AbstractThis study examines the spray characteristics of biodiesel, diethyl carbonate (DEC), and their mixtures in a common-rail injection system. Using the schlieren method, the spray tip penetration, spray cone angle, spray tip velocity, spray area, and spray liquid core ratio were observed with a high-speed camera. The test results show that the spray pressure and ambient pressure have significant effects on the spray characteristics. Increasing the spray pressure and decreasing the ambient pressure can increase the spray tip penetration, decrease the spray cone angle, and increase the spray area. After the addition of DEC to biodiesel, with increasing the mixing ratio, the viscosity and surface tension of the mixed fuel are reduced, but the density is increased. This increases the spray cone angle and spray area of the mixed fuel, and it reduces the Sauter mean diameter (SMD). The SMD of droplets were calculated, and it was found that DEC30 has the smallest SMD, and it is of the same order as that of diesel. An improved calculation model for the spray tip penetration of the DEC and biodiesel mixture under high injection pressure was obtained by modifying the exponent of an existing model. By comparing the linear relationship between the injection pressure and the spray tip penetration, it was found that the spray tip penetration of DEC10 has the largest increase.