Theoretical and Experimental Study of Fuel Injector Tip Wetting as a Source of Particulate Emissions in Gasoline Direct-Injection Engines

Abstract

Gasoline fuel film deposited on the tip of a fuel injector, i.e. injector tip wetting, has been identified as a significant source of particulate emissions at some operating conditions of gasoline direct-injection engines. The liquid film on the injector tip can be reduced by either mitigating the initial fuel film that deposits on the tip during injection or by evaporating all or most of the fuel film before ignition takes place. The former process requires a clear understanding of the dependence of the fuel film formation on injector design, operating conditions and fuel flow conditions through the injector nozzle, which impose difficulties in the understanding due to the complex and interrelated processes involved. The liquid film evaporation process, i.e. injector tip drying, however depends mainly on engine operating conditions, and less on hardware or fuel flow dynamics. Understanding of the physics of injector tip drying is therefore less ambiguous but remains a challenge. Clear understanding of the tip drying physics can lead to significant reductions in PN emissions due to tip wetting. This work developed an analytical model for liquid film evaporation on the injector tip during an engine cycle for the mitigation of injector tip wetting. The model explains theoretically how fuel films on the injector tip evaporate with time from end of injection to spark. The model takes into consideration engine operating conditions, such as engine speed, engine load, tip and fuel temperatures, gas temperature and pressure, and fuel properties. The model was able to explain for the first time the observed trends in particulate number (PN) emissions due to injector tip wetting at different operating conditions. Engine experiments were used to validate the theoretical model by correlating the film mass predicted at the time of spark to PN and tip deposit volume measurements at different conditions. A new experimental technique was developed to measure the volume of tip deposit for this purpose since tip deposits are good indicators of tip wetting. In addition, an evaporation time constant was defined and was also found to correlate well with measured PN for all conditions tested. Injector manufacturers can use this time constant to maximize liquid film evaporation by correlating the variables in the time constant equation to changes in hardware and calibration. The results indicate that the liquid film evaporation on the injector tip follows a first order, asymptotic behavior. Additionally, the initial film mass after end of injection was confirmed to increase linearly with the injected fuel mass, i.e. engine load. Furthermore, the observed increasing exponential trend in PN emissions with engine load was due to the exponential nature of injector tip drying. As the initial film mass after injection increased linearly with engine load, the film mass at the time of spark increased in an exponential manner. A parametric study was also performed to understand the influence of the different initial and boundary conditions on fuel film evaporation. The liquid film evaporation on the injector tip was found to be highly sensitive to most of the injector initial and boundary conditions including the initial film mass after end of injection, the wetted surface area, the available time for tip drying and the injector tip temperature. The initial film temperature had the least effect on film mass evaporation.