Page 372 - Advances in Eco-Fuels for a Sustainable Environment

Physicochemical fuel properties and tribological behavior of aegle marmelos correa biodiesel 327

the one that produces a smaller wear scar is said to have more lubricity. That is why
lubricity is also known as the antiwear property of a substance.
The lubricity of biodiesel fuel is important in fuel injection pumps (rotary and dis-
tributor). The parts in continuous motion are lubricated by the fuel itself and not by the
engine oil. Even the injectors are to some extent fuel lubricated. The lubricity indicates
the amount of wear and tear that occurs between two components in contact with each
other when entirely covered with the biodiesel. High lubricity biodiesel will ensure a
longer component life and reduced scarring. Similarly, low lubricity biodiesel may
cause high scarring, and it is undesirable. Lubricity has no direct relation whatsoever
with a viscosity of biodiesel. The factors present in the biodiesel that affects lubricity
are discussed below.

l Lubricity increases with chain length. As the hydrocarbon part is long, it does not compete
with the oxygen moieties during the hydrogen bonding.
l Lubricity increases with unsaturation (double bonds) but the lubricity-enhancing effect of
this is more than the former. Therefore, the lubricity characteristics of different biodiesels
are increased in the order of Jatropha, Karanja, and AMC.
Shorter chain lengths have very low molecular interactions, and the low-temperature stabil-
l
ity of the lubricant film makes it a poor lubricant.
The triglycerides of various fatty acids have more lubricity than their corresponding esters.
l
Some small remains of triglycerides can increase the lubricity in biodiesel. Hence, higher
saponification values mean improved lubricating properties. The saponification value of
AMC biodiesel is found to be higher than the Jatropha biodiesel. Karanja biodiesel has
the least saponification value among them.
l Carboxylic acid moieties have the highest lubricity followed by aldehydes, then alcohols,
esters, and ketones. The aldehyde or carbonyl group present in esters is solely responsible
for the lubricating properties. Ethers exhibit poor lubricating properties.
l FFA content in biodiesels is more responsible for lubricity properties. FFA content is directly
related to acid value, and hence the FFA value of AMC biodiesel is higher than Karanja and
lower than Jatropha.
l Sterically unhindered electron groups in doubly bonded carbon provide more lubrication.
The double bonds at the center of the chain have poor lubricating properties compared to
the ones at the end of the chain due to the formation of stable pi complexes such as organ-
ometallic complexes.
l The better lubricating properties of these moieties over others is that the ionic interactions
between the metal parts and lubricating molecules due to hydrogen bonding and Van der
Waal interactions within the lubricating molecules. With some fatty acids, even phy-
sisorption and chemisorption was observed.
l Fatty acid esters have lower lubricity because of the unavailability of free OH or COOH
groups to form hydrogen bonding with the metal substrate for ionic interactions. Byproduct
glycerol increases the lubricity to a very high value because of three OH groups.
Hence, we can conclude that the lubricity of biodiesels is influenced by so many fac-
tors. Each factor has its trends, and thus the lubricity of different biodiesels can be
compared qualitatively for conclusions. In this case, Karanja biodiesel has good
lubricity property over the other biodiesels, because it has a higher degree of
unsaturation and FFA content. Saponification values have low influence on lubricity
compared to FFA content because the lubricity characteristics of carboxylic acids are