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Development of a Bunker Norm for Ships

4. Fuel Oils – General Aspects

Gas Oil, MGO
Gas oil is a low-viscosity – often clear, yellowish to brownish – liquid with a specific density in the order of 0.85 to 0.89 g/cm3. The flash point is between 65 and 850C. The lower calorific value is normally from 44000 to 45000 kJ/kg. The kinematic viscosity is about 5 to 7 cSt at 400C. The sulphur content is about 1 to 1.5%. The ash content is less than 0.01%. The neutralisation figure is smaller than 0.1. The cetane number is higher than 45. Gas oil is used primarily for small engines or motors with high rpm.

Diesel Oil, MDO
Diesel oil, often referred to as Marine Diesel Oil (MDO) or Marine Diesel Fuel (MDF), has a specific density of about 0.9 g/cm3; the flash point is above 600C. The calorific value is about 44000 kJ/kg; the kinematic viscosity is about 8 to 11 cSt at 400C, and the sulphur content is about 2%. The colour is brownish. Because diesel oil is often delivered via the same pipelines and pumps as those used for fuel oil, so much fuel oil may be mixed with it that it is almost black. Diesel oil is used as engine fuel in medium-speed diesel engines.

Diesel oil on land
There are two readily available types of environmental diesel, viz. light diesel and ultra-light diesel. Both types are particularly notable for having a sulphur content of (max.) 0.05% according to the ASTM D 4294 method.

Both types are environmentally based fuels with a very low sulphur content and characterised by having extremely good ignition properties indicated by cetane number and/or cetane index, as well as cold-weather properties adapted to the time of the year. Environmental diesel is recommended for all types of diesel engine where the purity of the exhaust gasses is important for the environment.

It should be noted here that these oils have a flash point of not less than 560C, which does not tally with the demands for flash point on board ships, where the flash point temperature must be equal to or higher than 600C. Therefore, these two environmental diesel oils cannot be used in ships.

There other types of diesel oil that can be described as environmental diesel, e.g., fishing diesel (0.05%).

This type of diesel is a low-viscosity gas oil with a boiling point ranging from 2000C to about 3850C, and the specification and results for fishing diesel (0.05%) are as follows:

Density at 150C. g/l 820-860
Viscosity at 400C min. cSt 1.9
Viscosity at 400C max. cSt 3.7
Flash point, min deg.C 61
Sulphur, max. weight % 0.05
Water, max. mg/ kg 150
Ash, max. weight % 0.01
Conradsen Carbon Residue at 10% distillation residue, max. weight weight % 0.15
Cetane number, min.   47
Distillation T 95%, max. deg.C 385
Lower calorific value, typical Mj/kg 42.7

Cold-weather properties

Period:   Winter (1/10 - 31/3) Summer (1/4 - 30/9)
Cold Filter Plugging Point (CFPP) max.. deg.C -20 -5
Cloud Point max. deg.C -8 2

Fuel Oil, HFO
The name fuel oil, often referred to as heavy fuel oil (HFO) covers a large and wide area, which is indeed reflected in the residual grades list. Thus, fuel oil can occur both as a distillate product and as distillation residue from petroleum. The latter is then designated residual oil or residual grade. For sales purposes the oil companies also divide fuel oil into many different types. Marine Fuel Oil (MFO) is a very common term used by oil suppliers to describe the most inexpensive type of oil for merchant vessels. Its viscosity can be anywhere up to the maximum value, which varies from one country to the next and even from one port to the other within the same country. MFO is also known as "Bunker C".

Thin Fuel Oil (TFO) and Intermediate Fuel Oil (IFO) are terms used to identify mixed oils according to various viscosity requirements.

By this review of heavy fuel oil we wish to settle the widespread misunderstanding that fuel oil with a viscosity of 380 cSt is poorer, in operational terms, for the engines than fuel oil with a viscosity of 180 cSt. Today this is not correct, in fact it is quite the opposite. About 10 years ago this opinion was correct.

Nowadays a 180 cSt fuel oil often consists of a very heavy oil with gas oil or diesel oil added to achieve the viscosity of 180 cSt. This may result in poor combustion properties and can give rise to deposits of sludge in separators and filters.

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