Why an Oil's Base Number May Vary

Noria Corporation
Tags: oil analysis

"Our plant owns many generator sets. All of the engines are almost the same age. I noticed that some engine oils have experienced a rapid drop in the base number. For example, the base number of some engine oils reached 50 percent of their initial value just after 500 hours, while other oils are still at an acceptable level after 1,000 hours. The engines use the same oil and the same combustible. I think the decrease in the base number depends on the consumption of combustibles and lubricants (proportional to fuel or gas consumption and inversely proportional to the consumption of lubricants). Have you ever experienced such a situation? What else can explain such a reduction of the base number?"

In normal conditions, a reduction of the base number is expected due to the oil’s additives operating in the machine. These help to keep the engine cleaner and neutralize acids formed in the combustion chamber. If your lubricants, fuel, engine models and operating conditions are consistent across the fleet but different additive depletion rates are reported, you should investigate the units involved.

For example, be sure the same laboratory is used for all oil samples as well as the same test method. While lab test results generally are reliable, there is a natural variation accepted for each method. This means two or more tests may be run for the same sample with slightly different values reported. Do not disregard the possibility of an error being made when the test is performed. If you have doubts, ask the lab to confirm the results by conducting another test.

The lubricant consumption rate should also be considered. The higher an engine’s oil consumption rate, the higher the oil make-up rate, which results in an increase in the oil’s base number value.

Also, keep in mind that oils are manufactured within certain specifications, which have minimum and maximum values. Check to see if the oil’s base number is different from one batch of lubricant to another.

If fuel dilution is occurring in the engine due to fuel leaks or incomplete fuel burning, it can also reduce the base number concentration of the in-service oil.

In addition, you should evaluate the oil change interval. The more extended the oil change interval, the lower the base number tends to be when the oil is changed.

Finally, if the unusual base number patterns are seen in just a few units, analyze what is different about them. Likely, only one or two factors are involved. However, if the pattern is occurring randomly across the fleet, it may be the sum of many factors.