Page 466 - Advances in Eco-Fuels for a Sustainable Environment

Eco friendly biofuels for CI engine applications 419

atomization of diesel fuel during injection and hence this results in better combustion
of diesel fuel. These things thoroughly reduce the percentage of unburned carbon
emissions in the case of diesel fuel as compared to biodiesel.

15.3.6.2 Carbon monoxide
CO discharges are formed because of the incomplete combustion of petroleum fuels
that contain no oxygen in their molecular structure. In addition, the low in-cylinder
temperature causes a cooling impact that too favors the formation of CO emissions.
Increasing the engine load from zero to full load results in a decrease in the CO emis-
sion, which is because the air-fuel ratio decreases with an increase in load. Biodiesels
are naturally oxygenated fuel and the extra oxygen molecule present within the fuel
helps to achieve better combustion that leads to low CO emissions. Sufficient oxygen
content and high temperature of the biodiesel in the cylinder establish a reduction in
the formation of CO emissions. Thus, when compared to neat diesel, biodiesel give
fewer CO emissions. This is also because of the presence of oxygen in the biodiesel
that oxidizes CO to form CO 2 .

15.3.6.3 Oxides of nitrogen (NO x )
Oxides of nitrogen constitute two major components—nitrogen dioxide and nitrogen
oxide—inthe CI engineemissions.NO x emissionsare collectivelyreferredtoasnitrogen
oxide (NO) and nitrogen dioxide (NO 2 ) in the engine exhaust. A considerable amount of
around1%–2%oftheNO x emissionsinCIenginesisNO 2, whereasNOistheothermajor
constituent.FormationofNO x emissionsisgovernedbythreemajorfactors:oxygencon-
centration, in-cylinder temperature, and residence time of the gases at that temperature.
Varyingtheengineloadplaysanimportantroleinstudyingtheemissioncharacteristicsof
biodiesel. Emission studies showed that as the engine load is increased from zero to full
load in most cases, NO x emission is increased. Though the higher NO x emission can be
generated athigher power due tomore fuel injection, the increase in power output plays a
dominant role in NO x emission formation. Studies have shown that biodiesel produces
more NO x emission than neat diesel fuel under all loads due to the presence of excess
oxygencontentinthebiodieselthatincreasestheflame temperatureinoxygen-richzones
in the combustion chamber, thereby causing higher NO x emissions. The NO x emission
increases with the increase in the percentage of biodiesel in the blends, which is due
to the corresponding increase in the oxygen content of the fuel. The high density of bio-
dieselalsoplaysaroleintheformationofNO x emissions.TheNO x emissionsofbiodiesel
are higherthanthose ofdieselfuelbecause thehighcombustiontemperatureasa resultof
better combustion due to the presence of high molecular oxygen in the biodiesel. There
exists a slowburning rate and slow rise inpressureand temperature asa resultofa shorter
mixing time with a shorter ignition delay [18].

15.3.6.4 Smoke
Smoke emissions occur due to the incomplete combustion of the hydrocarbon fuel.
They also occur due to the formation of fuel-rich zones when more fuel is injected
in the combustion chamber. Smoke emissions are also influenced by other parameters