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The relationship between high temperature engine oil viscosity and its
load carrying capability is not well established. A new test method now
offered at Southwest Research Institute measures the bearing oil film thickness
(BOFT) of lubricants in a fired engine under various operating conditions.
BOFT is determined by isolating a main bearing from the engine block
and making electrical capacitance measurements between the bearing
and the crankshaft. The oil surrounding the radial clearance acts as the insulator.
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The bearing shell is isolated from the engine
block by a special nonconducting paper, and the crankshaft is grounded
by a sliding contact at the crank pulley, thus forming a cylindrical capacitor.
The capacitance is at a minimum when the two cylinders are concentric.
Eccentricity increases the measured capacitance. The ratio between the
eccentric and the concentric capacitance yields the value
of eccentricity. If Cr is equal to the radial bearing clearance,
the e is the eccentricity, the minimum oil film thickness (MOFT) is:
MOFT = Cr (l – e)
With the trend toward lower viscosity oils for fuel economy, a significant
unknown is whether all polymer-containing motor oils offer similar
bearing protection. We hope to find the answer to the question with the
new test method. |
