Diesel or Natural Gas?

Published March 1, 2000

Which fuel is the right choice for heavy trucks and buses?

It’s a question facing policymakers in California, at the U.S. Environmental Protection Agency, and at government agencies around the world, as well as executives at automakers and corporations that operate fleets of buses or trucks.

A new study comparing the two fuels, conducted by the Harvard Center for Risk Analysis (HCRA) at Harvard School of Public Health, finds advantages and disadvantages for each. Environmentally, natural gas is better at reducing particulate and NOx pollution. Diesel is better for reducing greenhouse gases.

Diesel is the fuel of choice now, but concerns about particulate pollution in diesel exhaust have prompted a move toward alternatives. The HCRA analysis finds that natural gas reduces emissions of fine particulates, those smaller than 2.5 microns. But natural gas may generate more ultrafine particles, less than .1 micron, than does diesel. Several studies indicate that ultrafine particles may have an even more dramatic impact on health than those in the fine category.

The HCRA study finds that because natural gas is primarily methane, a relatively simple molecule, it combusts more completely than do many other fuels, producing fewer emissions of several types, particularly NOx, an important contributor to ground-level ozone and the formation of fine particulates.

The advantages of diesel, by contrast, come from its efficiency. Diesel engines convert a large fraction of the available energy into useable work. As a result, diesel engines consume less fuel overall than they would if converted to natural gas. The HCRA study suggests that converting heavy trucks and buses from diesel to natural gas would increase emissions of C02, a significant greenhouse gas.

In addition, the study finds that more widespread use of natural gas would likely increase the escape of methane into the atmosphere. Methane is approximately 20 times more potent as a greenhouse gas than CO2.

The study finds that European regulators seem to be favoring diesel fuel as part of their effort to comply with the Kyoto agreements to stabilize CO2 and other greenhouse gas emissions. They are using tax incentives and emission standards to encourage the use of new cleaner-burning diesel fuels. European vehicle manufacturers appear to be increasing their application of “green” diesel technology that captures significant amounts of particulates.

The study finds that diesel has additional advantages, unrelated to the environment, over natural gas. Natural gas, which is a more flammable and explosive fuel to handle and store, presents a greater safety risk than does diesel. Diesel has a short-term cost advantage, but natural gas might end up with roughly the same costs if engines and refueling infrastructure become common.

For more information

find the full HCRA report on the Internet at http://www.hsph.harvard.edu/press/releases.