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

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Methodological Issues
The LDV analyses presented below are based on findings from the relevant literature
regarding the effectiveness and costs of the various efficiency improvement options. The
studies consulted frequently featured different assumptions and calculation methods,
including the type of vehicle(s) evaluated (e.g., compact, mid-sized, trucks), driving
patters (urban versus highway), and the technology package of the vehicle to which the
technology is compared. In addition, assumptions regarding vehicle miles traveled, retail
price markups, the treatment of fuel taxes and subsidies, acceptable payback periods and
discount rates vary between studies as well, all of which can have substantial impacts on
cost and cost-effectiveness estimates. Finally, the elements of the “technology packages”
evaluated in these studies were often different as well. As such, direct comparison of the
findings from these studies is difficult, and their subsequent use in this analysis must be
viewed as providing generalized estimates, rather than precise assessments of the
different technology costs and benefits.
Advanced Conventional Gasoline Vehicles
Overview
Engine and powertrain advancements can increase the efficiency of converting fuel energy
to vehicle movement. These technologies range from minor changes to current engine
characteristics to complete modification in the way fuel is burned. These technologies are
not available for retrofit, and will penetrate the marketplace only through the purchase of
new vehicles. Many of these technologies will be adopted to meet upcoming CAFE
requirements. These strategies also may be adopted on an accelerated schedule and/or at
a higher market penetration level in order to extend their benefits beyond the regulatory
base case scenario.
Some of the technologies discussed below can be applied to gasoline or diesel engines,
while others apply only to gasoline systems. Many improvements intended for gasoline
engines attempt to make them operate more like their more efficient diesel counterparts
by reducing throttling losses or increasing compression ratios. In effect, these
technologies attempt to capture diesel-cycle efficiencies without the increased pollutant
formation.
Technologies that vary air intake and exhaust valve timing, compression ratio, and engine
displacement all allow engines to operate more efficiently over a wider range of
conditions. Variable valve timing and variable displacement are technologies currently
available on a wide range of vehicles, but are still being advanced in ways that can
provide substantial fuel consumption benefits. Variable valve timing is particularly
common, being found on approximately half of all new vehicles, while variable
displacement is less common, being limited to larger six to eight cylinder engines with
larger pumping losses. (Bandivadekar et al., 2008). On the other hand, variable
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