Pilot injection impact on diesel PCCI combustion: an endoscopic study with a turbocharged engine

In this study, the effects of pilot injection on premixed charge compression ignition were investigated in a turbocharged diesel engine equipped with an endoscopic visualization system, and experiments were conducted under a speed of 1450 rpm at 25% and 50% loads. Results indicated that as the pilot injection mass and timing increased, the diffusion flame advanced and the flame area expanded. However, when the pilot timing advanced to 70 °BTDC, the flame luminance remained nearly constant. Moreover, with increased pilot injection mass, the peak values of in-cylinder pressure and the heat release rate of the pilot injection combustion were increased, brake-specific fuel consumption (BSFC) slightly increased, soot initially decreased but then increased, nitrogen oxide (NOX) was reduced by a maximum of 54%. The in-cylinder pressure peak value decreased with the advance of pilot injection timing, and the main combustion exhibited a lower heat release rate. Nevertheless, NOX increased significantly, and soot decreased initially but showed a slight increase at 25% load. RSM was used for finding the optimal pilot injection strategy to minimize BSFC and emissions. The optimal values found were a pilot injection timing of 50 °BTDC and a pilot injection mass of 4.4 mg for 25% load, and corresponding values of 58.9 °BTDC and 5.5 mg for 50% load.