Definition
In lubricant chemistry, a detergent is an additive — not a soap — that neutralizes acidic combustion by-products and keeps engine surfaces clean by suspending contaminants and preventing deposit formation. The key chemical function is the alkaline reserve provided by the metal-containing soap structure.
Common detergent types:
- Sulfonates — calcium or magnesium sulfonates; widely used in engine and marine oils
- Phenates — sulfurized calcium phenates; high TBN, good thermal stability
- Salicylates — ashless or metallic; cleaner combustion profile, growing use in low-ash engine oils
Detergents are characterized by their TBN (Total Base Number) — a measure of acid-neutralizing capacity. Overbased detergents with TBN values of 300–500 mg KOH/g are common in heavy-duty diesel engine oil formulations.