A Diesel fuel is any
fuel suitable for burning in diesel or compression ignition engines . Petroleum
diesel fuels may be distillates or blends of distillates and residual fuels.
In a compression ignition engine , air alone is drawn in to cylinder and
compressed until it is very hit (about 500 deg C) . At this stage , finely
atomized fuel is injected at a very high pressure, which is ignited by the heat
of compression and hence the term compression ignition (C.I) . A spark ignition
engine on the other hand , relies upon a carburetor to supply into the cylinder a mixture of
gasoline vapour and air, which after compression, is ignited by a spark.
The average compression ratio of a diesel engine is
much higher (about 15:1) than that of a gasoline engine (about 8:1) and this is
the reason for the higher thermal efficiency of the diesel engine (about 33% as
compared to about 25% of the gasoline engine) which makes for economy in
Two main grades of diesel fuel are marketed in India,
High Speed Diesel (HSD) and Light diesel oil (LDO). The former is a 100%
distillate fuel while the latter is a blend of distillate fuel with a small
proportion of residual fuel.
HSD is normally used as a fuel for high speed diesel
engines operating above 750 rpm i.e. buses, lorries, generating sets,
locomotives, pumping sets etc. Gas turbine requiring distillate fuels normally
make use of HSD as fuel. LDO is used for diesel engines, generally of the
stationery type operating below 750 rpm
When fuel is injected into the combustion chamber of a
diesel engine, ignition does not occur immediately. The interval between the
commencement of fuel injection and the commencement of combustion is known as
the " ignition delay" and is a measure of the ignition quality of the
fuel. This delay period depends on the nature of the fuel, the engine design,
and on the operating conditions. If the delay is too long, the engine may be
hard to start and when the accumulated fuel does ignite, the rate of pressure
rise may be so great that it causes roughness or diesel knock. The effects of
diesel knock are similar to the effects of knocking in gasoline engines, viz.
loss of efficiency and power output and a possibility of mechanical damage to
the engine if the knocking is prolonged.
The most accurate method of assessing the ignition
quality of a diesel fuel is by measuring its cetane number in a test engine,
the higher the cetane number the higher the ignition quality. The cetane number
of a fuel is defined as the percentage of cetane, arbitrarily given a cetane
number of 100, in a blend with alphamethyl - naphthaline (cetane number -0 ),
which is equivalent in ignition quality to that of the test fuel.
Defined simply, viscosity means resistance to flow or
movement. In metric system, centistoke is the unit for its measurement. It is
function of time taken in seconds for a given volume of oil to flow through a
calibrated viscometer under specified conditions. Viscosity depends on
temperature and decreases as the temperature increases, so no numerical value
has any meaning unless the temperature is specified.
Different fuels have different tendencies to crack and
leave carbon deposits when heated under similar conditions. This property is
normally measured by the Conradson or the Ramsbottom coke tests. In these
tests, a sample of the fuel is heated without contact with air under specified
conditions and the weight of carbon residue remaining after the test is
expressed as a percentage of the weight of the sample.
As a rule, the higher the viscosity of a liquid
fuel, the lower its volatility. Therefore provided the viscosity lies within
specified limits, a satisfactory volatility is automatically ensured. However,
the percentage recovered at some particular temperature e.g. 366 deg C, is
specified in the case of HSD mainly to control engine fouling due to incomplete
combustion of the higher boiling components.
This is significant because it governs the
amount of sulphur oxides formed during combustion. Water from combustion of
fuel collects on the cylinder walls, whenever the engine operates at low jacket
temperatures. Under such conditions, sulphurous and sulphuric acids are formed,
which attack the cylinder walls and piston rings, promote corrosion, and thus
cause increased engine wear and deposits.Total sulphur is expressed as a percentage of
the weight of the fuel sample.
It is important that diesel fuels shall be free
of these sulphur compounds which in themselves attack metal parts of the engine
or the fuel system. This characteristic is tested by the Copper Strip Corrosion
Test, a severe discoloration or pitting of the polished strip indicating the
presence of corrosive sulphur compound in the fuel.
This should be low in order that corrosion of
metals in contact with the fuel during storage and distribution is minimised.
INORGANIC OR MINERAL ACIDITY
Where diesel fuels are treated with mineral acid
as part of the refining procedure, traces of mineral acid remaining in the
final product would obviously be undesirable. Hence, zero limit is usually
specified for this property.
This is due to the naphthenic type which are
constituents of crude petroleum. Their presence in small amounts is not
necessarily an indication of improper refining or poor quality. Although much
weaker than mineral acids, they may attack galvanised metal and this is why the
use of galvanised containers for the storage of diesel fuels is not recommended.
Ash is a measure of the incombustible material
present in a fuel and is expressed as a percentage of the weight of the fuel
sample. In the case of distillate fuels, it usually consists of rust, tank
scale or sand which settles out readily. Blends of distillate and residual
fuel, e.g. LDO may additionally contain metal oxide derived from oil soluble
and insoluble metallic compounds. Ash is significant because it can give rise
to deposit problems such as abrasion, malfunctioning of injectors and high
temperature corrosion, particularly with residual fuels.
SEDIMENT AND WATER
These are absolutely undesirable contaminants
and should be as low as possible. The higher the specific gravity and viscosity
of a fuel, the greater the quantities of water and sediment it can hold in
suspension. Large quantities of sediment can affect the combustion of the fuel,
and if abrasive, may cause excessive wear of closely fitting parts of fuel
pumps and injectors. It may also clog filters and build up deposits in tanks
The pour point of a fuel is the lowest temperature at
which it will pour or flow when chilled under prescribed conditions. It is a
very rough indication of the lowest temperature at which a given fuel can be
readily pumped. However, since practical conditions are quite different from
those under which the laboratory test is conducted, many fuels can be pumped at
temperatures well below their laboratory pour point.
Sometime cloud point is measured. This is the
temperature at which paraffin wax begins to crystallise or separate from
solution when the fuel is chilled under prescribed conditions. This may settle
out in the fuel system and cause blockage of filters - leading to
malfunctioning or stalling of the engine.
COLD FILTER - PLUGGING POINT
The cold filter plugging point (CFPP) is defined as
the highest temperature at which the fuel, when cooled under prescribed
conditions, either will not flow through the filter (45 microns) or will
require more than 60 seconds for 20 ml to pass through. This is the temperature
at which wax crystals begin to cause blockage of filter.
This has no bearing on performance but is important
largely from the point of view of safety in handling the fuel and minimum
values are usually specified in the specification.
The flash point of High speed diesel is stipulated as
min. 32 deg C and thus it falls under the category of class 'B' of
petroleum products. While other diesel fuels have a flash point of min. 66 deg.
C and hence fall in the category of class 'C' of petroleum products.
This is defined as the ratio of the weight of a given
volume of oil to the weight of the same volume of water at a given temperature.
Another index for measuring this characteristic is by Density, mass per unit
volume at a standard temperature. Specific Gravity / Density is of limited
usefulness as a direct measure of diesel fuel quality. However, it provides a
convenient means of controlling product uniformity in refinery operations and
of converting volume to weight. Variations in specific gravity affect the
volumetric fuel consumption of an engine, since the higher the specific gravity
the higher the heat content in a unit volume of the fuel. However this is not
significant in use unless the variations are very large.
|SPECIFICATION FOR AUTOMOTIVE DIESEL FUEL (BS VI) (IS 1460-2005)
SPECIFICATION FOR LIGHT DIESEL OIL
|Characteristics||Unit||Bharat Stage II||Bharat Stage III||Bharat Stage IV||Bharat Stage VI†|
|Implementation date||2001 (selected cities), 2005 (nationwide)||2005 (selected cities), 2010 (nationwide)||2010 (selected cities), 2017 (nationwide)||2020?(nationwide)|
|Ash, max|| % mass||0.01||0.01||0.01||0.01|
|Carbon Residue (Ramsbottom) on 10% residue, max †|| % mass||0.3||0.3||0.3||0.3|
|Cetane Number (CN), min||–||48*||51||51||51|
|Cetane Index (CI), min||–||46*||46||46||46|
|Distillation 95% vol. Recovery at °C, max||°C||–||360||360||370|
|Flash point Abel, min||°C||35||35||35||35|
|Kinematic Viscosity @ 40 °C||cst||2.0-5.0||2.0-5.0||2.0-4.5||2-4.5|
|Density @ 15 °C||Kg/m3||820-860 (820-870)*||820-845||820-845||820-860|
|Total Sulfur, max||mg/kg||500||350||50||10|
|Water content, max||mg/kg||0.05% vol||200||200||200|
|Cold filter plugging point (CFPP)|
a) Summer, max
b) Winter, max
|Total contaminations, max||mg/kg||–||24||24||24|
|Oxidation stability, max||g/mg3||–||25||25||25|
|Polycylic Aromatic Hydrocarbon (PAH), max|| % mass||–||11||11||11|
|Lubricity, corrected wear scar diameter (wsd 1,4) @ 60 °C, max||µm (microns)||460||460||460||460|
|Copper Strip corrosion for 3 hrs |
@ 50 °C
|Rating||Not worse than No. 1||Class I||Class I||Class I|
BIS Standard for LDO
LDO of oil Marketing companies
Calorific Value, in Kcal/kg
9900 - 10500
Ash% Mass, Max
Carbon Residue (Ramsbottom) on 10% residue % by Mass, Max
Density in Kg/M3
To be reported
Kinematic Viscosity @ 40 Deg.C. in cst
Polluting Non renewable
Total Sulphur % by mass. Max
Water content, % by volume, Max
Flash Point in Deg.C Min