Cylinder Oil Base Number and Fuel Sulphur

Cylinder oil base number (BN) is the single most consequential formulation parameter for two-stroke slow-speed engine cylinder lubrication. It expresses the alkaline reserve available to neutralize sulphuric acid produced inside the cylinder during combustion, measured in milligrams of potassium hydroxide equivalent per gram of oil (mg KOH/g) per ASTM D2896. Match it correctly to the fuel’s sulphur content and the engine runs with controlled corrosive wear. Get it wrong in either direction and the consequences range from accelerated liner corrosion to calcium-deposit bore polishing, both of which shorten the interval between costly overhauls.

The January 2020 global sulphur cap under MARPOL Annex VI Regulation 14, mandated by IMO Resolution MEPC.280(70), compressed the permitted sulphur range of compliant bunkers from 3.50% to 0.50% for most vessels, and to 0.10% in Emission Control Areas (ECAs). That single regulatory event obsoleted two decades of BN70 cylinder-oil practice for the majority of the world fleet and forced a switch to BN40 or BN25 oils, a transition that exposed new failure modes: deposit-driven bore polishing from over-alkalinity in low-sulphur service, and incomplete acid neutralization during fuel quality excursions. This article covers the chemistry behind the matching, the grade architecture, the post-2020 OEM guidance from MAN Energy Solutions and WinGD, drain-oil monitoring, and the operational edge cases that still catch engineers off guard.

Cylinder lubrication feed rates interact directly with BN selection because the two variables together determine the total alkalinity delivered to the liner per unit of work. The cylinder oil feed rate optimization article treats feed-rate adjustment methodology in detail; the present article treats BN selection as the starting constraint. The cylinder lubrication systems for two-stroke engines article covers the hardware that actually delivers the oil to the liner surface, including the alpha lubricator and pulse-injection systems described in pulse lubrication systems for marine engines.

Sulphuric acid formation in the cylinder

The combustion reaction sequence

Sulphur in marine fuel is mostly organic, bound into hydrocarbon structures. During combustion it oxidizes in two stages. First, fuel sulphur reacts with oxygen to form sulphur dioxide:

At the temperatures in a slow-speed diesel cylinder (peak around 1,600 to 1,800 K for a modern MAN ME or WinGD RT-flex), a fraction of the SO2 undergoes further oxidation to sulphur trioxide, catalyzed by metal oxide surfaces in the combustion space:

The SO2-to-SO3 conversion fraction is typically 1 to 5% under diesel combustion conditions. It rises toward the upper end when liner metal-oxide surfaces are abundant (older liners with heavy wear deposits), when combustion is incomplete at low load, and when charge-air temperatures are low. WinGD’s technical papers on RT-flex engines note that SO3 conversion efficiency varies with load, with the highest per-unit-energy acid load occurring during maneuvering and port approaches rather than full-load sea passages.

As the cylinder gases cool toward the liner walls, SO3 reacts with water vapour when the local temperature drops below approximately 250 to 280°C (the acid dew point for typical marine combustion gas compositions):

This sulphuric acid condenses on any liner surface whose metal temperature falls below the dew point. The coolest zone on a slow-speed engine liner is the lower portion of the stroke, near and below the scavenge port belt. Wall temperatures there can be as low as 100 to 140°C under normal operating conditions, well below the acid dew point, making that region the primary site of cold corrosive attack.

Acid production rate as a function of fuel sulphur

The sulphuric acid production rate scales directly with fuel sulphur content and fuel consumption rate. For a given engine operating at specific fuel oil consumption (SFOC) of, say, 175 g/kWh on 3.50% sulphur HFO, each kilowatt-hour of output produces roughly 175 × 0.035 = 6.1 g of sulphur burned. With a SO3 conversion fraction of 3%, the H2SO4 load per kWh is:

where $w_S$ is the sulphur mass fraction in the fuel, $f_{\text{SO}_3}$ is the SO3 conversion fraction, $M_{\text{H}_2\text{SO}_4} = 98\,\text{g/mol}$, and $M_S = 32\,\text{g/mol}$. For the 3.50% HFO example:

Switch the same engine to 0.50% VLSFO (SFOC is comparable because combustion efficiency doesn’t change materially with sulphur content alone) and the acid load drops to roughly 0.08 g/kWh, a factor of 7 lower. That reduction is the direct physical basis for the shift to lower-BN cylinder oils after 2020: there is simply far less acid to neutralize.

Cold corrosion mechanism and wear rate

Sulphuric acid attacks grey cast iron liners through an electrochemical dissolution mechanism. The corrosion product is ferrous sulphate (FeSO4), which is soluble in the acidic film and is carried away by the oil, exposing fresh metal for continued attack. Corrosion rates measured on engines running HFO at 3.5% sulphur without adequate cylinder oil BN have been reported in the range of 0.5 to 1.5 mm of liner diameter loss per 1,000 hours of operation. For context, the typical liner replacement criterion on a slow-speed engine is a diameter increase of 0.5 to 1.0% of bore, corresponding to around 2.5 to 5 mm on a 500 mm bore engine. An unprotected liner could reach that limit in under 5,000 hours, less than a single drydock interval.

On low-sulphur fuels the acid load is smaller, but cold corrosion doesn’t disappear. Engines at reduced load or frequent port cycles spend more time in the cold part of the load range where liner temperatures fall and acid dew-point excursions become more frequent. Cold corrosion on VLSFO engines is concentrated in port-manoeuvring cycles rather than sea passages.

What BN measures and how it is determined

ASTM D2896 potentiometric titration

The ASTM D2896 test is the industry standard for measuring base number in petroleum products, including cylinder oils. The procedure dissolves a known mass of oil in a chlorobenzene-glacial acetic acid mixture and titrates with perchloric acid in glacial acetic acid. The endpoint is detected potentiometrically by a glass-calomel electrode system. The result is reported in mg KOH/g, meaning each gram of oil contains alkalinity equivalent to that many milligrams of potassium hydroxide.

ASTM D2896 measures total base number, including both the strong and weak bases present. The alternative test, ASTM D4739 (potassium hydroxide titration in alcohol), gives a lower number because it excludes the very weak bases. For cylinder oil specifications, D2896 is the universal reference. A commercial cylinder oil labeled “BN70” has a nominal D2896 value of around 70 mg KOH/g, though actual values typically range from 65 to 75 depending on formulation batch.

Chemistry of the alkaline reserve

The alkaline reserve in cylinder oil is carried primarily by calcium-overbased sulphonate or salicylate detergents. These are colloidal systems: surfactant molecules (with hydrocarbon tails and polar heads) form inverted micelles that encapsulate a core of calcium carbonate (CaCO3) nanoparticles. The CaCO3 content is what gives the oil its high BN. A BN70 oil typically carries around 2.5 to 3.5% calcium by mass. A BN40 oil carries around 1.5 to 2.0% calcium. A BN25 oil drops to around 0.8 to 1.2%.

When the oil film contacts sulphuric acid, the CaCO3 inside the micelles is consumed in the neutralization reaction:

Calcium sulphate (CaSO4) is insoluble. It forms fine particles dispersed in the oil. These particles, if produced in quantity beyond what the oil can disperse, can agglomerate into hard deposits on the piston crown, ring grooves, and liner surface. That is the core problem with over-alkalinity in low-sulphur service: excess CaCO3 reacts not with acid (there isn’t enough) but with residual sulphur compounds and combustion gases, producing calcium sulphate and calcium carbonate deposits that can mechanically damage the honing pattern and cause bore polishing.

The BN-per-feed-rate alkalinity delivery calculation

The total alkalinity delivered to the liner surface per unit of engine output is:

where $A$ is the alkalinity supply in mg KOH per kWh, $\text{BN}$ is in mg KOH/g, and $Q_f$ is the cylinder oil feed rate in g/kWh. For a BN70 oil at 1.2 g/kWh, $A = 70 \times 1.2 = 84$ mg KOH/kWh. For a BN40 oil at 1.0 g/kWh, $A = 40\,\text{mg KOH/kWh}$. For a BN25 oil at 0.8 g/kWh, $A = 20\,\text{mg KOH/kWh}$.

The acid to be neutralized per kWh is set by the fuel sulphur content and SFOC as described above. For a rough check, 1 mg KOH of alkalinity neutralizes approximately 0.87 mg of H2SO4 (from the molar mass ratio: H2SO4 = 98, KOH = 56; molar ratio 2:1 for complete neutralization, so 2 × 56/98 = 1.14 g KOH per g H2SO4, inverted: 0.88 g H2SO4 per g KOH).

For the 0.50% VLSFO example above (acid load around 0.08 g/kWh = 80 mg/kWh), the required alkalinity supply is roughly 80 / 0.87 ≈ 92 mg KOH/kWh, achievable with a BN40 oil at around 2.3 g/kWh, or more efficiently with BN40 at 1.2 g/kWh which gives 48 mg KOH/kWh. The discrepancy shows that the feed rate does not need to fully neutralize every molecule of acid in thermodynamic equilibrium: the oil film on the liner acts as a buffer with residence time, and not all condensed acid reaches the metal surface. In practice, MAN Energy Solutions’ baseline feed rate recommendations for BN40 on 0.50% VLSFO are in the range of 0.6 to 1.0 g/kWh depending on engine size and operating profile.

BN grade structure and selection table

The cylinder oil market organizes BN grades in a small number of nominal steps. Actual commercial products from suppliers such as ExxonMobil (Mobilgard series), Shell (Alexia series), Castrol (Cyltech series), and Chevron (Veritas series) are formulated to hit these nominal grades:

The “Category II” designation in the BN40 row is important. MAN Energy Solutions’ service letters issued from 2020 onward draw a distinction between Category I cylinder oils (legacy high-BN, high-detergent formulations) and Category II oils, which are purpose-engineered for VLSFO and ULSFO service. Category II oils at BN40 are not simply diluted Category I oils; they use different detergent balance and additive packages to reduce the calcium deposit tendency at low acid loading. MAN’s guidance specifies that on engines running 0.50% or lower sulphur fuel, only Category II oils should be used regardless of whether the nominal BN happens to match.

WinGD’s cylinder oil guidance for RT-flex and X-DF engines similarly distinguishes between formulations for high-sulphur service (requiring stronger alkalinity buffering) and low-sulphur service (requiring better dispersancy and lower calcium content). For WinGD X-DF dual-fuel engines in gas mode, where sulphuric acid production is essentially zero, even BN40 can produce problematic calcium deposits if the feed rate isn’t reduced substantially; WinGD’s guidance for gas-mode operation points toward BN15 to BN25 depending on the liquid-mode sulphur level when the engine switches fuels.

Post-2020 OEM guidance: MAN Energy Solutions

The Category II transition

MAN Energy Solutions issued several service letters in 2020 and 2021 establishing the Category II framework. The core argument is that BN and acid neutralization are necessary but not sufficient criteria for cylinder oil selection on post-IMO-2020 fuels. Excess alkalinity, expressed as high residual BN in drain-oil samples and visible as grey-white calcium deposits on piston tops and liner surfaces, was observed on engines that had transitioned to VLSFO but continued using BN70 oil, even at reduced feed rates.

The MAN guidance specifies the use of BN40 Category II oil for engines operating on 0.50% sulphur fuel. For engines operating on 0.10% ULSFO or MGO, BN25 or lower is specified. The feed-rate baseline for BN40 Category II on 0.50% VLSFO is given in MAN’s ACC (Adaptive Cylinder oil Control) tables, which vary the feed rate between approximately 0.6 and 1.2 g/kWh depending on engine load and sulphur content.

Adaptive cylinder control feed-rate tables

MAN’s Adaptive Cylinder oil Control (ACC) system, fitted to ME-class engines, adjusts the injection timing and quantity of cylinder oil based on load signal and, where sulphur monitoring is fitted, actual fuel sulphur. The feed-rate tables in ACC are keyed to BN grade: the system has separate tables for BN25, BN40, and BN70. Operating on the wrong table for the installed BN grade is a known source of either acid damage (too little oil for the BN) or deposit damage (too much oil for the BN).

Scrubber vessels and BN70 retention

Vessels with exhaust-gas cleaning systems (EGCS, commonly called scrubbers) operate on HFO with up to 3.5% sulphur under MARPOL Annex VI Reg.3.1 equivalence. For these vessels, the BN70 grade remains the appropriate choice. MAN’s guidance explicitly excludes scrubber vessels from the Category II transition. The practical implication is that fleet operators with a mix of scrubber and non-scrubber vessels need to maintain two cylinder oil grades aboard or at the bunker planning level. Cross-contamination, where BN40 is accidentally used on a scrubber vessel or BN70 is left aboard a VLSFO vessel after a flag change or scrubber failure, is a real operational risk. The egcs sox scrubber calculator can assist with scrubber compliance threshold checking.

Post-2020 OEM guidance: WinGD

RT-flex and X-series engines

WinGD (Winterthur Gas & Diesel) has issued cylinder oil guidance covering both the RT-flex two-stroke diesel family and the X-DF dual-fuel series. For RT-flex engines on VLSFO, WinGD’s position is aligned with MAN: BN40 formulated for low-sulphur service, not legacy BN70 diluted or operated at reduced feed rate.

WinGD’s technical papers published from 2020 onward note two failure patterns that emerged during the VLSFO transition. The first is the deposit pattern already described: excess calcium sulphate and carbonate from over-alkalinity accumulating in the piston top land groove and the ring pack zone, leading to bore polishing and loss of the cross-hatch honing that retains oil on the liner surface. A polished bore running without honing generates insufficient oil-wedge film thickness and can cause adhesive wear (scuffing) under thermal spikes at high load.

The second pattern is what WinGD calls “abnormal wear” on engines that switched to low-BN oil without adjusting the feed rate upward to compensate. A BN40 oil at the same volumetric feed rate as a BN70 oil delivers 43% less alkalinity per litre. If the feed-rate tables weren’t updated (a commissioning error found on some early retrofits), the net alkalinity supply dropped below the acid load even on 0.50% VLSFO during manoeuvring when SO3 conversion efficiency rises.

X-DF dual-fuel cylinder oil selection

The WinGD X-DF engine, burning natural gas in lean-burn mode with a small pilot oil injection, produces trace quantities of sulphuric acid (from the pilot fuel) but essentially no acid from the gas itself. WinGD specifies that in sustained gas mode, cylinder oil feed rate should be approximately 0.3 to 0.5 g/kWh, substantially lower than liquid-mode rates. If the engine uses BN40 oil in gas mode at a liquid-mode feed rate, it delivers around 40 × 1.0 = 40 mg KOH/kWh against a negligible acid load, and the excess CaCO3 has nothing to react with. It sits on the liner and piston surfaces, agglomerates, and forms hard deposits. On X-DF engines that switch frequently between modes, WinGD’s preference is for a BN25 oil managed with a dual feed-rate table: a higher rate in liquid mode and a lower rate in gas mode, reducing the calcium deposit risk without requiring an oil grade change at every mode switch.

CIMAC recommendations on cylinder lubricants

The International Council on Combustion Engines (CIMAC), Working Group 8 (Fuels, Lubrication & Emissions), published an updated guideline on cylinder oils in 2020. The CIMAC guidance is not manufacturer-specific and provides an independent industry baseline.

CIMAC WG8 defines cylinder lubricant performance requirements in terms of neutralization capacity (BN and reserve alkalinity), dispersancy (ability to keep combustion residues in suspension), film strength (anti-wear additive performance under high pressure at the ring/liner contact), and compatibility with fuel types. The 2020 edition explicitly addresses the VLSFO challenge: VLSFO is a blend of residual and distillate components with potentially high asphaltene content and variable viscosity at injection temperature. Cylinder oils formulated for HFO had detergent packages optimized for HFO’s combustion chemistry; the same package on VLSFO can produce different deposit morphology.

CIMAC WG8’s practical recommendations include:

• Testing cylinder oil performance against actual bunker samples rather than relying on nominal fuel sulphur alone, because VLSFO quality is more variable than HFO quality within the 0.50% cap.
• Establishing a drain-oil monitoring protocol with monthly sampling minimum and BN trending over time rather than single-point pass/fail.
• Accepting that feed-rate optimization is iterative: the correct rate on day 1 after a bunker change is not necessarily the correct rate at steady-state.
• Considering the interaction between cylinder oil detergent type and VLSFO’s asphaltene load, since high-asphaltene VLSFO can consume more of the oil’s dispersancy reserve than low-sulphur distillate would.

CIMAC also notes that the ASTM D2896 test measures total BN but does not distinguish between alkalinity that is readily available for neutralization (the outer layers of the colloidal micelle structure) and alkalinity that is kinetically slow to react. Two oils with the same D2896 BN can behave differently in the cylinder if their colloidal structures differ. This is one reason why CIMAC recommends qualifying a new cylinder oil grade against field results from comparable engines, not just from bench BN measurements.

Drain-oil BN monitoring

Sampling location and method

The primary monitoring point for cylinder oil condition is the scavenge box drain on each cylinder unit. Oil thrown from the liner surface by the piston rings drains into the scavenge space and exits through the drain valve. This drain oil is a mixture of fresh cylinder oil that has passed through the lubrication system without contacting combustion products, and oil that has been on the liner surface and has partially consumed its BN through acid neutralization.

The drain sample is collected in a clean sample bottle after a stabilization flush of a few seconds to remove any pooled oil from previous cycles. The sample is submitted to a laboratory for ASTM D2896 BN measurement. Most onboard cylinder oil management programs also measure viscosity, water content (by crackle test or Karl Fischer), and metal content by spectroscopic analysis. Metal content, particularly iron, is the wear indicator: elevated iron (above around 50 mg/kg in drain oil) flags accelerated liner wear.

Interpreting residual BN

A useful heuristic from CIMAC and from the OEM guidance is that a drain-oil BN of 5 to 15 mg KOH/g indicates that the alkalinity supply is adequate: there is some reserve remaining (BN is not zero, so acid hasn’t overwhelmed the system) but most of the BN has been usefully consumed (BN is not high, so there isn’t large excess). If drain BN consistently exceeds 25 to 30 mg KOH/g on a BN40 oil, the oil is not being fully utilized and deposit risk is elevated. If drain BN is consistently below 5 mg KOH/g, the neutralization system is under-supplied.

These thresholds are starting points, not hard rules. CIMAC WG8 (2020) notes that the appropriate drain BN target depends on the BN of the fresh oil, the feed rate, the fuel sulphur content, and engine-specific factors like liner temperature profile and load profile. An engine running at chronic low load (harbour duty, slow steaming at 30% MCR) generates a different drain BN distribution than the same engine at 85% MCR on a deep-sea passage.

Trending over time

Single-point drain BN measurements are less informative than trends. If drain BN is falling consistently over successive monthly samples on unchanged fuel and feed rate, it can indicate that fuel sulphur is increasing (perhaps a contaminated bunker or a change in refinery source), that the liner is developing a cold spot (perhaps a cooling-water distribution issue), or that the cylinder oil is being degraded faster than expected. Conversely, a rising drain BN trend on unchanged parameters can indicate reduced acid load (cleaner fuel) or reduced combustion efficiency (incomplete combustion, fuel spray degradation), which merits engine investigation independently of the BN picture.

MAN’s ACC-equipped engines log feed rate, cylinder oil consumption, and, on instrumented vessels, liner temperature data. The combination of drain BN trend and liner temperature trend is the most complete picture of the acid-neutralization state of the engine.

Over-alkalinity: deposits, bore polishing, and calcium accumulation

The bore polishing failure mode

Over-alkalinity in a two-stroke cylinder produces a failure mechanism that is distinct from, and in some ways as damaging as, cold corrosion. When the alkalinity supply substantially exceeds the acid load, the excess CaCO3 in the oil film has no acid to consume. It accumulates as fine particles on the liner surface. At cylinder pressures and temperatures, these particles are pressed against the liner wall by the piston rings and act as an abrasive between the ring face and the liner bore. Over time this abrades the cross-hatch honing grooves that retain the oil film.

A polished bore is visible at piston overhaul as a mirror-like finish where honing should show cross-hatch marks. The loss of honing means reduced oil film retention, reduced load-bearing area, and increased metal-to-metal contact at the ring-liner interface. Polished bore is not self-limiting: the same over-alkalinity that created it continues to worsen the surface. Rectifying bore polishing requires re-honing, which is only possible with the liner removed, i.e., at a drydock.

MAN’s service documentation for the ME-C and ME-B series engines notes that bore polishing on VLSFO service is almost always associated with continuing use of high-BN oils or high feed rates after the fuel switch. The fix is straightforward in principle: reduce feed rate or switch to a lower-BN oil. But the damage already done to the liner surface doesn’t reverse.

Calcium deposit accumulation on piston tops and ring grooves

A related consequence of over-alkalinity is the accumulation of calcium-containing deposits on the piston crown and in the upper ring groove. These deposits are grey or white in appearance, hard, and often layered. They form when residual calcium compounds in the cylinder oil film are baked onto the hot metal surfaces. Ring-groove deposits can cause ring sticking, which prevents the ring from conforming to the liner bore and creates blow-by paths for combustion gases. Blow-by raises thermal load on the liner and reduces combustion efficiency.

WinGD’s technical papers describe calcium deposit morphology in detail and note that deposits from high-BN oils on low-sulphur fuel are distinct from deposits formed in normal HFO service. HFO deposits tend to be carbonaceous (dark, soft initially). High-BN low-sulphur deposits are calcium-rich (white or grey, hard). The two types respond differently to cleaning at overhaul and should not be confused during engineering assessment of piston condition.

Quantifying the deposit risk

The practical threshold cited in MAN’s service letter guidance for Category II oils is that calcium delivery to the liner should not exceed approximately 5 mg Ca per kWh in low-sulphur service. At BN40 (roughly 1.5% Ca by mass) and a feed rate of 1.0 g/kWh, calcium delivery is approximately 15 mg Ca/kWh, three times the target. That is why the baseline feed rate on BN40 for 0.50% VLSFO is closer to 0.6 to 0.8 g/kWh in MAN’s ACC tables, keeping calcium delivery around 9 to 12 mg/kWh, approaching the target range. A BN25 oil at 0.6 g/kWh delivers around 5.4 mg Ca/kWh, hitting the target more closely.

Fuel sulphur variability and the MARPOL compliance context

ISO 8217 and fuel sulphur specification

Marine fuels supplied under ISO 8217:2017 carry a sulphur specification tied to the compliance zone. For VLSFO intended for use outside ECAs, the maximum sulphur is 0.50% m/m per MARPOL Annex VI Reg.14. For fuels used in North American, North Sea, Baltic, and US Caribbean ECAs (where the 0.10% cap applies per Reg.14.4), the maximum is 0.10% m/m. Distillate fuels (DM grades in ISO 8217) supplied for ECA compliance typically run 0.001 to 0.050% sulphur. Residual fuels (RM grades) meeting the global cap typically run 0.10 to 0.49% depending on refinery source and blend.

The key point for BN selection is that “0.50% VLSFO” is not a single product. The sulphur can be anywhere from traces to 0.50%. A vessel that bunkered consistently on Singapore-refined VLSFO at 0.30% sulphur and then takes delivery of a Rotterdam-blended VLSFO at 0.49% sulphur has more than doubled its acid load per unit fuel mass. If the feed rate and BN weren’t adjusted, the drain BN will fall. Good practice is to re-analyze fuel sulphur at each bunker and adjust ACC settings accordingly if the engine is fitted with the system.

ECA compliance and 0.10% fuel

The 0.10% sulphur cap in ECAs under MARPOL Annex VI Reg.14.4 requires either 0.10% fuel or EGCS equivalence. For vessels switching to 0.10% MGO or ULSFO in ECAs, the acid load drops by a further factor of 5 compared to 0.50% VLSFO. A BN40 oil at normal VLSFO feed rates in ECA service is almost certainly over-alkaline. Most operators running ECAs with significant dwell time (North Sea, Baltic routes) have established reduced feed-rate protocols for ECA entry, some using manual ACC table switching, some relying on the sulphur-adaptive function if installed.

The eca fuel cost premium calculator assists with the economic planning around ECA fuel switching; the BN and feed-rate adjustment is the operational complement to that cost calculation.

MEPC.280(70) and the global cap entry into force

IMO Resolution MEPC.280(70), adopted at the Marine Environment Protection Committee’s 70th session in October 2016, formally amended MARPOL Annex VI to change the global sulphur limit from 3.50% to 0.50% with effect from 1 January 2020. The resolution did not specify which fuels or technologies ships must use; it specified the sulphur content of fuel in use. EGCS equivalence (closed-loop systems treating exhaust) was already permitted under Reg.4, and the combination of Reg.14 and Reg.4 allowed scrubber vessels to retain HFO.

The practical consequence of MEPC.280(70) for cylinder lubrication was immediate. Between 2019 and 2021, the major engine OEMs, lubricant suppliers, and the CIMAC working group all issued updated guidance on BN selection for the new fuel landscape. The cylinder oil market shifted substantially from BN70 as the dominant grade to BN40 within approximately 18 months of the cap entering force.

Dual-fuel and alternative fuel considerations

LNG operation

On WinGD X-DF and MAN ME-GI dual-fuel engines operating in gas mode on liquefied natural gas, the fuel is essentially sulphur-free (natural gas specifications typically run below 0.001% sulphur by mass). Acid production from the gas itself is negligible. The only sulphur source is the small pilot fuel injection used to initiate combustion, typically 1 to 3% of total energy in gas mode. Cylinder oil demand in gas mode is therefore almost entirely lubrication and anti-wear, with very low acid neutralization requirement.

Both MAN and WinGD specify a BN of 15 to 25 for sustained gas-mode operation, with feed rates of 0.3 to 0.6 g/kWh. The challenge for dual-fuel vessels is the transition between modes. When switching from gas mode to liquid-mode HFO or VLSFO, the acid load rises rapidly while the BN on the liner surface may be at gas-mode levels. A pre-conditioning step, increasing feed rate above the liquid-mode steady-state rate for the first 30 to 60 minutes after the mode switch, bridges the gap. MAN’s guidance for ME-GI engines specifies this pre-conditioning protocol explicitly.

Methanol-fueled two-stroke engines

MAN Energy Solutions has produced methanol-capable ME-LGIM (liquid gas injection methanol) variants. Methanol contains no sulphur, so acid neutralization requirements are essentially zero. The cylinder oil requirements shift to pure lubrication: preventing adhesive wear between ring and liner, maintaining viscosity under the different thermal and chemical environment of methanol combustion (higher water content in combustion products, different deposit chemistry).

Specialist methanol cylinder oils with BN10 to BN17 are emerging from major lubricant suppliers. These formulations have high film-strength additives and dispersancy but low calcium content, minimizing deposit risk. The IMO’s Interim Recommendations for alternative fuels (relevant to methanol under MARPOL Annex VI Reg.16 and the IGF Code) don’t specify cylinder oil formulation directly, leaving that to engine OEM guidance.

Ammonia

MAN Energy Solutions announced the ME-LGIA (liquefied gas injection ammonia) engine concept, with first commercial deliveries expected in the mid-2020s. Ammonia contains no sulphur. Combustion of ammonia produces nitrogen oxides (NOx) but no SOx. The cylinder lubrication challenge with ammonia is chemically different: ammonia is itself a base, and its combustion products include water and nitrogen. The concern is not acid attack but potential reactions between ammonia slip (unburned ammonia) and cylinder oil components, and the need for film strength under the different load and temperature profile of ammonia combustion.

Industry development of ammonia cylinder oils was at an early stage as of 2024, with engine trials running experimental formulations. The BN framework developed for sulphur-bearing fuels is largely irrelevant for ammonia; the performance criteria are being redefined by the OEMs and CIMAC in parallel with engine development.

Multi-cylinder engine monitoring and fleet-level management

Per-cylinder drain-oil sampling

A slow-speed main engine typically has 6 to 14 cylinder units (for example, a MAN ME-C or ME-B engine in the 6S90ME-C class has 6 units on a 90 cm bore). Drain-oil sampling should cover each cylinder unit individually, not a mixed composite from the scavenge manifold. BN depletion can vary substantially between cylinders on the same engine because:

• Cylinder liner temperature varies with cooling-water distribution (units near the coolant inlet run cooler and see more cold corrosion potential).
• Fuel injector spray pattern asymmetry means some cylinders receive richer or leaner local combustion zones.
• Cylinder oil feed rate can vary between units if lubricators are not calibrated against each other.

A cylinder showing drain BN consistently 30% lower than the fleet average is a flag for investigation: check the feed-rate calibration on that lubricator, check liner wall temperature if sensors are fitted, and check injector condition.

Fleet BN management for mixed-fuel operations

Large shipping companies operate fleets where individual vessels bunker at different ports, sometimes receiving different sulphur levels from the same nominal grade. Centralizing drain-oil analysis through a shore laboratory with a common database allows trending that a single vessel engineer cannot do alone. Some operators have established fleet-wide BN management protocols with automated alerts when drain BN falls outside a 5 to 20 mg KOH/g target band.

The cylinder lubrication systems for two-stroke engines article covers the hardware aspects of lubricator calibration and the differences between quill-based and the newer accumulator-based injection systems that affect per-unit feed-rate control precision.

Limitations

Several factors limit the precision of BN-to-sulphur matching in practice:

Fuel sulphur variability within delivered bunkers. ISO 8217 permits the sulphur content of a RM (residual marine) grade fuel to vary within a specification band. A bunker batch delivered in multiple parcels from different refinery sources can have sulphur variation of ±0.10 to ±0.15% within the same declared grade. The BN selection is made for the nominal declared sulphur, but the actual sulphur drives the actual acid load.

SO3 conversion fraction uncertainty. The 1 to 5% range for SO2-to-SO3 conversion is engine- and condition-specific and can’t be measured directly aboard without specialized gas-sampling equipment. Engines with heavy oxide deposits on combustion surfaces convert more SO2, those with fresh surfaces less. The exact conversion factor changes BN requirements by a factor of 2 to 5.

ASTM D2896 measures available, not delivered, alkalinity. The BN number measures the alkalinity in the bulk oil as collected; it doesn’t directly measure how much of that alkalinity actually reaches the liner surface and contacts the acid film. Oil temperature, shear rate in the lubricator delivery channels, and the oil film dynamics on the liner surface all affect the fraction of alkalinity that is useful.

Drain-oil BN is a lagging indicator. By the time a drain-oil sample is analyzed at a shore laboratory and the result is returned to the vessel, several days to weeks have passed. An acute corrosion event from a bad bunker can progress substantially before the monitoring system catches it. Onboard rapid-test kits (colorimetric BN tests) give a same-day estimate but with lower precision than laboratory D2896.

Category II oils are not interchangeable between OEMs. MAN’s Category II designation and WinGD’s equivalent guidance are not formally harmonized. An oil qualified as Category II by MAN’s testing protocol has been tested on MAN engine geometries and load profiles. The same oil used on a WinGD RT-flex engine may or may not behave identically. In practice, major lubricant suppliers test against both OEMs, but the qualification status should be verified for each engine type aboard.

Dual-fuel mode transition timing is vessel-specific. The pre-conditioning feed-rate ramp when switching from gas to liquid mode is specified by the OEM but needs to be validated for the actual vessel with its specific engine load profile and thermal inertia. The specified protocol is a starting point.

See also

• Cylinder Lubrication Systems for Two-Stroke Marine Engines
• Pulse Lubrication Systems for Marine Engines
• Cylinder Oil Feed Rate Optimisation
• Cylinder Liner Design for Two-Stroke Engines
• Piston Ring Pack Design for Two-Stroke Engines
• Scavenge Port Geometry and Timing
• WinGD X-DF Dual-Fuel Architecture
• EGCS SOx Scrubber Calculator
• ECA Fuel Cost Premium Calculator
• Cylinder Oil Feed Rate MAN ACC
• Lube TBN Depletion Calculator