Flow directed spark stretch and flame propagation in a high-tumble production engine

Abstract

This study performs endoscopic high-speed imaging to enhance the fundamental knowledge of in-cylinder flow structure and flame development process in a selected high-tumble production engine. The endoscopic high-speed particle image velocimetry (eHS-PIV) was performed for varied engine speeds and intake valve closing (IVC) timings to evaluate their impact on the in-cylinder flow structure in a motored engine condition. On another endoscope engine sharing the same hardware, high-speed flame imaging was conducted to visualise spark stretch and flame propagation. The flow and flame measurements were repeated for over 100 cycles and the ensemble-averaged results are compared. The eHS-PIV showed that a strong tumble vortex is generated during the piston compression with the flow directed towards the exhaust side. As the piston reaches top dead centre (TDC), however, a complex flow breakup involving multiple flow components occurs. This is followed by lateral flow vectors travelling back towards the intake side, which is termed as the bounce-back flow. For a tested engine speed range of 1700–2700 revolutions per minute (rpm), 2500 rpm shows the most significant bounce-back flow as a result of competition between the remaining exhaust-ward tumble flow strength and the newly formed bounce-back flow strength. At a retarded IVC timing, the flow loss leads to a weakened tumble flow and subsequently no bounce-back flow formation to maintain the exhaust-ward TDC flow direction. From the comparison between the flow results and spark/flame high-speed images, a strong positive correlation is found between the TDC flow direction and spark plasma stretch, and subsequently the flame propagation direction. The findings indicate that the TDC flow direction should be considered as a key parameter in the engine design and operating condition settings.

The effect of intake port shape on in-cylinder flow field and turbulence distribution in a high-tumble production engine with endoscope accesses

“…For all three port shapes, a tumble vortex (illustrated by red arrows) is finally visible with its centre skewed towards the exhaust side, which is surrounded by the remained surging flow head similar with previous observations made for high-tumble engines. 21,[35][36][37] The near-TDC flow fields show significant variations due to the port design. For example, port I maintains the tumblesurging flow structure while the surging flow head develops in a vertical direction.…”

“…15,28–34 High-speed PIV was also performed in a running production engine using rigid endoscopes (borescopes) for both the laser and camera access. 35–37 These studies achieved major new findings about the high-tumble engine flow. For example, the tumble flow during the piston compression develops with its centre skewed towards the exhaust side and the leading head of the tumbling flow is observed to have higher vector magnitude, which therefore was identified as a surging flow structure.…”

“…36 The drastic change of flow direction also affects the spark stretch and in turn flame development direction. 37 The present study aims to enhance our understanding of high-tumble SI engine flow and turbulence by varying intake port shape. While the flow structure details have been reported, how they change with the intake port shape and how it leads to varied turbulence distribution is lacking.…”

“…36 The drastic change of flow direction also affects the spark stretch and in turn flame development direction. 37…”

Flame detection via active plasma probing

“…Optical diagnostics either uses schlieren imaging to record the flame front movement, 8 or uses a laser to perform either PIV or species analysis to trace the movement of the flame front. 9,10 Such kinds of methods have high temporal and spatial resolutions, thus making them ideal for mechanism study on flame structure and propagation. 7 Ion sensors, on the other hand, utilize ions generated during the combustion process to detect the flame front arrival.…”