The current ongoing rise in environmental pollution is leading research efforts toward the adoption of propulsion systems powered by gaseous fuels like hydrogen, methane, e-fuels, etc. Although gaseous fuels have been used in several types of propulsion systems, there are still many aspects that can be improved and require further study. For this reason, we considered it important to provide a review of the latest research topics, with a particular focus on the injection process. In advanced engine systems, fuel supply is achieved via enhanced direct injection into the combustion chamber. The latter involves the presence of under-expanded jets. Under-expanded jets are a particular kind of compressible flow. For this reason, the review initially provides a brief physical explanation of them. Next, experimental and numerical CFD investigation techniques are discussed. The last section of this manuscript presents an analysis of the jet’s structure. The injection parameters commonly used are examined; next, the characteristics of the near-nozzle field are reviewed and finally, the far-field turbulent mixing, which strongly affects the air–fuel mixture formation process, is discussed.

The direct injection of a gaseous fuel in internal combustion engines involves under-expanded supersonic jets and complex air/fuel fluid-dynamics. Furthermore, with the high pressure ratios between the injector and the cylinder, the gaseous flow usually becomes choked even inside the injector. Knowledge of all these phenomena is essential to achieve a deeper understanding of the air–fuel mixing process that follows, influencing combustion and pollutant formation. In this framework, this study deals with the development and validation of a fully explicit, density-based solver for supersonic compressible flows, using the OpenFOAM library and featuring Runge–kutta fourth order time discretization and the Kurganov central flux splitting scheme. This methodology was applied to analyze the inner and the external flow of an innovative, multi-hole, high pressure injector for heavy-duty vehicle applications. The adoption of multi-hole patterned injectors in gaseous fuel combustion systems is believed to be an efficient way of achieving a better air/fuel mixture and, therefore, improving the combustion reaction. The present work aims to evaluate the reliability of the aforementioned mathematical approach for such kinds of complex flows and, especially, provide a comprehensive characterization of the multi-jet spray. It was found that shock waves in the internal-nozzle deeply modify the flow development and the external Mach disk as shock cells move the mixing activity on a lateral shear layer. It was also observed that a methane cloud grows downstream and, although flammable conditions are present, it later inhibits air recirculation toward the near nozzle zone.

The valorization of residual biomass plays today a decisive role in the concept of “circular economy”, according to which each waste material must be reused to its maximum extent. The collection and energy valorization at the local level of biomass from forest management practices and wildfire prevention cutting can be settled in protected areas to contribute to local decarbonization, by removing power generation from fossil fuels. Despite the evident advantages of bioenergy systems, several problems still hinder their diffusion, such as the need to assure their reliability by extending the operating range with materials of different origin. The Italian project “INNOVARE—Innovative plants for distributed poly-generation by residual biomass”, funded by the Italian Ministry of Economic Development (MISE), has the main scope of improving micro-cogeneration technologies fueled by biomass. A micro-combined heat and power (mCHP) unit was chosen as a case study to discuss pros and cons of biomass-powered cogeneration within a national park, especially due to its flexibility of use. The availability of local biomasses (woodchips, olive milling residuals) was established by studying the agro-industrial production and by identifying forest areas to be properly managed through an approach using a satellite location system based on the microwave technology. A detailed synergic numerical and experimental characterization of the selected cogeneration system was performed in order to identify its main inefficiencies. Improvements of its operation were optimized by acting on the engine control strategy and by also adding a post-treatment system on the engine exhaust gas line. Overall, the electrical output was increased by up to 6% using the correct spark timing, and pollutant emissions were reduced well below the limits allowed by legislation by working with a lean mixture and by adopting an oxidizing catalyst. Finally, the global efficiency of the system increased from 45.8% to 63.2%. The right blending of different biomasses led to an important improvement of the reliability of the entire plant despite using an agrifood residual, such as olive pomace. It was demonstrated that the use of this biomass is feasible if its maximum mass percentage in a wood matrix mixture does not exceed 25%. The project was concluded with a real operation demonstration within a national park in Southern Italy by replacing a diesel genset with the analyzed and improved biomass-powered plant and by proving a decisive improvement of air quality in the real environment during exercise.