Properties of Diesel Fuel

Diesel is defined as a fuel comprises of complex hydrocarbons containing carbon atoms from C12 – C20. It mainly includes aromatics, paraffins, and naphthenes. The boiling range of diesel fuel is 70 – 360 °C [1]. Diesel fuel is a major fraction of the refinery products obtained from the crude oil obtained via fractional distillation. Diesel fuel, also called diesel oil, is liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and then injection of fuel. Therefore, diesel fuel needs good compression ignition characteristics.[5] . The chemical composition of diesel fuel is quite complex with a huge number of hydrocarbons in it. Euro specifications of diesel fuel are as follows;

Euro Specifications of Diesel Fuel

The major physical properties of diesel fuel are:

1. Density

The presence of different classes of hydrocarbons helps to estimate the density of diesel fuel. In diesel fuel, the density increases with the increase in the number of carbon atoms present in it. Thus, the increasing order of density begins with paraffins, naphthenes, and aromatics. Typically, the density of diesel fuel ranges from 0.820 ~ 0.845 g/ml.

2. Distillation Curve

The most significant characteristic of diesel fuel is the distillation curve representing the mixture of hundreds of hydrocarbons in the fuel having different boiling points.  The distillation curve is drawn between the temperature and volume percentage of the diesel fuel. The higher the distillation point, there be heavier the components in the diesel fuel and as a result poor combustion in the engine. The 95 % distillation point in Euro 5 diesel is 360 °C.

3. Ignition Quality

It is defined as the characteristic parameter expressing the quality of the fuel. In diesel fuel, the Cetane number or Cetane index is used for this purpose. The cetane number of  Euro 5 diesel fuel is maintained at 51.

4. Cetane Number

The Cetane number is defined as the performance meter of the diesel fuel. The higher number represents the better performance of the diesel. It depends on the type of hydrocarbons present in diesel fuel. In paraffins, the Cetane number increases with the increase in carbon number. The Cetane number of naphthenes ranges from 40~70 while the aromatics have 0 ~ 60. Generally, the Cetane number of diesel fuel is estimated via a standard testing method of Cetane number for diesel fuel oil ASTM D-613.

5. Appearance and Odor

The first and foremost characteristic property of diesel fuel oil is an appearance that is a strong indication of the contamination test in it. Thus, the visual inspection of the diesel fuel is a requirement before delivery. Diesel is a colorless liquid and any dark color shows the presence of impurities or the high distillation of the diesel fuel. The odor of the diesel fuel should be acceptable i.e. no unpleasant odor, thus, the presence of a pleasant odor in the fuel represents the indication of mercaptans in the fuel oil.

6. Neutralization Number

It is a measure of the tendency of a fuel to corrode metals. It is a measure of the total acidity and inorganic present in the fuel oil. The general method of measurement is the Corrosivity test ASTM D-130.

7. Cetane Index

It is defined as the estimation technique to approximate the Cetane number for any fuel i.e. it utilizes the specific engine for approximation of the Cetane number for fuel oil. Certain standard techniques are utilized for the measurement of the Cetane index:

  • Estimation of Cetane index of distillate fuels ASTM D-976
  • Estimation of Cetane index via four variable equation ASTM D-4737
8. Viscosity

Viscosity is defined as the resistive force to the flow of fluid. It is the critical characteristic defining the fluidity of fuel. It is related much to the carbon number present in the fuel and did not get much affected by the type of hydrocarbons present in the fuel. It is measured by the standard method of ASTM D-445.

9. Flash Point

A Flash point is defined as the minimum temperature point where any fuel generates an ignitable mixture of vapors. This characteristic property plays a key role in the operation of diesel fuel along with its handling and storage. It also assists in the determination of contamination in the fuel oil. Generally, it is measured by the Penskey-Martens Closed Cup Tester ASTM D-93. The minimum flash point for Euro-5 diesel is 55 °C.

10. Cloud Point

Cloud point is defined as the temperature at which wax gets separated from the oil when it is subjected to the lowest temperature. It assists in measuring the practical performance of the fuel oil. Generally, measured by the standard method of cloud point of petroleum products ASTM D-2500.

11. Pour Point

The temperature at which the fuel oil has the characteristic ability to flow when cooled at standard conditions of cooling. It is measured by the test method of ASTM D-97.

12. Lubricity

The smoothness of oil is defined as lubricity which is a significant characteristic property of diesel fuel defining its performance in the engines. The fuel with poor lubrication will destroy the engine. In general, the lubricity is estimated via two methods:

  • SLBOCLE (Scuffing load ball on cylinder lubricity evaluator method)
  • HFRR (High-frequency reciprocating rig method)
13. Sulfur

Sulfur contents in fuel are highly dependent on the origin of crude oil. It is a contamination to fuel which is reduced via the hydro-treatment of diesel fuel. It is a regulation to use low sulfur content fuel or almost zero sulfur fuel in automobiles for better engine performance. The maximum sulfur content in Euro-5 diesel is maintained at less than 10 ppm and the Euro-5 diesel is called Ultra-Low Sulfur Diesel. The standard test methods for sulfur content estimation are:

  • Sulfur in petroleum product by X-Ray Spectrometry ASTM D-2622
  • Sulfur in petroleum products by Energy Dispersive X-Ray Fluorescence Spectroscopy
  • Sulfur in light hydrocarbons and fuel oils by Ultraviolet Fluorescence ASTM D-5453
14. Carbon Residue

It is defined as the burnt carbon contents deposit via fuel burning or the decomposed carbonaceous material of diesel fuel oil, termed as carbon residue. It is estimated via Rams bottom Carbon Residue of Petroleum products ASTM D-524 method.

15. Heating Value

The total amount of heat energy released by the combustion of diesel fuel is defined as its heating value. It is generally expressed in terms of gross or net calorific value. It is highly dependent on the amount of water present in the exhaust.

16. Wear Scar or lubricity

It is the ability to reduce friction between moving parts (fuel pumps and injectors) of a diesel engine. There must be a minimum level of lubricity in the fuel to avoid excessive wear and to save the engines from being damaged.  A lubricity additive has to inject into diesel to control the lubricity.

  1. “Diesel and gasoline,” Advanced Motor Fuels
  2. E. L. K. A. K. G. E. Lois, “Fuels,” in Encyclopedia of Physical Science and Technology,  Maryland,USA,   ACADEMIC PRESS, 2003, pp. 275-314.
  3. “Cetane number,” Energy Education
  4. J. G. Speight, Handbook of Petroleum Product Analysis, 2nd ed

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