Environmental Engineering Reference
In-Depth Information
methanol, an oxygenated fuel also used in SI engines, has an energy density of only 20 MJ/kg, a 55%
reduction. Despite this energy density reduction, a benefit of oxygenated fuels is that they require
less air (oxygen) during combustion and provide carbon monoxide (CO) and PM emission reduc-
tions. The use of oxygenated fuels in SI engines was mandated as part of the 1990 Clean Air Act
(CAA) (U.S. EPA 2004) in regions of CO emission nonattainment because oxygen in the fuel assists
with more complete oxidation of the carbon to CO
2
, thereby reducing CO emissions (CFDC 2008).
Specific energy on a unit volume basis relative to gasoline is given in column E of Table 10.2.
Examining the alcohol liquid fuels, it is seen that all have a lower energy density, which indicates
that under the same fuel conversion efficiency, their specific fuel consumption (mass or volume of
fuel per unit output) will be higher. Neat ethanol has 63% of the energy density of gasoline, and E85
has 70% of the energy density of gasoline. This is the cause of the reduced fuel economy (MPG)
observed in FFVs. The alternative CI fuels have energy densities that are more similar to petroleum
diesel, except for DME. The gaseous fuels have low energy densities on a volumetric basis because
of their low mass densities, even when compressed. As Table 10.2 shows, hydrogen, even when
compressed to 34.5 bar (5000 psi), has a lower energy density, only one tenth that of gasoline. This
brings to light that we should not be paying for fuels based upon a volumetric basis (dollars per gal-
lon), but on an energy basis (dollars per MJ), which can be computed based on the ratio in column E.
Columns F and G characterize the specific emission of CO
2
of these fuels. Column F is the
mass-specific CO
2
produced per unit mass of fuel consumed, and column G is the energy-specific
CO
2
in grams of CO
2
per MJ of fuel energy. This energy-specific CO
2
provides a useful metric for
comparing the energy/CO
2
tradeoff of the fuels. Examining the values for the liquid fuels, their
energy-specific CO
2
values cover a relatively small range and are within 7% of gasoline. On first
approximation, engine efficiency will not change significantly for these different fuels when used
in the same IC engine technology. Thus, a good estimate of the relative CO
2
impact for alterna-
tive fuels on a tank-to-wheels analysis would be the ratio of the biofuel to the reference fuel (JRC/
IES 2007). For example, comparing ethanol to gasoline would be 71/72 = 0.99, or a 1% estimated
reduction in tank-to-wheels CO
2
. Therefore, to produce less CO
2
at this stage of the life-cycle, aside
from the solution of decreasing the miles driven, the efficiency at which the IC engine converts the
chemical energy in the fuel to useful mechanical work at the wheels needs to be improved and/or
the amount of mechanical work required to propel the vehicle must be reduced. One may note that
although some gaseous fuels, and in particular hydrogen, have a high energy content per unit mass
(120 MJ/kg for H
2
compared with just 44 MJ/kg for gasoline), they still cannot deliver the same
vehicle range as liquid fuels such as gasoline or diesel because of the low energy per unit volume
in the gas state. Technologies do exist to store hydrogen at low temperatures (-253°C or -423°F) in
a liquid state (BNL 2008) and at high pressures. However, in addition to adding considerable cost,
these solutions require a significant amount of additional energy to compress and cool the hydrogen,
and this extra energy must be taken into account when evaluating the overall energy requirements
throughout the life-cycle of hydrogen (see Figure 10.3 for comparisons of vehicle driving ranges for
several types of fuel). From Figure 10.3, it is apparent that the typical CI liquid fuels of diesel, FT
diesel, and biodiesel yield the largest distance range, followed by liquid SI fuels of gasoline, buta-
nol, and ethanol. The gaseous SI fuels (CH
4
, hydrogen, and biogas) yield the lowest driving range.
These trends all relate back to the energy content of the fuel on a volume basis, determined by
the energy contained in the fuel on a mass basis and the fuel density. The gaseous fuels, although
having a reasonably high energy content, have low densities that yield reductions in driving ranges.
CI and SI fuels have comparable energy contents and densities that provide similarities in driving
ranges, with exceptions to this including methanol, ethanol, and DME, each of which has a lower
energy content relative to the other fuels considered. It should also be noted that electrical energy
storage can be used, but considering that gasoline has a volumetric energy density that is over
60 times greater than that of a nickel metal hydride battery (Komatsu et al. 2008), the issue becomes
the ability to physically store enough energy onboard the vehicle. Furthermore, electric vehicles
currently need batteries that are 12 times the size of conventional plug-in hybrid batteries to provide
