Numerical Analysis of GDI Engine Cold-Start at Low Ambient Temperatures

The paper investigates the low-temperature cranking operation of a current production automotive Gasoline Direct Injected (GDI) by means of 3D-CFD simulations. Particular care is devoted to the analysis of the hollow cone spray evolution within the combustion chamber and to the formation of fuel film deposits on the combustion chamber walls. Due to the high injected fuel amount and the strongly reduced fuel vaporization, wall wetting is a critical issue and plays a fundamental role on both the early combustion stages and the amount of unburnt hydrocarbons formation. In fact, it is commonly recognized that most of the unburnt hydrocarbon emissions from 4-stroke gasoline engines occur during cold start operations, when fuel film in the cylinder vaporize slowly and fuel can persist until the exhaust stroke.In view of the non-conventional engine operating conditions (in terms of injected fuel amount, engine speed, ambient and wall temperature and almost null fuel atomization and breakup), an understanding of the many involved phenomena by means of an optically accessible engine would be of crucial importance. Nevertheless, the application of such technique appears to be almost unfeasible even in research laboratories, mainly because of the relevant wall wetting.CFD analyses prove then to be a very useful tool to gain a full insight of the overall process as well as to correlate fuel deposits to both the combustion chamber design and the injection strategy. In order to better understand where, and how thick, these wall films are formed during the intake and compression, a detailed description of the spray interaction with both the piston wall and the intake valves was performed by the authors in a previous paper [ 1 ]. Subsequently, a wide set of injection strategies was simulated in order to better understand the physics of spray/wall interaction and to minimize the formation of deposits in the combustion chamber most critical locations [ 2 ].In order to limit the overall number of modeling uncertainties (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) the spray model was at first validated against experimental data under low injection pressure, and results from the comparison were reported in [ 1 ].In the present paper, cold start operations at decreasing ambient temperatures are modeled and results are analyzed in terms of both fuel film distribution on the combustion chamber walls and resulting fuel/air mixture distribution within the combustion chamber. The use of CFD simulations prove to be useful to investigate and understand the influence of both combustion chamber design and injection profile on the amount and distribution of fuel deposits, showing a high potential to address future engine optimization