The reason a diesel engine can not run on straight vegetable oil (SVO) is that the oil is
too thick to run through the fuel lines and filters of the engine at the
required rate, at ambient temperature . The design of the engine, especially
the fuel system and combustion chamber, and the type of vegetable oil used will
dictate at what ambient temperature the engine will start.
Even if the fuel system can pump the
thicker oil there is risk of damage to the engine. SVO will not be injected
into the cylinders as effectively as diesel fuel, the injector will not provide
as fine a fuel mist with this less viscous oil. A mist of larger droplets will
not burn as effectively causing difficulty with starting and incomplete
combustion which will lead to build ups of carbon and vegetable oil residue. As
deposits build in the engine they will cause the engine to run poorly because
of low compression due to sticking piston rings or coked valves and poor
injection spray pattern from build ups on the injector nozzle. A bad spray
pattern will produce different combustion activities with a greater amount of
incomplete combustion and possible piston damage due to increased heat on the
piston surfaces, this can end in a holed piston.
Engine carbon build ups can be a problem
with diesel fuelled engines which have not been run hard/hot enough. It is
advisable to work diesel engines hard fairly regularly to burn this carbon from
the engine, more so when running on vegetable oil. A standing start to high
speed pedal to the metal run or a very long uphill haul has been shown to help.
Extended periods at low engine load or tick over are best avoided when
possible. Combustion is less complete from when the engine is first started
until it reaches operating temperature as the piston rings do not seal as
efficiently until the various engine components have expanded with the
combustion heat. It is good practice to drive fairly hard upon first starting
the vehicle to bring it up to operating temperature as soon as possible.
The greater effort required to pull the
vegetable oil through fuel lines and filters can damage the fuel injector pump
or the extra force required to move this oil through the pump causes extra
stress on the timing belt/chain which could give out prematurely, causing
costly engine damage.
Some engines have been found to be able
to operate reliably under given conditions without modification when fuelled
with certain oils.
a.Engine Modification
Engines have been designed to overcome
potential problems and run on vegetable oil. Heating the fuel system allows the
SVO to flow and an improved injector and combustion chamber design give better
combustion.
b.Fuel Modification
Another solution is to thin the
vegetable oil so that it behaves in a similar way to diesel fuel and can be
used in existing diesel engines. There are a number of options below.
-Biodiesel
Biodiesel is a fuel made through
transesterification. About 80% vegetable oil is mixed with 20% alcohol and a
catalyst. A chemical reaction occurs transesterification) and you are left with
biodiesel and glycerin which separate out when allowed to settle. Biodiesel in
operation is comparable to fossil diesel in most ways.
-Micro Emulsions and Fuel Blends
Vegetable oil can be mixed with
alcohols, diesel fuel, petrol, surfactants, cetane improvers, water and
solvents in varying mixes and proportions to form a micro emulsion or a blend
with suitable operating qualities
-Heating the Oil
Above 70 C vegetable oils have a
viscosity approaching that of diesel fuel. The design of an engines fuel system
and combustion chamber will dictate at what temperature vegetable oil can be
reliably run as a fuel.
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