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Vapor to Liquid Ratio Calculation (Specification for Automotive Spark Ignition Gasoline)

  • Method ASTM D4814
  • Repeatability Variable, see method
  • Reproducibility Variable, see method
  • Blending non-linear
  • Additive Correction No

Covers the establishment of requirements of liquid automotive fuels for ground vehicles equipped with spark-ignition engines.

This specification describes various characteristics of automotive fuels for use over a wide range of operating conditions. It provides for a variation of the volatility and water tolerance of automotive fuel in accordance with seasonal climatic changes at the locality where the fuel is used. For the period May 1 through Sept. 15, the maximum vapor pressure limits issued by the United States (U.S.) Environmental Protection Agency (EPA) are specified for each geographical area except Alaska and Hawaii. This specification neither necessarily includes all types of fuels that are satisfactory for automotive vehicles, nor necessarily excludes fuels that can perform unsatisfactorily under certain operating conditions or in certain equipment.

The spark-ignition engine fuels covered in this specification are gasoline and its blends with oxygenates, such as alcohols and ethers and where gasoline is the primary component by volume in the blend. This specification does not include fuels like E85 (85% ethanol 15% gasoline blend stock).

Drivability index is a calculation that uses the 10%, 50%, and 90% D86 distillation points to classify the volatility of the gasoline to be analyzed. This is very important to the cold starting and operation (drivability) of the fuel depending upon the climate it is being used in. Ethanol also plays a big a big role in this calculation as well. The method was modified to include calculations for both ethanol and non-ethanol blended gasoline.

Vapor to liquid ratio is another measure of volatility. It is measured by either the calculation contained in D4814 or the instrument D5188 which is now the referee. The calculation uses the vapor pressure and distillation points to estimate the temperature at which there are 20 parts vapor to 1 part liquid which is the domestic standard for v/l. Cargos in other countries tend to ask for the v/l ratio at a specific temperature (for example V/L @ 60°C). V/L is important to ensure that the gasoline will atomize properly in the combustion chamber when the climate is cold. When it is hot and the v/l is too high the fuel can vaporize in the fuel lines and cause the
engine to run lean which can cause damage.