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Eco friendly biofuels for CI engine applications 423

temperature inside the chamber [26]. Alcohol blends exhibited a longer ignition delay
as compared to diesel fuel because of the low cetane number of alcohol. Among alco-
hol blends, higher alcohols have a longer delay period than lower alcohols, even
though they have higher cetane numbers. This is because of the lower oxygen content
and higher kinematic viscosity of higher alcohols. Viscosity of the fuels affect the
ignition delay by influencing the atomization process and the fuel-air mixing. So,
lower alcohols that are richer in oxygen and have a lower kinematic viscosity have
shorter ignition delays.

Heat release rate
The longer ignition delay and richer oxygen content of alcohols result in alcohol
blends having higher HRRs than diesel fuel. As discussed with in-cylinder pressure,
there are two major reasons for alcohol blends having higher maximum HRR values
relative to diesel fuel. First, the presence of oxygen-rich alcohols in the fuel blends
leads to improved combustion, increasing the HRR. Second, the fuel that gets accu-
mulated in the chamber during the time between fuel injection and the start of com-
bustion undergoes rapid combustion and releases heat rapidly. Alcohol blends, having
a longer ignition delay, result in higher HRR than diesel fuel.

15.4.1.3 Emission characteristics of alcohols

Various emission parameters such as CO, carbon dioxide, nitrogen oxide, hydrocar-
bon emissions, and smoke opacity are used to determine the emission characteristics
of the engine. These parameters are discussed in detail in the following section.

Hydrocarbons
Hydrocarbon emissions depend on the combustion of the fuel. Unburned HC emis-
sions (UHC) are generated, owing to incomplete combustion. The unburned HC in
the emission are reduced when different aliphatic alcohols blended with mineral diesel
fuel are used to run compression ignition engines, and it is because of a lower carbon
to hydrogen ratio throughout the engine operation. Another reason is that alcohols
have rich oxygen content, which aids the combustion process to occur efficiently.
Also, alcohol blends have lower viscosity than mineral diesel, which causes better
atomization of fuel during injection and this helps in better combustion of fuel. These
things reduce the percentage of unburned carbon emissions in the blends of alcohol
and diesel relative to that of neat diesel.

Carbon monoxide
Major parameters influencing CO formation are air/fuel ratio, properties of the blend,
fuel atomization rate, geometry of the combustion chamber, engine loading condi-
tions, speed at which the engine is running, and operation parameters such as injection
pressure and injection timing. Among these parameters, the one that influences CO
formation the most is the air/fuel ratio [27]. Sufficient oxygen content and high tem-
peratures in the cylinder instigate a reduction in the CO formation. The addition of
oxygen-rich alcohols consequently reduces the formation of CO. Another reason