Properties of Gasoline Fuel

Gasoline or petrol is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most petrol engines or spark-ignited internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of crude oil in petroleum oil refinery enhanced with a variety of additives. The typical gasoline composition percentage of hydrocarbons (% volume) is as follows: 4-8% alkanes; 2-5% alkenes; 25-40% isoalkanes; 3-7% cycloalkanes; l-4% cycloalkenes; and 20-50% total aromatics (0.5-2.5% benzene). Gasoline streams from various plants of the petroleum refinery are blended to produce, petrol of desired specifications.

Gasoline is mainly used as a transportation fuel around the world. Gasoline uses include cars, motorbikes, sport utility vehicles, light trucks, small air crafts, boats, small vehicles, equipment and tools used in the construction industry, electricity generators, etc.

Gasoline is typically a blend of various refinery streams. The most common refinery components in the gasoline pool, depending upon the refinery configurations are as follows;

  • FCC gasoline from the Fluid Catalytic Cracking Unit has good octane and vapor pressure but is often high in sulfur and olefins.
  • Reformate from the Catalytic Naphtha Reformer having high octane and low vapor pressure, but high aromatics and Benzene.
  • Alkylate from the alkylation unit bears good octane and vapor pressure with no aromatics, olefins, or sulfur.
  • Isomerate from the Isomerization Unit is moderately good in octane, with low aromatics, no benzene, and low sulfur, but high vapor pressure.
  • Light naphtha from the Naphtha Hydrotreating Unit- Low octane and high vapor pressure

Following are Euro 5 specifications of gasoline fuel. Also, view the previous blog about “Euro Specifications of Gasoline Fuel”

Euro 5 Specifications of Gasoline

The major physical and chemical properties of gasoline are:

1. Volatility

The evaporation of gasoline fuel in the intake channel and cylinder is affected by the fuel’s volatility. When the fuel’s volatility is low, a suitable air-fuel combination may be formed, but when the fuel’s volatility is high, vapor bubbles can develop in the suction channel and block the flow of fuel as the local temperature increases.

2. Octane Number

Octane number (ON) is defined as the property of fuel to knocking resistance or how well the fuel itself ignites. Naphthene, alcohols, and aromatics, in addition to cyclic molecules, have high octane ratings. The octane rating of gasoline is measured according to a pair of scales that range from 0 to 100. Typical heptane (C7H16) is believed to have an octane rating of 0, whereas isooctane (C8H18) is assigned a value of 100. Gasoline has an octane rating of 80–90.

3. Density

The density of gasoline may be estimated based on the types of hydrocarbons present. The specific mass and density of the fuel decrease with increasing hydrogen content in the molecule. This means that paraffins, naphthenes, and aromatics are listed first in increasing density order. Gasoline has a typical density of = 700-800 kg/m3.

5. Boiling and solidification points

Not well-defined because they are mixtures. (e.g. when heating a previously subcooled sample at constant standard pressure, some 10% of the weight of gasoline is in the vapor state at 126 °C, and some 90% when at 440 K).

6. Composition

Gasoline composition has changed in parallel with the Spark Ignition gasoline engine development. Lead tetraethyl, Pb(C2H5)4, was used as an additive from 1950 to 1995 as an anti-knocking agent. Further, benzene and aromatics are also controlled within limits as per Euro specifications. Gasoline contains C4~C12 hydrocarbons.

7. Viscosity

A fluid’s viscosity is its resistance to being pushed about. The fuel’s viscosity is defined largely by this crucial property. 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. ASTM D-445 is used as the standard for this measurement.

8. Neutralization Number

The neutralization number may be thought of as a barometer for the degree to which oxidation has degraded old oils, while also providing some insight into the acidity or alkalinity of fresh oils.

 9. Energy Density

The specific composition of gasoline results in a high energy density. This high energy density is what makes gasoline such a valuable fuel, as a relatively small volume of fuel can provide a large amount of useful energy. The energy density of gasoline fuel is 34.2 MJ/L.

10. Flash Point 

The flash point of gasoline is the lowest temperature at which a volatile substance evaporates to form an ignitable mixture with air in the presence of an igneous source and continues burning after the trigger source is removed. Diesel fuel’s operation, management, and storage are all greatly aided by this trait. Generally, it is measured by the Penskey-Martens Closed Cup Tester ASTM D-93. Gasoline has a flash point of 40° C at 1 atm and an auto-ignition temperature of ~300°C.

11. Cloud Point 

The cloud point is defined as the temperature at which a cloud or a haze of wax crystals starts to appear in the fuel under test conditions. A clogged fuel system may be the result of these crystals accumulating in filters. It’s helpful for gauging how the fuel oil really performs in the field.

12. Flammability

The flammable range of gasoline is the ratio of the flammable liquid to air that would create a volatile mixture. The flammability range of gasoline is between 1.4 and 7.6%. If the ratio of gasoline to air is less than 1.4%, then the mixture is too thin to burn.

13. Latent Heat of Vaporization 

The latent heat of vaporization is defined as the heat required to change one mole of liquid at its boiling point under standard atmospheric pressure. At 25°C, the latent heat of the vaporization of gasoline is between 350~500 kJ/kg.

14. Air/Fuel

The air-to-fuel ratio defines the amount of air needed to burn a specific fuel. For gasoline fuel, the stoichiometric air-fuel mixture is about 14.7:1 i.e., for every one gram of fuel, 14.7 grams of air is required.

15. Reid Vapor Pressure 

The Reid vapor pressure test is used to determine the front-end volatility of products in the gasoline through heavy reforming naphtha boiling point range. It correlates with the normal butane content of the sample and the RVP of gasoline blends is adjusted by adding or removing normal butane. The Reid Vapor pressure of gasoline at 450oC is 8-15 psi.

16. Carbon Residue 

Carbon residue is a measure of the amount of coke residue left when a hydrocarbon stream is completely distilled. Gasoline is a petroleum-derived product comprising a mixture of liquid aliphatic and aromatic hydrocarbons, ranging between C4 and C12 carbon atoms with a boiling range of 30–225°C.

17. Distillation Profile

Gasoline is a complex mixture of different hydrocarbons with different properties. Consequently, when heated, different components will vaporize at different temperatures. How much gasoline vaporizes at low temperatures (front-end volatility), medium temperatures (mid-range volatility), and high temperatures (tail-end volatility) will affect performance across a wide range of dimensions and is known as the distillation profile of the fuel. Its final boiling point normally ranges from 160~200 °C.

18. Color and Odor

Generally, the paraffinic hydrocarbons possess the mildest odor and the aromatics the strongest odor. The odor level is related to the chemical properties and volatility of the constituents. Odors due to the presence of sulfur compounds or unsaturated constituents are excluded by the specification because they are monitored separately. Normally, naphtha is colorless (water white), but naphtha containing higher amounts of aromatic constituents may be pale yellow. Colors like green, red or yellow are also added to differentiate the different grades of gasoline. 

19. Benzene and Aromatics

Benzene and aromatics in gasoline are strictly controlled due to environmental regulations. As in Euro5 gasoline, Benzene is restricted to less than 1 Volume %  and Aromatics less than 35 vol.%.

20. Copper Corrosion

The copper corrosion test shows the level of corrosion in gasoline. Copper corrosion of gasoline products is maintained within “Class 1” (1a or 1b). Higher than one copper corrosion is not accepted due to a rise in the corrosion ability of gasoline which is normally due to sulfur compounds in the product.

21. Existent Gum

 During storage, some classes of hydrocarbons present in gasoline products, particularly olefins and diolefins, are able to slowly react, at ambient temperatures, with the oxygen in the air. The formed oxidation products are responsible for the formation of gums. The existent gum value is the gum actually present in the fuel and is measured as the weight of residue obtained after controlled evaporation of a standard volume of fuel. The accelerated gum test (ASTM D873, IP 138) is a safeguard of storage stability and predicts the possibility of gum forming during storage and decomposition of the antiknock additive.

22. Oxidation Stability

The oxidation stability of gasoline is tested in the laboratory to find the potential of gum formation in the gasoline product. The gasoline sample is tested in a pure oxygen environment for a specific time (240 minutes as per  D-525).

References

  1. Speight, Handbook of petroleum product analysis. 2015
  2. “Alternative Fuels Data Center: Fuel Properties Comparison.
  3. www.wikipedia.com
  4. www.mckinseyenergyinsights.com

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