Using the right lubricant in an engine, and knowing when to change it, are essential for keeping your vehicle on the road and gaining maximum efficiency from the engine.

Vehicle users across the world currently use total acid number (TAN) and total base number (TBN) as methods to monitor the condition of the lubricant in their engines. As an engine lubricant ages, acid contamination builds up in the oil. These acids are formed as a result of the fuel combustion process and the harsh operating conditions inside an engine, and this build-up is tracked by measuring the lubricant's TAN.

Engine lubricants are formulated to control acid build up and, by doing so, the lubricant protects the engine against acidic corrosive wear; failure to protect against acidic corrosive wear in the engine leads to loss of efficiency and premature failure. To achieve acid control, the lubricant is formulated to contain active ingredients which can neutralize acids; this is the so called 'base' content. Correspondingly, the amount of base in a fresh lubricant can be confirmed by measuring the lubricant's TBN.

ASTM Methods

One method used to measure TAN is ASTM D664, which is officially called Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration.

In ASTM D664, the acid content of the engine lubricant is measured by subjecting the engine lubricant to a chemical reaction which removes the acid - a process referred to as neutralization. The extent of this acid neutralization is measured with a technique called titration-something we may remember from science experiments at school-and the results of the titration are converted to a TAN value. The units of TAN are mgKOH/g, which describe the concentration (strength) of the acid in the engine lubricant.

TBN is measured by one of several similar methods, including ASTM D2896, ASTM D4739 and ISO 3771. These methods are all titrations and they measure the base content of a lubricant in a similar way to the TAN measurement. The units of TBN are also mgKOH/g, and every unit of TBN in the new engine lubricant can remove 1 mgKOH/g TAN of acid contamination formed during service.

The next article in this series will cover how to understand the lifetime of a lubricant via the use of used oil analysis (UOA).

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什么是 TAN 和 TBN？