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Public Transit Buses: Diesel or CNG?

March 12, 2012

Recently, Pittsburgh’s mayor, Luke Ravenstahl, announced that his city would buy four new garbage trucks fueled by compressed natural gas (CNG) rather than diesel because, among other reasons, it would improve local air quality. Like Pittsburgh, many municipalities are dealing with an aging fleet of vehicles and weighing the environmental and economic costs associated with updating their fleet.

CATF commissioned a study on the environmental impacts of both new diesel and new CNG transit buses that concluded that while both have much smaller negative environmental impacts than the older buses currently in use, a new CNG bus is not necessarily better for air quality or climate impact than a new diesel bus. Specifically, the study found that while new CNG buses may have marginally lower particulate matter and volatile organic compound emissions, they may have higher greenhouse gas and nitrogen oxide emissions. Additionally, CNG engines are simply less efficient than diesel engines at traveling the same distance, which must be taken into account.

Importantly, most transit agencies have limited funding available for purchase of new buses. In a capital-constrained environment, the higher purchase price of CNG buses may limit the number of new CNG buses that can be purchased compared to new diesel buses, thus reducing the number of older diesel buses that can be retired, despite the potential for life-cycle cost savings as discussed above. For every $10 million of capital funding, a transit agency could purchase approximately 26 new diesel buses or 21 new CNG buses (and associated fueling infrastructure), and retire an equivalent number of old buses. Given that a greater number of older, high emitting buses could be retired, fleet-wide emission reductions of NOx, PM, and HC per dollar of capital funding could be 47%, 23%, and 11% higher, respectively, if new diesel buses are purchased than if new CNG buses are purchased. For climate impacts, assuming GWP20 factors to assess the short-term climate impact of CH4 and BC, annual fleet-wide reductions in GHG emissions would be 62% less from purchasing 21 new CNG buses than from purchasing 26 new diesel buses with $10 million in capital funding. However, assuming GWP100 factors to assess the long-term climate impact of CH4 and BC, annual fleet-wide reductions in GHG emissions (MT CO2-e) would be 18% greater from purchasing 21 new CNG buses than from purchasing 26 new diesel buses.

These climate results are dependent upon two very important assumptions. The first is the leakage rate of methane from natural gas production and transportation through pipelines. The method of natural gas production currently in use also produces non-trivial methane leakage and methane is twenty-five times more efficient at climate forcing than carbon dioxide. So, while the municipality using a CNG bus may not note fugitive methane leaking from the tailpipe, the methane leaked as a result of the production and transportation of the CNG that is running the bus will nevertheless be part of its environmental impact. Data on methane leakage rates, however, is poor and some studies have suggested that it might be lower than assumed. Additionally, CATF supports EPA setting performance standards for the oil and gas industry that could result in the reduction of methane leakage from gas production. Therefore this assumption could change in the future, potentially changing the conclusion as well. For example, if the leakage rate assumed in the CATF study were reduced by half, then these two types of engines would have comparable climate impacts.

The second variable assumption of the study, global warming potential (GWP), also has a significant impact upon the conclusion. Generally, GWP100 — the global warming potential likely over the span of 100 years – is the standard measurement for evaluating the impact of a pollutant. However, the climate forcing of two of the particular pollutants examined – methane and black carbon — is realized within a twenty-year span (GWP20). Using GWP20 in the analysis gives the advantage to new diesel buses. If the analysis employed GWP100, the climate impacts of the diesel and CNG buses would be comparable.

What’s driving the interest in CNG buses? Cost. The price of CNG fuel now is so low that even though the CNG buses are more expensive and less efficient than new diesel buses, the payback period is only a few years. Moreover, under the federal transportation funding formula, the federal government shoulders 80 percent of the cost of a new transit bus (compared to 20 percent for the local transit agency) while the transit agency typically pays 100 percent of the fuel cost. That creates a powerful incentive for a transit agency to choose CNG for their new buses, even though that may not necessarily be the best outcome for public health and the environment.

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