A numerical investigation of VVA influence on the combustion phase for premixed combustion engine under partial load conditions

Nowadays, the vehicle hybridization and the use of non-conventional fuels for heavy-duty applications brings to a new beginning in the use of spark ignition (SI) engines. For a standard intake system, the premixed fuel/air mixture is controlled by the injection of fuel after the throttle valve. Then, the geometry of the intake system, with the intake duct, the intake valves and the cylinder head shape, influences the characteristics of the flow within the cylinder up to the combustion process. The new technology of fluid-power and electrical actuations gives the opportunity to decouple the intake and exhaust valve actuations with respect to the standard cam shaft distribution. The Variable Valve Actuation (VVA) concept is not new, but its application is now affordable and flexible enough to be applied to partial load conditions. In this work, the intake, compression and combustion processes of an SI engine are studied by means of a three-dimensional numerical approach based on a finite volume approach. In this model, the Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) equations are solved together with a k-ϵ model for turbulence and an Extended Coherent Flamelet Model (ECFM) for combustion. The 4-valve engine is equipped with two symmetrical intake valves as well as two symmetrical exhaust valves. Two strategies are studied under partial load conditions: a standard valve lift profile for both intake valves, and a single intake valve lift profile, to provide the same overall fresh mass in the cylinder of the 2-valve opening. The valve timing has been kept constant for both strategies, with an Early Intake Valve Closing (EIVC) approach due to the partial load conditions. The intake flow characteristics and their influence on the combustion process are analyzed and a comparison between the two strategies is carried out. The results show flow structures quite different between the single valve opening and the standard 2-valve opening. The asymmetry of the intake flow, induced by the single valve approach, leads to an increase of the swirl ratio with respect to 2-valve opening. The highest swirl ratio of the single valve case is sustained till spark ignition occurs. At spark timing, the Turbulent Kinetic Energy (TKE) is greatly influenced by the valve strategy, leading to higher values for the single valve lift case with respect to the standard two valves lift. Moreover, the results show that single valve opening provides a faster combustion in lean mixture conditions than the standard lift.