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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
close to 30 years (ORNL, 2008, Table 3.12). Therefore substantial penetration of new OEM
technologies can take several years after commercialization.
On the other hand, OEM strategies should have a disproportionately large impact on fuel
consumption since older vehicles travel significantly less on average than new vehicles. A
10-year-old truck travels only about 40 percent of the distance annually of a new truck,
and a 20-year-old truck less than 20 percent. 44
In addition to advancements related to vehicle efficiency, diesel HDVs are in the process
of adopting control technologies in order to meet increasingly stringent NOx and PM
emission standards. The latest EPA heavy diesel standards require 90 percent reductions
in PM in 2007 and 90 percent NOx reductions by 2010 relative to prior emission standards
(U.S. EPA, 2006). The technologies used to meet these standards, such as particulate filters
and NOx absorbers, have their own energy requirements, and add weight to the vehicles.
As such, these technologies may reduce fuel efficiency in some cases by about one percent
for particulate filters, and roughly five percent for NOx control using exhaust gas
recirculation, and other recent strategies (U.S. DOE, 2009b; Greszler, 2008). Newer
emission control technologies may increase fuel efficiency to the extent that they allow the
diesel engine to operate more efficiently.
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The effectiveness of different efficiency
strategies also will vary with vehicle age due to the fact that vehicles may change
operators and duty-cycles after a number of years. For example, Class 8 trucks commonly
spend the first four to six years of their life in long-haul service, which entails higher
speed operation (and greater mileage accumulation) than other service types (Lutsey,
2008). After this time, these trucks are often moved into lower mileage, urban/short-haul
applications. These different service types and operation modes obtain substantially
different benefits from different efficiency technologies. For example, aerodynamic and
rolling resistance strategies obtain their maximum benefit at highway speeds, while
hybrid technologies are best suited for lower speed urban drive cycles.
The following sections discuss resistance reduction strategies, weight reduction, engine
improvements (including hybrid power systems), and strategies to reduce vehicle
accessory loads. (Alternative fuel and fuel cell strategies are discussed in tandem with
their associated fuel type in Section 2.0.) The baseline emission estimates used below are
based on the AEO Reference case for 2030, which estimates fleet average fuel efficiency on
an annual basis. Annual incremental emission reductions and total costs for each
technology option are based on a variety of projections found in the literature. The costeffectiveness
of packages of multiple technology strategies (e.g., hybrid electric vehicles
with advanced aerodynamics) are discussed separately at the end of the HDV section.
44 Based on mileage accumulation data from EPA’s MOBILE6 emission factor model.