1994-10-01

A Mathematical Model for the Calculation of Blow-by Flow and Oil Consumption Depending on Ring Pack Dynamic Part I: Gas Flows, Oil Scraping and Ring Pack Dynamic 941940

In the present paper a mathematical model on ring pack behavior is presented. The program considers the aspect of gas flow into and from the inter-ring volume and the relative ring dynamic. Furthermore a proper mass balance on the oil film has been considered to automatically evaluate both starvation and the oil accumulation in front of the inlet boundary of each ring. The model can give quite accurate predictions of the gas flows and the oil film thicknesses. It may be considered the first step for the simulation of oil mist formation and evaporation that are the most important phenomena for oil consumption prediction.UBRICATING OIL gives a strong contribution to particulate formation in diesel engines. Moreover it influences the unburned hydrocarbon emission of spark ignition engines because of the absorption/desorption phenomenon between the unburned fuel and the lubricating oil films [1, 2].Therefore the design of ring pack has become increasingly important both for lubricant conservation and exhaust emission control. At this scope little lubricant as possible must be consumed in engine operations.The mechanism of oil consumption in the internal combustion engines is very complex so that nowadays is not completely understood. Anyway, in the study of the global phenomenon it may be useful to distinguish between two sequential stages, which are often named: oil rising and oil disappearing.The term “oil rising” is adopted to mean the ensemble of mechanisms that promote the transport of oil into the combustion chamber. The “oil disappearing” is related to phenomena such evaporation and combustion that transform the oil into products carried out by the flow of exhaust gas.The paths of oil flow through the rings are three (fig.1): 1. the oil flow as oil film between the piston ring and the liner 2. the oil flow through the ring gap 3. the oil flow through the ring groove. The driving forces that are generally considered responsible of flows are: the difference of pressure between the lands, the inertia forces and the transport of oil as fog or vapor in the gas stream. The relative importance of mechanisms depends on several factors including the geometry of the ring pack, the mass of rings, their shape, elastic loads and so on.Some calculations of oil consumption have been carried out according to the hypothesis that the skirt clearance remains fully filled with lubricant oil at any moment. This hypothesis seems to produce reasonable trends, even if the absolute values obtained by calculation are several times bigger than those measured [3]. Other authors consider that the most important mechanisms that allow the rising of the oil in the combustion chamber are: the flow between liner and ring and the transport of oil in the gas stream as vapor or fog [4]. Unfortunately the mechanism and the location of entrainment of oil to form a mist is currently unknown and no effective mathematical description of this phenomenon has been suggested.So, probably, the mechanisms of oil consumption must be more deeply explored in order to provide reasonable prevision by calculation.In this paper, which relates on a work in progress, the most important physical schematization of phenomena connected to the oil rising have been briefly reviewed. Moreover an extensive description of the developed mathematical model and its most significant results that can be obtained are presented.

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