- Detergents and dispersants are engine oil's cleanliness additives — chemically distinct but synergistic, together ~30–50% of the additive treat rate.
- Detergents are metallic soaps (Ca/Mg sulfonates, phenates, salicylates) that neutralise acids; their reserve is TBN (fresh oil 6–12 mg KOH/g; overbased grades 300–500).
- Dispersants are ashless PIBSI polymers that keep soot and sludge suspended until the next drain; bis-PIBSI and borated grades perform better.
- Key trade-off: more detergent raises TBN but also sulfated ash (SAPS), which low-SAPS specs (ACEA C2/C3, API FA-4) limit.
Detergents and dispersants are the two main “cleanliness additives” in engine oil. They are chemically distinct and perform different roles, but work synergistically to keep engine surfaces deposit-free and extend oil drain intervals. Together, they typically represent 30–50% of the total additive treat rate in a finished engine oil.
What Are Engine Oil Detergents?
Engine oil detergents are metallic soaps — typically sulfonates, phenates, or salicylates of calcium or magnesium. The term “detergent” is slightly misleading: unlike household detergents that remove dirt with water, engine oil detergents primarily neutralise acidic combustion by-products before they can attack metal surfaces and form lacquer or varnish deposits.
The key property of a detergent is its Total Base Number (TBN) — the reserve alkalinity available to neutralise acids. A fresh engine oil might have a TBN of 6–12 mg KOH/g. As the oil ages, TBN depletes. When TBN drops to around 2 mg KOH/g, the oil is considered spent for acid-neutralisation purposes.
Overbased detergents contain a core of colloidal calcium carbonate (or magnesium carbonate) stabilised by the surfactant soap. This carbonate reserve gives the detergent its high TBN — overbased calcium sulfonates can reach TBN values of 300–500 mg KOH/g.
CheMost offers a comprehensive range of engine oil detergents and TBN boosters, including low-base, medium-base, and highly overbased calcium sulfonates and phenates.
What Are Dispersants in Engine Oil?
Dispersants are ashless (metal-free) polymeric additives that prevent soot, sludge, and oxidation by-products from agglomerating and depositing on engine surfaces. The dominant dispersant chemistry is polyisobutenyl succinimide (PIBSI) — a polyisobutylene (PIB) “tail” connected through a succinic anhydride bridge to a polyamine “head group”.
The polyamine head is polar and adsorbs onto soot particles or oxidation debris. The long PIB tail keeps the adsorbed particles dispersed in the oil, preventing them from clumping into sludge. A good dispersant holds contaminants in suspension until they can be removed at the next oil change.
Bis-PIBSI (bisuccinimide) provides better dispersancy than mono-PIBSI at equivalent treat rates. Borated dispersants — where some of the amine groups are reacted with boric acid — offer improved antioxidant and antiwear performance at higher temperatures.
Explore CheMost’s range of ashless dispersants, including mono-PIBSI, bis-PIBSI, and borated grades for PCMO, HDDO, and marine applications.
From the labFormulating motor oil or transmission fluid? CheMost supplies the chemistry.View engine oil packagesDetergent vs. Dispersant: Key Differences
| Property | Detergent | Dispersant |
|---|---|---|
| Chemistry | Metallic soap (Ca/Mg) | Ashless polymer (PIBSI) |
| Primary function | Acid neutralisation + surface cleanliness | Soot/sludge suspension |
| Ash contribution | Yes (sulfated ash) | No |
| TBN contribution | High (especially overbased) | Low (amine groups only) |
| Molecular weight | Low (300–800 Da) | High (1,000–3,000 Da) |
How Treat Rates Are Balanced
In a typical PCMO additive package, detergents contribute 2–5% of the finished oil, while dispersants contribute 4–8%. For HDDO (heavy-duty diesel), dispersant levels are higher (6–10%) due to the greater soot load from diesel combustion.
Increasing detergent TBN helps with acid control but raises sulfated ash content — which must be limited in low-SAPS specifications (ACEA C2/C3, API FA-4). Formulators compensate by using low-overbased detergent combinations and relying more on dispersants for sludge control.
Frequently Asked Questions
Can I use more detergent instead of dispersant?
No — they perform different functions. Detergents neutralise acids and prevent high-temperature deposits (lacquer, varnish). Dispersants suspend soot and low-temperature sludge. Increasing detergent treat rate does not compensate for insufficient dispersancy, and vice versa.
What is sulfated ash and why does it matter?
Sulfated ash (SAPS) is the inorganic residue left when an oil is burned and the organic matter is oxidised. It comes primarily from metallic detergents (calcium and magnesium). High ash content can cause diesel particulate filter (DPF) plugging and is limited in modern heavy-duty and passenger car specifications.
What does “overbased” mean for a detergent?
An overbased detergent contains more base (calcium carbonate) than required to neutralise the organic acid used to make it. The excess base is held in colloidal suspension by the soap molecule. Overbasing allows detergents to carry large acid-neutralising reserves at manageable treat rates. TBN values of 300–500 mg KOH/g are typical for highly overbased sulfonates.
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