The continuous demand of even-higher efficiency in internal combustion engines has led to a growing interest in the study of their internal losses’ mechanisms. In particular, a large amount of friction losses is registered at engine’s bearings, whose correct definition deeply affects the achievement of the desired performances. The design of the layout of engine’s lubricated contacts is one of the first design choices to be addressed and a modification of their quotas usually involves an almost complete engine’s redesign. The developing of simulation methodologies able to evaluate the adequacy of the design choices concerning the lubricated contacts is therefore fundamental, in order to considerably reduce the number of trial-and-errors steps usually required in the design process of an engine. This activity deals with the problem of simulating lubricated contacts, mainly focusing on the developing of a proper approach to the analysis of the asperity contact problem. The hydrodynamic problem will be tackled adopting a mass-conserving algorithm, which uses a linear complementary formulation of the Reynolds equation to calculate the hydrodynamic pressure distribution within the lubricant film and to identify the cavitated region at given oil film height. Great attention will be paid to the methodology used to deal with cases of mixed lubrication, in which a partial direct metal-to-metal contact occurs. Two different methodologies will be investigated, the first based on a complementary formulation of the asperity contact problem and the latter based on the theory introduced by Greenwood and Williamson and then deepened by Greenwood and Tripp. This theory introduces a direct relationship between the asperity contact pressure and the gap between the two mating surfaces, obtained by statistically evaluating the probability that two asperity peaks come into contact and approximating each peaks’ contact according to the Hertzian theory. The derived methodology is then applied to the elastohydrodynamic analysis of the conrod small-end/piston-pin coupling of a high performance Ducati motorcycle engine. In order to calibrate the asperity contact model based on the Greenwood and Tripp theory, some non-standard roughness parameters are necessary, useful to describe the entire profile from a statistical point of view. The meaning and value assumed by these parameters will be investigated by directly measuring the roughness profiles of the two mating surfaces and implementing a procedure able to calculate them, starting from their definition. It will be shown how, by adopting different calibrations of the asperity contact model, it is possible to identify similar critical areas, in good agreement with empirical evidences of small end failures occurred during preliminary tests conducted at an early stage of the engine design process and under severe loading conditions. However, the absolute values of the various quantities involved are significantly different, making it difficult to identify admissible threshold values capable of defining the critical state of the components.
La continua domanda di una sempre maggiore efficienza dei motori a combustione interna ha comportato un crescente interesse nello studio dei meccanismi alla base delle loro perdite interne. In particolare, una grande quantità di potenza dissipata si sviluppa all’interno dei cuscinetti, dalla cui corretta definizione è strettamente dipendente il raggiungimento delle prestazioni desiderate. Il layout dei vari contatti lubrificati è una delle prime scelte progettistiche ad essere affrontata e una modifica delle loro quote solitamente comporta una riprogettazione quasi completa del motore. Risulta pertanto evidente l’importanza dello sviluppo di metodologie di simulazione capaci di prevedere l’adeguatezza delle scelte progettistiche riguardanti i contatti lubrificati, al fine di ridurre notevolmente il numero di step di trial-and-errors solitamente necessari nella progettazione di un motore. In questo lavoro viene affrontato il problema della simulazione di contatti lubrificati, con particolare interesse allo sviluppo di un adeguato approccio all’analisi del problema del contatto diretto. Il problema idrodinamico verrà affrontato impiegando un algoritmo basato sulla conservazione della massa, che impiega una formulazione complementare lineare dell’equazione di Reynolds per calcolare la distribuzione di pressione idrodinamica all’interno del film di lubrificante e per individuare la zona cavitata del cuscinetto a una data altezza di meato. Maggiore attenzione verrà posta alla metodologia impiegata per affrontare i casi di lubrificazione mista, in cui si verifica un parziale contatto diretto di metallo su metallo. Verranno indagate due diverse metodologie, la prima basata su una formulazione complementare del problema del contatto diretto e la seconda basata sulla teoria introdotta da Greenwood e Williamson e in seguito approfondita da Greenwood e Tripp. Tale teoria introduce una relazione diretta tra pressione di contatto diretto e gap tra le due superfici, ricavata valutando statisticamente la probabilità che due picchi di asperità entrino in contatto e approssimando ogni contatto di asperità secondo la teoria di Hertz. La metodologia sviluppata verrà impiegata per l’analisi elastoidrodinamica dell’accoppiamento occhio piccolo di biella – spinotto di un motore motociclistico Ducati ad alte prestazioni. Al fine di calibrare il modello di contatto basato sulla teoria di Greenwood e Tripp è necessario conoscere alcuni parametri non-standard di rugosità, impiegati per descrivere dal punto di vista statistico l’intero profilo. Il significato e il valore assunto da tali parametri verranno indagati misurando direttamente i profili di rugosità delle due superfici a contatto e implementando una procedura in grado di calcolarli, partendo dalla loro definizione. Si mostrerà come adottando differenti calibrazioni del modello di contatto diretto sia possibile identificare le medesime zone critiche, in buon accordo con alcune evidenze empiriche di cedimento dell’occhio piccolo di biella avvenute durante test preliminari condotti in severe condizioni di carico. I valori assoluti delle varie grandezze in gioco, tuttavia, risultano essere rilevantemente diversi, rendendo difficile l’identificazione di valori soglia ammissibili in grado di definire lo stato di criticità dei componenti.
Analisi elastoidrodinamica di contatti lubrificati in motori a combustione interna: gestione dei problemi di cavitazione e contatto diretto
FERRETTI, ANDREA
2020
Abstract
The continuous demand of even-higher efficiency in internal combustion engines has led to a growing interest in the study of their internal losses’ mechanisms. In particular, a large amount of friction losses is registered at engine’s bearings, whose correct definition deeply affects the achievement of the desired performances. The design of the layout of engine’s lubricated contacts is one of the first design choices to be addressed and a modification of their quotas usually involves an almost complete engine’s redesign. The developing of simulation methodologies able to evaluate the adequacy of the design choices concerning the lubricated contacts is therefore fundamental, in order to considerably reduce the number of trial-and-errors steps usually required in the design process of an engine. This activity deals with the problem of simulating lubricated contacts, mainly focusing on the developing of a proper approach to the analysis of the asperity contact problem. The hydrodynamic problem will be tackled adopting a mass-conserving algorithm, which uses a linear complementary formulation of the Reynolds equation to calculate the hydrodynamic pressure distribution within the lubricant film and to identify the cavitated region at given oil film height. Great attention will be paid to the methodology used to deal with cases of mixed lubrication, in which a partial direct metal-to-metal contact occurs. Two different methodologies will be investigated, the first based on a complementary formulation of the asperity contact problem and the latter based on the theory introduced by Greenwood and Williamson and then deepened by Greenwood and Tripp. This theory introduces a direct relationship between the asperity contact pressure and the gap between the two mating surfaces, obtained by statistically evaluating the probability that two asperity peaks come into contact and approximating each peaks’ contact according to the Hertzian theory. The derived methodology is then applied to the elastohydrodynamic analysis of the conrod small-end/piston-pin coupling of a high performance Ducati motorcycle engine. In order to calibrate the asperity contact model based on the Greenwood and Tripp theory, some non-standard roughness parameters are necessary, useful to describe the entire profile from a statistical point of view. The meaning and value assumed by these parameters will be investigated by directly measuring the roughness profiles of the two mating surfaces and implementing a procedure able to calculate them, starting from their definition. It will be shown how, by adopting different calibrations of the asperity contact model, it is possible to identify similar critical areas, in good agreement with empirical evidences of small end failures occurred during preliminary tests conducted at an early stage of the engine design process and under severe loading conditions. However, the absolute values of the various quantities involved are significantly different, making it difficult to identify admissible threshold values capable of defining the critical state of the components.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14242/80050
URN:NBN:IT:UNIMORE-80050