Computational study of mixture preparation in direct-injection spark-ignition engines with advanced valvetrain system

Abstract

Nowadays, ground transportation is a major source of air pollution with an impact on the greenhouse effect and on the public health, especially in urban areas. In this work, an investigation of the capabilities of a Continuous Variable Valve Lift system with the use of computational fluid dynamics was carried out. The aim was to understand the benefits and drawbacks of several valvelift strategies at various engine loads. This required validation of the commercial code STAR-CD and the models associated with engine simulations as well as implementation of a new flash-boiling model for fuel injection at low engine load. A generalised approach was adopted for sub-cooled and flash-boiling sprays by minimising the differences of the implementation. The Single Valve Actuation (SVA) was proved beneficial compared to the Dual Valve Actuation (DVA) because of the increase of the turbulent kinetic energy (TKE) and thus, the reduction of the combustion duration which allowed extension of the dilution limit. Moreover, it increased the mixture homogeneity because of the introduction of a strong swirl motion. However, the enhanced flow motion increased the heat transfer from the gas to the walls which offset the benefit of faster combustion. In addition, the difference in homogeneity was affecting the emissions depending on the dilution level. The benefits and drawbacks were balanced with the use of two differential intake valve lift strategies. The heat transfer could be reduced while the TKE and mixture homogeneity improved in comparison to the DVA. Lastly, the cyclic variability of the DVA and SVA were studied at high EGR levels. The lower in-cylinder velocity variability and lower variability of the laminar burning velocity due to lower residual mixing variability with the SVA denoted the lower combustion variability seen in the experiments