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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 into cylinder and compressed until it is very hot (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.