A Largely Frontier Area
For over half a century, the diesel engine has driven the marine industry by evolving with the times. Diesel engine technology today is a mix of theoretical knowledge and practical experience. Nevertheless, processes such as combustion are not precisely modeled causing unpredictable complications such as cylinder liner deposits, the causes of which are not accurately identified.
In the 1980s, service engineers reported transparent yellow-brown resinous deposits on piston liners of four-stroke, medium-speed marine engines. These are the dreaded cylinder liner lacquers / deposits. They can also be almost invisible, in the form of transparent-glaze or bore-glaze.
An acute rise in lubricant consumption without any leakages is an important symptom of liner deposits. The lack of data makes everything probabilistic. Consequences are however expensive and catastrophic, exactly why researchers developed a patented additive to fight the challenge.
Causes & Effects
Liner deposits consist of inorganic salts of zinc and calcium and organic polymerized hydrocarbon. Irregular case records, frequent vessel refueling that precludes the identification of the exact culprit-fuel, difficulty in acquiring deposit samples, and failure of field surveys prevent the derivation of proper conclusions.
Effects however are expensively conspicuous:
- piston sticks to cylinder
- acute rise in lubricating oil consumption
- excessive soot pollution
- collateral damage
- maintenance expenses and downtime losses
Piston sticks to cylinder as liner deposits collect in the piston grooves designed to hold lubricants. Liner deposits interfere with this piston-cylinder lubricating mechanism. More lubricating oil is mixed with the combustion gas and thrown out through exhaust. This increases the consumption of lubricating oil.
Various studies hold engine design, engine load, fuel composition, and lubricating oil composition responsible. Some blame it on incomplete fuel combustion at low loads while some condemn a mismatch between the rate of formation of oil-insoluble material (on or within the oil film) and the oil dispersing power and exchange rate.
- Gas Oil with High Polyaromatics Content and Final Boiling Point (FBP) above 4500C
- Residual Fuel Oil with High Polyaromatics Content
- Gas Oil with Low-Sulphur Content up to 0.7% m/m (m/m means this percentage is in terms of weight)
- Residual Fuel Oil with Sulphur below 1.0% m/m
- LUBRICANT OIL QUALITY
- ENGINE CONDITIONS VIZ. DESIGN /OPERATION / MAINTENANCE :
- High Mean Effective Pressure (BMEP) Rating above 18-20bar
- Injection Angle and Quality
- Engine Overload with Maximum Continuous Rating (MCR) of 100%
- Engine Design:
- Cooling Efficiency
- Liner Temperature
- Type of Engine Operation
- Type of Turbocharger
One reason why low-sulphur fuel is more lacquer-prone is its combustion produces too little sulphuric acid to dissolve and wash away the formed lacquer. Fuels with higher percentage of aromatics (40% plus) and lower percentage of paraffinics are more lacquer-prone because polycyclic aromatic hydrocarbons (PAHs) increase a fuel’s FBP.
Fuels with FBPs between 4200C-4500C are more lacquer-prone than those with FBPs around 3400C. At 4200C-4500C temperatures, PAHs readily oxidize into quinones with cylinder walls providing catalytic iron.
Quinones resist further oxidation even at such high temperatures and have very high melting points. This means, they remain stuck on the cooler parts of the cylinder walls as stable solids. Again, quinones are insoluble in fuel and lubricants. Sulphuric acid dissolves them but low-sulphur fuels are ineffective here.
Preventive & Curative Measures
Complete curing requires expensive liner re-honing after dismantling the engine. Temporary measures:
- washing with vinegar or acetic acid
- switching the engine to high-sulphur fuel – the formed sulphuric acid dissolves lacquer
- maintaining liner temperature so they are hot enough to prohibit fuel condensation
- using lubricants compatible with low-sulphur fuels
- proper positioning and cleanliness of fuel injectors avoids incomplete combustion and fuel condensation on liners
- tuning the turbocharger counters incomplete combustion
Inventors Robert William Allen and Michael James Attfield developed a patented additive (US6070558 A: published on June 6, 2000) made of:
- at least one diesel detergent; and
- at least one cetane improver
The additive lowers liner deposits in marine diesel fuels with:
- over (preferably) 4200C temperature for 90% volume recovery during distillation
- above (preferably) 35% aromatics
- below 60% low-saturates
- under 3% low-olefins
- cetane number beneath 40
- under (preferably) 0.2% sulphur
Detergents avert deposit build-up in the fuel injector system while cetane boosts ignition characteristics. Cetane Number measures the ability of a diesel fuel to self ignite. Higher values indicate faster starting.
Diesels must have larger cetane number for diesel engines run on compression ignition – the piston compresses and heats up air into which atomized diesel is injected. There are no spark plugs as in gasoline engines. Diesels must therefore self ignite at high temperatures.
Environmental-friendly regulations are here to stay in the marine industry. Soon, high-sulphur fuel will be a thing of the past. The sooner we adapt to changing times, the better.
Visit our blog for loads of great stuff on how the marine industry is evolving to the needs of the 21st Century. Contact Kemplon Engineering for apex large scale custom metal fabrication and marine fabrication services.
^ Lacquer Deposits on Cylinder Intake Ports (Source: http://www.cashmanequipment.com/bently/publications/appnotes/app19.php)