The Direct Transition of Fuel Sprays to theDense-Fluid Mixing Regime in the Contextof Modern Compression Ignition Engines

Fuel supercriticality has recently received significant attention due to the elevated pressures and temperatures that directly-injected (DI) fuel sprays encounter in modern internal combustion (IC) engines. This paper presents a theoretical examination of conventional and alternative DI fuels at conditions relevant to the operation of compression ignition (CI) engines. The focus is to identify the conditions under which the injected liquid fuel can bypass the atomization process and directly transition to a diffusional mixing regime with the chamber gas. Evaluating the microscopic length-scales of the phase boundary associated with the injection of liquid nitrogen into its own vapor, it is found that the conventional threshold based on the interfacial Knudsen number (i.e. Kn = 0.1) does not adequately quantify the direct transition between sub- and supercriticality. Instead, a threshold that is an order of magnitude smaller is more appropriate for this purpose. Extending the analysis to a range of diesel fuel surrogates (e.g. n-heptane and n-dodecane), and alternative engine fuels that can be blended for use in CI engines (e.g. dimethyl ether and propane), it is then found that the local Knudsen numbers associated with the injection of conventional liquid fuels are significantly higher than those previously calculated in the literature, suggesting that those fuels will not directly transition to a dense-fluid mixing regime for all engine relevant conditions. However, the results show that the immediate transition to a single-phase regime may be relevant to lighter alternative fuels like DME and propane, which is attributed to the significantly lower critical temperature of these fuels, as well as their higher miscibility with the gas in the chamber.