Abstract
In this paper, the in-cylinder flow characteristics for reduced maximum valve lifts were examined in a single-cylinder four-valve optical SI engine in which the combustion system is suitable for DISI (Direct Injection Spark Ignition) operations. It is expected that the investigation would be helpful for understanding and improving DISI combustion when a VVL (Variable Valve Lift) system is employed. The results show that in-cylinder flow fields achieved from CFD numerical simulations and PIV (Particle Image Velocimetry) measurements are in good agreement each other. When a reduced max valve lift (from 6.8 mm to 1.7 mm) was used, stronger in-cylinder swirl ratio and turbulence intensity could be generated, though the reduced max valve lift produced a weaker incylinder tumble flow. Especially in the induction stroke, the turbulence intensity of 1.7 mm max valve lift can create over two times turbulence intensity than 6.8 mm max valve lift. This should be very helpful for air-fuel mixing process. In addition, 1.6 mm max valve lift can help to form evener distribution and higher average value of turbulence kinetic energy at the end of the compression process, which would be beneficial for maintaining an ideal flame propagation process.
Keywords: In-cylinder flow, VVL (Variable Valve Lift), PIV, CFD, DISI engine