A Comparison of the Total Capacitance and Total Resistance Techniques for Measuring the Thickness of Journal Bearing Oil Films in an Operating Engine

Bearing oil film thickness (BOFT) values were determined for the front sain bearing of a four-cylinder engine at different engine speeds, loads, and oil temperatures using two different measurement techniques. The total resistance technique assumes the oil film can be modelled as a simple ohmic resistor. The total capacitance technique assumes the oil film can be described as a simple capacitor. A comparison of results determined using both methods for a set of single-grade oils demonstrates that, although the methods agree quantitatively for certain combinations of engine test conditions and oils, the level of precision with the total capacitance technique is greater than with the total resistance technique. This difference is attributed to the fact that the oil dielectric constant, which is needed for calculating BOFT values in the total capacitance method, can be measured more precisely than can the oil conductivity, which is needed in the total resistance method. Of the two methods, the total capacitance technique is superior for measuring bearing oil film thickness in an operating engine.