Study compares emissions from low-emission diesel and natural gas school buses

20 November 2002 (revised 6 December 2002)

International Truck and Engine Corporation (International) has released a report from a study comparing exhaust emissions from school buses in compressed natural gas (CNG), low emitting diesel, and conventional diesel engine configurations. It was concluded that emissions from CNG 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.

The study—conducted by the Southwest Research Institute (SwRI)—was sponsored by International, which has begun selling a low-emitting, particulate filter equipped diesel engine certified to EPA 2007 PM and HC emission standards, along with ConocoPhillips, a producer of the ultra low sulfur fuel that enables the use of the new diesel technology. The report was presented at the recent SAE International Truck & Bus Meeting in Detroit, MI.

The same diesel bus, an American Transportation Corporation RE bus powered by a 2001 model year International DT 530 engine, was tested in both the conventional and the low emitting diesel (Green Diesel Technology®) configurations to minimize the effects of vehicle-to-vehicle variation. The low emitting diesel configuration used a catalyzed diesel particulate filter (CDPF) and a low-NOx engine control module (ECM), and was fueled with ultra-low-sulfur fuel provided by ConocoPhillips. The natural gas school bus was a 2000 model year Blue Bird All-American powered by an 8.1-Liter John Deere natural gas engine. The natural gas bus was in the same size category as the diesel bus and approximately the same engine power rating. Both vehicles were well within their warranted service lives. Emissions were tested over the chassis dynamometer City-Suburban Heavy Vehicle Cycle. A test consisted of three consecutive runs of the cycle, consuming about 85 minutes and 21 miles. The average of three of these tests was considered to be the emission value for each bus.

The study found the following:

• Compared to low-emitting diesel, the CNG exhaust had higher levels of six toxic air contaminants (TAC) listed by the California Air Resources Board (ARB)—acetaldehyde, acrolein, benzene, formaldehyde, methyl ethyl ketone, and propionaldehyde—and did not have lower emissions of any TAC.
• Compared to conventional diesel, natural gas had higher emissions of several TACs (acetaldehyde, acrolein, 1,3-butadiene, formaldehyde, propionaldehyde), although it also had lower emissions of several TACs.
• Several hydrocarbons not identified as TACs by ARB also were detected. Natural gas had the highest emissions of total aldehydes and ketones and other hydrocarbons.
• For all the PAH compounds, the highest emission level was associated with the conventional diesel configuration. The low-emitting diesel and natural gas had similar low emissions of PAHs.
• Low-emitting diesel had the lowest emissions of NOx, NO, PM, SOF, THC, NMHC, methane, and CO, but had the highest emissions of NO₂ and CO₂.
• The natural gas vehicle had the highest emissions of NOx, NO, THC, NMHC, methane and CO, but the lowest emissions of CO₂ and lower emissions of NO₂ than low-emitting diesel.
• Although the NOx emissions from the low-emitting diesel configuration were significantly lower than the other two configurations, it produced more NO₂ because of the CDPF.
• The low-emitting diesel configuration reduced the average total PM emissions to about one-fifth the level obtained from the natural gas configuration and about one-eighteenth the level for conventional diesel.
• THC and NMHC levels were essentially zero for the low-emitting diesel configuration.
• Although the NMHC levels for both conventional diesel and natural gas were similar, THC emissions for natural gas were 24 times the level observed for conventional diesel. The bulk of the THC in natural gas exhaust was methane.

Remarkably similar conclusions have been reached in the ongoing comparison of CNG and diesel urban buses conducted by the California ARB. The first stage of the research, released in April, found that many emissions, including several air toxics of carcinogenic potential, were higher in CNG than in low emission diesel.

These findings challenge the notion that natural gas engines are inherently cleaner than diesel. The overall emission performance depends not on the fuel alone, but on the entire fuel-engine-aftertreatment system. With recent advances in technology, the diesel engines are often cleaner than natural gas engines, once the benchmark for low emission powerplant.

For a complete report, available from International as a pdf file, e-mail cathy.hope@nav-international.com.

Source: Navistar International Corporation