Environmental Engineering Reference
In-Depth Information
Gasoline is produced by fractional distillation of crude oil. It is a complex
mixture of about 500 hydrocarbons in the C
4
-C
12
range, belonging to four com-
pound groups: alkanes, cycloalkanes, alkenes, and aromatics. Straight chain or
branched alkanes are present in greatest amounts (about 50%), with smaller
quantities of cyclic alkanes and aromatics and trace amounts of alkenes. The most
important motor properties of a gasoline are its octane quality, which is respon-
sible for its resistance to autoignition, and volatility. This is expressed by distil-
lation behavior and vapor pressure, and permits obtaining of an ignitable air-fuel
vapor in the combustion chamber in all operative conditions, but can also affect
exhaust gas and evaporative emissions. Benzene and lead additives were formerly
used to control antiknocking properties of gasolines, but because of their toxicity
they are today substituted by isomerate higher aromatics, ethers (methyl tert-butyl
ether), alcohols.
Gasoline should be ideally converted to water and carbon dioxides during
combustion, however, also undesired compounds are present in engine exhaust gas
because of different phenomena occurring in the combustion chamber [
40
]. These
compounds can be grouped as total unburned hydrocarbons (THC), non-methane
unburned hydrocarbons (NMHC), carbon monoxide (CO), nitrogen oxides (NO
x
),
and particulate matter (PM). While improved engine technology has led to some
reduction in exhaust emissions, their dramatic reduction has been possible, thanks
to the introduction of exhaust gas treatment catalytic systems. Actually, more than
95% of these pollutants are converted (oxidized or reduced) on three-way catalytic
converters currently installed into the exhaust system of gasoline vehicles [
43
],
while their residual concentration can be affected by fuel reformulation in the
refinery. Several studies have been carried out about the effect of alternative
reformulations of gasoline on air quality; in particular the reduction of total aro-
matics and sulfur has been successfully investigated [
41
].
The gasoil for Diesel engines derives from the middle distillate fraction of
crude oil distillation. Unlike gasoline, it need to be easily decomposed at tem-
perature and pressure values reached in the combustion chamber, as a consequence
its composition is mainly based on alkanes and cycloalkanes (about 80-90%),
while the presence of different unsaturated compounds (aromatics and olefins)
permits other engine requirements to be met, such as low temperature behavior and
heating value. A high cetane number is beneficial not only for self-ignition and
starting behavior, but also for emission reduction and noise. As sulfur in the fuel
increases the emissions of sulfur dioxide and particulate matter, its content in
commercial Diesel fuels has been limited at very low levels (50 ppm in Europe
since 2005) and tends to be almost completely eliminated (10 ppm in Europe since
2009). Apart from the direct benefits on particulate emissions a sulfur content
reduction from 50 to 10 ppm could also increase the performance and durability of
oxidizing catalytic converters and give benefits in the form of reduced
sulfur-induced corrosion and slower acidification of engine lubricating oil.
Regarding the effect of gasoil hydrocarbon composition on exhaust emissions the
correlation between mutagenicity of exhaust gas and aromatic content has been
also assessed [
44
].
