Visualization of High-Power Density Diesel Engine Spray and Combustion

摘要

An experimental visualization study of spray and combustion processes under simulated high-power-density diesel (HPD) combustion conditions is carried out. The scope of work includes characterization of high-pressure injection on test rig, macroscopic and microscopic characterization of non-evaporative sprays in atmosphere ambient condition or in pressurized chamber, and direct visualization of spray and combustion processes inside a single-cylinder optically accessible research engine. The results provide very interesting and dynamic information on spray structure, showing spray angle variations, primary breakup processes, and spray asymmetry under different combinations of injection systems and nozzle geometry. The injection systems tested included electronic unit injector (EUI), hydraulic unit injector (HEUI), and high-pressure common rail (HPCR) with both valve-covered orifice (VCO) and mini-sac nozzles. The near-field spray behavior is shown to strongly depend on the nozzle geometry, needle lift dynamics, and injection pressure, which is a function of the injection system. Some complementary numerical simulations using new transient schemes (space-time method) and primary breakup models (turbulent and cavitational) to better correlate spray and injector design are also discussed. Combustion visualization inside an optically accessible diesel engine equipped with HEUI injection system capable of both single and multi-stage injections was also successfully conducted. Nozzle hole flow area, injection pressure, and length of rate-shaping pipe are the dominating parameters that control the injection rate. The results showed that ignition delay and heat release of pre- mixed combustion is significantly reduced with increase of charge air density and temperature, and with pilot injection.