EXPERIMENTAL STUDY OF GASOLINE DIRECT INJECTION UNDER SPRAY COLLAPSE CONDITIONS

The pressing and unresolved issue of environmental pollution and the increasing concerns on energy supply have led to the production of vehicles with improved fuel economy and reduced emissions, becoming even more necessary. This thesis shows that the gasoline direct injection (GDI) engine will continue to develop in the coming years, becoming a key part of the automotive propulsion market. The work presented has been carried out at CMT - Centro Motores Térmicos, a research centre located in Valencia, fully involved in the development of future combustion engines. The objective pursued is to analyse the behaviour of different kinds of fuel under varying operating conditions, such as temperature and pressure, of either the injected fuel or the combustion chamber. The importance of the study regarding the spray collapse lies in the fact that this phenomenon affects the atomization of the spray, because of its impact on fuel-air mixing. Spray collapses were observed, in this work, under the elevated ambient pressure conditions, but in other investigations also under flash boiling conditions. However, it was found flash boiling may be also beneficial, since it leads to a reduction in the size of the fuel particles, with the advantage to improve the mixing between fuel and air, increasing the effciency of the combustion and reducing pollutant emissions. Nevertheless, the effect of these phenomena on the behaviour of GDI injectors and their sprays is yet not fully understood and difficult to predict. In the present work, experimental studies have been performed in an optically accessible constant volume vessel. The ambient pressure of the vessel ranged from 3 to 21 bar and the ambient temperature varied between 573 and 800 K. The tests have been carried out with injection pressure of 100, 200, 280 bar. High-speed imaging using DBI and the Schlieren measurement technique were utilized to investigate the spray morphology. The tested fuels were iso-octane and n-heptane. Two different kinds of fuel have been used in order to evaluate the effect of the volatility on the flash boiling process. Normal-heptane tests have been carried out mostly to compare these experiments with others in the scientific literature. It was observed that thermo-physical properties of the chamber are the ones that have the most significant effect on the characteristic of the spray. The results highlighted which are the determining physical quantities and how they influence the morphology of the spray.