No abstract

Steady flow tests are widely used to evaluate the performance of intake ports in generating swirl flow in diesel engines. Such test data, however, may deviate largely from the real in-piston-bowl swirl ratio due to the complex unsteady air motion in the compression stroke. In this study, a new method is proposed to predict the unsteady in-piston-bowl swirl ratio of diesel engines from steady flow test data by focusing on three key steps, including the swirl field at intake valve close timing, swirl enhancement due to squish flow, and swirl decay during the compression stroke. Experimental results on an optically accessible diesel engine under non-firing conditions show that, at intake valve close, the relationship between the swirl ratio and the vertical location was approximately linear and the mean swirl ratio could be fitted by a Bessel function; the correlation between the swirl decay coefficient and surface-to-volume ratio was built by fitting the experiment data. Furthermore, the in-piston-bowl swirl ratio during the compression stroke could then be derived according to the conservation of angular momentum.

In a direct-injection spark-ignition (DISI) optical engine with a constant motored speed of 800 rpm, particle image velocimetry measurements on the in-cylinder bulk flow and the boundary layer flow near the cylinder wall were performed during the intake and compression strokes. Two different tumble flow levels were explored using a flap inserted in the intake port. The near-wall flow pattern under the effect of the bulk flow is more stable when the flap is fully closed with a higher intensity tumble. The serious optical distortion in the near-cylinder wall region under high magnification was addressed, and a high resolution of 76 μm × 76 μm within a field of view of 5 mm × 5 mm was achieved in six different measurement regions. Using inner scaling parameters (friction velocity uτ and viscous length-scale δυ), the dimensionless ensemble-averaged wall-tangential velocity profiles exhibit good consistency with the law-of-the-wall in the viscous sublayer but considerably departure in the logarithmic region. According to the low friction Reynolds number, the absence of the logarithmic layer was mainly attributed to the complete overlap between the viscosity-affected inner region and the outer region of the boundary layer. The near-wall velocity profiles were also normalized by three different outer scaling parameters. The collapse is significantly improved for y/ δ >0.07 when the velocity profiles are normalized by δ/ δ*(1− u/ u∞). The fluctuation RMS velocity in the wall-normal direction is greater, particularly for the high tumble flow level condition.