Wang et al. [12] studied the effects of injector hole size on flame structure and soot formation of a diesel engine. The experiments were performed in a high pressure, high temperature constant volume combustion vessel. Three injection pressures of 100 MPa, 200 MPa and 300 MPa, and two injector nozzles with diameters of 160 mm and 80 mm were used. With the conventional injector nozzle (160 mm), ultra-high injection pressure generates appreciably lower soot formation. With the micro-hole nozzle (80 mm), impinging spray flame shows much smaller size and lower soot formation at the injection pressure of 100 MPa. The soot formation is too weak to be detected with the micro-hole nozzle at injection pressures of 200 MPa and 300 MPa.