In-cylinder flow was optimised in a three-valve twin-spark-plug SI engine in order to obtain good two-zone fuel fraction stratification in the cylinder by means of tumble flow. First, the in-cylinder flow field of the original intake system was measured by Particle Image Velocimetry (PIV). The results showed that the original intake system did not produce large-scale in-cylinder flow and the velocity value was very low. Therefore, some modifications were applied to the intake system in order to generate the required tumble flow. The modified systems were then tested on a steady flow rig. The results showed that the method of shrouding the lower part of the intake valves could produce rather higher tumble flow with less loss of the flow coefficient than other methods. The optimised intake system was then consisted of two shroud plates on the intake valves with 120° angles and 10mm height. The in-cylinder flow of the optimised intake system was investigated by PIV measurements. Mean velocity distribution, tumble intensity and centre and their cyclic variations, as well as the velocity fluctuation were analysed. The PIV results showed that the optimised intake system provided strong tumble motion, very low velocity component in the direction of the tumble rotational axis, less but still obvious cyclic variations of tumble ratio and tumble centre, very high fluctuation kinetic energy especially near the end of the compression stroke. These flow features would no doubt be beneficial to fuel fraction stratification and the subsequent combustion process.
