Combustion Visualization of DI Diesel Spray Combustion inside a Small-Bore Cylinder under different EGR and Swirl Ratios

An experimental setup using rapid compression machine to provide excellent optical access to visualize simulated high-speed small-bore direct injection diesel engine combustion processes is described. Typical combustion visualization results of diesel spray combustion under different EGR, swirl, and injection pressure and nozzle conditions are presented. Different swirl intensities are achieved using an air nozzle with variable orientations and a check valve to connect the compression chamber and the combustion chamber. Different EGR ratios are achieved by pre-injection of diesel fuel prior to the main observation sequence. Clear visualization of the high-pressure fuel injection, ignition, combustion and spray/wall/swirl interactions is obtained. The injection system is a high-pressure common-rail system with either a VCO or a mini-sac nozzle. High-speed movies up to 35,000 frame-per-second are taken using a framing drum camera to record the combustion events. Combustion luminosity was used to qualify the combustion and sooting processes. Heat release analysis was also carried out. The results confirm earlier engine combustion results, which show that EGR decreases the heat-release rate and increases the soot formation. Wall interaction with spray plume is identified as a major soot formation contributor. Higher injection pressure does not always provide better soot emission results at high EGR conditions, due to wall interactions. Therefore, the soot-NOx trade-off relations should be optimized with injection pressure and swirl ratio in small-bore high-speed diesel engines.