Environmental Engineering Reference
In-Depth Information
2.0
B0
B100
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0%
10%
20%
30%
40%
EGRP
CO
2
FIGure 10.9
Impact of EGR on PM emissions with petroleum diesel fuel (B0) and 100% biodiesel (B100).
need to be changed to be compatible with the alternative fuel as is the case in enabling a gasoline
vehicle to run on ethanol or methanol blends or a diesel engine to run on biodiesel. Piston rings and
valve seats are often upgraded to be compliant with the alternative fuel, which can also add cost.
Electronic determination of the type of fuel currently in use can be required. In some cases this can
be done with relatively low cost through the use of “virtual sensors,” which are software algorithms
operating within the vehicles controller that utilize inputs from existing sensors to determine fuel
type. There is an additional cost for increased engineering effort because of the additional calibra-
tion and federal certification required. However, if production volumes are sufficiently high, this
increased cost is low on a per vehicle basis.
One issue with some fuels is their poor cold-start characteristics, on the basis of their fuel
properties. In the case of diesel engines, DME has a higher cetane number than petroleum diesel
(Table 10.2), which promotes ignition by reducing the required compression energy for autoignition.
In conjunction with its low boiling point, this helps reduce ignition delay, improve fuel-air mixing,
and therefore minimize cold-start issues. On the other hand, biodiesel exhibits fuel gelling in colder
environments, a characteristic common in petroleum ultralow sulfur diesel (ULSD). In the case of
SI engines, ethanol fuel results in different cold-starting issues. Ethanol does not vaporize signifi-
cantly in low-temperature conditions because of lower vapor pressure and higher heat of vaporiza-
tion (17 kPa vapor pressure vs. 62 kPa at 100°F for gasoline, and 900 kJ/kg heat of vaporization
vs. 400 kJ/kg for gasoline) and hence the fuel-air mixture is not rich enough (attributed to limited
vaporization) to combust (Davis et al. 2000). Hence, cold-start issues are common to alcohol fuels.
Adding gasoline to the mixture (e.g., yielding E85) helps to reduce, but not overcome, this issue
because the vaporized gasoline can assist with initial combustion; however, cold-start emissions can
be higher (Davis et al. 2000).
10.3 summary
In this chapter, we discussed the effect and interdependencies of fuels, engine type, aftertreat-
ment, and vehicle technologies with a focus on biofuels and where they fit into current and future
technologies. The energy densities on a volumetric basis for the oxygenated biofuels are lower than
their comparable petroleum fuels. Furthermore, combustion of a carbon-based fuel, be it gasoline,
ethanol, diesel, or biodiesel, in the presence of oxygen, produces CO
2
. When normalized, the CO
2
produced from combustion on an energy basis for the petroleum and bio-based liquid fuels are
within ±7% of gasoline.
