The ACEA 2021 Light Duty Engine Oil Sequences introduces an additional TBN test for all categories, affecting all oil marketers. 

Total Base Number, commonly referred to as TBN, has been part of engine lubricant specifications for around 80 years and continues to remain so today.

During the process of combustion, acids accumulate in the crankcase of the internal combustion engine (ICE). Left untreated, acid build up in the engine lubricant leads to acidic corrosion which can result in engine damage and ultimately catastrophic failure. Correctly-formulated performance additives neutralize these harmful acids, reducing the potential for acid corrosion throughout the oil drain interval. TBN gives an indication of the amount of base available to neutralize the acids and their unwanted effects.

The TBN of an engine lubricant is traditionally measured in both fresh as well as aged oils.

• To measure the TBN of fresh oils, the ASTM D2896 “Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration” has historically been used.
• For aged oils, the ASTM D4739 “Standard Test Method for Base Number Determination by Potentiometric Hydrochloric Acid Titration” has historically been used.

In order to satisfy the requirements of the European Automobile Manufacturers’ Association (ACEA) members, the latest ACEA 2021 Light Duty Engine Oil Sequences require fresh oils to be measured by the existing ASTM D2896 test method, as well as an additional requirement to be measured by the ASTM D4739 test method.

The ASTM D4739 test is a titration method used for determination of basic constituents, or alkalinity reserve, in petroleum products. In the ASTM D4739 test method, the fresh oil sample is dissolved in a mixture of toluene, isopropanol, chloroform and a small amount of water.  Measurements are made by the addition of fixed amounts of hydrochloric acid (HCl) to the lubricating oil at fixed time intervals, and the amount of acid required to neutralize the base content of the lubricant is monitored. The measurement of base is reported as TBN, expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralize all basic constituents present in 1 gram of sample.

For the new ACEA A7/B7 and ACEA C6 categories introduced in the ACEA 2021 Light Duty Sequences, limits have been prescribed for the ASTM D4739 test. Although the ASTM D2896 test is still required, the values are report only and there are no limits are in-place.

For existing light duty categories, the ASTM D4739 test is also required but again, as report only test with no limits in place. Prescribed limits for the existing ASTM D2896 test remain unchanged from the ACEA 2016 Oil Sequences.

Our View

The addition of the ASTM D4739 test for fresh oils brings change for the engine lubricant formulator, with considerations such as availability of testing and quality assurance needing to be factored into the product development process.

Protecting against acidic corrosion is more complicated than solely focusing on TBN. It is critical to understand the many facets of corrosion control within ICEs, including the challenges hybrid engines may impose; ensuring it is effectively measured and ultimately formulated against.

For the engine lubricant, the requirement remains for a carefully developed formulation that delivers robust protection across all areas of the engine and aftertreatment system, alongside the tangible efficiency gains demanded in today’s high-performing vehicles.

For more information on the ACEA 2021 upgrade for light duty engines, please contact your Lubrizol representative.