Characteristics Of In-cylinder Flow In A Disi Engine With Variable Valve Lift

For helping three-way catalysts to achieve necessary conversion efficiency, new developed DISI engines have reduced the utilisation of lean burn but used more stoichiometric operation which is normally accomplished by throttle load control. In the mean time, researches and developments on VVA (Variable Valve Actuation) have promoted its application on SI engines for replacing throttle to reduce pumping loss. In this study, experimental investigation on the in-cylinder air motion with VVA for obtaining non-throttled load control was carried out for understanding the air-fuel mixing and engine performance.The optical engine used for those investigations is a modified Cagiva Fox350 gasoline engine which is of single-cylinder four-valve. Four different MVLs (Maximum Valve Lift) were conducted, while Particle Image Velocimetry (PIV) was employed for achieving those in-cylinder flowfields.The results show that those tumble motions of in-cylinder flow are strong enough to be observed, which swirl motions are very weak and only can be identified under some special condition. It has been noted that in the middle of intake stroke, two reverse rotating closed and opened swirl structures separately exist under low and high MVLs conditions. With the progress of compression process, significant decay of swirl flow velocities was observed for low MVLs, while this was not found for high MVLs. At the end of compression stage, the flow velocities increase apparently in the lower measured planes but retain unchanged in higher measured planes for different MVLs.With regard to tumble motion, two reverse rotating vortexes for the low MVLs and a single clockwise rotating tumble structure for high MVLs generated in the symmetric plane in the middle of intake process,. In the early stage of compression stroke, with the piston moving up to TDC, tumble flows for high MVLs break down and form new tumble structures. For the low MVLs, there is no new tumble structure produced in the compression process. Later in the compression stroke, tumble motion for MVL 6.8mm can be observed in cylinder and the averaged tumble ratio can be up to 0.45. Its averaged high frequency fluctuation kinetic energy can be 2~3 times higher than that of the MVL 1.7mm.For achieving large scale flow fields, experiments for measuring swirl motions were further carried out by increasing curtain area of one intake valve. The results show that reduced MVL could result in the increase of averaged swirl ratio at all measured crank angles. In the middle of intake stroke, two reverse rotating vortexes are produced in the cylinder under different MVLs. In the late stage of intake process, two vortexes evolve into a single counterclockwise swirl flow for MVL 1.7mm and the peak of averaged swirl ratio reaches 1.9 for high MVLs. For low MVLs such as 1.7mm, the averaged swirl ratios have a apparent increase at the compression end stage, and the averaged high frequency fluctuation kinetic energy can be up to 0.5 which is 4~5 times to MVL 6.8mm.For extracting turbulence and cycle-to-cycle variation from velocity field, low-pass filtering was used and the cut-off frequency is within 70Hz~120Hz. When MVL is reduced, the maximum averaged fluctuation kinetic energy increases significantly at all crank angles. With those analyses, it was found that, at the end of compression stage, the differences of swirl velocities in different measure planes come from the different functions of tumble fields. Turbulence and cycle-to-cycle variation obtained significant increase from the break of the turbulence. The formation and strengthen of tumble flow can increase the intensity of turbulence but have no obvious effect on cycle-to-cycle variation.