Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Fossil Diesel
Despite the growth of alternative fuels and technologies, diesel is still the dominant fuel
for the transit bus sector. Because of the high-energy density of diesel fuel and the
thermodynamics of compression-ignition engines, diesel-powered vehicles tend to have
good fuel efficiency. However, they emit have historically emitted relatively high
amounts of certain types of air pollutants, including NOx and particulate matter.
Consequently, Federal regulations have increased emission standards for heavy-duty
vehicles and required the use of ultra low-sulfur diesel, both of which increase the capital
and operating cost of a transit bus somewhat (Clark et al, 2007). Since 2004, U.S. EPA’s
National Clean Diesel Campaign has been seeking to improve the environmental
performance of diesel engines in many sectors through both regulatory and voluntary
measures, and as a result, diesel engines are cleaner than ever (U.S. EPA, 2009b).
Compressed and Liquefied Natural Gas
While natural gas generally combusts more cleanly than diesel and gasoline, there are no
clear greenhouse gas-reduction benefits from CNG or LNG buses (Clark et al, 2007) as
total GHG emissions are about the same on a per-mile basis as for diesel buses. This is
especially true if CNG is mishandled and leaks occur, since CNG is composed of 85-
99 percent methane (IEA, 2002) a potent greenhouse gas. CNG has historically been costcompetitive
compared to diesel fuel, but transit buses using CNG require an expensive
fueling infrastructure consisting of high-pressure storage, compressors, and dispensers.
The median costs of modifying CNG/LNG-ready bus depots and building refueling
stations have been found to be around $875,000 and $2 million respectively (in 2007
dollars). However, once a fueling infrastructure is established, the operating costs of a
CNG/LNG fleet can be lower than for other technologies, due to lower overall fuel and
maintenance costs (Clark et al, 2007). However, CNG and LNG have lower energy
content than diesel fuels, reducing the total driving range compared to diesel. There were
6,600 CNG and 1,000 LNG transit buses operating in the United States as of January 2005
(CCAP, no date).
Electric and Hybrid-Electric Systems
Electric and hybrid-electric vehicles are steadily growing in popularity across transit bus
fleets. Pure electric vehicles, powered by on-board batteries, are quiet and do not directly
emit pollutants into the atmosphere, characteristics that make them highly desirable in
communities where a premium is placed on local air quality. Hybrid diesel-electric and
gasoline-electric vehicles are powered in combination by an internal combustion engine
and an electric motor. These buses can obtain 10 to 50 percent better fuel efficiency than
their conventional diesel counterparts, with similar reductions in GHG emissions (IEA,
2002; Greene and Schafer, 2003; CCAP, no date). Actual in-service fuel efficiency
improvements depend strongly on the duty cycle of the bus; hybrids are most
advantageous when used on routes with more stop-start cycles, hills, and time where the
bus is at rest (such as in traffic or at passenger stops). While hybrid diesel-electric systems
are gaining in popularity, vehicles using this technology are still relatively expensive
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