPF) and a re-calibrated engine control module (ECM). For the low-emitting engine, the model was CG275 and the ECM used the PED5AT00 calibration. For the conventional diesel configuration, the DT530 ECM was re-calibrated to the model year 2002 NOx emission certification level and the CDPF was removed.

The natural gas bus was a 2000 model year Blue Bird school bus with a rear engine. This school bus was powered by a Deere 8.1L natural gas engine, and is in service in an Arizona school district.

Why wasn’t an “oxidation catalyst” used on the natural gas school bus?

This was a test of school buses now in service. Oxidation catalysts are not standard equipment for new natural gas buses, are not required by California regulations that call for the use of natural gas school buses, and are not supplied by either of the two major manufacturers of natural gas school buses, including the natural gas bus tested.

In any other research, have oxidation catalysts been shown to reduce the pollution from a natural gas bus?

In a test conducted by the California Air Resources Board (CARB) in 2002-2003, the addition of an oxidation catalyst to a natural gas transit bus using ultra-clean, premium natural gas did show significant reduction in emissions of some toxic compounds, but not to levels that were below those from the same model bus in a low-emitting diesel configuration. With regard to other toxic compounds, a CARB test of a trap-equipped, advanced technology diesel transit bus, using ultra-low-sulfur diesel fuel, showed it was as clean or cleaner than the catalyst-equipped natural gas vehicle. Measurement of emissions of benzene, formaldehyde and acetaldehyde showed that the diesel bus emissions were one half to one fifth those of the catalyst-equipped natural gas bus.

What fuels were used in each school bus in the Southwest Research Institute research? Are they the same fuels that are ordinarily used in the types of vehicles tested?

The conventional diesel configuration used a D-2 fuel meeting specifications for EPA emission certification testing. The low-emitting diesel configuration required an ultra-low sulfur (less than 15 parts per million) diesel fuel to enable proper operation of the CDPF. This fuel, provided by ConocoPhillips, meets 2006 diesel fuel specifications and is typical of current ultra-low sulfur diesel fuel production. The compressed natural gas mixture was typical of that found in the natural gas market today. The natural gas mixture consisted of methane with small amounts of heavier hydrocarbon gases added to meet the requirements of the California Air Resources Board for certification of natural gas engine emissions.

Is ultra-low-sulfur diesel fuel commercially available?

A number of oil refining and marketing companies (such as BP, ConocoPhillips and Equilon) are providing this fuel commercially. It is now being sold in California, Ohio, Michigan, Indiana, Kentucky, Texas, New York, Washington State and Pennsylvania. In California, it is being sold in more than 100 service stations by BP. More is on the way. The federal government requires ultra-low-sulfur diesel fuel to be widely available nationwide beginning in mid-2006.

How was the toxicity of natural gas emissions vs. diesel measured?

Each bus was tested on a chassis dynamometer using the same test weight, road load, and driving cycle to compare exhaust emissions and emissions chemical characterization among the engine configurations. Many of the emissions that were measured were at very low concentrations, and considerable care was taken to review all data to ensure that values were reporte with confidence and with appropriate footnotes. To eliminate potential contamination of samples taken for the low-emitting diesel and the natural gas configurations, the CVS (constant volume sampling) dilution tunnel and sampling apparatus were cleaned prior to testing each of those configurations.

Are there economic benefits associated with the use of low-emitting diesel technology, compared with natural gas technology?

Yes. A new natural gas school bus costs approximately 30 percent more than a new Green Diesel Technology® school bus. In addition, natural gas school buses get on average one-third less mileage than diesel buses. (This is an important cost factor, particularly considering the volatility of natural gas prices.) Natural gas maintenance costs are also higher than those of new diesel vehicles. Other areas of higher cost associated with natural gas include the cost of building natural gas refueling facilities and the cost of modifying maintenance facilities to accommodate the volatility and flammability of natural gas.

What has International Truck and Engine Corporation done to advance the development of low-emitting diesel engines?

With 70 years in the diesel engine business, International has led the way in clean-air technology. In 1989, International demonstrated the first smokeless diesel engine — five years ahead of regulatory requirements established by the U.S. Environmental Protection Agency (EPA). In 2001, International introduced Green Diesel Technology® school buses, which meet 2007 federal and California standards for particulates and hydrocarbons and reduce those emissions to levels at or below levels of vehicles fueled by natural gas.

Is low-emitting diesel technology safer and more practical than alternative power sources?

Diesel fuel has a number of significant safety advantages over alternative fuels. For example, it has a much higher flash point than gasoline or natural gas, making it less likely to explode when exposed to heat sources.

Could hydrogen fuel cells offer potential environmental benefits in school buses?

Yes, hydrogen fuel cells have great promise. Development for practical use, however, is still relatively far in the future. Fuel-cell technology will require new technology breakthroughs, new infrastructure, substantial government support and a big unknown — marketplace acceptance. At the 2002 Automotive News World Congress, an Energy Department official said it would take “decades” before fuel-cell cars reach their marketplace potential.

Why did International Truck and Engine Corporation sponsor the new research at Southwest Research Institute?

There was a gap in the research comparing real-world emission data among the three school bus configurations. International wanted accurate, independent, peer-reviewed research to assist customers and public policy makers in their consideration of available options in school buses. International was joined in the research by ConocoPhillips, who wanted independent testing of the environmental benefits of ultra-low sulfur diesel fuel in school buses now in service. Southwest Research Institute is one of the leading research facilities qualified to conduct this kind of research.

In short, what technology offers the greatest potential to clean up the environment cost-effectively? Is it diesel or natural gas?

Low-emitting diesel technology is far more cost-effective in reducing emissions of particulate matter. A staff report from the California Air Resources Board found that $30 million invested in Green Diesel Technology® school buses would result in 24 fewer tons of PM emissions and 172 fewer tons of nitrogen oxide emissions than the same amount invested in natural gas buses. The potential of low emissions from diesel school buses is an outstanding option for those making decisions about student transportation.