A comprehensive analysis of natural gas direct injection and of mixture formation in spark ignition engines

The transportation sector is becoming increasingly linked to environmental problems, with a technology relying heavily on the exploitation of not renewable energies: the combustion of hydrocarbons in internal combustion engine (ICE). Since this is still expected to be the main technical solution to fulfil the mobility demand, more secure and sustainable fuel sources are needed. Natural gas (NG), which is composed primarily of methane, is regarded as one of the most promising alternative fuels because of its interesting chemical properties with high H/C ratio and high research octane number. The use of NG allows the reduction of pollutant emissions, while the gap in performance with respect to gasoline engines can be recovered by means of turbocharging devices. NG engine performance can be further improved by adopting a direct injection (DI) concept that can also extend the fuel-lean operating limit of normal engine operation, compared to port fuel injection. Within this framework a research activity was carried out at the Politecnico di Torino within the InGAS Collaborative Project (VII FP), Sub-project A2, dealing with the development of a turbo DI CNG engine, optimized for mono-fuel CNG operation. The present thesis focuses on the development of a methodology for the numerical simulation (by means of 3D CFD) of the CNG direct injection process, in view of its application to internal combustion engines, and on the analysis of the physical processes related to the jet formation and development and the formation of the air-fuel mixture. The results of the numerical analysis are then correlated with the outcomes of an experimental activity performed on the developed multi-cylinder engine and on an optical access single-cylinder engine, in order to obtain more detailed knowledge of the DI CNG engine behaviour. The thesis is divided into six chapters. After a short introduction related to the transportation sector environmental impact (Chapter 1, section 1.1) an overview of the CNG Vehicle technology (section 1.2) is presented. The results achieved during the InGAS CP (SP A2) are summarized in section 1.3. Chapter 2 focuses on the development of the numerical model for CNG direct injection simulation in an engine combustion chamber. Firstly the theoretical background of supersonic underexpanded jets is summarized (section 2.1), followed by a literary review of previous investigations regarding the numerical simulation of gaseous direct injection in internal combustion engines (section 2.2). In section 2.3, the test case of a two-dimensional compressible flow issuing from a nozzle is examined to characterize the underexpanded-jet phenomenon independently from any geometrical constraint, imposed by the specific application. The main guidelines drawn in this preliminary study are then applied to the development of the finite volume numerical model of the engine, that is thoroughly described in section 2.4. Chapter 3 is devoted to the validation of the numerical model. Section 3.1 describes the planar laser induced fluorescence (PLIF) technique, employed for the experimental investigation of the mixture formation process in the optical access SCE, whose setup is presented in section 3.2. Finally the post-processed results of the PLIF experiments are used to validate the developed numerical model in section 3.3. A comprehensive analysis of the mixture formation process in the InGAS DI-CNG engine is discussed in Chapter 4, with reference to relevant operation conditions and injection strategies. The homogeneous operation at partial and full load is analyzed in section 4.1 and 4.2 respectively. The lean stratified operation at partial load is studied in section 4.3. Chapter 5 is related to the injection system development and focuses on the optimization of the control phase of the InGAS injector, paying specific attention to its behavior at small injected-fuel amounts. The development of a CFD model of the actual injector geometry is described and the dynamic behaviour of the injector is analyzed. Finally, Chapter 6 draws the main conclusions of this study.