:: Fuels - Automotive Fuels --- ID :: 10151


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 operation.


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.


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.


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 and piping.


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.


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.


1 Acidity (inorganic) MGKOH/GM Max NIL IS 1448, P:2 NIL

2 Acidity, total mg KOH /gm To report Is 1448, P:2 0.05

3 Ash percentage by mass, max 0.01 IS 1448,P:4/ISO 6245 0.01

4 Carbon residue (Ramsbottom) on 10% Residue, percent by mass,max 0.3 IS 1448, P:8/ISO10370 0.10

5 Cetane Number, Min Or 51 IS 1448, P:9/ISO 5165

Cetane Index, Min 46 ASTM D 4737/ISO 4264 54

6 Pour Point , 7C, max 37 C , for Winter IS 1448, p: 10/D5949 Or D5985 0

157C, for summer +9

7 Copper Strip corrosion for 3 hours @100 deg.c Not worse than No.1 IS 1448, P15/ISO 2160 No.1

8 Distillation,% V/V, Recovered

At 360 7C, Min 95.0 IS 1448, P:18/ISO 3405 96

9 Flash Point ( Abel) deg. C. Min 35 IS 1448, P:20 39

10 Kinematic Viscosity, cst @ 407C 2.0 to 4.5 IS 1448, P:25/ISO 3104 3.5

11 Total contamination, mg/m3, max 24 EN12662 0.9

12 Density @ 15 Deg. C kg/m3 820-845 Is 1448,P:16 or P:32/D 4052 /ISO 3675 OR ISO 12185 0.830

13 Total Sulphur, mg/kg,max 350 IP 336 OR D 4294/ISO 14596 OR ISO 8754/P:83/D 2784/D 5433/D2622/D 3120 320