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Physical and Chemical Characteristics
 
Liquid fuels release their stored chemical energy by combustion. However, what actually combust is the vapors from the fuel not the liquid it self. A good fuel must ignite easily, burn well and not explosively, yield a lot of heat, produce low smoke and ash, be inexpensive and easy to store and transport. How well a liquid fuel meets these criteria is determined by its physical and chemical characteristics. These include:
 
  • flash point: lowest temperature at which a liquid can vaporize to form an ignitable air-vapor mixture, lowest amount of vapor necessary to ignite with an ignition source is applied, useful in defining fire hazards of liquids, defines:
    • flammable: flash point below 60.5oC 140.9oF) or 37.8oC (100.0oF)
    • combustible: flash point above 60.5oC 140.9oF) or 37.8oC (100.0oF)
  • fire point: temperature at which a fuel will continue to burn for at least 5 seconds after ignition by an open flame, below the fire point, evaporation of the liquid fuel is too slow to maintain combustive conditions
  • cloud point: temperature at which dissolved solids are not completely soluble, precipitation of these solids gives the fuel a cloudy appearance, results in clogging of filters and injectors thereby degrading performance
  • pour point: temperature at which a liquid becomes semi-solid and loses its ability to flow
  • viscosity: measure of the resistance of a fluid to deform under stress, i.e. its ability to flow, low viscosity fluids flow easily whereas high viscosity fluids do not flow as freely
 
  The combination of these variables determines a particular fuel's suitability for different applications. For example, a fuel with low cloud and pour points would be suitable for use in cold conditions. In contrast, a fuel with a high fire point is inherently more safe to handle than one with a low fire point.
 
 
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