Waste Vegetable Oil As A Diesel
Index of Waste Vegetable Oil As A Diesel Replacement Fuel
Properties of Triglycerides as Fuels
Oils and their melting point and Iodine Values
In Car installation
Problems Costs Exhaust emissions and Cost efficiency
Saturated oils and possible improvements
Properties of Triglycerides as Fuels
A large amount of research has gone into examining Diesel’ s dream of using
raw vegetable oils as fuels and when one speaks of growing crops for liquid fuels
it is often assumed that the oil will be used after only basic extraction and
Work has been conducted to examine these oils as fuel replacements or
additives. For example in the late 1970’ s and early 1980’ s, research was
undertaken at Murdoch University (Perth, Australia) into the use of eucalyptus
and other plant oils as a fuel component.  In New Zealand, there are
considerable problems with the disposal of surplus tallow from the processed
meat industry and a large amount of work was conducted in the early 1980’ s on
the use of tallow as a fuel.
Experience has shown that the use of unsaturated triglyceride oils as a fuel may
cause significant problems that can affect the viability of their fuel use. But this
is not always the case and in many circumstances these problems can either be
dealt with or are acceptable to the user.
While power output and tailpipe emissions using plant or animal oils are in most
cases comparable with those when running on petroleum diesel fuel, the main
problem encountered has due to the higher viscosity of the triglyceride oils and
their chemical instability. These can cause difficult starting in cold conditions, the
gumming up of injectors and the coking-up of valves and exhaust.
The viscosity of plant and animal fats and oils varies from hard crystalline solids
to light oils at room temperature. In most cases, these ‘ oils’ or ‘ fats’ are
actually a complex mixture of various fatty acids triglycerides, often with the
various components having widely varying melting points. This may give the oil
or fat a temperature range over which solidification occurs, with the oil gradually
thickening from a thin liquid, through to a thick liquid, then a semi-solid and finally
to a solid.
High melting points or solidification ranges can cause problems in fuel systems
such as partial or complete blockage as the triglyceride thickens and finally
solidifies when the ambient temperature falls.  While this also occurs with
petroleum based diesel, particularly as the temperature falls below about ~ -10 to
-5° C for ‘ summer’ formulations and ~ -20 to -10° C for ‘ winter’ diesels, it is
relatively easy to control during the refining process and is generally not a major
Many vegetable oils and some animal oils are ‘ drying’ or ‘ semi-drying’ and
it is this which makes many oils such as linseed, tung and some fish oils suitable
as the base of paints and other coatings. But it is also this property that further
restricts their use as fuels.
Drying results from the double bonds (and sometimes triple bonds) in the
unsaturated oil molecules being broken by atmospheric oxygen and being
converted to peroxides. Cross-linking at this site can then occur and the oil
irreversibly polymerises into a plastic-like solid.
In the high temperatures commonly found in internal combustion engines, the
process is accelerated and the engine can quickly become gummed-up with the
polymerised oil. With some oils, engine failure can occur in as little as 20 hours.
The traditional measure of the degree of bonds available for this process is given
by the ‘ Iodine Value’ (IV) and can be determined by adding iodine to the fat or
oil. The amount of iodine in grams absorbed per 100 ml of oil is then the IV. The
higher the IV, the more unsaturated (the greater the number of double bonds) the
oil and the higher is the potential for the oil to polymerise.
While some oils have a low IV and are suitable without any further processing
other than extraction and filtering, the majority of vegetable and animal oils have
an IV which may cause problems if used as a neat fuel. Generally speaking, an
IV of less than about 25 is required if the neat oil is to be used for long term
applications in unmodified diesel engines and this limits the types of oil that can
be used as fuel. Table 1 lists various oils and some of their properties.
The IV can be easily reduced by hydrogenation of the oil (reacting the oil with
hydrogen), the hydrogen breaking the double bond and converting the fat or oil
into a more saturated oil which reduces the tendency of the oil to polymerise.
However this process also increases the melting point of the oil and turns the oil
As can be seen from Table 1, only coconut oil has an IV low enough to be used
without any potential problems in an unmodified diesel engine. However, with a
melting point of 25° C, the use of coconut oil in cooler areas would obviously lead
to problems. With IVs of 25 – 50, the effects on engine life are also generally
unaffected if a slightly more active maintenance schedule is maintained such as
more frequent lubricating oil changes and exhaust system decoking.
Triglycerides in the range of IV 50 – 100 may result in decreased engine life,
and in particular to decreased fuel pump and injector life. However these must be
balanced against greatly decreased fuel costs (if using cheap, surplus oil) and it
may be found that even with increased maintenance costs that this is
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