Environmental Engineering Reference
In-Depth Information
CO
2
(molecular weight of 44.01). For modern IC engines that are regulated by toxic emission stan-
dards and operate under normal conditions, the conversion of the chemical energy to sensible energy
is typically greater than 97% (Heywood 1988). For on-road vehicles, over 99.9% of the carbon in the
fuel is converted to CO
2
through combustion and aftertreatment devices such as catalytic converters
(Heck and Farrauto 2002). However, because of several losses and limitations, not all of that released
chemical energy results in usable work transmitted to the output shaft of the engine. Therefore fuel
consumption and CO
2
emissions are directly coupled and need to be considered jointly.
There are two common measures of fuel usage in vehicles. The United States uses a rating of
MPG whereas Europe and many other countries use a rating of liters per 100 kilometers (L/100
km). Per above, CO
2
specific emissions in grams of CO
2
per kilometer (g(CO
2
)/km) is proportional
to L/100km for a given fuel. The relationships between these metrics are shown in Figure 10.4 for
gasoline on the basis of the properties in Table 10.2. MPG is a measure of efficiency in that it is
the useful output in miles divided by the input in gallons of fuel, whereas L/100 km is a specific
consumption metric (input over output). It is then clear that they are inversely proportional (L/100
km = 234.2/MPG). Following through, the g(CO
2
)/km-speciic emissions metric is proportional to
L/100 km and is dependent on the fuel properties [density (column C) and CO
2
/fuel (column F) in
Table 10.2]. Here we note that because of the inverse relationship between MPG and L/100 km or
g(CO
2
)/km, when changes are discussed, a specific percentage change in one does not correspond
to the same percentage change in the other. For example a 42% increase in MPG from 35 to 50 cor-
responds only to a 30% decrease in L/100 km or g(CO
2
)/km of CO
2
emissions.
In addition to the specific emission scale in Figure 10.4, also shown are the related standards and
targets for CO
2
and CO
2
-equivalent greenhouse gases (GHGs). With this figure, comparisons can be
made between U.S. Corporate Average Fuel Economy (CAFE) standards, including the newly adopted
U.S. CAFE standard (Sissine 2007), European CO
2
-speciic emissions targets (Brink et al. 2005),
and California CO
2
equivalent GHG specific emissions standards (California Legislation 2002; ARB
2005). From this figure we can see that these standards, including the CAFE fuel economy standard,
are in application all regulating CO
2
-speciic emissions. Additionally, although the measured emis-
sions are determined on different vehicle test cycles, it can be seen that the U.S. 2016 MPG CAFE reg-
ulation is set at a higher CO
2
-speciic emission level than the European targets and California limits.
From the previous discussion, it can be seen that regulating fuel economy not only provides
benefits regarding reduction in oil consumption, but also directly reduces CO
2
emissions. With the
implementation of the latest LDV CAFE standards, U.S. GHG emissions could decrease by 960
million metric tones, saving 1.8 billion barrels of oil over the lifetime of the vehicles sold between
2012 and 2016 (U.S. EPA 2010c). This is a measureable savings for the economy and the environ-
ment, as well as providing increased energy surety by reducing reliance on imported oil. However
these estimates do not account for increases in miles driven that have been observed with increased
vehicle fuel economy. Although the original CAFE standards in 1975 increased vehicle fuel economy
by 37.5% from 1980 to 2007 (U.S. DOT 2004), over this time the total driven miles in the United
States increased by 199% (U.S. DOT RITA 2010), resulting in an overall increase in fuel consump-
tion. Hence, fuel consumption is not decreasing as rapidly as desired from these CAFE standards.
One method proposed to compensate for this trend is to increase fuel costs via gasoline taxes as an
example, to motivate the demand for fuel-efficient vehicles meeting stringent CAFE standards while
also reducing miles driven on the basis of the increased cost of fuel (NRC 2002, 2010a).
Various methods have been proposed to improve fuel economy for SI and CI engines, as will
be discussed further for both engine technologies in subsequent sections. Options to reduce fuel
consumption applying to SI and CI engines include vehicle improvements such as mass reduction,
decreased rolling resistance and friction, improved aerodynamics, advanced materials and body
designs, transmission modifications (Jones 2008; NRC 2010b), and hybridization. In the case of
MDVs and HDVs, proposed methods are similar to those mentioned, but other options include auto-
mated manual transmissions, wide-base low-rolling resistance tires, and intelligent transportation
systems that include appropriately training drivers, modifying truck size and weight restrictions,
