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Slow Steaming and Engine Cleanliness

Sustained slow steaming at loads below 50 percent MCR produces several engine cleanliness challenges that do not arise at design-load operation. Lower combustion temperatures generate more deposits in piston ring grooves, exhaust valves, scavenge boxes, turbochargers, and air coolers. Without active management, deposits accumulate over weeks and months, eventually causing performance degradation, scavenge box fires, or component failures. This article covers deposit accumulation mechanisms, the scheduled high-load (decarbonisation) run protocol, water washing, and the maintenance procedures that protect engine condition during sustained slow-steaming operation. Visit the home page or browse the calculator catalogue for related operations engineering tools.

Contents

Background

Modern slow-speed two-stroke marine engines are designed to operate efficiently at 75-85 percent MCR. At this operating point:

  • Combustion temperatures are high, producing complete fuel combustion
  • Scavenge air supply is adequate
  • Turbocharger operates at design efficiency
  • Deposit formation is minimal

When the same engine operates at 30-40 percent MCR for extended periods (typical slow steaming), conditions change in ways that promote deposit formation:

  • Combustion temperatures drop, leading to incomplete combustion
  • Scavenge air pressure falls, reducing turbulence and mixing
  • Turbocharger operates below design point, reducing efficiency
  • Cooler exhaust gases promote acid condensation

The cumulative effect, over weeks of sustained low-load operation, is:

  • Carbonised deposits in piston ring grooves, narrowing the grooves and risking ring sticking
  • Carbon and varnish deposits on cylinder cover surfaces and exhaust valve seats
  • Oily deposits on scavenge port surfaces, contributing to scavenge box fires
  • Soot accumulation in scavenge receivers and exhaust manifolds
  • Compressor and turbine deposits in turbochargers, reducing efficiency
  • Air cooler fouling from particulate accumulation

This article describes deposit mechanisms and the operational protocols that manage them.

Deposit mechanisms

Cylinder deposits

Inside the cylinder during low-load operation:

  • Combustion temperature: drops from ~1500°C peak at full load to ~1300°C peak at 30% MCR
  • Combustion duration: extends from ~30°CA at full load to ~50°CA at 30% MCR
  • Local rich mixtures: from poorer fuel-air mixing, produce soot

Soot and unburned hydrocarbons mix with cylinder oil and condense on:

  • Top piston ring grooves (most heavily exposed)
  • Cylinder cover central area
  • Exhaust valve seat upper surface
  • Scavenge port edges (lower cylinder)

Deposits accumulate at typical rates of 0.5 to 2 mm thickness per 1,000 hours during sustained slow steaming.

Scavenge box deposits

Below the cylinder, in the scavenge box:

  • Cylinder oil overflow from the cylinder mixes with combustion residues
  • Soot particles from incomplete combustion settle on surfaces
  • Sulphuric acid condenses from cooled exhaust gases
  • Water condenses from cool exhaust below the dew point

The result is an oily, acidic, sooty residue that:

  • Coats scavenge port faces and edges
  • Accumulates on liner surfaces below the ports
  • Pools at the bottom of the scavenge box
  • Provides fuel for scavenge box fires

Turbocharger deposits

In the turbocharger:

  • Compressor side: oil mist and salt residue from charge air
  • Turbine side: soot, unburned hydrocarbons, salt deposits
  • Diffuser, nozzle ring, blades: progressive accumulation

Turbocharger deposits increase pressure drop, reduce efficiency, and eventually cause surge or balance issues.

Air cooler deposits

The air cooler (between turbocharger compressor and scavenge receiver) accumulates:

  • Salt deposits from charge air containing sea spray
  • Oil mist from the turbocharger
  • Particulate matter from the engine room atmosphere

Cooler fouling raises charge air temperature, reducing trapped mass and engine efficiency.

Operational consequences

Performance degradation

Cumulative deposits cause measurable performance decline:

  • SFOC rise: 1-3 g/kWh increase over months of slow steaming, often eliminated by decarbonisation runs
  • Compression pressure decline: from ring sticking and combustion chamber deposits
  • Peak pressure variation: cylinder-to-cylinder spread increases
  • Exhaust temperature rise: from poorer combustion completion
  • Turbocharger output decline: gradually reducing scavenge air pressure

Risk of scavenge box fires

Heavy oil and soot accumulation in the scavenge box can ignite:

  • Heat sources: hot blow-by gas during compression, hot piston rings
  • Fuel: accumulated cylinder oil residue, hydrocarbon deposits
  • Oxidiser: scavenge air

Scavenge box fires can be:

  • Self-extinguishing: brief flame in bottom of scavenge box, no significant damage
  • Sustained: damages scavenge box internal surfaces, may propagate to other cylinders
  • Catastrophic: very rare, can cause major engine damage

Modern engines include scavenge box fire detection (CO sensors, temperature sensors) and inerting systems.

Component damage

Sustained deposit accumulation can cause:

  • Ring sticking: rings become locked in grooves, lose sealing function, cause blow-by
  • Exhaust valve burning: deposits prevent proper seating, gas leaks erode the valve
  • Compressor surge: turbocharger compressor unable to maintain stable flow
  • Turbocharger imbalance: deposit asymmetry causes vibration

Decarbonisation runs

The principal protocol for managing slow-steaming cleanliness is the decarbonisation run (also called engine cleaning run or scheduled high-load run):

Procedure

The engine is operated at high load (typically 80-90% MCR) for an extended period (typically 30-60 minutes). At high load:

  • Combustion temperature rises, burning off accumulated soft deposits
  • Scavenge air supply rises, sweeping out scavenge box deposits
  • Turbocharger operates at design point, partially clearing its own deposits
  • Higher gas velocities through the air cooler may clear some deposits

Frequency

Recommended decarbonisation run frequency:

  • Weekly for ships in extended slow steaming (~30% MCR)
  • Biweekly for moderate slow steaming (~40-50% MCR)
  • Monthly or as needed for ships with mixed load profiles

Some operators use noon-report data to trigger runs based on observed performance trends.

Practical considerations

Decarbonisation runs require:

  • Schedule flexibility: deviation from planned voyage speed for the run duration
  • Bunker margin: high-load runs consume more fuel
  • Crew vigilance: runs are good times for engine-room performance checks
  • Pre/post comparison: PMI data, exhaust temperatures, turbocharger pressures should be logged before and after

Ineffective for hard deposits

Decarbonisation runs are effective for soft, freshly accumulated deposits. Once deposits harden into carbonised crust, runs alone are insufficient. Mechanical cleaning at overhauls is required for hardened deposits.

Water washing

Turbocharger compressor washing

Periodic water washing of the turbocharger compressor removes salt and particulate deposits:

  • Frequency: weekly to monthly during slow steaming
  • Procedure: water injected into compressor air inlet while engine running
  • Quantity: typically 5-50 litres of fresh water per wash
  • Effectiveness: 1-3% restoration of compressor efficiency typically

Turbocharger turbine washing

Less common but available for some turbochargers:

  • Water injected into exhaust before turbine
  • Less effective than compressor washing because turbine deposits are typically harder
  • Required for very heavy fouling

Air cooler washing

Air cooler washing is an offline procedure (engine stopped):

  • Cooler casing opened
  • Tube bundle cleaned with high-pressure water spray or chemical solvents
  • Fins and tubes inspected for damage
  • Reassembled and pressure tested

Done at major overhauls or when fouling is severe.

Scavenge box management

Routine inspection

The scavenge box is inspected during each piston overhaul (every 16,000-24,000 hours), but slow-steaming operation may warrant more frequent inspection:

  • Visual check: through inspection covers when engine is stopped
  • Endoscope examination: through dedicated ports on some engines
  • Photographic record: comparing successive inspections

Cleaning

When deposits become heavy, scavenge box cleaning is performed:

  • Engine stopped and cooled
  • Inspection covers removed
  • Mechanical scraping or chemical cleaning of accumulated residues
  • Disposal of removed material per environmental regulations

Drainage

Scavenge box drains should remain clear:

  • Periodic check that drain lines flow freely
  • Cleaning of drain pipes when blocked
  • Verification of drain tank levels

Cylinder lubrication adjustments

Feed rate

Cylinder oil feed rate requires adjustment for slow steaming:

  • Below standard rate: increased risk of cold corrosion
  • Standard rate: produces deposits at low load that don’t arise at design load
  • Slightly elevated rate: may help with acid neutralisation but increases deposits
  • Specialty slow-steaming oil: formulated for this regime

The optimum varies; experienced operators determine the right balance for their specific operating profile.

Skip-cycle operation

Some engines support skip-cycle lubrication: dosing every 2nd, 3rd, or 4th cycle instead of every cycle. Skip-cycle:

  • Reduces total oil consumption
  • May reduce oil-related deposits
  • Requires careful matching to engine load and acid load

Oil sample monitoring

Drip oil sampling becomes especially important during slow steaming:

  • Increased frequency (weekly vs monthly)
  • Watch for iron content trends
  • BN depletion patterns
  • Visual inspection of sample colour and consistency

Long-term protocols

Maintenance schedule adjustments

Engines in extended slow steaming may require:

  • More frequent piston overhauls: every 14,000 hours instead of 24,000
  • More frequent turbocharger inspections: focus on deposit accumulation
  • More frequent air cooler cleaning: at scheduled intervals
  • Adjusted spare parts inventory: more rings, gaskets, exhaust valves

Service letters

Engine manufacturers issue service letters with slow-steaming guidance:

  • Recommended operating envelopes
  • Cylinder oil specifications
  • Lubrication adjustments
  • Inspection schedules

These should be consulted and applied.

Operator training

Crew training on slow-steaming-specific procedures:

  • Recognising deposit signs
  • Performing decarbonisation runs effectively
  • Sample interpretation
  • Emergency response to scavenge box events

Fleet-wide programmes

Large operators run fleet-wide slow-steaming management programmes:

  • Standardised procedures across ships
  • Performance data aggregation
  • Shore-side analysis and feedback
  • Continuous improvement cycles

Industry experience

The shipping industry has accumulated 15+ years of slow-steaming experience since 2008. Key conclusions:

  • Slow steaming is sustainable with appropriate engine management
  • Decarbonisation runs are essential and should be scheduled, not deferred
  • Cylinder oil management is critical, with feed rate optimisation specific to load profile
  • Component lifetimes are not significantly reduced with good cleanliness practices
  • Scavenge box fires remain rare with proper protocols

See also

References

  • MAN Energy Solutions. (2023). Slow Steaming Engine Management Service Letter. MAN Energy Solutions.
  • WinGD. (2023). Slow Steaming Operation Guidelines. Winterthur Gas & Diesel.
  • DNV. (2020). Slow Steaming and Engine Health: Industry Guidance.
  • Lloyd’s Register. (2020). Engine Operating in Low Load: Best Practices.
  • Castrol Marine. (2022). Slow Steaming Cylinder Oil Programme. BP Castrol.