Study of fuel spray characteristics using a high-pressure common rail diesel injection system by the method of Schlieren

Abstract

The supercritical atmosphere created by the increase in temperature and pressure in the diesel engine cylinder is the key factor that causes changes in the fuel atomization process. In this study, the differences in spray morphology of n-heptane under different ambient temperature and pressure conditions were investigated using a constant volume combustion bomb combined with the Schlieren method. Based on the Peng-Robinson equation of state (PR equation), several transport characteristics models are constructed to predict the transport characteristics of n-heptane under different operating conditions. During the experiment, the injection pressure and fuel temperature were kept constant (80 MPa, 300 K), the ambient pressure was varied from 2 to 5 MPa, and the fuel temperature was changed from 400 to 800 K, spanning from subcritical to supercritical state. Three different spray regions have been identified based on the fuel temperature and spray geometry: liquid phase region, gas-liquid interface mixing layer, gas phase region. The results show that in a subcritical environment (400 K, 2 MPa), the fuel injection and atomization process is dominated by evaporation. The degree of heat exchange between the fuel and the medium gas is very low, while a small amount of phase change occurs. When the fuel is in low (600 K, 3 MPa) and medium supercritical (600 K, 4 MPa) ambient atmosphere, turbulence becomes the dominant factor in the fuel atomization process. The mixing layer at the gas-liquid interface at the edge of the liquid jet increases significantly. When the fuel is in a high supercritical (700 K, 4 MPa; 800 K, 5 MPa) atmosphere, the jet takes on a shape similar to that of a gas jet, and the mixing layer at the gas-liquid interface becomes the main part of the spray projection area.