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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
HDV operators are subject to different economic influences and constraints than LDV
owners. Profitability margins within the trucking industry are low and operating costs
due to fuel are relatively high (about 33 percent of costs per mile in 2006) (21 st Century
Truck Partnership, 2009). Accordingly, market forces will encourage some development
and adoption of fuel efficiency improvements for U.S. trucks. In addition, voluntary
initiatives such as EPA’s SmartWay Transport Partnership have helped promote the
adoption of energy efficiency improvements among many of the largest freight carriers in
the United States. Adoption of SmartWay certified vehicles and trailers is estimated to
result in a 10 to 20 percent reduction in fuel use per truck compared to conventional
truck/trailer combinations (U.S. EPA, 2007b). In spite of these factors, HDVs have only
witnessed modest gains in fuel efficiency over the past several decades, improving by
about 0.5 percent per year for single unit (straight) trucks, and 0.2 percent per year for the
larger combination (tractor/trailer) trucks between 1970 and 2006 (ORNL, 2008, Tables 5.1
and 5.2). As such, substantial opportunities for further efficiency improvements remain.
Energy consumption and therefore fuel use by HDVs varies substantially depending on
vehicle size and weight. As a general rule Class 8 tractor-trailer rigs tend to travel more
miles per year (about 46,000 on average) and have lower fuel efficiency (about 5.7 mpg)
than lighter vehicles. For comparison, HDVs less than 26,000 pounds gross vehicle weight
average about 13,000 miles per year with a fuel efficiency average of 8.0 mpg. This
disparity leads to a disproportionate impact for Class 8 trucks, which comprise about
41 percent of HDV registrations but are responsible for about 78 percent of total HDV fuel
consumption (ORNL, 2008, Table 5-4). As such, identifying opportunities for reduced fuel
consumption among Class 8 HDVs is a primary focus of this section.
Unlike LDVs, the most appropriate way to measure the efficiency of HDVs is usually in
terms of fuel use per unit of work performed (e.g. ton-miles hauled or horsepower-hours
per gallon). Although a simple miles per gallon metric is utilized here for consistency
with other sections, it provides a somewhat crude and imprecise measure of efficiency for
these sources.
A vehicle’s operation-cycle (e.g., frequency of starts and stops, acceleration requirements,
average speed, percent of time at idle) also has a direct impact on fuel consumption and
GHG emissions. The frequent acceleration and braking events commonly encountered in
urban operations result in very poor fuel efficiency. For example, full-sized transit buses
operating under central business district conditions average only 3.2 mpg (21 st Century
Truck Partnership, 2009). Operation at very high speeds and extended idle time also
decrease fuel efficiency for freight trucks.
Energy audits for two key HDV categories—a Class 8 truck and a typical medium-duty
truck—are presented in Figures 3.6 and 3.7. These figures help identify the primary
energy requirements for conventional diesel HDV operation as well as opportunities for
reducing fuel consumption. The target energy consumption levels represent the goals for
the 21 st Century Truck Partnership, a partnership between the Federal government and
dramatic shift to gasoline engines is not anticipated for the medium duty vehicle market
(Reinhart, 2009).
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