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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 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.

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.

In order to develop modern two-stroke engines with low fuel consumption, respectively with low exhaust emissions, two alternative development areas - the mixture formation and the scavenging system - have been correlated. For a satisfying mixture formation without fuel losses by scavenging, the direct injection seems to be one of the best solution for the high speed two-stroke engine of the future. On the other hand the modern development of two-stroke scavenging systems shows a large field of application and improvement methods of cross and loop scavenging [1]. Based on the specific optimisation factors of the injection system, respectively of the scavenging system, the aim off this common work of the Universities of Pisa and Zwickau is to correlate both the optimisation fields in an advantageous mixture formation process.