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The fuel direct injection in SI engines is demonstrating a remarkable potential regarding the reduction of consumption and pollutant emission. Nevertheless, the management of the mixture formation “in-cylinder” - in conditions of a short duration and of a complex fluid dynamic configuration imposes both an accurate modeling and an exact control of the process. The problem gains on complexity when considering the use of alternative fuels which becomes more and more a subject of actuality. The paper presents a comparative analysis of mixture formation process and engine performances, when applying direct injection of gasoline, respectively of ethanol in a four-stroke single cylinder SI engine. The modulation of the injection rate shape is the result of a fuel high pressure wave, generated in a pressure pulse direct injection system.

The actual trends in development and series application of mixture formation techniques for SI engines converge irrevocably to a process after scavenging, by direct injection, the reason being the higher thermal efficiency in a wide operation range of the engine, leading to substantially lower bsfc and pollutant emission. After numerous and successful research projects of direct injection for two-stroke engines, the most of series applications are being introduced for four-stroke automotive engines. A main reason for this profitable way consists in the better fluid dynamic conditions and in the longer time for mixture formation in the case of the four-stroke process.

The paper presents the results of a down-sizing concept implicating gasoline direct injection, which is applied to a four-stroke four-valve SI engine with a displacement of 500 ccm per cylinder. The typical features of a down sized engine such as a high level of engine speed, high power density at low fuel consumption and a low level of pollutant emission form the main targets of this study. Numerical models of the process stages have been developed in 1D and 3D CFD codes. The accurateness of the models has been proved using experimental results. The main work consisted on the application of a direct injection system to the engine. The compact engine design and the high compression ratio have been maintained resulting in a combustion chamber design without any cavities or bowls. To obtain accurate results, the simulation work has been carried out using two different CFD-codes (FIRE and VECTIS); the results have been analyzed and compared.

The internal mixture formation within SI engines using fuel direct injection has a significant potential regarding the reduction of bsfc and pollutant emission. However the short time available for injection and spray distribution, as well as the complexity of the fluid dynamic conditions, amplified in a wide load and speed range, form a different base for the combustion process than using external mixture formation. The intend of the present study is to develop a method for modeling and optimization of mixture formation and combustion using a general approach for the fuel direct injection, which consist in the modulation of the injection rate, independently on the engine speed. In the first stage of modeling, the optimum combination between mixture formation elements as fuel pressure history, injection timing, spray characteristics, injector location or combustion chamber design is of great importance, forming the conditions for the subsequent combustion process.