Near-TDC Flow-Field Analysis in a High-Tumble Production Spark-Ignition Engine Using Endoscopic High-Speed Particle Image Velocimetry

The latest-generation spark-ignition (SI) engines implement high-tumble flow design to achieve unprecedented high brake thermal efficiency of over 40%, which will continue to play an important role in both conventional and future electrified vehicles. To maximize the potential of high-tumble SI engines, there is a clear need for in-cylinder flow and flame analysis conducted timely in a realistic environment. For the first time, this study meets this need by performing innovative endoscopic imaging of flow fields and flame inside the cylinder of a selected production engine using a particle image velocimetry (PIV) laser and high-speed camera system operated at 35 kHz. Through this time-resolved, two-dimensional measurement of the realistic in-cylinder phenomenon, many new findings have been achieved. Regarding the tumble vortex, its center is seen more shifted to the exhaust valve side, which is related to an asymmetric "surging" flow structure formed during the upward motion of the piston in the compression stroke. When the piston approaches the top dead center (TDC), the tumble center is not clearly defined anymore, but a new lateral exhaust-to-intake flow forms as the surging flow bounces back off the exhaust valve side of the pent roof. This wall-guided "bounce-back" flow does not form when the intake valve closure (IVC) is retarded for the realization of the Atkinson Cycle, and thus the surging flow is reduced and peaks at a later timing. The new in-cylinder flow structures observed from the high-tumble engine directly impact the flame development. From the high-speed natural combustion luminosity imaging performed using the same camera endoscope, the flame centroid is found to shift toward the intake valve side, which is consistent with the bounce-back flow direction at the advanced IVC timing.