Cylinder Oil
The large bore MAN B&W Model 12K80MC-S 2-stroke engines used by the Cabras
3&4 generation units are an example of large crosshead type engines. In large crosshead
type engines, the cylinder is isolated from the crankcase and a separate cylinder
lubrication system is fitted which supplies a measured quantity of oil to each liner.
The MAN B&W Model 12K80MC-S engine has 12 cylinders each with a cylinder liner.
Each engine cylinder liner has eight lubricating points, where cylinder oil is injected into
the liner via non-return valves from a mechanically driven piston pump lubricator. The
injection of cylinder oil takes place during the engine piston upward stroke. Two-stroke
engines fire-each revolution of the crank-shaft.
Cylinder liners require adequate lubrication in order to reduce piston ring friction and
wear and to neutralize the sulfur in the residual fuel used by the plant. The oil film also
acts as a gas seal between the liner and rings and as a corrosion inhibitor.
Practically all the injected cylinder oil is burned during the cylinder firing. The cylinder
oil is thereby consumed as part of the combustion process that produces electric power.
Therefore, cylinder oil has to be injected at an appropriate rate considering the amount of
sulfur content in the fuel oil and the engine load.
Cylinder oil is a “use once consumable.” It is injected into the cylinder at a feed rate to
give optimum protection against acid corrosion and microsiezure (scuffing). Typical feed
rates for a modern slow speed diesel engine are about 1.3 g/kWh. Note that the cylinder
is the combustion chamber. Therefore, cylinder oil is meant to be burned as part of the
combustion process in the production of electric power.
The oil used to lubricate the piston rings and liner must be thermally stable and be able to
retain an oil film at the high surface temperatures found in the cylinder. The oil must
possess anti wear characteristics, dispersants and detergents to ensure minimum deposits
on the piston and the ring grooves. In addition, because of the high sulfur content in
residual fuel oils, cylinder oil is high alkaline oil with a typical Total Base Number of 70
in order to prevent liner corrosion by sulfuric acid produced by the combustion process.
A specification for typical cylinder oil for an engine burning 3% sulfur fuel is shown
below.
SAE Number 50
Specific Gravity kg/dm3 (15°C) 0.942
Flash Point (°C) 241
Pour Point (°C) -9
Viscosity cSt at 40 °C 247
Viscosity cSt at 100°C 21
Viscosity Index 100
Total Base Number (mg KOH/g) 70
The oil has a high TBN of 70 to neutralize the acids formed by the combustion of the
sulfur in the fuel. These alkaline additives make up about 30% of the oil.
The viscosity of the oil is relatively high (21 cSt at 100 °C). This is so that it can lubricate
effectively at the higher liner temperatures. The oil has a viscosity index of 100 which
means it has a low tendency to change viscosity with temperature.
The heating values for both the high sulfur fuel oil and the cylinder oil are similar. Each
quantity of cylinder oil combusted contributes almost the same amount of energy as the
same quantity of high sulfur fuel burned by the plant.
Cylinder oils use dispersants for three reasons.
• They carry other additives in the base oil, mainly BN-additive.
• They carry dirt debris, which enters the lubricating oil and neutralized products.
• They prevent the dirt debris from sticking to the surfaces.
Generally, the higher the base number of the oil, the higher the dispersion.
The dispersancy of the oil is a delicate balance. Cylinder oil should have high
dispersancy in order to keep the engine clean – especially in heavy fuel operation. If the
dispersancy is too low, the dirt and asphaltenic contaminants will precipitate to form
black sludge and carbon deposits.
Soot and coke create problems in diesel engines by reducing both combustion and heat
transfer efficiency, and causing environmental pollution by stack solids and soot
emissions. Dispersants keep soot and combustion products in suspension in the body of
the oil charge and therefore prevent deposition as sludge or lacquer. These dispersants
become depleted with time; one important reason for regular oil changes in heavily
contaminated systems.
Cylinder oils have detergency properties. Detergents serve to hold the acid-neutralizing
compounds in solution in the oil. They are usually alkaline and react with the strong acids
(sulfuric and nitric) which form during the combustion of the fuel and which would cause
corrosion to the engine internals if left unchecked. Neutral detergents are also used to
impart anti-corrosion, anti-wear and even extreme pressure properties to oil.
Cylinder oil has several functions or effects:
• Reduces deposits and sludge formation
• Reduces lay-up time required for overhauls by cleaning the engine
• Reduces liner and ring wear. And provides excellent anti-scuffing control
• Provides superior piston and liner cleanliness to increase combustion efficiency
and extend periods between piston overhauls
• Minimizes the corrosive effect of high sulfur fuel combustion
• Provides heating value that is directly used to produce electrical energy.
Cylinder Oil Features and Benefits (Mobil and Shell Products)
Cabras 3&4 and MEC 8&9 Slow Speed Diesel Power Plants use either Mobilgard 570 or
Shell Alexia 50 cylinder oil products.
From Mobilgard 570 Brochure
Features Advantages and Potential Benefits
Excellent thermal and Reduced deposits and sludge formation
oxidation stability
Extended oil life Cleaner engine reduces lay-up time required for
overhauls
Exceptional anti-wear Reduced liner and ring wear. Excellent anti-scuffing
properties control
Outstanding detergency Superior piston and liner cleanliness increases
capability combustion efficiency and extends periods between
piston overhauls
High TBN Level and Wide fuel sulfur capability. Minimizes the corrosive
Retention effect of high sulfur fuel combustion
High Viscosity Index & Reduced cylinder oil consumption
Stable Base Stocks
From Shell Alexia 50 Brochure
Features/Benefits
• unique additive technology - contains outstanding neutralizing properties to
counter corrosive acids resulting from the use of high sulfur fuels
• minimize deposit formation - on critical parts such as cylinder ports, pistons,
piston rings, ring grooves and under piston spaces
• low cylinder and piston ring wear - cylinder wear rates below 0.05 mm per 1000
hours have been experienced
• excellent engine cleanliness - keeps engines clean – even under extended intervals
• storage stability - product has demonstrated excellent stability in storage under
varying conditions
• sight-glass fluid compatibility - compatible with all normal lubricator sight glass
fluids
• service experience - over 35 years in close co-operation with ship-owners
