Optimizing Oil Drain Intervals Using BN vs. AN

Polaris Laboratories

Safely extending oil drain intervals by using fluid analysis to monitor oil condition and engine wear has always been a significant means for reducing a maintenance department's consumable costs. But in today's economic environment, it is becoming even more critical to achieve optimal drain intervals and, at the same time, minimize engine damage.

Advanced oil formulations have allowed the industry to make great strides in extending drains. The CI-4 and CI-4 Plus oils used in diesel engines manufactured prior to 2007 had a significant impact. Although many felt this progress would be jeopardized by the CJ-4 oils that were later introduced with a lower starting base number (BN), better CJ-4 additive technology actually enabled many fleets to increase drain intervals even further.

While fluid analysis is the best way to safely determine an optimal drain interval, the appropriate testing should be done by a reputable laboratory that uses reliable testing methods.

Safely extending diesel engine drain intervals requires the following tests:

  • Elemental analysis

  • Wear metals

  • Contaminant metals

  • Additive metals

  • Fuel dilution percentage by gas chromatography

  • Soot percentage by Fourier transform infrared (FTIR) spectroscopy

  • Viscosity at 100 degrees Celsius (ASTM D-445)

  • Water by crackle

  • BN ASTM D-4739

  • Oxidation/nitration by FTIR

(Definitions of each test can be found at www.polarislabs1.com/testlist.htm.)

Consistent Relationship
In the past, standard practices for determining optimal drain intervals using fluid analysis have required testing the oil for BN and acid number (AN). The theory was that when new, an oil's BN is high and its AN is low; and the longer the oil is used, BN decreases while AN increases. The point at which they meet is the optimal time to change the oil. It is important to note that ASTM D-4739 should be used when testing the BN of used (in-service) oils as opposed to ASTM D-2896, which may be used when testing BN in new oils. ASTM D-4739 uses a weaker acid for titration than does ASTM D-2896 and, therefore, produces slightly lower BN results.

Figure 1. Determining Optimal Drain Intervals Using Acid Number (AN)

Historical test data shows the relationship between BN and AN to be quite consistent. AN just begins to increase when BN depletion reaches 50 percent. As the BN drops below 50 percent, AN begins increasing rapidly. So in reality, BN depletion can reach about 65 percent before it becomes necessary to change the oil.

Consider a CJ-4 engine oil with a starting BN of 9.0. AN will hold steady at around 1.75 to 2.0 until BN depletes to approximately 5.5, at which point AN begins to increase. The two will meet at around 3.15 to 3.5. As a result, most laboratories do not require both an AN and a BN to make extended drain recommendations.

Figure 1 represents BN/AN test results across a fleet of more than 450 pick-up/delivery trucks. AN remains fairly steady until the BN depletes from 12 to about 6. The two meet at between 50 and 65 percent depletion of the BN. The AN is significantly higher than the BN after the BN has reached 65 percent depletion, which indicates that the oil's ability to neutralize acids has dropped significantly. The oil should be changed to prevent corrosive engine wear from occurring.

Monitoring BN, viscosity, oxidation and nitration simultaneously and changing both the oil and filters when contamination from dirt, coolant, fuel dilution or soot reaches critical alarm limits is ultimately the best way to determine optimal engine oil drain intervals regardless of the time on the oil.

About the Author

The Polaris Laboratories provides complete testing and analysis for oils, fuels, coolants and water-based fluids. The company is headquartered in Indianapolis and has additional laboratories in Houston and Salt Lake City. For more information, visit www.polarislabs1.com.

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