Page 471 - Advances in Eco-Fuels for a Sustainable Environment

424 Advances in Eco-Fuels for a Sustainable Environment

underpinning the low CO emissions when fueling the engine with alcohol blends is
that they have a lower C/H ratio relative to that of diesel. Ethanol in CI engines results
in high CO emissions when compared to that of blends of other alcohols and diesel
because of the high heat of vaporization of ethanol. High heat of vaporization reduces
the combustion temperature inside the cylinder [28, 29]. Moreover, at high engine
loads, operating conditions are rich in the combustion chamber, which increases
the CO content in the engine exhaust emissions owing to the larger quantity of fuel
injected. On the other hand, the fuel lean environment in the cylinder removes heat
from the products of combustion. This lowers the temperature in the cylinder, which
finally inhibits oxidation of CO.

Oxides of nitrogen (NO x )
Formation of nitrogen oxides depends on the temperature inside the cylinder, the gas
residence time it takes for the reaction to occur, and the oxygen content in the chamber
[30]. The addition of alcohol to diesel fuel at low engine loads does not significantly
affect the NO x emissions but causes a slight increase in NO x at higher engine loads
owing to the lower cetane number of blends. Ignition delay is increased due to the
lower cetane number of alcohol-blended fuels, which leads to higher peak cylinder
pressure owing to the combustion of fuel that gets accumulated in the cylinder during
the delay period. High peak cylinder pressures lead to higher maximum temperatures,
which in turn leads to increased nitrogen oxide emissions. The maximum temperature
attained inside the engine cylinder is further increased by the high oxygen content in
alcohol blends [30]. Additionally, the shorter duration of combustion of alcohol fuel
blends does not give sufficient time for cooling, resulting in high NO x emission [31].
At low loads, the evaporation of alcohols results in a cooling effect that has a dom-
inating role over the high oxygen content, thereby reducing the in-cylinder tempera-
ture and hence low NO x is emitted. As the engine load is increased, more fuel is
injected, progressively higher temperatures are attained, and the cooling due to evap-
oration is compensated for. The rich oxygen content of alcohols starts controlling the
NO x emissions from the engine [24, 32, 33].

Smoke
Smoke formation depends on the condition of the engine, the type of combustion
chamber of the engine, and the physical and chemical properties of the fuel [33]. Basi-
cally, oxygen deficiency leads to smoke formation and oxygen becomes scarcer with a
reduction in air/fuel ratio. It occurs locally in the engine cylinder. The addition of alco-
hols to diesel fuel results in a reduction of smoke emissions, with the reason being that
alcohols contain high oxygen content and have low C/H ratio. This increased oxygen
content in the fuel blends improves combustion efficiency, thereby reducing the
amount of smoke in the engine exhaust. Also, the more viscous and dense the fuel
is, the poorer is the fuel atomization and hence the higher the smoke content in the
exhaust. Among alcohol blends, higher alcohols being more viscous and dense with
greater carbon content cause higher smoke emission than lower alcohols [28, 34, 35].