Download Diesel Fuels

Dr. Fatma Ashour
The term ‘diesel fuels’ comprises
•
Gas Oil’ (Solar)
o
Diesel Oil
Both fractions are heavy distillates obtained
from crude oil.
 The main difference between gas oil and
diesel oil is that the former, is completely a
distillate fraction while the latter may include
some residual oils.

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The usual boiling range of gas oil varies from
210 to 350C.
In the case of diesel oil, the initial boiling
point is higher than that, and the final boiling
point cannot be reached under atmospheric
pressure.
In general, gas oil has a lighter color and a
lower specific gravity (below 0.85) than diesel
oil.
Both gas oil
and diesel oil
are used to
operate ‘diesel
engines’ which
are
I.C.E.
without spark
ignition.
The diesel engine cycle consists of the following strokes:
Air Suction stroke: in which air is
sucked inside the cylinder.
Air Compression stroke: in which
the air is compressed adiabatically
and its temperature therefore rises
& the fuel is then injected. After the
fuel has burned, the expansion
proceeds adiabatically, driving the
piston back.
It is important to note that the fuel
is injected at a high pressure (after
the compression stroke) in the form
of fine droplets by means of an
‘atomizer’ (or injector).
The compression ratio in diesel engines is
much higher than that in gasoline engines.
Since the fuel is not compressed with the air
but injected after the compression, this could
be done and gives a higher efficiency than
lower compression ratios.
 Air is compressed with a compression ratio
typically between 15:1 and 22:1 resulting in
40-bar (4.0 MPa; 580 psi) pressure compared
to 8 to 14 bars (0.80 to 1.4 MPa) (about 200
psi) in the petrol engine.
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The performance of diesel fuels obviously
differs from that of gasoline.
In the case of diesel fuels, it is essential that
the fuel ignites easily-from the heat of
compression of air-without using any spark.
Good diesel fuel will be composed mainly of
Paraffins, with preferably little or no branched
hydrocarbons and aromatics.

The performance or ignition quality of a diesel
fuel is measured by one of the following
methods:
A- Measuring the Angle of Delay


It is defined as ‘the angle travelled by the crank
during the period between the fuel injection to
the start of combustion’ of the engine.
Low grade diesel fuel will require a longer period
of time between the injection of the fuel and the
starting of ignition (i.e.: a big Angle of Delay).
B- Diesel knock
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In diesel engines, the injection of the fuel and its
ignition as well as the expansion of the hot
combustion gases, all take place in one stroke the power stroke.
If the period between the injection of the fuel
and its ignition is long (low grade diesel fuel) this
leaves a relatively short period for the ignition
and the expansion.
Consequently, the power is generated over this
small period of time. The impulsive force acting
on the piston will be so high resulting in what is
known as the ‘diesel knock’
(i.e.: explosive ignition resulting in non uniform
pressure distribution on the piston).
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The performance or ignition quality of a diesel
fuel is measured by the ‘Cetane Number’. Two
reference fuels compounds are used for the
determination of the octane number, these are;
n-cetane (n-hexadecane, n-C16 H34) which is
normal paraffin with a long chain, and therefore
expected to ignite easily. It is given an arbitrary
cetane number of 100.
The -methyl naphthalene which is a ring
structure having a side-chain and therefore
expected to resist spontaneous ignition (i.e. poor
diesel fuel constituent). It is therefore given an
arbitrary cetane number of Zero.
A standard C.F.R. engine is used for that test

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The engine is first operated using the fuel under
test, then with mixtures of n-cetane and methyl naphthalene. The composition of the
latter
mixtures
(n-cetane
and
-methyl
naphthalene) is varied until a performance similar
to that obtained by the fuel tested obtained, as
measured by the ‘angle of delay’ on the ‘card
diagram’.
The cetane number may be defined as “the
volume percent of n-cetane in a mixture of ncetane and  -methyl naphthalene which gives
the same performance (or ignition quality) as the
diesel fuel sample under test”.

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The performance characteristics of diesel fuels
may be improved by the addition of certain
additives, such as alkyl nitrites, nitrates, nitroand nitroso-compounds as well as peroxides.
It is important to note that, owing to the
difficulty of measuring the cetane numberthrough the angle of delay-several substitute
characteristics have been suggested, the most
important of which are:
1. Aniline Point
2. Diesel Index


This is defined as the “minimum temperature at
which two equal volumes of the sample and
aniline are completely miscible with one another,
this simplified method gives an indication about
the extent of “aromaticity of the fuel”.
Accordingly, a highly paraffinic fuel will have a
high aniline point, since there is a great
difference between the molecular structure of
aniline (an aromatic) and a paraffin. The opposite
is true for high aromatic contents, since an
aromatic is more easily soluble in another
aromatic than is a paraffin.
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It has been observed that hydrocarbons of
low specific gravities have lower aniline
points than hydrocarbons with higher specific
gravities.Thus the specific gravity as well as
“the paraffinicity of the fuel” affect the aniline
point.
when the aniline point of a diesel fuel (in F)
is multiplied by its (A.P.I) gravity/l00), which
is inversely proportional to the specific
gravity, a value representing the real
paraffinicity of the fuel is obtained.
D.I. = Aniline Point(˚F )x ˚API
100
It has been found that, the diesel index of a
diesel fuel is approximately equal to its
cetane number.
 The “diesel index” is sometimes used in the
specifications of diesel fuels instead of the
cetane number.
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Diesel engines may be classified into the following 3
groups according to their speeds:
High speed engine: (speeds above 800 r.p.m). A high
cetane number diesel fuel is necessary to run such an
engine. The fuel is usually specified to have a minimum
cetane number of 52 which could only be obtained by
using gas oils.
Intermediate speed engine: (speeds between 300 and 800
r.p.m.) The minimum cetane number required for such
engines is about 40. Such fuels used may be gas oil or
diesel oil.
Slow speed engines: (speeds below 300 r.p.m). No limit is
specified for the cetane number of the fuels for such
engines (usually diesel oil is used).
N.B. The lowest cetane number of diesel fuels is in the
range of 20-25.

Carbon Residue

Sulphur Content, a maximum of 1.5- 2 %

Pour Point, a maximum of –10 C
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Viscosity, a minimum of 30 secs Red. I at 100 F is
specified for gas oil and a maximum of 45 secs. for
gas oil and 60 secs for diesel oil.
Flash point, above 150 F - using the “PenskyMartens” closed cup tester for safety purposes.
However, this limit may be reduced to 130 F (at least
in countries like Egypt).
Fats
Oils
Soybean
Animal Fat
Edible Oils
Rapeseed
Non-edible
Oils
Jatropha
Waste
Vegetable Oil
WVO
Reaction
Mixing
Separator
Washing
1.
Fossil fuels contribute to global warming by
increasing GHG emissions
Fossils
Biofuels
Biofuels reduce GHG emissions (closed carbon
dioxide cycle)
2.
Energy Security:

Decrease dependency on crude oil

Decrease imports of crude oil

Energy diversification
3.
Renewable
4.
Less toxic
5.
Sustainable use of otherwise problematic waste
products
6.
Economic Benefits





Economic use of wasteland
Useful co-products
Carbon Credit Opportunities
Additional distribution channel for agricultural
products
Decentralized production and thus strengthen rural
economies

The properties of biodiesel (viscosity, ignition properties)
are similar to those of fossil diesel.

Cetane number and lubricating effect of biodiesel are
significantly higher than fossil diesel.

The toxicity of biodiesel is lower compared to fossil diesel.

Biodiesel can be blended with petrodiesel at any ratio.

Bioethanol Production Africa 2006: 606,000 liters
= 1.2% global production (US + Brazil: 75% global
production)