Environmental Engineering Reference
In-Depth Information
with the ethanol, while the aromatics remain compatible with other hydrocarbons in
diesel fuel.
The addition of ethanol to diesel lowers the viscosity and lubricity of the final
blend. Lower fuel viscosities lead to greater pump and injector leakage, reducing
maximum fuel delivery and ultimately power output. Hot restart problems may
occur also as a consequence of the insufficient fuel injected at cranking speed (when
fuel leakage in the high-pressure pump is amplified) due to the reduced viscosity of
the hot fuel. Fuel viscosity also affects the atomization and spray characteristics in
the combustion chamber [184].
With the inverse relationship of octane number and cetane number, ethanol
exhibits a low cetane rating (inferior to 10). Hence, increasing concentrations of
ethanol in blends proportionately lower the cetane number.
An increase in fuel consumption approximately equivalent to the reduction in
energy content of the fuel can also be expected when using ethanol/diesel fuel
blends. The energy content in the blends decreases by approximately 2% for each
5% of ethanol added, by volume, assuming that any additive included in the blend
has the same energy content as diesel fuel [185]. In any case, no noticeable dif-
ferences in engine performance, compared to diesel fuel, have been reported with
ethanol contents up to 10% [186].
8.3.2.2 Effect of Bioethanol on Spark Ignition Engines
Performance Properties
Extensive research efforts have been devoted to investigate the effect of ethanol as
a pure fuel and its blends with gasoline on engine performance [187]. Currently,
the ethanol consumption in the Brazilian transportation sector represents 44% of
the overall gasoline consumption used for transport [188]. Nevertheless, the use
of ethanol and ethanol-gasoline blends on spark ignition engines may originate
some problems. Upon increasing the ethanol content in the fuel (up to 10%), the
heating value of the blends decreases and then the Reid vapor pressure (RVP, a
common measure of the volatility of gasoline) increases (which indicates indirectly
increased evaporative emissions, while CO tailpipe decreases) to a maximum and
then decreases. The heating value of ethanol is lower than gasoline. Therefore, in
order to achieve the same energy output, 1.5-1.8 times more ethanol is needed. This
leads to higher volumetric fuel consumption compared to petrol, which causes an
increase of the BSFC [174]. Other problem related to the use of ethanol-gasoline
blends is the phase separation, in the presence of water. Alcohols with 3-8 carbon
atoms have better water solubility in blends [189].
Bioethanol has a higher octane rating mixture than branched alcohols, but is far
more volatile owing to the formation of minimum temperature azeotropes with the
hydrocarbons of gasoline and thus presenting a higher vapor pressure than mixtures
of ethers in gasoline [190].
The rapidly increasing use of ethanol as a biofuel in blends with gasoline pro-
vides an opportunity to expand its further use as petrol-fuel replacement, with the
potential to expand markets for agricultural commodities used to produce ethanol.
