CFD Investigation of Wall Wetting in a GDI Engine under Low Temperature Cranking Operations 2009-01-0704

The paper reports a numerical activity on the investigation of the spray evolution within the combustion chamber of an automotive DISI engine under low-temperature cranking operations. In view of the high injected fuel amount and the strongly reduced fuel vaporization at cold cranking, wall wetting becomes a critical issue. Under such conditions, fuel deposits around the spark plug region can affect the ignition process, and even prevent engine start-up. In fact, due to the low injection pressure at engine start-up, the fuel shows almost negligible atomization and breakup, and the spray structure at the swirl-type injector nozzle is characterized by a single column of liquid fuel, strongly limiting the subsequent vaporization and enhancing the fuel-wall interaction.

In order to properly investigate and understand the many involved phenomena, experimental visualization of the full injection process by means of an optically accessible engine would be a very useful tool. Nevertheless, the application of such technique, far from being feasible from an industrial point of view, appears to be very difficult even in research laboratories, due to the relevant wall wetting at cranking conditions.

CFD analyses prove therefore to be the sole chance to gain a full insight of the overall process, to correlate spark plug wetting to both the combustion chamber design and the injection profile and eventually address either design modifications or changes in the injection strategies. In order to limit the overall number of modeling uncertainties, and to validate the spray model under actual cranking conditions, comparisons with available experimental data at low temperature and low injection pressure were performed and are reported in the paper. Despite the CFD software continuous improvement and development, low-temperature cranking conditions proved to be an open challenge for the in-cylinder numerical simulations, due to the simultaneous presence of many physical sub-models (spray evolution, droplet-droplet interaction, droplet-wall interaction, liquid-film) and the very low motored engine speed. Furthermore, the high injected fuel quantity as well as the reduced fuel atomization and vaporization lead to very high concentration of liquid fuel droplets in the computational cells, posing a serious challenge to the adoption of a lagrangian approach to the injection simulation. Nevertheless, the use of a properly customized and validated numerical setup led to a good comprehension of the many involved phenomena as well as of the effects of injection strategy modifications on both the air/fuel and fuel/wall interaction.