Minimum oil film thickness MOFT measurements have been carried out in big-end bearings of V6 and in-line four-cylinder gasoline engines during engine operation. MOFT decreases with increasing crankshaft speed above 2000 rlmin. The most severe practical steady-state operation is high-speed cruising. Maximum shear rates are in the region of 107s-* at 4000 rlmin.the dynamic viscosities at a shear rate of 1O6s-' correlate significantly better with monograde MOFT data than with multigrade data; the correlation parameters for mono-and multigrade data are also significantly different. Although the dynamic viscosity measurement correlates with multigrade data better than the low-shear rate kinematic viscosity, the differences are not always significant at the 95% confidence level. Some other rheological parameter or combination ofparameters may be better than either kinematic or dynamic viscosities.Abstract minimum oil film thickness, MOFT, big-end, bearing, engine, vis-
cosity, rheology KeywordsUntil recently, the most widely used method of measuring journal bearing performance in operating gasoline or diesel engines was t o measure bearing weight loss at the end of a test, which inevitably involved operating under severe conditions of load and speed in order to obtain metal-to-metal contact.'" Such tests are the ultimate arbiter for assessing long-term durability performance. They are less than ideal however for studying the effects of oil rheology on journal bearing performance because of: (a) poor test repeatability; and (b) complications arising from operating under boundary/mixed lubrication conditions where chemical effects of the dispersanthnhibitor package complicate interpretation of the results in terms of oil rheology.'T2 Poor repeatability also makes bearing weight-loss
