Slow Steaming and Engine Cleanliness

Why low load is hard on a two-stroke

A slow-speed two-stroke is tuned for a narrow band. Most large bore engines are matched for best efficiency around 70 to 85 percent of maximum continuous rating, and the combustion, scavenging, and turbocharging all assume that operating point. Run the same engine at 30 percent MCR for weeks at a time and almost every parameter that keeps the cylinder clean moves the wrong way.

The driver is air. The single turbocharger on a modern uniflow engine is a fixed-geometry machine, so its delivery pressure falls roughly with the square of engine speed. Drop from 100 to 50 percent power and scavenge-air pressure can fall from about 3.8 bar absolute to under 2 bar; below 25 percent MCR the turbocharger barely contributes and the engine leans on its auxiliary blowers. Less air means less excess oxygen, lower in-cylinder turbulence, and poorer atomized-fuel mixing. The result is incomplete combustion, soot, and unburned hydrocarbon that ends up as deposit. For the wider operating context see slow steaming and engine derating for slow steaming.

Combustion temperature drops with load too. Peak gas temperature can fall from roughly 1500 C near full load toward 1300 C around 30 percent MCR, and combustion stretches later into the expansion stroke, from near 30 crank-angle degrees of burn duration up toward 50. A late, cool burn is a dirty burn. It deposits carbon on the piston crown, leaves varnish on the cylinder cover, and feeds the soot load that the gas path then has to carry. None of this is a fault in the engine. It is the engine being asked to work far from where it was matched, which is exactly what charter-driven speed orders demand.

Cold corrosion: the dominant low-load risk

The mechanism that does the most damage at low load is cold corrosion of the cylinder liner. Residual and many distillate fuels carry sulfur. During combustion that sulfur oxidizes to sulfur dioxide & a fraction to sulfur trioxide; the trioxide combines with water vapor to form sulfuric acid. As long as the liner wall and the scavenge air stay hot enough, that acid stays as vapor and leaves with the exhaust. Let the wall cool toward the acid’s condensation point and liquid sulfuric acid plates out onto the running surface, where it attacks the iron directly.

The number that governs this is the sulfuric-acid dew point, and it sits far above the water dew point. For typical marine combustion gas the acid dew point lands in the 120 to 160 C range, climbing with both the SO3 concentration and the moisture content. Higher fuel sulfur raises the SO3 partial pressure & lifts the dew point; this is why a high-sulfur 3.5 percent fuel condensed acid more readily than a compliant fuel does. At low load, two things fall toward that dew point at once: the scavenge-air temperature drops because the air cooler now over-cools a small charge mass, and the liner wall runs cooler because there is less heat to reject. When the lower liner cools below roughly 140 to 150 C, acid condenses on the very surface the rings sweep.

The damage is not gradual wear; it is corrosive attack. Operators have seen liner wear rates jump from a healthy 0.05 mm per 1000 hours to several tenths of a millimeter per 1000 hours once cold corrosion sets in, with the characteristic “clover-leaf” pattern of wear lobes between the lubricator quills. A liner that should last 60,000 to 90,000 hours can be condemned in a fraction of that. This is the failure mode that turned slow steaming from a fuel-saving idea into an engine-management discipline. Wear tracking through drip-oil iron is covered in cylinder liner wear monitoring, and the acid-versus-base balance in cylinder oil base number and fuel sulfur matching.

What the 0.50% sulfur cap changed

MARPOL Annex VI dropped the global fuel sulfur limit from 3.50 to 0.50 percent on 1 January 2020, with 0.10 percent inside Emission Control Areas. Lower sulfur means less SO3 and a lower acid dew point, so on paper the cold-corrosion threat eased. In practice it shifted the problem to the other side of the lubrication window. The cylinder oil’s job is to neutralize acid with its alkaline reserve, measured as Base Number in mg KOH per gram. A high-BN oil that suited 3.5 percent fuel now over-bases against 0.50 percent fuel; the surplus calcium carbonate that does not get consumed neutralizing acid deposits as hard, abrasive ash on the piston top land and ring grooves. So the modern slow-steaming engine runs a tighter line: enough alkalinity & feed to stop acid attack at low load, but not so much that calcium ash builds and the rings start to stick. The fuel-quality background sits in MARPOL Annex VI; ISO 8217 sets the sulfur, ash, and micro-carbon limits the bunker must meet.

Micro Carbon Residue is the laboratory proxy for a fuel’s tendency to form combustion-chamber deposits, and at low load, where burn-out is already poor, a high-MCR fuel makes piston-crown and ring-groove carbon worse. ISO 8217 caps MCR by grade; checking the bunker delivery note value against the grade limit is the first deposit-control step before the fuel ever reaches the engine.

Gas-path & exhaust-side fouling

What does not burn in the cylinder leaves as soot, and at low load there is a lot more of it. The exhaust side of the engine then has to carry that load through the turbine, the exhaust-gas economizer, and the uptakes, and each of those becomes a fouling and, in two cases, a fire problem.

Exhaust-gas economizer soot fires

The exhaust-gas economizer (also called the exhaust-gas boiler) sits in the uptake and recovers heat from the main-engine exhaust to make steam. At design load the exhaust enters it around 250 to 300 C and gas velocities keep the finned tubes swept clean. At deep slow steaming the exhaust can drop toward 200 C or lower, gas velocity collapses, and unburned soot & oil mist settle on the tube banks instead of blowing through. Wet, oily soot on a hot tube is the ingredient list for a soot fire. A small “soot fire” raises tube-bank temperature and can escalate to an “iron fire,” where the steel tubes themselves oxidize at 1000 C and upward, destroying the unit and risking the uptake structure. The standard defenses are regular soot blowing while at sea, keeping the boiler water side circulating so tubes are never dry, and not letting the engine idle at very low exhaust temperature for long unbroken periods.

The fouling factor card above relates the heat-transfer penalty to the soot layer building on the gas side; the same deposit that costs you steam output is the fuel for a soot fire, so the economizer’s falling steam production is itself an early fouling alarm. You can put numbers to the heat-transfer loss with the exhaust-gas boiler fouling calculator.

Turbocharger fouling and surge

The turbocharger fouls on both sides. The turbine collects soot, unburned hydrocarbon, and sulfate on the nozzle ring and blades; the compressor collects oil mist and salt drawn through the air filter. Both narrow the effective flow area and shift the machine on its map. At low load the compressor is already operating near the left side of its map, close to the surge line, and a fouled, narrowed compressor pushes it across that line. Surge is the violent flow reversal you hear as a deep cough or bark from the turbocharger; repeated surging hammers the thrust bearing and can crack blades. The defenses are turbine and compressor water washing on a regular cycle and, for the matching problem itself, turbocharger cut-out, covered below. Scavenge-pressure trends are the early-warning signal; estimate the expected value with the scavenge-air pressure calculator and watch for drift below it.

Piston, ring, and scavenge-space deposits

Inside the engine the same poor burn leaves carbon on the piston crown and, worse, packs the ring grooves. A ring that sits in a groove choked with carbon & calcium ash loses its freedom to follow the bore; it sticks, stops sealing, and lets hot combustion gas blow past. That blow-by carries flame and unburned oil down into the scavenge space below the piston. The scavenge space already collects drained cylinder oil, soot, and condensed acid, so a blow-by event can ignite that residue: a scavenge-box fire. Most are brief and self-extinguishing, a flash that scorches the space; a sustained one warps the space, damages the piston rod gland & stuffing box, and can propagate between units through a common scavenge receiver. Detection is by local temperature sensors and, on many engines, a scavenge-space CO sensor; the response is to slow down, raise cylinder oil feed to the affected unit briefly, and let the fire burn out under reduced load before inspecting.

Cylinder lubrication strategy at low load

Cylinder lubrication is the main lever the engineer has against cold corrosion, and at low load it has to be re-thought rather than left at the design setting. Feed rate is dosed in grams of oil per kilowatt-hour, and the modern electronically controlled lubricator can vary feed independently of engine speed. Three quantities have to match: the feed rate, the oil’s Base Number, and the actual acid load coming from the fuel’s sulfur at the present load.

Set feed too low and there is not enough alkaline reserve laid on the liner to neutralize the condensing acid; cold corrosion accelerates. Set it too high, especially with a high-BN oil on low-sulfur fuel, and the unconsumed calcium deposits as hard ash that drives ring sticking and bore polishing. The window between these is real and it narrows at low load, because the acid load per stroke is lower but the wall temperature is also lower, so the acid that does form is more likely to condense. OEM lubricators address this with feed-rate factors keyed to fuel sulfur, and with anti-corrosion or “load-change-dependent” dosing that lifts feed transiently when the engine changes load. The matching detail is set out in cylinder oil feed rate optimization.

Base Number selection follows the fuel. The market standardized on roughly 40 BN oils for the high-sulfur era; the 0.50 percent cap pushed the mainstream toward 25 to 40 BN, and 0.10 percent ECA fuel toward 10 to 25 BN oils, so the alkaline reserve is not wasted as ash. Many ships now bunker two cylinder oils & blend on board, or run a single mid-BN oil with feed-rate trimming, to track a fuel mix that changes every time the vessel crosses an ECA boundary. The right answer is the one that holds drip-oil iron low without building ash, which is why drip-oil sampling frequency goes up during sustained slow steaming, from monthly toward fortnightly or weekly, with the iron and residual-BN trends read together. Sample-side condition checks tie into lube oil TAN and water condition.

Skip-cycle dosing, where the lubricator injects every second, third, or fourth revolution instead of every revolution, lowers total consumption and can reduce ash build at low load, but only if it is matched carefully to load and acid load. Inject too rarely and the oil film between doses thins enough that a patch of liner sees acid with no fresh reserve. The setting is engine-specific and is one the OEM lubrication guidance covers directly.

Mitigations on the engine

Turbocharger cut-out and exhaust-gas bypass

The cleanest fix for the air shortage at low load is to give the remaining turbocharger more work. On engines with two or three turbochargers, turbocharger cut-out blanks off one machine’s gas and air sides so the rest run nearer their efficient point, restoring scavenge pressure and combustion air at part load. On single-turbocharger engines the equivalents are an exhaust-gas bypass, which routes some exhaust around the turbine to manage matching, and an exhaust waste-gate or a swing of the turbine geometry where fitted. All of these aim at the same target: keep enough air mass and turbulence in the cylinder at low load that the burn stays complete and the gas path stays clean. They are normally selected automatically by the engine control system as a function of load.

Variable injection timing, slide valves, and auto-tuning

Variable Injection Timing lets the engine advance or retard fuel injection with load. At low load, controlled timing keeps peak combustion pressure up where the burn is hotter and cleaner, rather than letting it sag with reduced fueling. Variable exhaust-valve timing does the same on the gas-exchange side, trimming the effective compression and the scavenge overlap to suit the load. Modern slide-type fuel valves replaced the old conventional valves precisely because they leave no sac volume to dribble fuel after injection ends; that dribble was a direct source of low-load soot and tip deposits. On top of these, engine auto-tuning continuously balances cylinder peak pressures and injection across units, so no single cylinder runs cold and dirty while its neighbors carry the load. The capabilities differ by engine generation, which is part of why two ships of similar size can have very different low-load cleanliness.

Part-load optimized derating

If a ship’s real operating profile is slow, the durable fix is to re-match the engine for it. Derating, or part-load optimization, retunes the turbocharger matching, injection, and timing so the efficient point moves down toward the speed the ship actually runs. A part-load-optimized engine running at its new design point is clean and fuel-efficient there, instead of fighting deposits at a load far below its old match. The trade is flexibility & top speed: a derated engine gives up some of its upper power band, and a vessel that later needs to sprint may regret it. The economics and method are in engine derating for slow steaming.

The derating card frames the relationship between the limited rating and the original MCR; the same percentage de-rate that improves part-load cleanliness is also what shows up in the attained efficiency index, so a derate is rarely a pure engineering decision. Run the numbers with the MCR derating calculator.

Specific fuel-oil consumption is sensitive to scavenge-air temperature, which is exactly the quantity that drifts at low load when the air cooler over-cools a small charge mass. A cooler that runs the air too cold raises the cold-corrosion risk; one that fouls and runs the air too hot loses efficiency and trapped mass. Holding scavenge-air temperature in its target band, often near 40 to 50 C at the cooler outlet, is therefore both an efficiency and a cleanliness control. The SFOC air-temperature sensitivity calculator puts a figure on the penalty.

Load-up runs, soot blowing, and performance monitoring

Even a well-tuned engine benefits from a periodic load-up run: an hour or so at 80 to 90 percent MCR raises combustion temperature enough to burn off soft deposits, lifts scavenge air enough to sweep the scavenge space, and brings the turbocharger and economizer back to swept conditions. These runs clear soft, recent deposits; once carbon has baked into a hard crust, only mechanical cleaning at overhaul removes it, so the discipline is to run them on schedule and not defer them. A reasonable cadence is weekly under deep slow steaming near 30 percent MCR, fortnightly around 40 to 50 percent, and as-needed on mixed profiles, triggered by performance trends rather than the calendar alone.

Soot blowing on the economizer should run on its own timer at sea, more often the lower the exhaust temperature. Turbocharger water washing, turbine and compressor, runs on a weekly-to-monthly cycle. Underpinning all of it is performance monitoring: pressure-indicator (PMI) traces for cylinder peak and compression pressure, exhaust-gas temperature spread between units, turbocharger speed and scavenge pressure against expected, and economizer steam output. A widening exhaust-temperature spread, a falling scavenge pressure, or a sagging steam output is the fouling telling you before the boroscope does. The exhaust-to-intake temperature ratio is one such trend; track it with the exhaust-to-intake ratio calculator.

Brake thermal efficiency derived from SFOC is the bottom-line health number: deposits, fouling, and a poor low-load burn all show up as a falling efficiency before any single instrument flags a fault. Tracking it across a slow-steaming voyage separates a one-off bad reading from a real fouling trend, and a step recovery after a load-up run confirms the deposits were soft. Estimate it from measured consumption with the brake thermal efficiency calculator.

How the air shortage propagates through the engine

It helps to follow the air through the engine at 30 percent MCR and see why each station fouls. The turbocharger compressor draws ambient air, but with the turbine starved of exhaust energy the compressor delivers maybe half the pressure ratio it makes at full load. The auxiliary blowers cut in below roughly 25 to 35 percent load to make up the deficit, and they are sized to maintain combustion, not to flush the cylinder the way full scavenge pressure does. So the charge mass per cycle is small and its velocity through the scavenge ports is low.

Low port velocity is the first problem. Uniflow scavenging relies on a fast, organized swirl that pushes the burnt charge up & out through the exhaust valve while the fresh charge fills from below. Drop the velocity and the scavenge becomes lazy: residual gas stays behind, the fresh charge dilutes with exhaust, and the next combustion starts with less oxygen than the indicator card assumes. That residual dilution is a quiet driver of soot, separate from the temperature effect, and it is why simply burning the same fuel mass at low speed does not give the same clean burn.

The small charge mass is the second problem, and it shows up at the air cooler. The cooler is sized to pull a large charge mass down to its target temperature at full load. Pass a small mass through it and the same cooling surface over-cools the air, so scavenge-air temperature at the cooler outlet can fall well below its target band. Cold scavenge air feeds straight into the cold-corrosion mechanism: it pulls the liner wall temperature down and shifts the local surface below the acid dew point. This is the counter-intuitive part of low-load operation. The instinct is that a cool engine is a safe engine, but a cool liner is the one that condenses acid. Some makers fit scavenge-air heaters or recirculate jacket water to the cooler precisely to hold the air temperature up at low load, and where they are fitted the engineer’s job is to make sure they are working, not bypassed.

The third problem is the exhaust side, already covered, but it closes the loop: poor scavenge and a cool burn put more soot into a slower, cooler exhaust stream that cannot carry it, so the soot settles in the turbine, the uptake, and the economizer. Every station downstream inherits the upstream shortfall. This is why no single fix is enough on its own and why the mitigations stack: restore the air with turbocharger cut-out or bypass, restore the burn with timing control, hold the air temperature up, and clean what still settles.

Inspection and the boroscope cycle

Cleanliness is verified, not assumed. The scavenge ports give the cheapest window into the running surface, and on most engines they can be inspected through the scavenge-space doors with the piston positioned to expose the liner below the ports. What the engineer reads there tells the story: a light, even brown film is a healthy oil film; a dry, scuffed, grey surface with vertical wear marks between the lubricator quills is cold corrosion in progress; a black, lacquered, or carbon-crusted ring pack warns of sticking rings and bore polishing. A camera record from each inspection, compared port by port over months, catches a trend long before a single look would.

Boroscope inspection through the scavenge ports or dedicated access checks the ring pack, the ring grooves, and the lower liner without pulling the unit. During sustained slow steaming the sensible cadence tightens from the routine overhaul interval toward something keyed to running hours at low load, because the deposit and corrosion mechanisms run on low-load hours, not calendar time. A ship that spends most of its time near 30 percent MCR accrues cold-corrosion exposure far faster per calendar month than one on a mixed profile, and the inspection plan should reflect the actual load history, which the engine-management system logs.

The drip-oil sample is the other half of the picture. Iron content in the scavenge-drain oil is the direct proxy for liner wear, and residual Base Number in the same sample tells whether the oil’s alkaline reserve is being consumed faster than it is being laid down. Read together, a rising iron with a low residual BN says the feed is too lean for the acid load: lift it. A flat iron with a high residual BN on low-sulfur fuel says the feed is too rich: trim it before the ash builds. Sampling weekly during deep slow steaming, against monthly on a normal profile, is the cost of running the engine far from its match, and it is cheap insurance against a liner set.

Operating envelope and the OEM service letters

Engine makers publish the low-load envelope and the cleanliness procedures in their service letters, and those documents, not general guidance, are the controlling reference for a given engine. MAN Energy Solutions and WinGD each issue letters covering cylinder lubrication feed-rate factors, the acid-control dosing logic, the minimum recommended continuous load, the load-up-run procedure, and the cylinder-oil Base Number selection against fuel sulfur. The letters are engine-family specific because the turbocharger matching, the lubricator capability, and the timing control differ across families, and a feed-rate factor correct for one engine can be wrong for another.

A recurring theme in the guidance is the minimum unbroken low-load period. The makers do not forbid deep slow steaming, but they set a recommended interval at which a load-up run should interrupt it, and they tie the cylinder-oil feed rate to both the load and the fuel sulfur rather than to engine speed alone. The other recurring theme is load-change management: the moment of greatest cold-corrosion risk is often not steady low load but the transitions, when the wall temperature lags the load and the acid balance is briefly off. Load-change-dependent lubrication, which lifts feed transiently during maneuvering and load changes, is the maker’s answer to that, and it is one of the features that separates a modern electronically controlled lubricator from an older mechanical one.

Crew familiarity with these documents is part of the program. The procedures are not difficult, but they are specific: the right load-up-run duration and target load, the right washing cycle, the soot-blowing interval keyed to exhaust temperature, the cylinder-oil Base Number for the fuel in the tank, and the inspection cadence for the current load history. A standing instruction that names the actual numbers for the actual engine, drawn from the actual service letters, is worth more than any general article, and the engine-room performance log is where the trends that trigger each action are read. Maintenance interval planning sits in engine maintenance scheduling overview.

Economics, CII, and the speed order

Slow steaming exists because resistance climbs roughly with the square of speed and propulsive power with the cube, so a 10 percent speed cut can drop main-engine fuel by close to a quarter. The IMO Fourth GHG Study 2020 documented how widely the world fleet used reduced speed after 2008 for exactly this reason. The same physics now sits inside regulation: the Carbon Intensity Indicator rates a ship A to E on grams of CO2 per deadweight-mile, and reduced speed is the fastest way to improve a rating, so the commercial and the regulatory pressure both point at lower speed. The interaction is set out in slow steaming and CII.

The catch is that the CII math rewards the operating point that is hardest on the engine. A ship pushed to a low CII band by deep slow steaming is precisely the ship most exposed to cold corrosion, economizer soot fires, and turbocharger surge. The cleanliness program is what lets the commercial decision stick: the cost of weekly load-up runs, more frequent washing, two cylinder oils, and tighter monitoring is small against a condemned liner set or a gutted exhaust-gas boiler. Treated as a system, slow steaming is sustainable; treated as just a throttle position, it quietly destroys the engine. The wider engine context is in two-stroke marine diesel engine fundamentals and uniflow scavenging.

Deposit and fouling sites at a glance

The locations and their dominant low-load mechanism, for inspection planning:

• Top piston ring grooves: carbon & calcium ash from poor burn-out and over-alkaline oil; risk is ring sticking and blow-by.
• Lower cylinder liner: condensed sulfuric acid below the acid dew point; risk is cold-corrosion wear and clover-leaf patterns.
• Piston crown & cover: soft carbon and varnish from late, cool combustion; clears on load-up runs while still soft.
• Scavenge space: drained oil, soot, and acid pooling; risk is scavenge-box fire on blow-by.
• Turbine side of turbocharger: soot and sulfate on nozzle ring & blades; risk is efficiency loss and surge.
• Compressor side: oil mist and salt; risk is surge margin loss at low load.
• Air cooler: salt and oil mist raising charge-air temperature; risk is lost trapped mass and a knock-on SFOC penalty.
• Exhaust-gas economizer: wet oily soot on finned tubes at low exhaust temperature; risk is soot fire escalating to iron fire.

Limitations

OEM thresholds are not universal. The load percentages, scavenge pressures, exhaust temperatures, and acid dew points cited here are representative of large modern uniflow two-strokes; the exact numbers, feed-rate factors, and low-load envelopes differ by engine model and generation, and the controlling figure is always the maker’s own service letter and operating guidance for the specific engine, not a general value.

Engine capability varies widely. Turbocharger cut-out needs more than one turbocharger; variable injection timing, variable exhaust timing, slide valves, and auto-tuning are present on newer electronically controlled engines and absent or limited on older camshaft engines. Two vessels of similar size can have very different low-load cleanliness purely because of which control features their engines carry, so a procedure that works on one will not transfer unchanged to the other.

The central trade-off has no single right answer. Derating improves part-load fuel economy and cleanliness but surrenders top-speed flexibility; high-BN oil protects against acid but builds ash on low-sulfur fuel; a deeper de-rate that helps the engine can move the attained efficiency index in a direction the commercial side dislikes. These are judgment calls that depend on the ship’s real trade and its charter, not on a formula.

Condition data has gaps. Drip-oil iron, PMI traces, scavenge pressure, and steam output are lagging and noisy indicators; a clover-leaf wear pattern may be well advanced before the iron trend turns, and a boroscope between overhauls sees only part of the liner. The figures from any single calculator on this site are estimates for screening and trend-watching, not a substitute for direct measurement and the maker’s limits.

Finally, the operational reality is that charter-driven speed orders, weather routing, and port schedules set the load, and the engineer manages cleanliness around a speed profile chosen for commercial reasons. The cleanliness program described here is what makes that externally imposed profile survivable, but it cannot override a sustained order to run at a load the engine was never matched for.

See also

• Engine Derating for Slow Steaming
• Slow Steaming
• Slow Steaming and CII
• Cylinder Oil Feed Rate Optimization
• Cylinder Oil Base Number and Fuel Sulfur Matching
• Cylinder Liner Wear Monitoring
• Two-Stroke Marine Diesel Engine Fundamentals
• Uniflow Scavenging in Two-Stroke Marine Engines
• Piston Ring Pack Design
• Exhaust Valve Actuation in Two-Stroke Engines
• EGR Retrofit on Two-Stroke Marine Engines
• Engine Maintenance Scheduling Overview
• MARPOL Annex VI

Related calculators

• Exhaust-Gas Boiler Fouling
• MCR Derating
• SFOC Sensitivity to Air Temperature
• Brake Thermal Efficiency from SFOC
• Fuel MCR / Micro Carbon Residue
• Scavenge Air Pressure: 2-Stroke Estimate
• Exhaust-to-Intake Temperature Ratio
• Lube Oil TAN / Water Condition Check
• Main Engine Slow-Speed 2-Stroke System