In a head-to-head comparison of diesel fuel vs. natural gas in heavy trucks, the Harvard Center for Risk Analysis found, as you might expect, advantages and disadvantages for each fuel, depending on the criteria. The center issued a report in January that compared the fuels based on emissions (particulates, nitrogen oxides and greenhouse gases), safety, performance and cost. The study was commissioned by Navistar International, a proponent of diesel fuel, but officials at the center say the report was unbiased. Although the report focuses on heavy-duty trucks, many of the findings apply equally well to school buses. Here’s what the three Harvard researchers (Edmund Toy, John D. Graham and James K. Hammitt) reported. . .

Pollution — Particulate: Because natural gas burns cleaner than diesel fuel, it produces fewer emissions of fine particulates, which have been linked to adverse health effects such as lung diseases like bronchitis and asthma. However, the use of regenerative particulate traps with low-sulfur diesel fuel, known as "green diesel," could significantly reduce the emission of fine particulates. Edge: Natural gas

Pollution — Nitrogen oxides: Natural gas engines have an advantage over diesel engines when it comes to nitrogen oxide (NOx) emissions. Although advances in "green diesel" technology could reduce NOx emissions, the tradeoff is a likely increase in particulate emissions. Edge: Natural gas

Pollution — Greenhouse gases: Because diesel engines are generally more efficient than natural gas engines, they burn less fuel and emit less carbon dioxide, which contributes to the greenhouse effect (global warming). Moreover, natural gas is primarily methane (another greenhouse gas), which can escape into the atmosphere during refueling. Edge: Diesel

Safety: Clearly, the use of natural gas poses greater safety concerns than diesel. Natural gas is highly flammable and must be stored under extreme pressure and temperatures, both with compressed natural gas (CNG) and liquefied natural gas (LNG). Diesel, although less flammable, has its own safety concerns, including the potential that it could leak into the environment and expose workers to toxic chemicals. Edge: Diesel

Performance: Diesel engines have performance benefits such as power and high fuel economy. In contrast, natural gas vehicles have poor fuel economy and a more limited driving range. In addition, the limited infrastructure for natural gas vehicles usually requires that fleet vehicles return to a central facility for refueling. Edge: Diesel

Cost: The cost of natural gas vehicles, including school buses, is typically higher than their diesel-powered counterparts. One study found that natural gas transit buses cost from $45,000 to $75,000 more than diesel buses. However, prices of natural gas buses would likely decrease if production was increased. In addition, the cost of diesel vehicles is likely to increase with the adoption of "green diesel" technologies. Fuel costs also have to be considered. The cost of diesel fuel fluctuates widely, making it hard to compare against natural gas. A gallon of LNG is generally less expensive than a gallon of diesel, but a valid comparison would require an analysis of energy equivalents. (Natural gas contains less energy per unit volume than diesel.) In addition, a transition to natural gas would require an investment in fueling stations, which can cost as much as $500,000. Maintenance facilities would also need to be upgraded to improve ventilation and leak detection systems. Edge: Diesel

Conclusions
Policy-makers need to consider the trade-offs that the use of each fuel necessitates. Neither fuel has a decided edge. A broader assessment might include fuel options such as liquid petroleum gas, alcohol fuels (methanol and ethanol) and hydrogen.