Diesel fuel has been the fuel of choice for California trucks for decades, but recent actions from the state's regulators may increase the viability of using alternative fuels, such as natural gas. Although touted as a clean alternative, is natural gas really a better choice?

Chicago-based Navistar International recently commissioned the Harvard Center for Risk Analysis (HCRA), Boston, to analyze diesel and natural gas fuels used by heavy-duty trucks. Preliminary findings show that diesel and natural gas affect environmental quality, health, safety, truck performance and economics differently, and either choice involves trade-offs.

Engines and the Environment. Air pollution from diesel engines is one of the biggest concerns, especially in areas that struggle to meet air-quality standards. Because it mixes more uniformly in the ignition chamber, natural gas causes more complete combustion and fewer pollution emissions than diesel fuel. Though evidence is inconclusive, using natural gas could lead to fewer particulate and nitrogen oxide emissions, but it may increase "ultra-fine" particulate and greenhouse gas emissions.

Conversely, using diesel engines could reduce greenhouse gas emissions such as carbon dioxide and methane. In addition, technological advances are leading to cleaner, "green" diesels. Fischer-Tropsch diesel fuel, for example, contains no sulfur and claims to reduce pollutant and particulate emissions significantly.

Why all this focus on heavy-duty truck fuels? "Heavy-duty trucks comprise about 10 percent of nitrogen oxide emissions in the United States," says Edmond Toy, a Ph.D. candidate at Harvard University, Cambridge, Mass., working on the HCRA study. Nitrogen oxide helps form groundlevel ozone, smog and fine particulates. Advances in green diesel technology, such as exhaust gas recirculation (EGR), may reduce some nitrogen oxide emissions, but controlling all harmful diesel emissions is difficult. To simultaneously reduce nitrogen oxide and particulate emissions poses engineering challenges.

In spite of trade-offs, many still believe that diesel fuel is a viable alternative to natural gas. Why? Because diesel engines are efficient. They operate at high compression ratios and convert a large percentage of the fuel's available energy into usable work. Diesel engines' higher fuel efficiency generally lowers the carbon dioxide emissions that contribute to the greenhouse effect.

Another central subject for this debate is methane emissions. According to Toy, "methane is approximately 20 times more potent as a greenhouse gas than carbon dioxide." One study suggests that using natural gas instead of diesel in heavy-duty vehicles increases greenhouse gas emissions by 5 percent to 10 percent.

Health and Safety Concerns. Natural gas characteristics potentially make it a greater safety hazard than diesel. Not only is natural gas highly flammable, but its vapors at low temperatures are dense and can form clouds of flammable vapor concentrations. The National Fire Protection Association gives natural gas the highest hazard ranking for flammability, while designating diesel as moderately flammable. Diesel fuel is less flammable because it usually does not form ignitable mixtures unless it is heated.

In addition to its flammable nature, natural gas poses special storage hazards. For example, liquefied natural gas (LNG) storage vessels must be equipped with pressure-release valves to prevent the pressure buildup that can occur when LNG warms and changes to a gaseous state. Special care must be given to the transport, transfer and storage of compressed natural gas (CNG) and LNG to avoid leaks and tank ruptures.

Using diesel fuel, however, is not without health risks. "In some urban areas, diesel-powered trucks significantly contribute to ambient levels of particulates, comprising 10 percent to 30 percent of fine particulate emissions," Toy says. "Particulates reduce visibility and are associated with adverse health effects, such as excess mortality, aggravation of cardiopulmonary disease, exacerbation of bronchitis and asthma, and perhaps lung cancer."

For these health risks, diesel emissions are the subject of continued controversy and scientific research. The Health Effects Institute, Cambridge, Mass., an independent scientific organization, has questioned the scientific validity of California's quantitative estimates of cancer risk from inhalation of diesel exhaust. However, it is uncertain whether particulates from natural gas engines are carcinogenic.

Performance and Cost Considerations. Heavy duty diesel truck engines outperform those powered with natural gas, though natural gas engines may perform well in less demanding applications. Diesel engines are powerful enough to haul heavy loads and climb steep hills, and their high fuel economy allows drivers to travel further between refuelings. Drivers of LNG heavy-duty trucks frequently report that LNG engines are less powerful than diesel engines.

In addition, natural gas vehicles have a smaller driving range because of the limited number of CNG and LNG refueling stations. As a result, natural gas vehicles typically must return to a central facility for refueling. Drivers of natural gas trucks also report poorer fuel economy than diesel truck drivers.

"One gallon of LNG contains about 60 percent of the energy in a gallon of diesel fuel, and CNG contains even less energy per unit volume," Toy says. "The performance limitations of natural gas engines suggest that they may be a viable option only in certain niches, at least in the short run."

A heavy duty natural gas vehicle generally costs more than a diesel vehicle. According to a study of California LNG trucks, a conventional diesel truck may cost $70,000, but a natural gas truck costs more than $100,000. LNG truck prices might decrease if the market for natural gas vehicles increases substantially, but the cost of diesel vehicles is likely to increase with the adoption of green diesel technologies.

Neither diesel nor natural gas has a clear cost advantage when it comes to fuel. While a gallon of LNG generally is less expensive than a gallon of diesel, natural gas contains less energy than does diesel per unit volume.

Although LNG can be produced readily at competitive rates, the costs of supplying LNG often depend on transportation expenses. The incremental costs of installing natural gas stations can be substantial. Large investments in LNG infrastructure throughout the transportation system would be required to make natural gas a viable option for heavy duty trucks.

While natural gas prices rely on uncertain long-term investment, diesel prices rely on a volatile market. Diesel fuel costs fluctuate wildly; the national average price of diesel fuel increased about 30 percent last year. Because both diesel and natural gas prices are affected by unpredictable forces in worldwide energy markets, future fuel costs are unclear.

According to HCRA, policies to discourage diesel use currently are being written in California. Whether these policies will increase natural gas use in the heavy duty trucking industry or just add kindling to a heated debate, remains to be seen.