Gasoline direct injection (GDI) promises better fuel economy and superior performance in internal combustion engines when compared to conventional gasoline port fuel injection. GDI offers control on fuel injection with load and operating conditions, thereby ensuring stoichiometric spark-ignited engine performance, as well as higher compression ratios resulting in 15% more efficient fuel consumption. As the sources of particulates in GDI engines are mainly attributed to the incomplete combustion of hydrocarbon fuel-rich regions, a relevant topic of study is the formation of fuel deposits in the injector’s recesses. The geometry of interest here is an eight-hole counterbored (stepped) GDI injector during injection closing, when tip wetting can lead to sub-optimal combustion. Due to limited optical access, no experimental information about surface wetting is readily available from the recessed region inside the counterbores, and conventional computational fluid dynamics methodologies are unable to accurately account for fuel evaporation. For the first time, a sharp-interface method for the multiphase Navier-Stokes equations (the CLSVOF computer code), coupled to a new mass- and energy-conserving numerical method for surface evaporation, is applied to calculate the vapor concentration in the counterbores. The simulation shows substantial entrainment of hot gas from the combustion chamber into the injector due the rapid pressure decrease in the fuel passages during the closing transient; it also shows that the temperature increase at the liquid surface is mitigated by the cooling effect of evaporation. The sensitivity of the computational approach to fuel properties is assessed by comparing the results obtained for the injection of neat iso-octane and of the homogenous mixture of 30% ethanol and 70% iso-octane (E30). This study is concluded by relating the larger saturation pressure and lower energy of vaporization of E30 to the final liquid fuel deposit found in the counterbore.

Detailed evaporation modelling for gasoline direct injection: Iso-octane vs. E30

ACS Fall 2021

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