Abstract: The fuel dynamic transfer process, including fuel injection, fuel film deposition and evaporation in the intake port, was analyzed for spark ignition (SI) engines with port fuel injection (PFI). The influence of wall-wetting fuel film, especially its evaporation rate, upon the air-fuel ratio of in-cylinder mixtures was also discussed. According to the similarity principle, Fick’s law, the ideal gas equation and the Gilliland correlation, an evaporate prediction model of wall-wetting fuel film was set up and an evaporate prediction based dynamic fuel film compensator was designed. Through engine cold start tests, the wall-wetting temperature, which is the key input of the fuel film evaporate prediction model, was also modeled and predicted. Combined with the experimental data of the evaporation characteristics of ethanol-gasoline blends and engine calibration tests, all the parameters of the wall-wetting fuel film evaporate prediction model used in the fuel film compensator were identified. Square-wave disturbance tests of fuel injection showed that with the help of the fuel film compensator the response of the in-cylinder air-fuel ratio was significantly improved and the real air-fuel ratio always closely matched the expected ratio. The fuel film compensator was then integrated into the final air-fuel ratio controller, and the engine tests showed that the air-fuel ratio control error was less than 2% in steady-state conditions, and less than 4% in transient conditions. The fuel film compensator also showed good adaptability to different ethanol-gasoline blends.

Key words: Spark ignition (SI) engine, Ethanol-gasoline blend, Wall-wetting effect, Evaporate prediction, Fuel film compensation

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