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A very stringent problem in most of European cities is the individual mobility. This problem is mainly caused by traffic jam and arising from this are two particularly interesting environmental issues: pollution and noise [1]. Use of two wheeler vehicles does not represent the final solution to these problems, nevertheless they can supply a useful aid to ease them. Recently, two stroke engines are being replaced with four stroke engines. For small capacity engines this means a true reduction in exhaust emissions, especially unburned hydrocarbons (HC), but, on the other hand it involves a performance reduction, particularly for sudden accelerations, which constitute an essential requirement for these vehicles [2, 3, 4, 5]. Hybridisation can help to fill the gap between two stroke engines and cleaner, but less performing four stroke engines [6]. At the same time, it can help to lower fuel consumption, by means of a reduction in the revolution speed [2, 5].

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.

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.