eSource 116 Base Number Versus Acid Number in Today’s Heavy Duty Engine Oils
Current API diesel engine classification are represented by API CK-4 and API FA-4.
- API CK-4 oils provide better emission control system durability where particulate filters and other advanced aftertreatment systems are used. These formulations provide enhanced protection against oil oxidation, wear and particulate and deposits control.
- API FA-4 is formulated for certain XW-30 oils for use in select high-speed four-stroke cycle diesel engines designed to meet on-highway greenhouse gas (GHG) emission standards.
Base number is still needed in diesel engine oils to neutralize acids that form. Also the metalorganic additives which contribute to alkalinity reserve in the form of base number also provides important detergency properties to the engine oil. These are most commonly represented by the calcium and magnesium containing additives. Even though the base number starts out lower in today’s engines oils base number retention is actually better in today’s API diesel engine oil classifications. There is still some level of sulfur present, which can create highly corrosive sulfuric acid. And due to exhaust gas recirculation NOX compounds react and form a weaker nitric acid back in the oil. But the general rule used in years past of 50% reduction no longer has any real relevancy and the base number can go to very low levels while in service and not be a great concern. The level of base alkalinity is adequate to combat the level of acid creation in today’s diesel engine oils, even when the base number gets to low levels during service.
As base number takes on a less important role for monitoring engine oil service life, the role of testing for acid number, oxidation, and even nitration, in monitoring service life has taken on more relevancy. With less concern about neutralization of highly corrosive acids that depletes engine oil protection, better means to look at truer indicators of drain intervals and service life has become a discussion point in recent years.
Many of the additives historically used in diesel engine oil formulations are still in use only their proportions have changed to meet today’s performance requirements.
As noted, calcium and magnesium in today’s diesel engine oils are reduced due to lower base number requirements.
Antioxidants have increased to provide better oxidation resistance for extended drain
Phosphorus and general ash content have lowered to reduce deposit formation and coating of EGR activation sites.
Sulfur has decreased to lower sulfates, which contributes to accumulation of particulate deposits on engine surfaces and particulate emissions. Though sulfur content is lower, sulfur cannot be completely eliminated in today’s diesel engine oil formulations. Sulfur still plays a role as an antioxidant, and providing enhanced lubricity properties. See March eSource article 113 The Changing Role of Sulfur in Today's Engine Oils.
In summary the relevancy of monitoring base number in used diesel engine oil some not have the importance that it once had. Base number is still in use today’s as an indicator because it has always been around and people are used to seeing this test results. Lubricant manufactures are beginning to lean more towards looking at other test that are more relevant to monitoring the true service life of diesel engine oils.
David Doyle, CLS, OMA I, OMA II
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