ALS Staveley Services Fluids Analysis
Detergents and Your Reserve Alkalinity

Detergents and Your Reserve Alkalinity
by Ambrose Hughey, OMA I

Internal combustion engines’ lubricants are specifically blended to possess alkaline properties to combat the acidic byproducts that are created in the engine during combustion, any acidic ingressed contamination, and acidic byproduct resulting from lubricant oxidation. Making up the majority of the alkalinity reserve, detergent additives supply this alkaline function by providing an over-base condition. The amount of the over-base condition still remaining in the oil is termed the alkalinity reserve. The alkalinity reserve is regularly being depleted from the lubricant, as it is designed to be self-sacrificial in order to protect the lubricant and the oil-wetted components. Quantifying the alkalinity reserve will aid in determining the remaining useful life of the lubricant (the suitability for continued use of the lubricant).

Detergents also aid in the suspension of contaminants in the lubricant to prevent sludge and varnish buildup on oil-wetted components. Detergents help clean contamination from components’ surfaces by reacting their polar head with the contaminants. This results in a product that is soluble in the lubricant, allowing for easy contaminant removal when the oil is drained. Detergent additive levels may be depleted, as dilution can occur through contaminant ingression or capture of any agglomerated particles in the filter that contains some of the reacted detergent additives. The most common cause of additive package dilution in crankcase applications is attributable to fuel dilution resultant of ingression of fuel and combustion blow-by products.

While the detergent additives provide these important functions, such as reducing the harmful effect of combustion byproducts and assisting the prevention of sludge buildup, they also have setbacks. The metal-containing detergent additives cause ash formation as small amounts of the lubricant is burned during combustion. The ash formation directly leads to the formation of engine deposits and, therefore, ash levels are generally specified as a maximum by both the API/SAE and OEMs. The most recent commercial oil API/SAE classification, CJ-4, specifies a maximum sulfated ash content of 1.0%. This sulfated ash limit provides a threshold for ash-forming additives, such as detergents that are found in HDMOs. The lowered ash limit is necessary with the recent use of emission control devices such as diesel particulate filters (DPFs) and diesel oxidation catalysts (DOCs). The ash formation from the oil may cause increased maintenance through premature DPF plugging and poisoning of the catalyst in DOCs. Consequently, a need for the lowered ash limit is created.

The detergent additives are typically composed of compounds that contain calcium, magnesium, or barium sulfonates. Spectrochemical analysis determines the concentration of the metals contained in the lubricant, which will provide an indication of the detergent additive package levels. In-service lubricants will still contain approximately the same detergent additive metal concentrations. However, when comparing the detergent package’s metal concentration of the in-service oil to the new lubricant, a true representation of the reserve alkalinity of the lubricant is not obtained. This is attributed to the presence of detergent additives that have already reacted to create byproducts. These byproducts will no longer provide the alkaline function that they possessed before the reaction occurred. As this reacted metal is still present in the lubricant, the detergent additives shown on the oil analysis report will no longer be an accurate indication of the reserve alkalinity.

With all of this in mind, how do we know if the detergent package is still active? Sure, the detergent package’s metals are still present on the analysis report. But, are they still functional? The remaining alkalinity reserve of the in-service oil needs to be evaluated. This is accomplished by titrating a known portion of the sample to a method-specified endpoint using an acidic titrant. This titration will provide the means to quantify the alkalinity reserve of the oil. The reported result is expressed as the base number (BN). BNs are reported in terms of the amount of potassium hydroxide (KOH) it takes to neutralize the acid used to titrate the lubricant. As a result, one BN point will neutralize one acid number (AN) point. This allows for ANs and BNs to have synchronized units.

ASTM D2896 and D4739 are the two methods generally used to determine the BN. D2896 measures strong and weak alkaline constituents, whereas D4739 only measures the strong constituents. This is very important for comparison of the two values, as the D2896 BN values are routinely higher than D4739 values. D4739 is the more commonly used BN method for in-service oil analysis, as weak alkaline contaminants that may be present in engine lubricants can yield a falsely high BN when employing D2896. This could potentially lead to overextending oil drain intervals and damaging equipment.

The self-sacrificing alkalinity reserve is regularly being depleted from in-service lubricants. Quantifying the alkalinity reserve will aid decisions in determining the remaining useful life of the lubricant (the suitability for continued use of the lubricant). Adding a BN to your routine crankcase oil analysis is a great tool for monitoring and aiding decisions for potentially extending drain intervals for internal combustion engines’ lubricants.