Published 29 April 2026

Cylinder Oil Base Number and Fuel Sulphur Matching

Cylinder oil base number (BN) must be matched to the fuel sulphur content to ensure complete neutralisation of sulphuric acid produced during combustion. Modern slow-speed two-stroke marine engines burn fuels ranging from 0.0 to 3.5 percent sulphur, demanding cylinder oil BN values from 10 to 100. Matching is governed by the chemistry of sulphate detergent additives, the feed rate used, and the engine’s combustion characteristics. The IMO 2020 global sulphur cap drove a market-wide transition from 70 BN to 25-40 BN oils on most ships. This article covers the chemistry, grading, selection, and operational management of BN-fuel matching. Visit the home page or browse the calculator catalogue for related operations engineering tools.

Contents

Background

Sulphur in marine fuel oil oxidises during combustion to sulphur dioxide (SO2) and sulphur trioxide (SO3). Some of the SO3 reacts with water vapour to form sulphuric acid (H2SO4) in the cylinder gas. As the gas cools toward the cylinder liner walls (particularly above the scavenge port belt where wall temperatures are lowest), the sulphuric acid condenses on the metal surface. Without intervention, this acid attacks the iron of the cylinder liner and the piston rings, causing cold corrosion at rates that can exceed 1 mm of liner per 1,000 hours of operation.

The function of the cylinder oil is, in addition to providing lubrication, to neutralise this sulphuric acid before it can attack the metal. The cylinder oil is formulated with a substantial alkaline reserve in the form of sulphate detergents and similar compounds. The reserve, expressed as base number (BN, mg KOH per gram of oil), is a stoichiometric measure of how much acid the oil can neutralise.

Matching BN to fuel sulphur is therefore a fundamental engineering choice. Too little BN and acid escapes neutralisation, attacking the engine. Too much BN and the oil contains excess detergent that produces deposits, costs money, and may interfere with combustion. The matching has become more complex with modern low-sulphur fuels, where the appropriate BN may be a fraction of the historical norms.

This article covers the chemistry of acid neutralisation, the BN grades available, the matching process, and the operational considerations introduced by multi-fuel and dual-fuel operation.

Acid neutralisation chemistry

Sulphur to sulphuric acid

The reaction sequence in the cylinder is approximately:

Fuel sulphur burns to SO2: S + O2 → SO2
Some SO2 oxidises further to SO3: 2 SO2 + O2 → 2 SO3 (catalysed by metal oxides at high temperature)
SO3 reacts with water vapour: SO3 + H2O → H2SO4 (occurs as the gas cools below approximately 250 degrees Celsius)
H2SO4 condenses on cool surfaces below the acid dew point

The fraction of SO2 that becomes SO3 ranges from 1 to 5 percent. SO3 conversion is favoured by lower combustion temperatures, longer residence time, and specific catalytic surfaces.

For HFO at 3.5 percent sulphur burning at 200 g/kWh fuel consumption, sulphuric acid production is roughly 0.5 to 2.5 grams per kWh, depending on the SO3 conversion fraction.

Detergent neutralisation

Cylinder oil detergents are typically calcium-overbased sulphonates or salicylates. The active component is calcium carbonate (CaCO3) suspended within micelles formed by the surfactant molecules. When the oil contacts sulphuric acid, the CaCO3 reacts:

CaCO3 + H2SO4 → CaSO4 + H2O + CO2

The calcium sulphate (CaSO4) is insoluble and is dispersed in the oil. Excess CaCO3 remains as alkaline reserve.

BN as a stoichiometric metric

BN measures the total alkalinity equivalent in the oil. One milligram of KOH equivalent neutralises a known amount of acid. A 70 BN oil at 1.0 g/kWh feed rate provides 70 mg KOH equivalent per kWh, sufficient to neutralise approximately 86 mg of H2SO4 per kWh. For HFO at 3.5 percent sulphur, this is roughly the level needed for full neutralisation.

BN grades

Cylinder oils are graded on a coarse scale of nominal BN values:

Nominal BN	Typical fuel sulphur range	Application
100	4.0 - 5.0 percent	Limited use; some older HFO
70	2.0 - 3.5 percent	Standard HFO before IMO 2020
40	1.0 - 2.0 percent	Transitional; some scrubber operation
25	0.5 - 1.0 percent	VLSFO standard
17	0.1 - 0.5 percent	LSFO and some MGO
10 - 13	0.0 - 0.1 percent	MGO, ULSFO, LNG (in dual-fuel)

The actual BN of a specific commercial product can vary by 5 to 10 BN units from the nominal designation, reflecting formulation differences between suppliers.

BN above 100

Specialty oils above 100 BN exist for very-high-sulphur fuel applications. These are now rarely used in commercial marine practice because few fuels exceed 4 percent sulphur in the IMO 2020 era.

Multi-grade approaches

Some recent developments offer “multi-grade” cylinder oils designed to accommodate a range of sulphur contents from a single grade. Multi-grade oils typically operate in the 25 to 40 BN range and target ships that switch between fuels frequently.

IMO 2020 impact

Before 2020, the global sulphur cap for marine fuel was 3.5 percent. Most slow-speed two-stroke engines burned HFO at sulphur levels of 2.5 to 3.5 percent and used 70 BN cylinder oil with feed rates of 1.2 to 1.5 g/kWh.

The 2020 sulphur cap of 0.5 percent (lower in SECAs, 0.1 percent) drove a fleet-wide transition. Most ships moved from 70 BN to 25 or 40 BN cylinder oil. Some installed exhaust gas scrubbers and continued burning HFO; these ships retained 70 BN oils.

The transition exposed several issues:

Cold corrosion at low load with low-BN oil: insufficient alkalinity at part-load operation
Deposit formation patterns: low-BN oils formed different deposits than the 70 BN oils
Optimum feed rate shifts: lower BN sometimes called for slightly higher feed rate to maintain alkalinity supply
Switching between fuel modes: ships that occasionally burn HFO (in tow scenarios, scrubber failures, etc.) needed a strategy for BN

The transition is largely complete and stable. Most operators now have settled BN/fuel/feed-rate combinations that work for their typical operations.

Matching process

Standard recommendation

For a single-fuel operation, the matching process is:

Determine fuel sulphur content (from fuel certificate or onboard analysis)
Consult engine manufacturer’s BN/sulphur matching chart
Select the closest commercial BN grade
Set feed rate per the manufacturer baseline
Operate and monitor wear (per feed rate optimisation)
Adjust feed rate downward if wear is acceptable

Sulphur range strategies

For operations that span a range of fuel sulphur content, three strategies are common:

Strategy A: One BN grade, varying feed rate. Use a moderate-BN oil (e.g. 40 BN) and adjust feed rate based on actual sulphur content. Lower-sulphur fuel uses lower feed rate; higher-sulphur fuel uses higher feed rate. Total alkalinity supplied tracks fuel acid production.

Strategy B: Multiple BN grades. Carry two grades (e.g. 25 BN and 70 BN) and switch between them based on fuel. Switching takes time (settle the lubricator pumps, drain previous oil, fill new oil).

Strategy C: Single high-BN grade. Use a higher-BN oil (e.g. 70 BN) for all operations, accepting some over-alkalinity at low-sulphur conditions. Simplifies operation but accepts deposit risk and oil cost.

Strategy A is most common for ships operating exclusively on VLSFO or MGO. Strategy B is used by ships that frequently switch between scrubber HFO and compliant fuel. Strategy C is rare in modern practice.

Dual-fuel operations

For dual-fuel engines that switch between liquid fuel (HFO/LSFO/MGO) and gas (LNG/methanol):

Liquid mode demands BN matching to liquid fuel sulphur
Gas mode produces almost no acid; very low BN demand
Switching between modes requires either a single grade compromise or sophisticated multi-grade handling

Most modern dual-fuel ships use a 25 to 40 BN grade compromise, accepting slight over-alkalinity in gas mode and slight under-alkalinity in liquid mode at higher-sulphur fuels.

Drip BN feedback

Drip oil samples taken from the scavenge box drain provide direct feedback on BN consumption:

Fresh oil BN: known from product specification
Drip oil BN: measured by laboratory analysis
Depletion ratio: (Fresh BN - Drip BN) / Fresh BN

A healthy depletion ratio is 30 to 70 percent. Higher depletion (above 70 percent) suggests the fuel sulphur is high relative to the BN supplied, risking incomplete neutralisation. Lower depletion (below 30 percent) suggests over-alkalisation, with detergent surplus that may produce deposits.

Operators monitor drip BN trends over time and adjust BN grade or feed rate based on the data.

Operational issues with low-BN oils

The shift to low-BN oils (25 BN and below) for IMO 2020 compliance introduced several operational considerations:

Acid sensitivity

Low-BN oils have less margin for fuel sulphur fluctuation. A bunker that nominally meets the 0.5 percent specification but actually contains 0.7 percent sulphur can outrun a 25 BN oil’s neutralisation capacity. Fuel quality verification is more important than it was with 70 BN oils that had ample reserve.

Cold corrosion

Cold corrosion remains a risk even with low-sulphur fuels because some acid is still produced. Maintaining adequate liner cooling water temperature (above 75 degrees Celsius) and avoiding sustained low-load operation help mitigate cold corrosion on low-BN oils.

Deposit chemistry

Low-BN oils have different additive packages than high-BN oils. Combustion deposits formed in low-BN service have different morphology, often less coke-like and more powdery. Routine piston overhauls reveal these patterns; experienced operators learn to interpret them.

Cylinder oil consumption

Some operators found that low-BN oils consumed at slightly higher feed rates than 70 BN oils, partly offsetting the per-tonne cost savings. The economic case for IMO 2020 transition was not as clean as initially expected, but the regulatory requirement was binding regardless.

Specialty fuel matching

LNG dual-fuel

In gas mode, LNG produces essentially no acid. Cylinder oil demand is purely lubrication and detergency. Low-BN oils (10 to 17 BN) are appropriate; the same oils may also serve a low-sulphur liquid mode.

Methanol

Methanol contains no sulphur and produces no acid. Like LNG, demands are purely lubrication and detergency. Some methanol-specific cylinder oils are emerging, formulated for the different chemistry of methanol combustion (more water vapour, different deposit patterns).

Ammonia

Ammonia, expected to enter commercial marine service in the mid-2020s, will produce no sulphur acid but will produce nitrogen-bearing combustion products that may require novel cylinder oil formulations. Industry development is ongoing.

Manufacturer guidance

Engine manufacturers maintain BN matching charts that specify the recommended BN grade for each combination of:

Engine model
Fuel sulphur content
Operating profile

These charts are updated periodically based on field experience. Operators are advised to consult the latest manufacturer guidance rather than relying on historical practice.

References

CIMAC. (2020). Recommendations Concerning Cylinder Oils. CIMAC Working Group 8.
MAN Energy Solutions. (2023). Cylinder Lubrication and Oil Selection Manual. MAN Energy Solutions.
WinGD. (2023). Cylinder Oil Selection and Feed Rate Guidance. Winterthur Gas & Diesel.
Castrol Marine. (2022). IMO 2020 Cylinder Oil Selection Guide. BP Castrol.
Wakuri, Y. et al. (2003). Tribology in Marine Diesel Engines. Wiley.