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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.

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 manufacturers continually look for improved methods to design engines. Increased durability, enhanced engine performance, decreased emissions and low cost are all issues to consider during the early design stages. The purpose of this paper is to investigate the thermal flows and heat transfer phenomena occurring in the small engines and to suggest valid methods for their prediction to be used inside computer design software. A new approach to theoretically calculate the heat transfer based on the thermal vibrational convection theory is first proposed. The basic idea of this approach is that the heat transfer process can be correlated mainly with the thermal explosion and with the detonation wave produced by combustion. Later on, a simplified model based on energy balance method is investigated and its use in the engine computational software is proposed showing how it represents the best solution to develop a software procedure to simulate heat transfer.