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ural gas, and coal with oxygen (O
2
).
HC + O
2
→ H
2
O +CO
2
+ energy.
Consumers can reduce vehicle CO
2
emissions by us-
ing less fuel. This can be accomplished by select-
ing vehicles that have better fuel efficiency, using less
carbon-intensive fuels, or by driving fewer miles.
We use a well-to-wheels (WTW) approach to
quantify the CO
2
emissions from a vehicle fleet.
WTW CO
2
includes both the direct emissions from
the combustion of fossil fuel by the vehicle, also
known as tailpipe emissions or tank-to-wheel (TTW)
emissions, as well as the upstream, or well-to-tank
(WTT), emissions. WTT emissions are introduced
when the feedstock for the finished fuel is extracted
or grown, transported, and refined into a usable fuel or
used to generate electricity. The WTW emissions rep-
resent up to 80% of the vehicle life cycle CO
2
, while
raw materials, manufacturing and assembly, mainte-
nance, and end of life account for the remainder of
vehicle life cycle CO
2
emissions (Notter et al., 2010;
Ma et al., 2012). Conventional internal combustion
engine vehicles (ICEVs) have about 80% of the life
cycle CO
2
in the WTW phase (Notter et al., 2010;
Ma et al., 2012). With advanced technologies such
as HEVs becoming more prevalent, vehicle fuel effi-
ciency improves, reducing the WTW CO
2
. However,
the manufacturing or raw materials become more car-
bon intense; for example, the WTW share of life cycle
CO
2
for BEVs can decrease to 50-60% (Notter et al.,
2010; Ma et al., 2012).
Vehicle WTT and TTW CO
2
emissions are calcu-
lated based on the vehicle fuel economy (miles per
gallon, MPG) reported by the U.S. EPA and DOE
at fueleconomy.gov. Common liquid fuels are gaso-
line and diesel, which may be blended with the biofu-
els ethanol and biodiesel, respectively. A mixture of
10% ethanol and 90% gasoline (by volume) is called
E10. E10 is sold as gasoline in most U.S. states.
E85 contains 85% ethanol by volume and is used
only by flex-fuel vehicles (FFVs), which can oper-
ate on any blend from E0 (gasoline) to E85. Each
fuel has known TTW CO
2
emissions, calculated from
the physical and chemical properties of the fuel. The
WTT CO
2
emissions for each fuel are provided by
GREET 1.8d.0, a fuel life cycle assessment tool de-
veloped at Argonne National Labs (Wang, 1999).
Other GHGs are emitted in smaller quantities, primar-
ily during the WTT phase. The GHGs methane (CH
4
)
and nitrogen dioxide (N
2
O) have 25 and 298 times
the global warming potential (GWP) of CO
2
, respec-
tively, over 100 years (Solomon, 2007). Frequently,
the emissions of CH
4
and N
2
O are weighted by their
GWPs and combined with the CO
2
emissions to pro-
vide a single CO
2
-equivalent GHG metric (CO
2
eq).
Table 1 lists the WTT and TTW CO
2
eq factors for the
fuels used in the model in units of kg/gal.
Table 1: WTT and TTW fuel emission factors.
GHG (kg CO
2
eq/gal)
f
W T T
f
T TW
Gasoline 2.2 8.9
E10 (corn ethanol) 2.5 8.0
E85 (corn ethanol) 4.7 1.3
Diesel 2.47 10.0
B10 (soy biodiesel) 2.49 9.0
The factors in Table 1 include only fossil-based
GHG emissions. Renewable biofuels, like neat
ethanol E100, have no TTW fossil-based CO
2
emis-
sions because there is no net increase in atmospheric
CO
2
concentrations when the fuel is burned. The CO
2
is repeatedly emitted and reclaimed in a closed-cycle
in which the ethanol is combusted then absorbed from
the atmosphere as the biomass (corn) grows. Fossil
fuels like gasoline produce a net increase in atmo-
spheric CO
2
by removing carbon stored underground
and releasing it into the atmosphere with no mecha-
nism for returning it underground. Biofuels have only
WTT fossil-based CO
2
emissions.
For ICEVs, the annual metric tons of GHG emis-
sions are calculated as a function of fuel economy,
distance traveled, and GHG emissions factors in (1).
HEVs are treated as ICEVs since the small on-board
battery is recharged from the engine, not from an elec-
tric outlet.
GHG
W TW
= V MT
f
W T T
+ f
T TW
1000MPG
, (1)
where V MT is annual travel (miles); MPG is the
EPA label fuel economy (miles/gallon), f
W T T
is
the well-to-tank (fuel production) emission factor
(kg CO
2
eq/gallon) in Table 1, and f
T TW
is the
tank-to-wheel (fuel combustion) emission factor (kg
CO
2
eq/gallon) in Table 1.
BEVs have only WTT CO
2
emissions. Like liq-
uid fuels, electricity may come from both fossil and
renewable sources. Renewable sources include hy-
dropower, solar energy, biomass, and wind power and
have no WTT CO
2
. Fossil fuels’ carbon intensities
combined with the efficiency of the power plant de-
termine the electricity WTT CO
2
footprint. Table
2 lists the WTT CO
2
factors for electricity by feed-
stock fuel including 8% transportation and distribu-
tion losses (Wang, 1999).
The electricity used to charge the BEV battery
varies across the country depending on the regional
mix of fuels used in the power plants. The GREET
1.8d.0 database provides mixes for the Northeast and
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