Mixture Preparation in Spark-Ignition Engine and its Effect on Engine Performance and Combustion Characteristics

Engine combustion behaviors were investigated when the mixture condition at the intake port was varied. This experimental study was performed for several engine variables including types of cylinder head (gas motions), spark plug loction and MBT timing. Among the variables for the mixture condition at induction were the fuel/air mixture ratio (excess air factor) and the portion of atomized liquid fuel out of total fuel in the mixture. The engine operation was analyzed by obtaining the mean effective pressure, thermal efficiency, heat release history, stability of combustion, and lean misfire limit. When the mixture at the intake port contained about 30% of atomized liquid fuel out of the total fuel (R = 30), the engine exhibited highest mean effective pressure and thermal eficiency; the MBT timing for mixture with R = 30 was retarded, which indicates its short flame propagation period; the entire combustion period was unchanged although the heat release lag duration was shortened; the cyclic variation greatly deteriorated when the mixture was overall lean. The tendency in most of these observations was less apparent in the cylinder head having stronger gas motions and for spark plug located closer to the center of mixture volume. There was no measurable extension of lean misfire limit by having some atomized fuel in the mixture. This change in limit may be due to vaporization of fuel when the mixture is overall lean.