An experimental study was conducted to investigate the effect of the structure of the induction flow on the characteristics of early flames in a lean-stratified and lean-homogeneous charge combustion of compressed natural gas (CNG) fuel in a direct injection (DI) engine at different engine speeds. The engine speed was varied at 1500 rpm, 1800 rpm and 2100 rpm, and the ignition timing was set at a 38.5 crank angle (CA) after top dead center (TDC) for all conditions. The engine was operated in a partial-load mode and a homogeneous air/fuel charge was achieved by injecting the fuel early (before the intake valve closure), while late injection during the compression stroke was used to produce a stratified charge. Different induction flow structures were obtained by adjusting the swirl control valves (SCV). Using an endoscopic intensified CCD (ICCD) camera, flame images were captured and analyzed. Code was developed to analyze the level of distortion of the flame and its wrinkledness, displacement and position relative to the spark center, as well as the flame growth rate. The results showed a higher flame growth rate with the flame kernel in the homogeneous charge, compared to the stratified combustion case. In the stratified charge combustion scenario, the 10 SCV closure (medium-tumble) resulted in a higher early flame growth rate, whereas a homogeneous charge combustion (characterized by strong swirl) resulted in the highest rate of flame growth

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The study of flame commencement and growth in SI engine has attracted the attention of many researchers due to its strong dependence to the in-cylinder flow and its capability to influence flame propagation characteristics in SI engines. Optical visualisation has been one of the prominent techniques utilized in this study of the early flame properties. This work shows the application of Elliptic Fourier analysis (EFA) and image processing tools for the investigation of the early flame details from its shape features. An endoscopic ICCD camera was used to capture the flame images, at an interval of 2° CA starting from ignition onset for a period of 30° CAs (about 3 ms), from a CNG DI single cylinder SI engine operation in stratified and homogeneous charge conditions. The intake configuration was adjusted to acquire tumble or swirl induction, and engine speed was made to vary between 1500 and 2100 rpm. The sequences of collected images were processed to investigate wrinkles, distortion and growth rate of the early flame within the 3 ms combustion duration. Some tools of image processing, such as intensity enhancement, filtering, thresholding and boundary tracing, were applied. Once the flame boundary identified, it was chain coded, and elliptic Fourier function was utilised to characterise the contour. For this purpose, a computer programming code was proposed by the authors for automatic processing of the flame image data. The application of the elliptic Fourier analysis was found useful in the investigation of the early flame characteristics from its shape features; led to the portrayal of wrinkles and distortion levels quantitatively in a simpler way.

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