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Page 5e

Other Important Diesel Fuel Properties

Density
  • Specific gravity - The density of petroleum products is usually expressed as a specific gravity. The specific gravity is defined as the ratio of the mass of a volume of the fuel to the mass of the same volume of water. It is dependent on the temperature of both the fuel and the water. It will commonly be expressed as "sp gr @ 60oF/60oF" which means that both the fuel and water were at 60oF.
  • API - The API gravity is a widely used measure of a fuel's density. It is related to the specific gravity of the fuel by the following equation:
API = 141.5/[sp gr @ 60oF/60oF] - 131.5
Viscosity
Viscosity is a measure of a fluid’s resistance to flow. The greater the viscosity, the less readily the liquid flows. The viscosity of petroleum oils is a strong function of temperature with the viscosity decreasing as the temperature increases. ASTM D445 is a standard test procedure for determining the kinematic viscosity of liquids. It provides a measure of the time required for a volume of liquid to flow under gravity through a calibrated glass capillary tube. The kinematic viscosity is then equal to the product of this time and a calibration constant for the tube. The dynamic viscosity can be obtained by multiplying the kinematic viscosity by the density of the fluid.

Volatility

  • Distillation Curve - (ASTM D86) The distillation curve is determined by relating the fraction of a fuel sample which is removing by heating a fuel sample to progressively higher temperatures. Typically, the curve is characterized by the initial point, the temperature at which the first drop of liquid leaves the condenser, the temperatures at each 10% of the liquid and the end point.
  • Flash point - (ASTM D93) The flash point is the lowest temperature at which a combustible mixture can be formed above the liquid fuel. It is dependent on both the lean flammability limit of the fuel as well as the vapor pressure of the fuel constituents. The flash point is determined by heating a sample of the fuel in a stirred container and passing a flame over the surface of the liquid. If the temperature is at or above the flash point, the vapor will ignite and an easily detectable flash can be observed. The flash need not correspond to a sustained flame. The "fire point" is sometimes used to designate the fuel temperature that will produce sufficient vapor to maintain a continuous flame.

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