An experimental and numerical investigation of GDI spray impact over walls at different temperatures

Internal combustion engines performance greatly depends on the air-fuel mixture formation and combustion processes. In gasoline direct injection (GDI) engines, in particular, the impact of the liquid spray on the piston or cylinder walls is a key factor, especially if mixture formation occurs under the so-called wall-guided mode. Impact causes droplets rebound and/or deposition of a liquid film (wallfilm). After being rebounded, droplets undergo what is called secondary atomization. The wallfilm may remain of no negligible size, so that fuel vapor rich zones form around it leading to so-called pool-flames (flames placed in the piston pit), hence to unburned hydrocarbons (HC) and particulate matter (PM) formation. A basic study of the spray-wall interaction is here performed by directing a multi-hole GDI spray against a real shape engine piston, possibly heated, under standard air conditions. High temporal and spatial resolution images are collected to obtain information about spray penetration and impact over wall at different temperatures. Firstly, the spray dynamics is analyzed through a visible high speed camera; secondly, the impact on the piston is studied through both infrared thermography and surface temperature measurements by fast response thermocouples. The experimental study is devoted also to the validation of a properly developed 3D CFD spray simulation model that has the novelty of accounting for the conductive heat exchange within the piston. The CFD model is conceived with the scope of its future application within numerical calculations of entire GDI engine working cycles. The piston cooling by the subtraction of the latent heat of vaporization of gasoline needed for secondary evaporation is particularly relevant for a correct prediction of droplet splashing and deposition phenomena and of the actual equivalence ratio distribution within the combustion chamber, hence to accurately predict HC and PM formation. The obtained results serve to clarify the importance of considering local surface temperature variations during the spray impact and its link with a multi-component fuel evaporation model.