eMTBE = Methyl tertiary butyl ether (CH3OC4H9).
fMathPro 1999, Case 8 HHV and density reported other values estimated.
Source: Acurex 1996, ANL 1999, MathPro 1999.
Carbon content as weight percent or per MMBtu is used to determine CO2 emissions from fuel combustion. Higher heating values are used to relate fuel use to energy consumption for process efficiency calculations while lower heating values are used to compare vehicle fuel consumption. The molecular weight of fuels corresponds to vapor density and associated evaporative emissions. The values in Table 2-1 were used throughout the report.
2.1 Fuel Composition and Properties
Fuel composition and properties affect many aspects of the fuel-cycle analysis. Liquid fuel and vapor composition and properties are necessary to predict emissions from fuel transfer operations. The vapor pressure of fuels affects the mass emissions from vapor transfers. The composition of fuels affects the mix of toxic compounds from liquid spills as well as that of vapor emissions. Fuel specifications affect refinery energy requirements and to some extent emissions. Finally, the composition of fuels needs to be consistent with values used for energy content, vapor pressure, and vehicle fuel economy.
The following sections summarize the physical properties of the fuels considered in this study. Since the fuels in this study can be represented by a variety of formulations, presenting the potential range in fuel properties provides some insight into how the results of this study might be affected by different fuel properties.
Diesel fuel is used to fuel compression-ignited light-and heavy-duty engines. The popularity of diesel as a fuel for passenger cars has dropped in recent years while diesel is the dominant fuel for trucks. Unlike gasoline, diesel has a low vapor pressure and a low octane number. High quality diesel is characterized by a high cetane number. The ARB implemented a specification for clean diesel that took place in October 1993. This fuel required lower sulfur and aromatics (0.05 and 10 percent maximum, respectively) with an option to meet an alternative specification that results in equal emission benefits. Much of the diesel fuel sold in California after 1993 met alternative specifications that achieved the same emission reductions as the 10 percent aromatics formulation. Table 2-2 shows how the heating value, density, sulfur content, and cetane index for various formulations. The low aromatics and alternative formulations were sold in California in 1994.