Iniezione diretta di combustibili liquidi e gassosi nei motori alternativi a combustione interna

The adoption of direct injection systems in internal combustion engines seems to be a proficient way to reduce tailpipe pollutant emissions, fuel consumption and enhance vehicles performances. Indeed, in the last decades the ever-increasing concerns about the environmental pollution and the green-house effect demonstrate the absolute necessity to achieve, among the many human activities, a sustainable mobility too. In this scenario many solutions have been developed to enhance the environmental performance of the vehicles. The hybrid or fully electric vehicles alone are not the solution of this issue, at least in the short period. Consequently, the author decides to investigate internal combustion engines and, in particular, injection processes performed directly in combustion chamber. Direct injection, established practice in compression ignition engines, is challenging to be implemented in spark ignition engines due to the intrinsic nature of the latter but allow meeting the latest rules about pollutant emissions. Latest advancement in the field of fluid dynamic simulations and of experimental non-invasive optical techniques allow to deeply analyze injection processes and effectively adopt them in commercial vehicle. Therefore, in this work the concept of direct injection was intensively studied and declined for liquid and gaseous fuels. At the first category belong Gasoline Direct Injection (GDI) engines which are a promising technology that is giving a new life to gasoline engines. Thermal efficiency enhancement and engine downsizing are the key improvement achievable. Special attention will be paid to flash boiling sprays in GDI engines that is a topic of great scientific interest in the last years and a phenomenon capable to improve GDI technology. Among the gaseous fuel Compressed Natural Gas (CNG) was chosen in this study. CNG compared with gasoline has higher-octane number, higher hydrogen to carbon ratio, and lower energy-specific CO2 emissions. Furthermore, it can be produced in renewable ways and is more widespread and cheaper than conventional liquid fossil fuels. In this regard, the direct injection of CNG engines can be considered a promising technology for highly efficient and low-emission future engines. The jets produced by a single hole and a multi hole device were studied detailing the characteristics of under-expanded structures present in supersonic gaseous jets. A proper Computational Fluid Dynamic (CFD) method, featuring high order scheme discretization, was developed and tested. The results of the simulations have been validated by means of the experimental data collected.