Flame Development Study at Variable Swirl Level Flows in a Stratified CNG DI Combustion Engine using Image Processing Technique

This study aims to investigate the characteristics of flame development in a lean-stratified combustion of CNG in a single cylinder Direct Injection (DI) engine at a specific motor speed and fixed injection timing and air-fuel ratio by varying only the swirl level at the intake. The engine is set to run at 1800 rpm with half-load throttled. The ignition advance is adjusted at 21.5 BTDC and to create an overall lean and stratified mixture, injection timing is set at 61 BTDC with fuel/air equivalence ratio of 0.425. By positioning the swirl-control-valves just before the two intake valves, medium tumble, medium swirl and high swirl intake flow structures are created in the cylinder and this results variable flow conditions at ignition onset. An endoscope and CCD camera assembly is utilized to capture the flame images from the tumble plane parallel to the cylinder axis every 2 CA degrees after ignition onset for 30 CAs. Using the natural luminosity of the flame and thresholding of the image intensity at certain point the binary image and its boundary vertices are identified. Freeman chain coding and Elliptic Fourier Analysis (EFA) algorithms are utilized to characterize the flame boundary, such as wrinkles and distortions. The result of the study shows that flame growth rate and flame convection velocity are increasing with the swirl level rise. The total combustion duration is thus, shorter in swirl induced combustion than without. However, COV in IMEP which is identified from the cylinder p-v data, is greater in swirl induced flow cases than the tumble flow case. Degree of flame distortion and flame front wrinkles increases with flame size.