March 31, 2010


by David Doyle, CLS, OMA 1 & 2
Vice President & Operations Manager

Discussions have recently intensified regarding the relevance of monitoring base numbers in today’s CJ-4 in-service diesel engine oils. Some industry representatives believe that due to current formulations the base number may not be as relevant as it once was in monitoring service life.

The level of additives that contribute to the alkalinity reserve have indeed decreased in the quest for lower ash oils, resulting in a lower initial base number in new CJ4 oils. This was in response to compatibility with changes in engine designs for emission requirements using diesel particulate filters and other after-treatment devices. There is also lower sulfur content in both the diesel fuel and engine lubricant in CJ4 oils. Consequently, the diminished amount of sulfuric acid created during the combustion process has also decreased the required level of alkalinity reserve measured as base number.

This has led to some changes in the way in which we look at base number for in-service CJ-4 diesel engine oils. Some schools of thought lean toward less reliance on scrutinizing the base number as a significant factor in monitoring used diesel engine oils. Some believe that measuring acid number may be more relevant for CJ-4 oils. End users may now be concerned with the change in both acid and base numbers for their in-service oils.

ALS Laboratory Group works closely with customers to provide relevant test packages that maximize equipment reliability. Although some industry representative may feel that monitoring base number performance in today’s CJ-4 engine oils may not be as relevant as it once was, ALS experts believe that the way in which base number is analyzed and evaluated is what has really changed with today’s application.

Since new CJ-4 engine oils start out with a lower base number, and the level of sulfur available to create sulfuric acid during combustion has been reduced, condemning limits for in-service oil have also been reduced appropriately. Condemning limits for in-service CJ-4 oils are much lower then previous API formulations.

Another relevant factor is that it isn’t always a matter of how high the base number starts out for new engine oil. The importance is the rate at which the alkalinity reserve depletes, which is represented by the rate of decrease in the total base number. This is generally a matter of hard TBN versus soft TBN.

ALS Laboratory Group still regards base number as a relevant test parameter for extended drain intervals in used oil analysis when using API CJ-4 engine oils. What has changed is how we analyze it. Condemning limits have been adjusted accordingly. Today’s engines with exhaust gas recirculation, smaller sump sizes, and hotter temperatures can still create acid degradation constituents. NOx compounds that get re-circulated into the engine oil can create nitric acid, for example. End users will see lower values for their base numbers on test reports. Therefore, condemning limits are flagged at lower values, and the overall rate at which the base number depletes has also decreased.