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Development of a Simplified Instantaneous Friction Model of the Piston-Crank-Slider Mechanism of Internal Combustion Engines
The continuous increase of the green house emissions in conjunction with the limited and finite fuel resources make the improvement of efficiency of all engines, converting fuel chemical energy to mechanical energy, imperative. Even small increase in engine's mechanical efficiency, can be proved significant in economical and environmental terms. Towards this direction, the tribology studies of the Internal Combustion Engines, mainly used as propulsion, power generation and auxiliary drives, are considered important for the design of new engines and the improvement of existing ones. The objective of this paper is to present a simplified (analytical) model for the prediction of the instantaneous friction, developed on the main components of the piston-crank-slider mechanism of an internal combustion engine, including complete piston ring packs, piston skirts, connecting rod bearings and crankshaft main bearings. In contrast to the detailed numerical models, the developed simplified model requires limited computational resources and can be easily implemented or used in conjunction with existing codes, modeling thermodynamic engine processes or engine control algorithms. On the other hand, the proposed model compared with the semi-empirical friction formulas can be considered more general and non case dependent, since it is based on the physics of engine lubrication (short bearing theory) and not on statistical observations (case related) of motoring friction tests. The proposed model was used in the case of a typical, small, single cylinder, four-stroke engine. The obtained simulation results were compared with results from a detailed numerical engine friction simulation code, previously developed and published by the author [1]. Friction measurements were also conducted on simplified friction measurement test cell, consisting of an AC electric motor driving the piston-crank-slider mechanism of an internal combustion engine. Good agreement was found between simulation and experimental results, indicating that the simplified friction model is capable of adequately predicting the dynamic friction behavior of rotating and oscillating engine main components.
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