Use of palm oil as fuel for diesel engines.
Summary of the end of Study Report of Barthélemy De Theux
The first objective of this document is the study of the use of palm oil as fuel for diesel engines. This study is aimed for use African countries. The principal advantage lies in the low cost this oil in comparison to diesel oil and its easy availability.
However, the use of this oil raises some problems due to the difference in the physical and chemical properties between palm and diesel oil.
They are five differences:
- The viscosity
- The flash point
- The polymerization
- The chemical reactions
- The physicochemical reactions
2. Viscosity
Palm oil has a viscosity similar to that of colza oil, namely around 70
mm²/s (at 20C) and 40 mm²/s (at 40C).
To allow its use in a diesel engine, it should have a viscosity of about 4,5 to 5mm²/s. For that purpose, three methods are possible, either to heat it, to use a mixture of diesel and palm oil, or to make it undergo an esterification reaction.
In practice, we chose the first possibility because of its simplicity of implementation.
The palm oil - diesel mixture has the disadvantage of using diesel oil continuously, which is not what we set out to achieve.
As for the esterification reaction, the disadvantage is the cost of the process which increases the cost price of the fuel. This chemical reaction improves the viscosity of the oil
3. The flash point
The flash point corresponds to the temperature at which a fluid emits vapour that can ignite in contact with a flame or of a spark. For the diesel oil, it lies in the neighbourhood of 90C while for palm oil it is in the vicinity of 280-300C.
This property strongly influences the combustion: palm oil will ignite less easily than the diesel oil. In order to solve this problem, it can be shown that it is sufficient to increase the temperature of the end of the compression of the engine cycle by around 70 degrees. Several methods are possible to achieve this:
The first consists in acting on the parameters of the ignition advance.
This is a simple and cheap method: if one delays the ignition advance,
the compression cycle is longer, resulting in a rise in temperature at
the end of the compression cycle. The temperature can thus easily be increased
by 60C, but this will also produce a slight loss of output of the engine.
One can also notice that the slower the engine runs and the higher its
load, the hotter are the exhaust fumes. This is the result of the increased
internal temperature of the engine. It is therefore advantageous to run
the engine at a low speed but at full load in order to obtain the highest
possible temperature in the combustion chamber.
Thanks to various tests carried out at AAN in Germany, we can notice that in the case of an indirect fuel injection engine, a delay of the ignition advance of 5 to 8°V compared to the value for the diesel engine would be sufficient to obtain the necessary temperature with a good combustion.
In the case of a direct injection, an advance of only 2 to 3°V is to be considered. However, that is not enough to sufficiently increase the temperature of the oil. This type of engine is very often used in agriculture and in the industry. This is why we considered a method frequently used in Germany that further increases the temperature at the end of the compression cycle of the engine.
The engineers at AAN developed a piston in a material having particular thermal properties, the "Ferroone". This material makes it possible to keep a sufficiently high temperature to guarantee an optimal combustion, also at low speed. It is a very interesting method but rather expensive since it is necessary to completely change the dimensions of the piston, which is not economical when producing on a small scale.
Another method to obtain a suitable temperature is the re-injection of a certain proportion of burned gases: this method makes it possible to increase the temperature of the intake air causing an increase in temperature at the end of the compression. This is an applicable method thanks to the fact that the engine in partial load absorbs a significant excess of air that can be partly replaced by hot exhaust gases.
These are all methods that vary a little from one engine to another,
so that well defined rules and proportions cannot be given. This is why
engine tests will have to be carried out in order to obtain satisfying
results. The adjustment will be performed in a simple manner; this is
with a minimum of black fumes, no smell of oil and a good running sound.
4. Polymerization
A vegetable oil, containing double bonds, is rather sensitive to polymerization:
palm oil has the tendency, in the presence of oxygen acting as a trigger,
to add its double bonds thereby forming longer hydrocarbon chains. The
number of double bonds present in the oil varies strongly with its composition
which in turn varies according to the varieties of palm trees.
One can classify the various vegetable oils according to the percentage
of saturated bonds, mono-unsaturated, bi-unsaturated and tri-unsaturated.
Palm oil is situated in the part favourable from the point of view of
the saturated connections. It will therefore have a tendency to polymerize
less that the major part of other oils. This confers to palm oil an extra
advantage in its usage as fuel.
The most important effects of polymerization of oil can be found at the
injector. A phenomenon called cocking is produced there: it concerns the
formation of deposits around the nozzle of the injectors, when using a
natural vegetable oil as fuel, because the temperature of the injector
nozzles is lower than the flash point. This deposit has of course an influence
on the jet and, with its appearance, "trumpets" can be formed
that deviate the jet and destroy the quality of combustion. A trumpet
is a ball of deposit which appears during an incomplete combustion, due
to a partial plugging of the injector, with formation of free carbon and
its deposit around the cylinder head. Moreover, this formation is increased
when the applied load is small, due to a lower temperature existing in
the combustion chamber in this condition. It is therefore recommended
to work under full load.
To counter this inconvenience produced by polymerization, various tests were carried out. These experiments showed that the injector with only one nozzle, was best adapted for this kind of application. Moreover, research led to defining an ideal gauge of the injector in the neighbourhood of 0,2 to 0,3 mm², thereby avoiding the formation of a "trumpet" due to the phenomenon of "cocking".
It should be mentioned that a rinse with diesel oil .prevents the formation
of a too thick deposit. Hence the advantage of starting and stopping the
motor with diesel oil
5. Chemical Reactions
The main problem lies in the acidity of the palm oil and the basic character of the lubrication oil: there is inevitably a small quantity of fuel in the engine which mixes with the lubricating oil contained in the crankcase.
Most of the vegetable oil which passes through the segments, comes to
mix with the lubricating oil, and does this when the starting the cold
engine with vegetable oil: at this time, the segments on the piston do
produce sufficient sealing to avoid the passage of vegetable oil towards
the crankcase . Moreover, at the time of starting of the engine, combustion
is not really at its most efficient due to the fact that the temperature
of the combustion chamber is still at room temperature. Another significant
point is that during start-up, far too much fuel is injected and on top
of that, the gearbox is in neutral position. This excessive quantity reaches
sometimes even ten times the level required for a balanced chemical combustion
As stated previously, lubricating oil is chemically basic in the beginning.
When this basic oil mixes with another oil that is acid, in this case
the palm oil fuel, a chemical reaction occurs with the irreversible formation
of a kind of "soap", sometimes referred to as "mayonnaise".
This chemical reaction, rather strong and fast, grows exponentially and
occurs both during the operation of the engine, and when the engine is
at rest. This speed of this reaction increases with the temperature.
When this happens, the oil loses all its lubricating properties, its viscosity
increases and its basic character strongly decreases. Experiments show
that after approximately 300 operating hours the properties of oil start
to deteriorate. The percentage of fuel oil then reaches 1 to 2% of the
lubricating oil. It is therefore necessary to replace the oil at this
time. One could run the engine up to 400 hours, but at that stage, the
percentage of fuel oil reaches 4 to 5%of the lubrication oil and the viscosity
of the engine oil reaches 100 mm²/s at 20C, and the basic value has
lost 50%.
In order to measure the viscosity, we generally have the adequate equipment
in Europe, but this is not often the case in Africa. To solve this problem,
a small and very simple system can be imagined to determine the time at
which it is essential to drain the engine.
It is quite simple enough to have an inclined plan, provided with two
channels, in a material neither too rough nor too smooth. The method consists
in comparing, simultaneously and one next to the other, the flow of the
used engine oil with fresh lubrication oil. If the used oil runs slightly
faster than the new oil, it is still usable. That means that there is
fuel oil present in the engine oil, because in most cases the viscosity
of lubrication oil is lower than that of the engine oil, but the lubrication
properties are still present. On the other hand, if the used engine oil
runs much more slowly than the fresh oil, it means that the chemical reaction
is taking place and that the lubrication of the engine is not ensured
any more.
The first remedy for this state of affairs is wholly a question of discipline! When the oil has lost its necessary lubricating properties it is mandatory to replace it to avoid the risk of encountering many technical problems.
An additional remedy is to start the engine with the diesel oil so as to avoid an excessive passage of palm oil in the lubrication oil.
By making a rapid calculation, one can find that when one starts the engine with the diesel oil, the lubricating oil will lose its properties after approximately 1800 operating hours. That is to say approximately six times longer than when starting directly with palm oil. This constitutes an undeniable advantage.
The engineers of AAN have performed a lot of research on lubricating oil. Tests are being performed on the addition of surface-active agents to the oil in order to allow the introduction of the acid in the chemically basic oil. They are also considering the production of acid oil in order to avoid the chemical reactions between the fuel and lubricating oils. We suppose that this lubricating oil will probably be more expensive, considering its innovative character and its quite specific usage.
Another solution under consideration by AAN, is the use of the palm oil as both lubricant and fuel. They concluded that palm oil could be used as lubricant at the condition of not exceeding 20 to 40 operating hours. They thus managed to conceive a rather complex system in which vegetable oil is used initially as an engine lubricant to then pass in a heat exchanger after having been in contact with the hot parts of the engine and thereby reaching 100-110C. In this heat exchanger, the oil, that was used for lubrication, heats the entering fresh oil, and is therefore cooled at a temperature around 75-80C, before passing in the injection pump. In theory, the oil is used 250 times in the cycle as a lubricant and once as fuel.
The above summarizes the process, leaving aside the various circulating pumps that have to be installed, the oil levels, the temperature sensors…
This method of operation of the engine is without doubt the most favourable. It solves a great many problems. Admittedly, it requires important adaptations to the engine, hence a high cost. This method proves to be interesting for high power engines, starting with vegetable oil, and functioning with rather varied loads. For the smallest loads, it would be preferable to start with the diesel and to keep a strict follow-up of the properties of the lubrication oil.
6 The physico-chemical reactions
This concerns, in fact, various phenomena which are the result of a stagnation period of the vegetable oil. It is a phenomenon of decomposition of the Triglyceride (vegetable oil) in Glycerine and free fatty Acids
A vegetable oil can also give rise to a phenomenon of rancidity which
is always complex and is caused by enzymes or oxidation. Unsaturated oils
can, under the action of oxidants, transform into aldehydes which are
the cause of the rancid taste.
The consequences of these reactions are that oil which has remained at
rest for too long a period, could not longer be used as engine fuel.
Initially, this oil becomes very acid and will attack the copper and chromium
elements of the engine. This will result in a chemical corrosion which
will inevitably deteriorate these elements.
After that, this oil that contains many free fatty acids and aldehydes,
will give rise to crossings between the molecules resulting the formation
of "jelly" or "butter" that will completely block
the injection system.
Ideally one should use vegetable oil that has just been pressed, that
has undergone a reheating of the shortest possible duration (but sufficiently
to eliminate the greatest part of its water content) and has been submitted
to a fast heating to bring it to a sufficient viscosity.
The majority of the tests quoted in this work was carried out with colza oil. One can however apply, without problem, all these conclusions to palm oil thanks to the similarities between these two oils. It would obviously be very interesting to carry out these various experiments with palm oil, as often as necessary in order to be able to optimize the performance of the engine.