Fuels: Natural Gas
Based on the recent procurement activity and successful demonstration programs of the nation’s two largest transit operators (New York and Los Angeles), compressed natural gas (CNG) is emerging as the most likely successor to the conventional diesel engine. In the early 1990’s, liquefied petroleum gas (propane, or LPG) was the fuel of choice for AFV's; since then, the market preference has decisively switched to natural gas.
The American Public Transportation Association’s national survey of 300 transit agencies suggests that the number of CNG buses in use, and on order, far outweighed any of the other transit AFV types for 2001, at 7.5 per cent. Twenty-one percent of new orders, as of January, 2001, were for CNG vehicles.
CNG’s rising popularity has less to do with the cost-efficiency of the technology, than with federal and state prioritization of air quality. CNG has proven clean air advantages: it has been demonstrated to generate significantly less particulate matter and NOx, which makes it attractive to urban transit agencies working to reduce smog levels. While natural gas is inexpensive, physical conversion to CNG means a major investment in fueling infrastructure and new buses.
The emissions profile of natural gas, however, is mixed. While CNG comes out well in GREET’s emissions simulation, engine duty cycle performance tested by the Transportable Emission Testing Laboratory of the University of West Virginia shows lower efficiency in CNG buses in heavy duty application, resulting in higher GHG emissions. (See the Travel Matters chart, "Emissions Coefficients for Alternate Fuels"). As the Northeast Advanced Vehicle Consortium (NAVC) reports in their emissions testing of AFV’s, “CNG buses consume more fuel for the same output…canceling out nearly half of the CO2 benefit.”
CNG buses also suffer a “weight penalty” due to the larger and heavier fuel tanks required to maintain natural gas in a pressurized state, and in volumes sufficient to complete a typical round of service. Heavier vehicles consume more fuel.
Perhaps more importantly, CNG buses emit much higher amounts of methane (CH4) than diesel buses, which emit virtually none of the gas. Since methane has 21 times the global warming potential of CO2, a small volume of methane emissions can cancel out a much larger decrease in CO2 emissions. As NAVC reports, “even though the CNG buses emit less CO2, the impact from the released methane creates a larger GHG impact.” Here again, this difference is more pronounced in dynamometer tests than in computer simulations.
A comprehensive conversion of the national stock of transit buses to CNG would have numerous beneficial effects on the air quality of urban areas, but its beneficial impact on greenhouse gas emissions is uncertain. In either case, while current trends indicate a shift to natural gas by major transit agencies, those involved in alternative fuel programs at the nation’s two largest transit agencies view CNG as an intermediate step in the direction of still different technologies.
“Natural gas will have outlasted its usefulness in the near future,” says New York City Transit's Dana Lowell; "CNG is ultimately a transitional strategy," echoes LA's John Drayton. Expected improvements in the petroleum refining process, and complementary advances in engine technologies, may soon make diesel just as clean to burn, and more attractive in terms of capital and other costs, than CNG. The other likely competitor with CNG, both agencies anticipate, is the hybrid-electric transit bus.
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