The results of our WTW analysis of criteria pollutant emissions show that, as tailpipe emissions from
motor vehicles continue to decline, WTT activities could represent an increased share of WTW emissions,
especially for hydrogen, electricity, ethanol, and FT diesel. Thus, in order to achieve reductions in criteria
pollutant emissions by advanced vehicle technologies, close attention should be paid to emissions from
WTT, as well as TTW, activities.
Our study analyzed advanced vehicle technologies together with new transportation fuels, because vehicle
technologies and fuels together have become increasingly important in seeking solutions to transportation
energy and environmental problems. High-quality fuels are necessary to allow introduction of advanced
vehicle technologies. For example, low-sulfur gasoline and diesel are needed for gasoline lean-burn and
clean-diesel engines. The energy and environmental benefits of FCVs can be guaranteed only by using
hydrogen from clean feedstocks and efficient production pathways. In a way, the recent popularization of
WTW analyses reflects the new reality — that vehicles and fuels must be considered together in
addressing transportation energy and environmental issues.
Our study separates energy use into total energy, fossil energy, and petroleum energy. Separate results for
each of the three energy types shed light on the true energy benefits offered by various transportation
fuels. For example, some other studies that developed estimates for total energy use showed large
increases in energy use for biofuels. But those studies failed to differentiate among the different types of
energy sources. An energy pathway that offers a significant reduction in petroleum use may help U.S.
domestic energy supply and energy security concerns. In Section 4, we demonstrate that total energy
calculations can sometimes be arbitrary. For these reasons, we maintain that the type of energy sources, as
well as the amount of energy use, should be considered in evaluating the energy benefits of vehicle/fuel