"The analysis refutes contentions by some that NGVs are unnecessary because a new generation of 'clean diesel' engines emit fewer pollutants," Gable said. "Study after study documents that diesels in actual use still emit large amounts of pollutants. While gasoline engines for passenger cars have achieved impressive emission reductions during the past 20 years, diesel engines in new heavy-duty vehicles continue to produce high levels of pollution. In fact, research shows that new diesel transit buses still emit 80 times the nitrogen oxides and 60 times the particulate matter (PM) emitted by today's gasoline-powered vehicles even though they easily pass applicable standards."

Newly available data also show that PM emissions from diesel engines in heavy-duty vehicles in-use consistently exceed certification standards. Data from a series of tests by the NREL and the U.S. Department of Energy show that diesel trucks with engines certified to the current PM standard of 0.1 gram per brake horsepower-hour (g/BHP-hr) had average in-use PM emissions of 0.23 g/BHP-hr -- more than twice the standard. Many trucks were emitting 0.4-0.6 g/BHP-hr by the time they had accumulated 200,000 miles. The NREL data show that actual average natural gas engine emissions of vehicles in-use were well below the standard at 0.016 g/BHP-hr for both buses and trucks.

The debate over in-use emissions recently culminated in a lawsuit by the EPA, CARB and the U.S. Department of Justice against seven major U.S. diesel engine manufacturers. The suit alleged that many of the alternative emission-control devices (AECDs) on the engines were designed to maintain low emissions during official emission tests while later advancing fuel injection timing under on-road driving. The suit also charged that these so-called "defeat devices" helped improve fuel economy and reduce soot contamination of oil during driving, but more than doubled nitrogen oxide emissions. In the largest legal settlement in the history of the Clean Air Act, manufacturers denied their AECDs were defeat devices, but agreed to phase them out and eliminate them from future engines.

"The analysis suggests that after more than two decades of regulation, emission standards for heavy-duty diesel truck and bus engines considerably lag minimum standards that have long been mandated for passenger vehicles," Gable said. "Regulations lag even more for diesel engines used in locomotives, ships, construction equipment and other non-road equipment."

As the analysis indicates, natural gas engines can also be designed to achieve very low nitrogen oxide (NOx) emissions, and do not appear to incorporate the kinds of AECDs that caused increases in NOx emissions in diesel engines.

More than 60,000 natural gas transit buses, pickups and vans are already in operation in the United States. Heavy-duty natural gas engine manufacturers have or are pursuing certification to CARB or EPA ultra-low emission vehicle (ULEV) standards. These engines will easily meet the 2004 heavy-duty engine standards when they become effective. In addition, some heavy-duty natural gas engines have been certified to even lower NOx levels under California's emission credit program. The most recent models of heavy-duty natural gas pickups and vans have been certified to California's super-ULEV emission standard, with NOx emissions less than 0.3 grams per mile and non-methane organic gas (NMOG) less than 0.06 grams per mile. This is less than most light-duty gasoline vehicles.

One of the criticisms of natural gas vehicles is cost, which the analysis indicates is usually higher than the price of a comparable diesel vehicle. However, the analysis also shows that costs have declined in recent years, and that the cost differential is likely to be reduced further in the next few years as economies of mass production reduce natural gas engine and cylinder prices. Compliance with the 2004 emission standards is also expected to increase diesel engine and fuel costs. 

The GRI report summarizes a wide range of data from the review of studies on the effects of diesel pollutants on air quality and public health. Among the findings:

-- Diesel engines are among the main sources in urban areas of particulate matter less than 2.5 micrometers in aerodynamic diameter (PM2.5). The EPA has found that PM2.5 significantly increases the risk of a number of diseases and health conditions such as asthma and chronic bronchitis. In 1997, the EPA established new National Ambient Air Quality Standards for PM2.5. The stricter standards were suspended by a federal court, and were widely criticized by the diesel industry as too strict. However, had the standards been implemented, they still would have allowed PM2.5 pollution in urban areas to reach levels that the EPA calculates would increase the death rate by more than 10 percent during pollution episodes.

-- Preliminary results of a study by the South Coast Air Quality Management District of California indicate that 72 percent of the total cancer risk from toxic air contaminants in the district is attributable to diesel PM. Another 10 percent is due to 1,3 butadiene, 6 percent from benzene, and 12 percent from all other toxic air contaminants combined. Diesel and gasoline engines are primary emitters of 1,3 butadiene and benzene emissions in urban areas, whereas natural gas vehicles -- because of their cleaner burning fuel -- emit very little of either carcinogen.

-- CARB has classified diesel PM as a toxic air contaminant. Based on CARB risk factors and exposure data, EF&EE researchers calculate that the risk of lung cancer from the estimated average exposure to diesel PM in California is about 540 cases per million people. However, EPA data show that this cancer threat is only one of a number of risks from exposure to diesel PM. Other risks from PM2.5 exposure include pneumonia, asthma and other respiratory diseases, and greater risk of death from cardiopulmonary causes. After taking into account these other health risks of PM2.5, the EF&EE study calculates that the lifetime risk of premature death from exposure to diesel PM concentrations in California is about 4,250 cases per million, or one person in 235.

The GRI report is available at no charge and can be ordered directly from the GRI Document Fulfillment Center, 1510 Hubbard Drive, Batavia, IL 60510, by fax at 630-406-5995.

GRI manages a research, development and commercialization program that benefits the entire natural gas industry and its customers, as well as targeted RD&C efforts in partnership with individual organizations and consortia. GRI technologies improve the competitiveness of natural gas and provide cost savings and other benefits for customers.

EF&EE is an engineering and consulting firm, headquartered in Sacramento, with activities worldwide. EF&EE specializes in measurement, quantification, and control of mobile emissions; conventional and alternative fuels; and measurement planning and management of urban air quality.

SOURCE: Gas Research Institute (GRI)

CO: Gas Research Institute (GRI); Engine, Fuel, and Emissions Engineering