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A detailed prediction of injection and air–fuel mixing is fundamental in modern direct injection, spark-ignition engines to guarantee a stable and efficient combustion process and to minimize pollutant formation. Within this context, computational fluid dynamics simulations nowadays represent a powerful tool to understand the in-cylinder evolution of spray and air–fuel charge. To guarantee the accuracy of the adopted multidimensional spray sub-models, it is mandatory to validate the computed results against available experimental data under well-defined operating conditions. To this end, in this work, the authors proposed the calibration and validation of a comprehensive set of spray sub-models by means of the simulation of the Spray G experiment, available in the context of the engine combustion network. For a suitable validation of the proposed numerical setup in addition to the baseline condition, gasoline direct injection operating points typical of early injection with homogeneous operation, late injection with high ambient density and flash boiling with enhanced fuel evaporation were also simulated. Numerical computations were validated against a wide set of available experimental data by means of an accurate post-processing analysis taking into account axial liquid and vapor penetrations, gas-phase velocity between spray plumes, droplet size, plume liquid velocity, direction and mass distribution. Satisfactory results were achieved with the proposed setup, which is able to predict gasoline spray evolution under different operating conditions.

This paper presents a theoretical and experimental study of direct fuel injection at conditions relevant to spark ignition (SI) and compression ignition (CI) engines. The focus of this work is to identify the conditions under which fuel droplet formation should occur or be suppressed. An experimental investigation of the injection of sub- and supercritical propane into gaseous nitrogen is first discussed. This includes study of one case in which the fuel remained supercritical with respect to temperature and pressure throughout the injection event, and which appears to be the first time that truly supercritical hydrocarbon fuel injection is examined experimentally. A nondimensional parameter τ representing the ratio of the time scales of droplet formation and droplet evaporation is also proposed and used to explain the observed occurrence or suppression of fuel droplets at different conditions. While instants of τ < 1 suggest that droplets should always be observed in any plausible SI or CI engine design, τ > 1 also occurs during the bulk delivery of heavier hydrocarbons in CI engines. In such cases, this should justify simplified modeling of the spray as a dense fluid that mixes with its surroundings, ignoring droplet transport during the less important parts of the injection event.

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