The alkali value of lubricating oil represents the standard of alkaline substance content in lubricating oil. It is an index to determine the effective additive composition in oil products, to express the neutralization ability of lubricating oil.

For diesel engine lubricants, alkali value is due to the burning of sulfur-containing heavy oil and other fuel oils in diesel engines, especially ships, and this kind of diesel engine oil is mixed with fuel oil in the engine, resulting in the engine will produce acidic substances in the process of operation, so the significance of the existence of alkali value here is to neutralize acidic substances while preventing equipment corrosion.

For metal processing fluids, a certain alkali value can ensure the activity of additives in lubricating oil, such as non-ferrous metal corrosion inhibitors, stabilizers, etc., that is to say, many lubricating oil additives will easily fail or deteriorate in acidic environment, while alkaline environment can maintain the stability of such additives. The alkali value of many metal processing fluids remains around 9.0, too high to stimulate the skin of workers, too low to affect the activity of additives.