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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.

World-wide attention to environmental issues in recent years has resulted in a greater demand for cleaner engines, especially with regards to the two-stroke. Considering the techniques for reduction of exhaust emissions the direct injection of fuel into the combustion chamber adapted for a loop scavenged cylinder seems to be an advantageous method. This paper describes the application of advanced experimental and computational techniques to evaluate mixture formation produced on a commercial engine by means of a direct fuel injection strategy, namely a ram-tuned injection system. The injection system data are experimental while air flow and fuel air mix for the direct injection engine are calculated using a turbulent model of the three dimensional code FLUENT. Extension of a first work in this field is presented. In particular two possible strategies to simulate direct injection are tested. The influence of different boundary conditions on the scavenging process was examined too.

One of the characteristic features of two-stroke cycle engines is to have the cylinder filling phase with fresh charge over-lapped to the exhaust phase. This situation leads to an inevitable loss of fresh mixture through the exhaust port. The first part of this work deals with the possible methods of evaluating the percentage of fresh charge so wasted. The validity of the simplifying hypothe sis is verified by analysis both of gas composition inside the cylinder as a function of crankangle, carried out by electrovalve sampling, and hydrocarbon composition in the exhaust gases carried out by gaschromatogra-Phy. A calculation method for evaluating the short-circuiting loss in S.I. two-stroke engines fed with air only, with subsequent gasoline injection, is then described. In fact, this system seems to be a valid solution for the future development of this type of engine.

The future development of two-stroke engines will be conditioned by the drastic reduction of pollutant emission, especially of hydrocarbon. This goal is not achievable only by scavenging improvement, rather a new quality of mixture formation using direct injection is imposed. However, the internal mixture formation in a large range of speed and load, considering the scavenge flow particularities of two-stroke engines as well, appears as an extremely complex process. Thereby a numerical simulation is in this case very effective for the adaptation of a direct injection method at the engine. The paper presents a concept for modeling and optimization of the mixture formation process within a high-speed two-stroke engine with liquid fuel injection system. The injection system generates a pressure pulse which is not dependent on the engine speed.

The paper presents a concept for modeling and optimization of the mixture formation process during gasoline direct injection, using a high-pressure single fluid injection system which allows the modulation of the injection rate independently on the engine speed. Going from this favorable premise for the adaptation of the mixture formation to various load and speed conditions, the aim of modeling is to find the optimum combination between the adaptable elements as follows: form of the fuel pressure wave, injection timing, spray form, injector location, form of the combustion chamber. Moreover, the interaction between fuel and air flow within the cylinder during the mixture formation is considered as a determining factor for the combustion process, and forms thereby an important part of the modeling.

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.