The major problem associated with the use of pure vegetable
oils as fuels for diesel engines is caused by high
fuel viscosity in compression ignition. The vegetable oils
are all highly viscous, with viscosities ranging 10-20
times those of No. 2 Diesel fuel. Amongst vegetable oils
in the context of viscosity, castor oil is in a class by itself,
with a viscosity more than 100 times that of No. 2 Diesel
fuel (MSDS of No. 2 Diesel Fuel − PetroCard). Due to
their high viscosity and low volatility, they do not burn
completely and form deposits in the fuel injector of diesel
engines. Furthermore, acrolein (a highly toxic substance)
is formed through thermal decomposition of
glycerol.
Dilution, micro-emulsification, pyrolysis and transesterification
are the four techniques applied to solve the
problems encountered with the high fuel viscosity.
Amongst the four techniques, chemical conversion of the
oil to its corresponding fatty ester is the most promising
solution to the high viscosity problem. This process -
chemical conversion of the oil to its corresponding fatty
ester, and thus biodiesel − is called transesterification