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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.

Great attention to the efficiency benefits of spark ignition direct injection engine has been averted due to its problematic particulate emissions. In the present study, the fundamental knowledge of wall-wetting-induced spark ignition direct injection soot particles is enhanced through direct particle sampling from pool fire on the piston top surface and cylinder liner as well as from the exhaust stream. The sampled soot particles are imaged using transmission electron microscope, and the image post-processing for statistical morphology and internal structure analysis is performed to better understand the soot formation and oxidation processes. The experiments were performed in a single-cylinder optical spark ignition direct injection engine where diffusion flame luminosity was recorded using a high-speed camera through the cylinder liner window, with which the overall sooting level was understood, and the pool fire location was identified. Given the in-flame soot sampling experiments in the spark ignition direct injection engine were new, error analysis was conducted in terms of the number of fuel injections and engine run-to-run variations. This sampling technique then was applied for various injection timings in the intake stroke. The data analysis and physical interpretation was focused on a piston-wetting condition at the most advanced injection timing of 320 °CA bTDC and a liner-wetting condition at the most retarded injection timing of 180 °CA bTDC in the present study. Between these two different wall-wetting conditions, it was found that the piston-wetting condition has larger soot primary particles and soot aggregates. The internal carbon-layer fringe shows longer length, less tortuosity and smaller gap, indicating more mature and carbonised soot. This was consistent with more significant and wider distributed pool fire and thus longer soot residence time within the flames. When the exhaust soot particles were analysed, however, it was found that the reduction in soot aggregate size was much higher and the carbonisation was more progressed for the piston-wetting condition than those of the liner-wetting condition. This suggested higher soot oxidation later in the expansion/exhaust stroke for the piston-wetting condition, which potentially can be better utilised for engine applications.

Three cylinder heads with varied intake port shapes are experimentally investigated to evaluate intake flow structures and their influence on near top dead centre (TDC) flow fields and turbulence distributions in a motored high-tumble engine. The endoscopic high-speed particle image velocimetry (eHS-PIV) is implemented in multi-cylinder engines with two laser endoscopes and a camera endoscope installed in the cylinder head. For a range of engine load and speed conditions, particle seeded and laser illuminated high-speed movies are recorded for 100 cycles with which ensemble-averaged flow fields and spatial-filtered high-frequency flow magnitude distributions are analysed. The results show that a straighter intake port producing a more lateral flow direction results in a larger swinging arc, which is related to enhanced flow later in the compression stroke. The tumble vortex shows an asymmetric structure; the flow field during the compression stroke exhibits higher magnitude vectors at the leading head. This surging flow head becomes stronger with a straighter intake port, which leads to enhanced tumble vortex formation near TDC. As it becomes very strong, the flow vectors originally directed towards the exhaust valves bounce back towards the intake valves, causing a very complex flow structure involving multiple flow components. The result is enhanced turbulence throughout the late compression stroke including the spark timing. However, the impact of the straighter intake port shape on TDC turbulence becomes less significant at lower engine load and speed conditions as the lower intake air momentum limits the enhancement.

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