Combustion and Soot Visualization of Low Temperature Combustion within an HSDI Diesel Engine Using Multiple Injection Strategy

Low Temperature Compression Ignition (LTCI) combustion employing multiple injection strategies in an optically accessible single-cylinder small-bore High-Speed Direct-Injection (HSDI) diesel engine equipped with a Bosch common-rail electronic fuel injection system was investigated in this work. Heat release characteristics were analyzed through the measurement of in-cylinder pressure. The whole cycle combustion process was visualized with a high-speed digital video camera by imaging natural flame luminosity and three-dimensional like combustion structures were obtained by taking flame images from both the bottom of the optical piston and the side window. The transient in-cylinder late cycle soot distribution was obtained by applying a Backward Illumination Light Extinction (BILE) technique through side windows. Based on the flame luminosity and soot spatially integrated signal, new parameters were defined to evaluate the combustion performance and soot formation characteristics. The effects of operating load and injection pressure on combustion mode and soot formation were investigated. For the base case, no liquid fuel is found when ignition flame points appear, which shows low temperature Premixed Charge Compression Ignition (PCCI) combustion. Little soot is seen during the whole combustion process for the base case. For the higher load and lower injection pressure cases, liquid fuel being injected into low temperature premixed flame was observed, which is different from the conventional diesel combustion with liquid fuel injected into hot premixed flame. The heat release patterns of the second injection for all of the cases are similar to the low temperature UNIBUS combustion. Results show that increasing injection pressure will greatly reduce soot emissions. Lowering injection pressure is to some extent helpful for reducing combustion temperature, which benefits NO_x reduction.