Connecting Rod Bearing Operation with Aerated Lube Oil

The connecting rod big-end bearing is one of the most heavily loaded components of the lubrication system of high speed combustion engines. The bearing's oil supply has to be designed consciantious in order to ensure an immaculate reliability in operation.

The supply oil flow has to pass the main bearing and the rotating crankshaft before entering the connecting rod bearing. It is common knowledge that the centrifugal forces due to the crankshaft rotation influence the oil flow through the also rotating supply bore. The centrifugal forces effect a parabolic pressure profile along the supply bore. The oil pump has to ensure a certain pressure level in the main oil gallery (depending on the engine speed and the spherical positioning of the rotating bore) to overcome these centrifugal forces. If the oil pressure is lower than this certain level the bearing's oil supply will be interrupted - bearing damage is the consequence.

A second oil pressure limit for conrod bearing operation has been detected in the early eighties. When reducing the usual oil pressure level reproducable cavitation erosion marks could be observed on the bearing shells still before reaching the centrifugal pressure limit. This effect has been explaned by quick changes in excentricity at low oil pressure levels which cause cavitation effects in the bearings's oil film. Only topical research material makes the clarification of the problem's background and the attendant phenomenon possible: beside engine speed the amount of oil aeration is one of the strongest influence parameters on happening of the effect. ‘Aeration Cavitation’ within the rotaing oil supply bore is the real explanation of the second pressure limit. Dissolved air will be released in the rotating oil bore due to the pressure decrease caused by the centrifugal forces. The released air disturbs the oil flow - in consequence oil starvation occurs in the bearing.

Beside basics to the behavior of oil aeration the paper presents mainly the ‘Aeration-Cavitation’ phenomenon within the rotating supply bore and its effects on conrod bearing reliability. In that matter measurement results taken from the operating engine and photography of the phenomenon artificially produced within a transparent crankshaft model will be shown.