Published: Jan 2008
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THE VOLATILITY CHARACTERISTICS OF A SPARKignition engine fuel are of prime importance to the driveability of vehicles under all conditions encountered in normal service. The large variations in operating conditions and wide ranges of atmospheric temperatures and pressures impose many requirements on a fuel if it is to give satisfactory vehicle performance. Fuels that vaporize too readily in pumps, fuel lines, carburetors, or fuel injectors will cause decreased fuel flow to the engine, resulting in hard starting, rough engine operation, or stoppage (vapor lock). Under certain atmospheric conditions, fuels that vaporize too readily can also cause ice formation in the throat of a carburetor, resulting in rough idle and stalling. This problem occurs primarily in older vehicles. Conversely, fuels that do not vaporize readily enough may cause hard starting and poor warm-up driveability and acceleration. These low-volatility fuels may also cause an unequal distribution of fuel to the individual cylinders in vehicles with carburetors or throttle body fuel injection systems. Similar problems would not be expected in vehicles with port fuel injection systems. The volatility of automotive spark-ignition engine fuel must be carefully “balanced” to provide the optimum compromise among performance features that depend upon the vaporization behavior. Superior performance in one respect may give serious trouble in another. Therefore, volatility characteristics of automotive fuel must be adjusted for seasonal variations in atmospheric temperatures and geographical variations in altitude. ASTM D4814-04b, “Standard Specification for Automotive Spark-Ignition Engine Fuel,” defines the volatility requirements for gasolines sold in the United States. Volatility of fuels is varied for seasonal climatic changes and for conformance to U.S. EPA volatility regulations by providing six vapor pressure/distillation classes and six vapor lock protection classes for fuel [1,2]. The four common volatility properties, i.e., vapor pressure, distillation, driveability index,and vapor-liquid ratio at temperature or vapor lock index, are described below. The effect of these volatility parameters on the performance of the vehicle is also discussed.
Bonazza, B. R.
Principal Chemist, TI Automotive LLC, Caro, MI
Gibbs, L. M.
Chevron Fellow, Chevron Products Co., Richmond, CA