A large two-stroke engine and its turbocharger are matched to each other at one region of load, usually somewhere around three-quarters power. Slow steaming does not simply make the engine do less of the same thing - it moves the engine to a place the matching was never designed for. Too little air for the fuel, and too little heat to keep the liner above the temperature at which acid condenses on it. Almost every drawback in this question is one of those two facts working its way outward.
- 1Why speed reduction is attractive, and what "low load" costs
- power varies roughly with speed cubed
- no capital cost, so it is the default short-term lever
- EEXI reg 25 and CII reg 28 in force since 1 Jan 2023
- 2Combustion and thermal consequences
- reduced scavenge air pressure, air-fuel ratio falls
- low compression and maximum pressure
- poorer atomisation, afterburning
- 3Turbocharger and scavenge-side dangers
- operation near the compressor surge line
- auxiliary blowers running continuously
- scavenge space fouling and scavenge fire
- 4Cylinder condition - cold corrosion and lubrication
- liner wall below the acid dew point
- corrosive wear at top ring reversal
- BN and feed rate must match the duty
- 5Consequences beyond the main engine
- economiser sooting and soot-fire risk on load-up
- auxiliary boiler firing erodes the saving
- exhaust valves running cool
- 6Engineering measures to resolve the problems
- low-load or part-load tuning
- turbocharger cut-out, VTA or bypass
- cylinder oil BN and feed rate from drain analysis
- 7Operational and maintenance measures
- planned periodic load-up runs
- more frequent scavenge and economiser cleaning
- condition monitoring and shorter wear intervals
- 8Conclusion
- cheapest short-term carbon measure
- transfers cost from fuel to wear and fire risk
- controllable if actively managed
Write these headings first, then expand them in order. If time is short, get every heading down before you expand any of them.
- Why speed reduction is attractive, and what "low load" costs
- power varies roughly with speed cubed
- no capital cost, so it is the default short-term lever
- EEXI reg 25 and CII reg 28 in force since 1 Jan 2023
- engine optimised near 70-80% MCR
- Combustion and thermal consequences
- reduced scavenge air pressure, air-fuel ratio falls
- low compression and maximum pressure
- poorer atomisation, afterburning
- soot through the whole gas path
- Turbocharger and scavenge-side dangers
- operation near the compressor surge line
- auxiliary blowers running continuously
- scavenge space fouling and scavenge fire
- fouled nozzle ring reduces air further
- Cylinder condition - cold corrosion and lubrication
- liner wall below the acid dew point
- corrosive wear at top ring reversal
- BN and feed rate must match the duty
- over-lubrication gives deposits and bore polish
- Consequences beyond the main engine
- economiser sooting and soot-fire risk on load-up
- auxiliary boiler firing erodes the saving
- exhaust valves running cool
- manoeuvrability, barred speed range, hull fouling
- Engineering measures to resolve the problems
- low-load or part-load tuning
- turbocharger cut-out, VTA or bypass
- cylinder oil BN and feed rate from drain analysis
- raise liner temperature where design allows
- approved EPL or ShaPoLi with controlled override
- Operational and maintenance measures
- planned periodic load-up runs
- more frequent scavenge and economiser cleaning
- condition monitoring and shorter wear intervals
- update SEEMP and the maintenance plan
- Conclusion
- cheapest short-term carbon measure
- transfers cost from fuel to wear and fire risk
- controllable if actively managed
Memory cue. AIR (2, 3), WALL (4), REST (5), then fix the machine (6) and fix the operation (7).
Memorise the 8 headings. The core points are there to help you check whether an answer is too thin — they are not a mark scheme, and they are not 32 separate facts to learn individually.
1. Why speed reduction is attractive, and what "low load" costs
Propulsion power varies roughly with the cube of ship speed, so a modest speed reduction gives a large fuel and CO2 saving at no capital cost - which is why MARPOL Annex VI regulation 25 (EEXI) and regulation 28 (CII), in force since 1 January 2023, are often met by slowing down. The cost is that a two-stroke is optimised around roughly 70-80% MCR, and running far below that puts every balance - air to fuel, thermal and lubrication - outside its design condition.
2. Combustion and thermal consequences
- Reduced scavenge air pressure. The turbocharger is sized for high load, so at low load the air-fuel ratio falls and combustion is incomplete.
- Low compression and maximum pressure, giving late, slow burning and poor thermal efficiency.
- Poorer atomisation where injection pressure depends on speed - large droplets, afterburning, and cool nozzles prone to dribbling and trumpet deposits.
- Soot and unburnt fuel through the whole gas path - piston crown, exhaust valve, receiver, turbine nozzle ring and economiser.
3. Turbocharger and scavenge-side dangers
- Surging, from running close to the compressor surge line and aggravated by a fouled turbine side, with mechanical damage to rotor and bearings.
- Auxiliary blowers running continuously, because scavenge pressure never rises enough to cut them out.
- Scavenge space and port fouling by oil-soaked carbon - the classic precondition for a scavenge fire.
- Fouled nozzle rings and blades, reducing air supply further: a self-reinforcing loop.
4. Cylinder condition - cold corrosion and lubrication
This is the most damaging effect. Lower heat release lets the liner wall fall below the acid dew point, so sulphuric acid formed from fuel sulphur and water vapour condenses on it. The result is cold corrosion - rapid corrosive wear concentrated near the top ring reversal point. It is aggravated when cylinder oil BN is not matched to the duty, or when feed rate is cut for economy; the opposite error, persistent over-lubrication, gives deposits, bore polishing and eventually scuffing. Low-sulphur fuel does not remove the problem - it changes the correct oil BN.
5. Consequences beyond the main engine
- Exhaust gas economiser. Low exhaust temperature and flow cut steam production, so the auxiliary boiler must run - eroding the saving - while soot builds on the tubes. A later load increase can ignite it as a soot fire.
- Exhaust valves running cool, with deposits, poor rotation and seat blow-by.
- Manoeuvring: reduced propeller inflow gives less rudder authority, and prolonged running near a barred speed range must be avoided.
- Hull and propeller fouling increases at low speed and during warm-water idling, degrading the very efficiency sought.
6. Engineering measures to resolve the problems
- Low-load or part-load tuning, so injection and exhaust valve timing suit the new band. On electronically controlled engines this is a control change, not mechanical rework.
- Turbocharger cut-out on multi-turbocharger engines, or variable turbine area or an exhaust bypass, to restore scavenge pressure and move away from surge.
- Cylinder lubrication set from evidence: match BN to fuel sulphur and liner condition, and set feed rate from scavenge drain analysis (iron content, residual BN) and scrape-down inspection.
- Raise liner wall temperature where the design allows, by jacket water control or liner insulation, to keep the surface above the acid dew point.
- Where slowing down is used for EEXI compliance, implement it as an approved engine or shaft power limitation with a sealed, recorded override, documented in the technical file and accepted by class.
7. Operational and maintenance measures
- Planned periodic load-up runs to burn off deposits, taken in the passage rather than left to a manoeuvring situation.
- More frequent scavenge space inspection and economiser soot blowing and water washing, treating the boiler as a fire risk before any load increase.
- Condition monitoring - PMI pressure analysis, exhaust deviation, turbocharger speed against scavenge pressure, and liner wear at shorter intervals than the maker's baseline.
- Update the SEEMP and the maintenance plan to the new profile: the engine is running in a condition those intervals were not written for.
8. Conclusion
Speed reduction is the cheapest short-term carbon measure, but not a free one: it moves the engine outside its design envelope and transfers cost from fuel to wear, fouling and fire risk. Retuned, correctly lubricated and monitored, those risks are controllable; unmanaged, cold corrosion and scavenge or economiser fires are the predictable results.
Why this structure scores
The question has two explicit halves - drawbacks and dangers, then how they are resolved. A common failure is to write a long list of problems and then two lines of remedies. Give the second half real weight: roughly nine or ten marks of problems and six or seven of solutions. Grouping the problems by where they occur (combustion, turbocharger and scavenge, cylinder, everything else) rather than as an undifferentiated list also shows you understand the mechanism rather than having memorised a list.
The central mechanism - get this right and the rest follows
Almost every drawback traces back to one fact: the turbocharger is sized for high load and cannot deliver design air at low load. Less air gives incomplete combustion; incomplete combustion gives soot; soot fouls the turbine and economiser; a fouled turbine gives still less air. Separately, less heat release lowers liner wall temperature, and below the acid dew point that produces cold corrosion. If you can state those two chains, you can derive the whole answer in the exam without recalling a list.
Common mistakes
- Treating cold corrosion as a low-sulphur-fuel problem. It is a liner temperature problem. Low-sulphur fuel changes the correct cylinder oil BN; it does not remove condensation.
- Claiming a specific percentage of MCR as the universal definition of 'low load'. The band depends on engine type and tuning - say 'well below the design point of roughly 70-80% MCR' instead.
- Forgetting the exhaust gas economiser entirely. It is where a low-load soot fire actually starts, and it is worth marks.
- Answering only about the main engine when the question says 'risks associated with low speed operation' - manoeuvrability, hull fouling and auxiliary boiler load are part of the picture.
- Presenting engine power limitation as something a Chief Engineer simply sets. It is an approved arrangement recorded in the EEXI technical file, with a controlled override.
Examiner traps
"So slow steaming saves fuel - what is the problem?" The trap is to agree too readily. The correct engineering answer is that the fuel saving is real but partly offset by auxiliary boiler firing, increased maintenance and hull fouling, and it carries fire and wear risks that must be actively managed.
"Which regulation makes you slow down?" None does. EEXI (regulation 25) is a technical index and CII (regulation 28) is an operational rating; speed reduction is a common means of compliance, not a requirement. Saying "MARPOL requires slow steaming" is wrong.
Currency - what was true in January 2026
At the date of this paper, EEXI and CII were in force and had been since 1 January 2023. The IMO Net-Zero Framework was approved at MEPC 83 in April 2025 but its adoption was adjourned at the extraordinary MEPC session of 14 to 17 October 2025, to reconvene twelve months later. So in January 2026 the Net-Zero Framework was not adopted and not in force. Do not write that a global GHG fuel standard or carbon price is in force. If you mention the Framework, say it is approved and awaiting adoption.
Likely oral follow-up
- Why does cold corrosion concentrate at the top of the liner? That is where the wall is coolest relative to the gas and where the top ring reverses, so condensed acid is mechanically worked into the surface.
- How would you know cold corrosion is happening before you open up? Rising iron content in scavenge drain oil analysis with falling residual BN, and accelerating liner wear measurements.
- Why must you be careful increasing load after a long slow-steaming passage? Accumulated soot in the economiser can ignite. Soot blow first, and raise load in stages.
- What is turbocharger cut-out and when is it used? Isolating one turbocharger on a multi-turbocharger engine at low load to restore scavenge pressure and move away from surge.
- Does low-sulphur fuel mean you can use a low-BN cylinder oil? It means you should match BN to sulphur - but if the liner is running cold you may still need alkalinity, so decide on drain analysis, not on fuel grade alone.
Memory framework
Work outward from the cylinder: AIR - BURN - WALL - REST. Air: turbocharger cannot supply design air (steps 2 and 3). Burn: incomplete combustion, soot, fouling. Wall: liner too cold, cold corrosion, lubrication mismatch (step 4). Rest: economiser, exhaust valves, manoeuvring, hull (step 5). Then the two remedy steps are simply fix the machine (step 6) and fix the operation (step 7).
Regulation and source map
- MARPOL Annex VI regulation 25 - Required EEXI, which existing ships must meet (attained EEXI is regulation 23); engine or shaft power limitation is an accepted means of achieving it
- MARPOL Annex VI regulation 26 - SEEMP, with Part III for the CII plan
- MARPOL Annex VI regulation 28 - operational carbon intensity indicator and A to E rating
- IMO Net-Zero Framework - approved MEPC 83 (April 2025); adoption ADJOURNED at the extraordinary MEPC session, 14 to 17 October 2025, to reconvene twelve months later; not adopted, not in force
- Engine-designer service guidance on part-load / low-load tuning, turbocharger cut-out and cylinder oil feed rate - industry guidance, not regulation
- SOLAS II-1 and class requirements for scavenge and economiser fire precautions - the safety context for the fire risks described
Uncertainty and applicability
The regulatory statements in this answer are verified against IMO primary sources. The engineering content is established marine-engineering principle supported by engine-designer service guidance, and it is written to apply generally to large slow-speed two-stroke engines. It is deliberately not tied to one designer's figures: load thresholds, tuning options, permissible jacket water temperatures and cylinder oil recommendations differ between engine types and between tuning packages, and the ship's own maker documentation and class approval govern in every case. Where this answer says 'roughly 70-80% MCR' it is describing the usual optimisation region, not a specification. Whether a specific ship may apply engine power limitation, and on what conditions, depends on its own EEXI technical file and class acceptance.
Before you read the answer: can you name all 8 sections, in order? Say them or write them down, then reveal.
- Why speed reduction is attractive, and what "low load" costs
- Combustion and thermal consequences
- Turbocharger and scavenge-side dangers
- Cylinder condition - cold corrosion and lubrication
- Consequences beyond the main engine
- Engineering measures to resolve the problems
- Operational and maintenance measures
- Conclusion
1 Why speed reduction and what low load costs. 2 Combustion and thermal. 3 Turbocharger and scavenge side. 4 Cylinder condition and cold corrosion. 5 Beyond the main engine. 6 Engineering measures. 7 Operational measures. 8 Conclusion.
Problems grouped by location, then remedies split into machine and operation.
Reduced heat release lets the liner wall fall below the acid dew point, so sulphuric acid formed from fuel sulphur and water vapour condenses on it. The wear concentrates near the top piston-ring reversal point.
It is a liner temperature problem, not simply a fuel sulphur problem.
No. It changes the cylinder oil BN that is appropriate. Condensation still occurs if the liner runs below the acid dew point, so feed rate and BN should be set from scavenge drain analysis and liner wear, not from fuel grade alone.
A very common and confidently made error.
No. EEXI (Annex VI reg 25) is a technical index and CII (reg 28) is an operational rating. Speed reduction is a common means of complying with them, not a requirement in itself.
Examiners test whether you can separate a requirement from a means of compliance.
Approved at MEPC 83 in April 2025, but adoption was adjourned at the extraordinary MEPC session of 14 to 17 October 2025 for twelve months. It was therefore not adopted and not in force.
Stating it as in force is a currency error that dates an answer instantly.
Soot accumulated in the exhaust gas economiser can ignite. Soot blow and, if required, water wash before load-up, then raise load in stages while monitoring exhaust and boiler temperatures.
A soot fire can escalate to an iron fire if the water side dries out.
Low-load or part-load tuning of injection and exhaust valve timing; turbocharger cut-out on multi-turbocharger engines; variable turbine area or exhaust bypass; raising liner wall temperature by jacket water control or liner insulation.
Around 70-80% MCR, approximately and depending on engine type and tuning. Sustained running well below that band is what puts the air, thermal and lubrication balances outside their design condition.
Give it as an approximate band - a single universal figure would be wrong.
Engine or shaft power limitation is an approved arrangement recorded in the ship's EEXI technical file and accepted by class, with a sealed and recorded override available for safety. Ordering a slower speed is an operational decision with no such status.
Only the approved arrangement counts towards attained EEXI.
- 15-second recall
- A two-stroke is optimised near 70-80% MCR. Below that the turbocharger cannot supply design air, so combustion is incomplete: soot, fouling, surging, scavenge fires. Less heat release lets the liner fall below the acid dew point, giving cold corrosion at top ring reversal. Economiser sooting risks a soot fire on load-up. Remedies: low-load tuning, turbocharger cut-out or VTA, cylinder oil BN and feed rate set from drain analysis, liner temperature control, periodic load-up, more frequent scavenge and economiser cleaning. EEXI reg 25 and CII reg 28 are in force; the Net-Zero Framework is approved but NOT adopted.
- Keywords
- design point 70-80% MCRair-fuel ratiocold corrosionacid dew pointsurgingscavenge fireeconomiser soot firelow-load tuningEPL
- Critical numbers
- Two-stroke optimisation region: roughly 70-80% MCR (approximate, designer-specific)
- EEXI (reg 25) and operational CII (reg 28) in force from 1 January 2023
- Propulsion power varies approximately with the cube of speed
- Critical regulation
- MARPOL Annex VI regulation 25 (EEXI) and regulation 28 (operational CII) - speed reduction is a common means of compliance, not itself a requirement
- Major trap
- Saying MARPOL requires slow steaming, or that the IMO Net-Zero Framework is in force. It was approved at MEPC 83 but its adoption was adjourned in October 2025 and it was not adopted at the question date.