Limit of Fuel Injection Rate in the Common Rail System under Ultra-High Pressures

The common rail injection system with higher injection pressure can improve injection characteristics. However, relevant researches for injection characteristics under ultra-high pressures are insufficient. In this article, the results of experiments with a maximum injection pressure of 390 MPa for nine different injectors of four types are presented. The experiment showed the existence of supercritical pressure during injection. At pressures below the supercritical pressure, the injection quantity increases with increasing injection pressure, however, when the injection pressure is over supercritical pressure, the injection quantity does not increase. According to the experiment results, the supercritical injection pressure is about 300 ∼ 350 MPa. Under ultra-high pressures, fuel is strongly heated and the local sound velocity decreases, and the adiabatic flow velocity reaches the sound velocity. Under supercritical pressure, the injection rate ceases to increase and even begins to fall. The traditional equations for calculating the injection rate cannot correctly describe the injection under ultrahigh pressures. A new mathematic model with considering the fuel heating for describing the injection quantity of compressible fluid was developed, this model is not only suitable for calculating the injection quantity under ultra-high pressures, but under traditional injection pressures.