Page 469 - Advances in Eco-Fuels for a Sustainable Environment

422 Advances in Eco-Fuels for a Sustainable Environment

Brake thermal efficiency
When alcohols are blended with diesel and the fuel acquired is used in compression
ignition engines, the BTE of alcohol-blended fuels is lower compared to pure diesel.
The reason behind this is the specific gravity of the alcohol-blended fuel is less than
that of diesel. At a higher blend percentage, it can be noted that alcohols generate higher
brake power in comparison to mineral diesel. The reason underpinning this is the higher
oxygen content of alcohol-blended fuel leads to better combustion of the blend. As the
alcohol percentage increases in the blend, the specific gravity and the calorific value of
the blend are decreased and hence BTE decreases. Butanol, ethanol, and methanol fol-
low mineral diesel with respect to BSFC values. BTE is inversely related to BSFC,
therefore mineral diesel has the highest BTE because it has the least BSFC [22].
Brake specific fuel consumption
The BSFC generally decreases while the BTE increases with an increase in the engine
load. At lower engine loads, BTE does not show a major difference from diesel fuel.
However, because of the lower calorific value of butanol, BSFC increases slightly.
With increasing alcohol content in the fuel blends, the energy content of the blend
decreases and therefore BSFC increases. This is because more fuel is required to
deliver the same power output as diesel. Additionally, because the cetane number
of alcohols is lower, the ignition delay period of the fuel blends increases with the
increasing alcohol content, leading to an increase in BSFC [23, 24].

15.4.1.2 Combustion characteristics of alcohols
Different parameters such as in-cylinder gas pressure, HRR, and ignition delay that are
used to delineate the combustion characteristics of diesel engines are discussed in
detail in the following section.

In-cylinder peak pressure
Higher peak in-cylinder pressures are attained in the case of alcohol blends when com-
pared to diesel because of the availability of more oxygen in the fuel. The longer ignition
delay period also contributes to this higher peak pressure because the fuel accumulated
during the delay undergoes rapid combustion and adds to the pressure in the chamber [9,
25]. As the engine load increases, more fuel is injected into the cylinder and the
in-cylinder pressure increases. Consequently, the pressure in the cylinder increases with
increase in engine load and reaches a peak at high engine loads. Among alcohol blends,
combustion of higher alcohols resulted in higher cylinder pressures as compared to
lower alcohols. This can be attributed to the reduction in viscosity and increased vol-
atility that result with the addition of higher alcohols; these changes lead to enhanced
atomization and hence better fuel air mixing, improving fuel combustion.

Ignition delay
Cetane number, oxygen content, and viscosity of fuel are significant factors affecting
the ignition delay. The use of fuels with high cetane numbers results in shorter ignition
delay. And the engine load results in shorter ignition delay due to the increase in gas