Modern passenger car and heavy duty diesel commercial engine lubricants provide long-lasting active protection to your engine hardware, resulting in oil drain intervals (ODIs) for European vehicles as high as 30,000 kilometers or two years (whichever comes first) for passenger cars and 140,000 kilometers for heavy duty diesel commercial trucks. Protecting the hardware from damage, such as that caused by corrosive wear, keeps engines operating at maximum efficiency for as long as possible, enabling the lowest emissions and highest fuel economy possible, and keeping operating costs as low as possible.

Importance of Measuring Acid for Corrosion Protection

Keeping track of lubricant lifetime is a key consideration for maintaining engine durability. As the engine lubricant ages in service, contaminants build up in the lubricant and the lubricant’s physical properties change, which can harm the lubricant’s performance. Used oil analysis (UOA) is commonly utilised to evaluate the lubricant life in vehicles. Typical parameters measured by UOA include wear metal content, oxidation and nitration of the lubricant, viscosity change, soot content, and build-up of acid.

Current Use of TAN and TBN

Measuring TAN and TBN is used to quantify the build-up of acids in the engine lubricant. These acids form as a result of the fuel combustion process and the degradation of the engine lubricant under the harsh operating conditions inside an engine. At the beginning of an oil drain interval (ODI) the TAN will be low and the TBN will be high. Over the course of an ODI, TAN will increase as the acidic contaminants accumulate, while TBN will decrease as the active base content of the lubricant is consumed. Historically, the point at which TAN increases above TBN – the “TBN/TAN crossover” – was considered to be the point at which the lubricant had reached the end of its useful life.

Our View

TAN and TBN were first developed in the mid-20th century for vehicles running on high sulfur fuel. The high sulfur content leads to high levels of acidic contamination in the lubricant, therefore necessitating a way to measure the balance between acid and base in the lubricant. With the current use of ultra-low sulfur fuel and modern hardware it’s time to re-evaluate whether measuring TAN and TBN is the best approach to track build-up of acids in the lubricant, and ultimately protect against corrosive wear in the engine.

The third article in this series will cover the measurement of acids using the pH scale.

For more information, please contact your Lubrizol representative.