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In this paper various methods of studying in-cylinder heat transfer in small displacement two-stroke engines are presented and compared. First of all, the mistakes that can be caused by the application of steady heat transfer models to the study of such a complex phenomenon are considered. After the demonstration that an ordinary convective heat transfer model determines mistakes in the evaluation of the thermal load, three-dimensional analysis and improved heat transfer models are considered. The multidimensional code Fluent is used for the 3-D fluid mechanics simulation. Numerical analysis evidences the importance of the non-uniformity of the gas temperature inside the cylinder. The improved models considers the fact that heat transfer during compression and expansion is out of phase with bulk gas-wall temperature difference and that the great part of the heat transfer is linked to the combustion.

The chief aim of the work concerns the optimisation of a forced convection air cooling system and particularly of the external heat exchange surfaces, applied on a small internal combustion two-stroke engine. At the present time in the industrial practice, the design of cooling system and of fin arrays is developed by means of simple technical criteria, starting fiom the assumption to consider the thermal loading of engine about 30% of the power furnished by fuel. This paper describes an approach that combines theoretical and simulation techniques with experimental methods. Attention is primarily focused on the most important input data of this kind of optimisation represented by the thermal load of the engine. Then attempts are made to establish some general criteria for the optimum design. The proposed calculation methods have carried out the creation of a PC software, useful instrument for the design phase of a new engine or for the improvement of an old one.

The heat transfer process has always played an important role in internal combustion engine design. An area of importance is the thermal loading of engine structural components, and the optimisation of engine cooling system. The engine cooling system of a vehicle makes up a significant portion of the total component cost. It also places demands on other vehicle systems, and the quality of its design is evident to the customer in terms of the power that it consumes that for a two-stroke engine with forced convection air cooling can be also the 10% of the total brake power. An area of importance is the calculation of the thermal load of engine structural components, and the optimisation of engine cooling system. Optimisation of engine cooling requires the solution of the coupled problem of heat transfer from gases to walls and of heat convection from the structure (generally a finned surface) to the external environment.