Effect of Supercharging on the Intake Flow Characteristics of a Swirl-Supported Engine 2020-01-0794
Although supercharged system has been widely employed in downsized engines, the effect of supercharging on the intake flow characteristics remains inadequately understood. Therefore, it is worthwhile to investigate intake flow characteristics under high intake pressure. In this study, the supercharged intake flow is studied by experiment using steady flow test bench with supercharged system and transient flow simulation. For the steady flow condition, gas compressibility effect is found to significantly affect the flow coefficient (Cf), as Cf decreases with increasing intake pressure drop, if the compressibility effect is neglected in calculation by the typical evaluation method; while Cf has no significant change if the compressibility effect is included. Compared with the two methods, the deviation of the theoretical intake velocity and the density of the intake flow is the reason for Cf calculation error. For the transient intake condition, such increase of intake flow velocity with increasing intake pressure was found to be valid only at low engine speeds (2000 rpm). At high engine speeds (4000 rpm), however, flow velocity remains almost unchanged regardless of the intake pressure. This implies that flow velocity is determined by the effective pressure difference across the intake ports, which is synergistically controlled by the initial intake pressure difference and the piston wall confinement, during early intake process, while the intake velocity is restricted by the piston motion speed in the middle and later intake stroke. As such, the increased intake mass in cylinder is mainly resulted from the larger intake gas density rather than the higher flow velocity in supercharged engines. Furthermore, the supercharging may cause a high Ma for high engine speed round the valve seats and valve stems at low valve lift; while the engine speed is the main reason for high Ma intake flow under intake process at high valve lift.