Quote:
Originally Posted by ZDan
At low engine speeds, the hydrodynamic wedge that supports the conrod bearings isn't as strong. Consider the film strength of the oil in the bearing gap with no relative motion. Weak. Applying full torque is a bad idea at low engine rpm...
At high rpm when inertial loads dominate, the high speed gives a MUCH stronger hydrodynamic wedge to support the conrod.
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Agreed,
At low engine speeds it also depends on the temperature of the oil as the film thickness will be effected by viscosity as well as speed.
At higher rpm the shear loads on the oil are higher so there is more temporary viscosity loss in the oil(depending on its design) which can contribute to reduced film thickness particularly with higher loads.
Flow volume can be more important than oil pressure, in both cases as you need to keep feeding the wedge. So oil that is too thick at low speed can starve the bearing while oil that is too thin cannot provide the support.
Oil that is too thick at high speed can create too much oil drag and impact power and fuel economy.
It is a complex balancing act.