In-cylinder air motion characteristics with variable valve lift in a spark ignition engine. Part 1 : Swirl flow
While variable valve actuation or variable valve lift (WL) is used increasingly in spark ignition (SI) engines to improve the volumetric efficiency or to reduce the pumping losses, it is necessary to understand the impact of variable valve lift and timing on the in-cylinder gas motions and mixing processes. In this paper, the in-cylinder flow characteristics for reduced maximum valve lifts (MVLs) were examined in a modified four-valve optical SI test engine in which the combustion system is suitable for gasoline direct-injection (GDI) operations. Three different MVLs of 6.8 mm, 4.0 mm, and 1.7 mm were tested and particle image velocimetry was employed for the measurements. It is expected that the investigation will be helpful in understanding and improving GDI combustion when a WL system is used. The results showed that a reduced MVL could significantly enhance the in-cylinder swirl motion in all three measured horizontal planes. By comparing the swirl ratios for different MVLs, it can be found that a reduced MLV can considerably increase the swirl ratio during the intake and compression processes. In general, the swirl centre induced by a lower MVL is more stable and can stay closer to the cylinder centre. In addition, the reduction in the MVL can also increase both the high-frequency fluctuating kinetic energy and the low-frequency fluctuating kinetic energy. These may contribute to an improvement in the air-fuel mixing but also to an increase in the cycle-to-cycle variation.