Effect of Oil Rheology on Journal Bearing Performance Part 3—Newtonian Oils in the Connecting-Rod Bearing of an Operating Engine

The connecting-rod, big-end, bearing of a 2.8 litre, V-6, gasoline engine has been instrumented to allow measurement of oil film thickness during engine operation. The minimum oil film thickness at any given crankangle is measured as a function of crankangle by continuous measurement of the capacitance of the oil film in the bearing; a light-weight, aluminum-alloy, scissor linkage is used to support wires from the reciprocating big-end. Although absolute magnitudes of oil film thickness could be uncertain by as much as 50%, the technique provides excellent precision for measuring the relative effects of oil rheology and engine operating conditions on bearing oil film thickness, the standard error for repeat measurements being about 3%. Minimum oil film thickness increases when oil viscosity is increased and oil temperature decreased. Oil film thickness responds to changes in crankshaft speed and engine torque in a predictable manner. The thinnest oil films occur during the induction stroke where inertial rather than gas pressure effects are dominant. As a consequence, high crankshaft speed represents the most critical steady-state operating conditions for the connecting-rod big-end bearings of the engine instrumented, with engine torque being uncritical.