This article highlights the key takeaways from our recent webinar, titled: Measuring Lubricant Lifetime: Is TBN and TAN Data Providing the Full Story of How Lubricants Age Inside Your Engine?, which is part of our Lubrizol360 Webinar Series featuring Lubrizol experts discussing important industry trends.
Used oil analysis (UOA) remains the standard for determining aging of an engine oil. UOA measures the buildup of contaminants in the lubricant, including wear metal components, soot content and fuel dilution. It also measures the effect of lubricant degradation and the resulting viscosity change, oxidation and nitration. Yet another critical measure in understanding how an oil has aged in an engine is the acidification of the lubricant within the engine, which is typically measured as the Total Acid Number (TAN) and Total Base Number (TBN).
These measures have served the industry well over many years, but product and fuel changes require additional diligence. A flaw in UOA has been revealed because TBN is not an accurate measure of an oil’s ability to neutralize acid for modern lubricants and modern fuels. An additional measure must be taken to further confirm the integrity of a vehicle’s lubricant. Fortunately, that solution can be found in the ASTM D7946 (i-pH) testing regime, which is why we support adding the i-pH measurement to UOA. This will provide the most accurate way of determining how lubricants are aging in an engine and provide better information to end users.
The Importance of Monitoring Lubricant Acidification
Lubricant acidification can occur during fuel combustion. Acids form under the harsh operating conditions inside the engine and can degrade the lubricants over time. The acids are corrosive to ferrous metals used in engines and can corrode components, which may lead to a loss in engine performance and efficiency, as well as increased emissions. While using high-quality lubricants can ameliorate these effects, even the most advanced lubricants will degrade over time.
As ultra-low-sulfur fuels, aftertreatment systems and extended oil drain intervals (ODI) become more prevalent, protecting an engine’s hardware from corrosive wear and other damage has never been more critical. As a result, accurate monitoring of how well lubricants perform in modern engines is essential.
The current method of measuring acidification that employs the TAN and TBN titrations will not always accurately measure lubricant degradation, especially in low-sulfur fuel.
How TBN and TAN Fall Short
TAN is calculated by measuring how much base must be added to neutralize all the acid in the lubricant sample. If the acids in the sample are strong (such as sulfuric acid, more prevalent when high sulfur fuel was commonplace), the demarcation point will be clear and unequivocal. If the acids are weak (such as organic acids, more prevalent with the increased use of biofuel), the inflection point will be much murkier, leading to poor precision when measuring the amount of acid in the used oil.
All modern lubricants have overbased detergents added to them, whose function is to control the acid levels in the oil. The specific chemical structure of these overbased detergents is more important than the simple measure of how much TBN they contribute to the finished fluid. For example, the size and morphology of the overbased detergent particles can be tuned. This is important because the neutralizing ability of these overbased detergents is dependent on the surface area-to-volume ratio of the particles, since neutralization occurs on the particle surface after a random collision between colloidal base and acid. Neutralization is also affected by the overbased detergent surfactant type, and the metal carbonate composition. Therefore, TBN alone is likely to be an inaccurate measure of how well a lubricant will neutralize acid.
Why i-pH Matters
Measuring the acidity of an oil using ASTM D7946 (i-pH) is an alternative way to assess acidification. In contrast with TAN and TBN measurements, which can be affected by the way the tests are administered, the ASTM D7946 testing regime is more direct. Crucially, i-pH also differentiates between strong and weak acids, offering a more detailed analysis of how the oil acidity will affect engine components. After all, there is a clear relationship between iron wear and acid strength (measured by i-pH), which is often not observed with TAN or TBN measurements. Acidified oil with lower i-pH is more corrosive and can therefore be flagged more easily by end users. The early warning system provided by i-pH is enough to affect the way end users handle their lubricant needs.
Our View
While TAN and TBN are still prevalent measurement tools when it comes to measuring lubricant acidification, they have become less relevant as fuel and engine technology have evolved over the past decade. Therefore, to reflect the acidification of engine lubricants more effectively, another test must be deployed. We believe i-pH is that test because it provides the best indicators of corrosion risks.
For more information about why i-pH acidification measurements and their value as an alternative to the more traditional use of TAN and TBN, download the webinar slides here, or contact your Lubrizol representative today.
