تجزیه و تحلیل از مدل های شبیه سازی متنوع برای مجله علمی بلبرینگ موتور احتراق و نفوذ از وضعیت نفت

The paper deals with the comparison of different models, from simple to complex, for the simulation of non-stationary response of the journal bearings used in combustion engines. The variety of simulation models covers classical methods from Holland, Butenschoen and numerical methods based on the Hydrodynamic (HD), Elasto-hydrodynamic (EHD) and Thermo-elasto-hydrodynamic (TEHD) lubrication theory. Several crankshaft main bearings and connecting rod big end bearings are investigated. The comparison includes the following bearing parameters: Peak Oil Film Pressure (POFP), Minimum Oil Film Thickness (MOFT) and oil flow. Calculation time is compared, too. Estimation of oil viscosity is discussed over a typical temperature and pressure range found in combustion engines, including the influence of the fuel dilution as an example of the oil aging phenomena.

Performance and reliability of journal bearings has always been of central importance for the crank train operation. In this paper, the results of different models for the simulation of non-stationary response of the journal bearings used in combustion engines are compared. These simulation models include classical methods from Holland and Butenschoen and numerical methods based on the Hydrodynamic (HD), Elasto-hydrodynamic (EHD) and Thermo-elasto-hydrodynamic (TEHD) lubrication theory. Holland and Butenschoen algorithms are semi-analytical and work only with fully filled bearing, rigid shell and constant viscosity. HD and EHD finite volume methods can account for cavitation (filling ratio) and pressure-dependent viscosity. The HD algorithm implies rigid and EHD elastic bearing shell. The TEHD method implies elastic shell and works with pressure and temperature dependent viscosity. Generally, pressure and temperature have been considered as potentially important variables in numerical simulation but shear thinning and especially oil condition are often ignored. The latter can be dated back to the lack of cooperation between oil manufacturers, oil condition laboratories and simulation software developers. Research work on oil viscosity comprising temperature, pressure and shearing is rare. Within this work, several crankshaft main bearings and connecting rod big end bearings are investigated. Bearing loads are obtained from the simulation of the complete engine. Selected simulation results are discussed and several conclusions are drawn. All simulations presented in this paper are performed using AVL software EXCITE and EXCITE Designer.

The present analysis shows that for the accurate modeling of the bearing behavior the elasticity of the shell has to be ultimately considered. Hence only the EHD and TEHD methods can be applied there. However, these methods require much more input data and lead to long calculation times. Therefore classical methods by Holland and Butenschoen can also be useful if medium accuracy is sufficient and short simulation times are required. For the mean oil flow in crankshaft main bearings these methods often give satisfactory results and can be combined with the stationary simulation of the entire lubrication system. The influence of pressure and temperature on the oil viscosity has to be taken into consideration. This can be accomplished only within the TEHD algorithm. Rodermund formula proves to be applicable there. However, for different oils it is difficult to obtain reliable coefficients. Using only pressure-dependent viscosity (EHD calculation) is not always justified because it accounts only for the viscosity increase due to pressure rise without considering the viscosity decrease due to temperature rise. Analysis of the oil aging phenomenon due to the fuel dilution shows that within the realistic content of diluted fuel (up to 2%) the main simulation results change insignificantly. Only for the higher percentage of the fuel in the oil (5% and more), the influence on the oil flow becomes considerable. Numerical simulation considering the viscosity dependence on oil shear thinning around the bearing circumference is a matter of future investigation. Furthermore, the property changes of used oils caused by soot and oxidation and VI improver degradation will be studied.