Experimental and numerical investigation of the idle operating engine condition for a GDI engine

The paper investigates the idle operating condition of a current production turbocharged Gasoline Direct Injected (GDI) high performance engine both from an experimental and a numerical perspective. Due to the low engine speed, to the low injection pressure and to the null contribution of the turbocharger, the engine condition is far from the standard points of investigation. According to the low heat flux due to combustion, temperature levels are low and reduced fuel evaporation is expected. Consequently, fuel spray evolution within the combustion chamber and spray/wall interaction are key points for the understanding of the combustion process. In order to properly investigate and understand the many complex phenomena, a wide set of engine speeds was experimentally investigated and, as far as the understanding of the physics of spray/wall interaction is concerned, many different injection strategies are tested. Among the wide set of experiments, the present paper focuses on a restricted portion which is then numerically reproduced and further investigated. UV-visible imaging and spectral measurements are carried out in the engine to investigate the spray characteristics and flame propagation. Measurements are performed in the optically accessible combustion chamber realized by modifying the actual engine. The cylinder head is modified in order to allow the visualization of the fuel injection and the combustion process in the fourth cylinder using a high spatial and temporal resolution ICCD detector. The complete engine cycle is reproduced by means of 3DCFD simulations using a commercial code; due to the many physical submodels an ad hoc numerical methodology is validated and implemented. The CFD models are validated against experiments and particular care is devoted to the spray and wall film simulations. A lagrangian approach is implemented in order to simulate the GDI multihole spray. The experimental and numerical comparisons, in terms fuel mixing and flame front propagation, give a good understanding of the idle condition. CFD analyses prove to be a very useful tool to investigate and understand the effects generated by the direct injection into the combustion chamber and they integrate the information provided by the optical investigations.