Diesel fuels :-
Diesel fuels are petroleum fractions that lies between kerosene and lubricating oil. Properties of Diesel fuel are influenced by the crude source and the method of refining.
(a) smooth and efficient Burning :-
To burn well a Diesel fuel should atomise properly, ignite quickly and burn completely. Atomisation is controlled by surface tension and viscosity. Volatility and cetane number are important properties which effect ignition quality of a Diesel fuel. Specific gravity of the fuel is related to the energy content of the fuel and gravity affects the specific as well as volumaric fuel consumption of a Diesel engine.
(b) ignition quality :-
For a Diesel fuel smooth spontaneous ignition at relatively low temperature is essential. Ignition delay is the time period between start the injection to the start of combustion. Ignition delay great influence on the correct optimisation of a Diesel engine. If it is too long the rate of pressure rise, once it starts, can become so rapid that servre Diesel knock and engine damage can occur, if it is too short then there is not sufficient time for the mixing and smoking are produced.
(c) volatility :-
Evaporation and mixing of Diesel fuel with air is essential for ignition and burning. However, unlike spark ignition engine, this vaporisation and mixing takes place inside the engine, the volatility of a Diesel fuel is not as high as that for gasoline fuels.
(d) Cetane number :-
The cetane rating of a Diesel fuel is a measure of its ability to autoignite quickly when it is injected into the compressed and heated air in the engine. Through ignition delay is affected by several engine design parameters such as compression ratio, injection rate, injection time, inlet air temperature etc.
Requirement of Diesel fuel
The main desirable characteristics of Diesel fuel, some what in order of importance
(a) cleanliness :-
Carbon residue, contamination, sulphur etc.
(b) Ignition quality :-
Centane number or diesel index API gravity.
(c) Fluidity :-
Viscosity, pour point etc.
(d) volatility :-
Flash point, carbon residue.
Properties of Diesel fuel
(1) Diesel index :-
It is the method of expressing the quality of Diesel oils. It is the ratio of product of aniline point in farhenheight and API gravity at 60°F to the 100.
(2) Aniline Point :-
It is the lowest temperature at which the oil is completely miscible with the same volume of aniline. For a good Diesel fuel the aniline point is greater than 70°F.
(3) A. P. I. Gravity and specific gravity :-
The specific gravity is the ratio of the weight of a unit volume of oil to the weight of the same volume of water at 15°C.
(4) Flash point :-
Flash point is the temperature at which flammable liquid produce, with a standardized apparatus and procedure, a mixture of its vapour and air will autoignite to give a visible flash.
(5) Fire point :-
It is the temperature at which the flash will sustain itself as a steady Flame. In other word it is the temperature below which oil can be handled without Denger of fire.
(6) viscosity :-
Viscosity of a fuel is a measure of its resistance to flow.
(7) Cloud point :-
The temperature below which the wax content of the petroleum separates out in the form of a solid is called Cloud point.