Submitted to: Lubrication Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 20, 2007
Publication Date: March 15, 2008
Citation: Knothe, G.H. 2008. Evaluation of ball and disk wear scar data in the HFRR lubricity test. Lubrication Science. 20:35-45. Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils. The new petroleum-derived diesel fuels (petrodiesel) required by regulations possess poor lubricity, a property the diesel engine had relied on in the past. Biodiesel and minor components thereof possess good lubricity and can restore lubricity to petrodiesel. For testing lubricity of diesel fuels a procedure termed the high-frequency reciprocating rig (HFRR) lubricity tester exists. In this test, a steel ball scrapes over the surface of a steel disk, producing a wear scar. The wear scar produced on the ball is used to evaluate the lubricity of the sample. The smaller the wear scar, the better the lubricity. The present work, using numerous data generated by biodiesel and petrodiesel samples as well as mixtures thereof; shows, that while the best statistical correlation exist when using ball wear scar data, the disk wear scar data is almost competitive with the ball wear scar data. The wear scar on the disk is easier to recognize than the wear scar on the ball. Limits for standards besides the commonly used ball wear scar data were determined. This work shows that lubricity of diesel fuels, including the effect of biodiesel on petrodiesel lubricity, can be evaluated more easily than is presently done.
Technical Abstract: The high-frequency reciprocating rig (HFRR) lubricity tester has become a widespread method for determining the lubricity of diesel fuels. The test is a ball-on-disk method, in which a steel ball scrapes over a steel disk immersed in the liquid to be tested. According to standards the wear scar generated on the ball in form of the average of the x- and y-axis is used for evaluating the lubricity of the sample. Generally, the smaller the wear scar, the greater the lubricity of the sample. However, a wear scar is also generated on the disk. The size of the wear scar on the disk also depends on the lubricity of the sample. In this work, the wear scar data of the balls and disks of 230 samples related to testing of biodiesel and related compounds with petrodiesel were evaluated. Data comparisons for all wear scar combinations correlated well by linear regression. Although correlations are slightly better when using only ball wear scar data (r2 > 0.99), other wear scar data including that generated on the disk appear just as useable (r2 0.97-0.99) for evaluating lubricity by the HFRR test. The wear scars on the disk have the advantage of being more easily measurable and recognizable under the microscope, especially if the wear scars are small. Limits for all wear scar values corresponding to current limits for average ball wear scar data in standards are presented.