The Use of Variable Geometry Sprays With Low Pressure Injection for Optimization of Diesel HCCI Engine Combustion

A numerical study of the effects of injection parameters and operating conditions for diesel-fuel HCCI operation is presented with consideration of Variable Geometry Sprays (VGS). Methods of mixture preparation are explored that overcome one of the major problems in HCCI engine operation with diesel fuel and conventional direct injection systems, i.e., fuel loss due to wall impingement and the resulting unburned fuel. Low pressure injection of hollow cone sprays into the cylinder of a production engine with the spray cone angle changing during the injection period were simulated using the multi-dimensional KIVA-3V CFD code with detailed chemistry. Variation of the starting and ending spray angles, injection timing, initial cylinder pressure and temperature, swirl intensity, and compression ratio were explored. As a simplified case of VGS, two-pulse, hollow-cone sprays were also simulated. The results show that VGS is effective in minimizing wall wetting and allows wall wetting to be decoupled from ignition timing control. Using VGS, the initial gas temperature, boost/throttling pressure, and compression ratio are effective parameters in ignition timing control. Variation of swirl intensity is effective to ensure fuel vapor-air mixing, and to prevent the formation of rich regions with high NOx emissions. The results indicate that VGS is a promising methodology to be used to control diesel-fueled HCCI engine operation, and it deserves further experimental and numerical study.