|Tipo di tesi||Tesi di dottorato di ricerca|
|Autore||MASTRANDREA, LUCA NICOLÒ|
|Titolo||Analisi elastoidrodinamica di contatti lubrificati in componenti motore: analisi dei meccanismi di danno|
|Titolo in inglese||Elastohydrodynamic analysis of lubricated contact in engine components: Investigation of damage mechanisms|
|Settore scientifico disciplinare||ING-IND/14 - PROGETTAZIONE MECCANICA E COSTRUZIONE DI MACCHINE|
|Corso di studi||INGEGNERIA INDUSTRIALE E DEL TERRITORIO|
|Data inizio appello||2018-09-28|
|Disponibilità||Accesso limitato: si può decidere quali file della tesi rendere accessibili. Disponibilità mixed (scegli questa opzione se vuoi rendere inaccessibili tutti i file della tesi o parte di essi)|
|Data di rilascio||2058-09-28|
Durante gli ultimi decenni le prestazioni dei motori a combustione interna hanno visto un costante aumento. Contemporaneamente si ha sempre maggiore attenzione agli aspetti inerenti all’efficienza e all’affidabilità dei componenti.
In the present contribution a complementarity formulation of the elastohydrodynamic problem in presence of cavitation have been proposed. The elasto-hydrodinamic problem is tackled using a formulation based on the solution of the Reynolds equation, suitably recast in terms of two complementarity variables, namely the hydrodynamic pressure and the void-fraction. The correct prediction of the cavitation and the location of the boundaries between cavitated and active areas is guaranteed by the complementarity nature of the two chosen variables and the approach naturally guarantees mass conservation and unconditional convergence. The HL complementarity formulation of the Reynolds equation has been improved to handle the elastic deflections of the contacting bodies. The EHL problem is numerically solved using a weak formulation based on the Finite Element Method.When the load capacity of the fluid film is not able to sustain the external loads, a direct contact between the solid surfaces asperities occurs. In this case, the supporting load is provided partly by the hydrodynamic oil film pressure and partly by the asperities dry contact pressure. This lubrication regime is well known as the mixed lubrication regime. Therefore, the proposed algorithm have been further improved including the possibility to handle a direct contact between the surfaces asperities. In particular, the mixed lubrication problem has been described in terms of a complementarity problem, being the two complementarity variables the dry contact pressure and the gap between the mating surfaces. The proposed algorithm have been employed for the analysis of lubricated contact of different engine components in order to predict the reliability of this joints. In particular two of the mains causes of failure in lubricated contacts has been analyzed in detail, the wear due to direct metal to metal contact has been investigated in the analysis of the conrod small-end and the cavitation damage in the analysis of the conrod big end bearing. The conrod small-end bearing is one of the most critical engine parts from a tribological point of view. In the conrod small-end, the relative rotational speed is low and a complete rotation between the mating surfaces does not occur. Therefore, the dominant effect in the lubrication of the small-end is the film squeeze caused by the alternating combustion/inertial loads, acting on the coupling, while sliding hydrodynamic effects are negligible. On the other hand the phenomenon of the damage caused by the onset of cavitation bubbles within the lubricant fluid film in engine bearings has been analyzed in detail. With the increase of the engine performance, rotational speed and combustion/inertial loads, the cavitation of the lubricant in the conrod big-end bearing plays a crucial role in the bearing durability assessment. Despite the fact that an universally accepted theory does not exist, it is clear that the cavitation damage occurs due to the rapid and violent collapse of the vapour bubbles in proximity of a solid boundary. A cavitation damage index (CDI) has been proposed, based on the accepted hypothesis that the damage occurs due to the rapid and violent collapse of the vapour bubbles in proximity of the bearing inner surface, and it can be related to the bubble dimension evaluated just before the film reformation.