CFD optimisation of the in-cylinder flow patterns in a small unit displacement HSDI Diesel Engine for off-highway applications

The aim of the paper is to provide information about the in-cylinder flow field optimisation in a high speed, direct injection (HSDI) four valve per cylinder diesel engine for off-highway applications.Fully transient CFD analyses of different valve profile strategies for the intake and compression strokes are at first performed, in order to evaluate the effects on both engine permeability and in-cylinder flow field evolution. Modifications are applied to each intake valve separately: gradually stretched cam profiles are imposed so that strategies range from the standard operation, i.e. the adoption of a unique cam profile for the two intake valves, up to the limit case characterized by a 40 % difference between the intake valves maximum valve lifts for three different engine conditions. Organized flow structures (i.e., swirl) and turbulent flow patterns are investigated, in order to address rules for ad-hoc strategies aiming at finding the best trade off between engine performance and pollutant emission.The effectiveness of the valve strategies is validated by means of full injection and combustion simulations using state of the art models. At first, results for the base case are validated against experiments; subsequently, both full-load / peak-torque and mid-load / low-speed operations for the most promising cases are performed.Relative valve profile strategies prove to strongly influence the flow field within the combustion chamber, and therefore the subsequent spray evolution and fuel combustion, confirming the importance of an ad-hoc optimization in order to meet the best trade-off between performance and pollutant emissions.