Accurate modeling of fuel spray behavior at diesel engine conditions requires well-characterized boundary conditions Among those conditions, the spray cone angle is important due to its impact on the spray mixing process and flame lift-off locations, and subsequent soot formation. It is also a highly dynamic variable, but existing correlations of spray cone angle have been mainly developed for quasi-steady state and for use with diesel fuels at relatively low injection pressures. The objective of this study is to develop spray cone angle correlations for both diesel and a light-end gasoline fuel over a wide range of diesel-engine operating conditions that are capable of capturing both the transient and quasi-steady state processes. For this study, two important macroscopic characteristics of solid cone sprays, spray cone angle and spray penetration, were measured using a single-hole heavy-duty injector using two fuels at diesel engine conditions. It was observed that the spray cone angle experienced three stages, beginning with a large initial value, dropping to a quasi-steady state, and then finally rising again near the end of injection. Experimental spray data for the two fuels under injection pressures of 1000-2500 bar and charge gas densities of 10-167 kg/m3 were examined to construct the correlations. The model was able to capture the transient process and provide dynamic cone angles as inputs in engine simulations, with the charge gas density changing due to piston movement. The model showed good agreement with the experimental measurements, leading to improved prediction accuracy in CFD studies.