General Fuel Specifications: Relative density (specific gravity), Fuel composition, Specific heating value, Flash point, Viscosity, Surface tension and Freezing point

General Fuel Specifications

Different properties of fuels have,

(a) Relative density (specific gravity)

(b) Fuel composition

(c)Specific heating value

(d) Flash point

(e) Viscosity

(f) Surface tension

(g) Freezing point

(a) Relative density (specific gravity)

It is related to the measurement of the ratio of the weight of a given volume of fuel to the weight of the same volume of water, both at 20 degree celsius and 101.325 kPa

For gasoline, the relative density is around 0.72 to 0.78 - which is equivalent to an API (American petroleum institute) range of 65 to 50,

ρ = 700 −800 [kg/m3], for unleaded gasoline this value is higher due to the aromatics

For diesel fuel, ρ = 830 − 950 [kg/m3]

(b) Fuel composition

C and H: carbon content of aromatics is around 89 %, and of paraffins and naphthenes is around 86 %

Benzene: max allowable concentration is specified because it is highly toxic material, the level is 5 %

Sulphur content: HC fuels contain free sulphur, hydrogen sulphide and other sulphur compounds which are objectionable it is a corrosive element that can corrode fuel lines, carburettor and injection pump. It will unite with oxygen to form sulphur dioxide, which in presence of water at low T, forms sulphurous acid.

-It has low ignition T, promote knock in SI engines. limited to approx 250 ppm (50 ppm is aimed for low pollutant emitting vehicles)

Gum deposits: gasoline with unsaturated HCs forms gum in the engine, paraffin, naphthene and aromatic HCs also form some gum – it causes operating difficulties, sticking valves and piston rings, deposits in the manifold etc.

Water: both dissolved and free water can be present in gasoline, free water is undesirable because it can freeze and cause problems. Dissolved water is usually unavoidable during manufacture.

Lead: for leaded and unleaded gasoline max lead content is specified, lead causes pollution and destroys catalytic converters in the exhaust system.

Manganese: used for antiknock in gasoline (MMT), max amount is specified, 0.00025 to 0.03 gMn/L

Oxygenates: oxygenated compounds such as alcohols are used in gasoline to improve octane rating.

In USA gasohol (10% ethanol contains 3.5% oxygen), TBA and methanol up to 3.5% oxygen methanol up to 5% volume, MTBE up to 15% are used. In EC monoalcohols and ethers with atm boiling points lower than the final atm boiling point of gasoline in the standards can be used. Higher concentrations require modifications on the vehicles

- carburator or fuel injection system must be modified to compensate for the oxygen content of the fuel. Blends with 15% methanol can be used.

(c) Specific heating value

-Specific heating value, Hu is a measure of the energy content of the fuel per unit mass (kJ/kg or kcal/kg)

-Gaseous fuels sp heating value is given in terms of energy content per unit volume (kJ/liter or kJ/m3, kcal/m3)

-In IC engines lower heating value is given as the combustion products contain water in vapour form.

For gasoline and diesel fuel

Hu=42000-44000 kJ/kg or Hu=10200-10500 kcal/kg

-Heating value of the combustible air-fuel mixture is a decisive factor for engine performance.

(d) Flash point

-Flash point is the lowest temperature of a sample at which the fuel vapour starts to ignite when in contact with a flame (ignition source).

-Marcusson method – fuel container is slowly heated, while the fuel vapour is in contact with an open flame – T is measured

-For gasoline it is 25 degree celsius, diesel fuel 35 degree celsius and heavy diesel 65 degree celsius

(e) Viscosity

-Viscosity is an important parameter for CI engines, also influences fuel metering orifices since Re is an inverse function of fuel viscosity lower the viscosity, smaller the diameter of the droplets in the spray.

-Below certain limits, low viscosity increases the leaks in the fuel system. It is a strong function of T – must be given at certain T values

at 50 degree celsius, 1.5 – 5.0 Engler or 0.5 to 0.6 centistokes

(f) Surface tension

-Surface tension is a parameter which effects the formation of fuel droplets in sprays

-increasing the surface tension will reduce mass flow and air-fuel ratio in gasoline engines

-lower the value, smaller the droplet diameter

diesel fuel value is in the range of 0.023 – 0.032 N/m

and for gasoline it is 0.019 – 0.023 N/m

(g) Freezing point

-the precipitation of paraffin crystals in winter can lead to clogged filters. It can be prevented by either removing paraffins from the fuel or adding flow improvers (additives).

-Antifreezing properties are determined by its filterability.

-For gasoline freezing point is –65 degree celsius and for diesel fuel –10 degree celsius

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