High Speed Diesel
HSD (HIGH SPEED DIESEL)
major performance characteristics of Diesel fuel (HSD) somewhat in
order of importance are:
· Cleanliness Carbon
Residue, B.S.W, Sulphur.
· Ignition Quality Cetane
number/ Diesel Index.
· Fluidity and Atomisation Viscosity,
· Volatility Flash
Point and Distillation
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.
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 and piping