Fuel oil: what ships burn
Most ships have diesel engines that burn fuel oil from crude oil.
- Heavy fuel oil (HFO): the leftover after lighter parts are taken out. It is thick like tar, cheap, and has a lot of sulphur. It must be heated before use.
- Marine diesel oil (MDO): a thinner blend, easier to handle.
- Marine gas oil (MGO): light and clean, like road diesel. Costs more.
- Newer fuels: LNG, methanol and ammonia, which reduce pollution.
Fuel is stored in bunker tanks. Taking fuel on board is called bunkering.
Properties of fuel oil
- Calorific (heat) value: heat given by 1 kg of fuel when burnt. About 40 to 43 MJ/kg for ship fuels.
- Viscosity: how thick the oil is, or how much it resists flowing. Measured in centistokes (cSt). Heating lowers it. Engines need about 10 to 15 cSt at the injector, so HFO is heated to roughly 100 to 150 °C.
- Density: HFO is about 0.99 t per m³, MGO about 0.85. Needed for purifier setting and for tank volume.
- Flash point: the lowest temperature at which the oil gives enough vapour to flash with a flame. Ship fuel must be 60 °C or more for safety.
- Pour point: the lowest temperature at which oil still pours.
- Sulphur: when burnt it makes SO₂, which causes acid rain. The global ship limit is 0.50% sulphur, and 0.10% in special emission control areas. Ships can also use scrubbers.
- Water and sediment: must be removed or the injectors and pistons are damaged.
From tank to cylinder: treating the fuel
Fuel path: bunker tank then settling tank (water and dirt sink) then purifier (a centrifuge that spins out the rest) then service tank then filter then heater then injection pump and injectors. Each step makes the fuel cleaner or hotter, so it burns well and does not wear the engine.
Lubricants: why oil is needed
Even smooth-looking metal is rough when you zoom in. Two surfaces sliding dry catch on each other, make heat and wear. A lubricant (lube oil or grease) puts a thin film between them. Its jobs:
- Reduce friction and wear by separating the surfaces (the oil film).
- Cool by carrying heat away from bearings and pistons.
- Clean by carrying dirt and soot to the filter.
- Seal the gap between piston rings and the cylinder wall.
- Protect from rust and acid.
Kinds and properties of lubricants
- System oil for bearings and gears. Cylinder oil for the cylinder liner in large slow engines; it is alkaline (high TBN) to neutralise the acid made by sulphur. Grease for places where oil would run off.
- Additives improve oil: detergents (clean), dispersants (hold dirt), anti-wear, anti-foam and anti-rust.
- Viscosity is the main property. It is chosen by grade (for example SAE 30 or 40). Oil gets thinner when hot and thicker when cold. A good oil changes little: this is the viscosity index.
- Used oil is tested often in labs for water, wear metals and viscosity, and cleaned by a purifier, to know when to change it.
If oil is too thin, the film breaks and metal touches metal. If too thick, it wastes power.
Try it: heat the oil
In the 3D, choose Heavy fuel oil and move the temperature slider from 20 to 120. Watch the dots speed up and the flow percent rise. Then turn the lube oil Off and On. Try high temperature with oil On: what happens to the film? At home, warm and cool a spoon of cooking oil or honey and see how it pours.
Key formulas and definitions
- Heat from fuel = mass x calorific value
- Fuel per hour = SFOC (g/kWh) x power (kW)
- Volume = mass / density
- S + O₂ → SO₂ (32 g sulphur gives 64 g SO₂)
- Engine efficiency = 3.6 MJ / (SFOC in kg/kWh x calorific value in MJ/kg)
Worked examples
1. How much heat does 2 tonnes of fuel give? (calorific value 42 MJ/kg)
2 t = 2000 kg. Heat = 2000 x 42 = 84,000 MJ.
2. An engine of 8000 kW has SFOC 180 g/kWh. How much fuel does it burn in one hour?
8000 x 180 = 1,440,000 g = 1440 kg = 1.44 tonnes per hour.
3. How many cubic metres is 99 tonnes of HFO (density 0.99 t/m³)?
Volume = 99 / 0.99 = 100 m³.
4. 100 tonnes of fuel contains 3.5% sulphur. How much SO₂ is made when it burns fully?
Sulphur = 3.5 t. S (32) becomes SO₂ (64), so mass doubles: 7 tonnes of SO₂.
5. SFOC is 175 g/kWh and calorific value is 42 MJ/kg. Find the engine efficiency.
Energy in fuel per kWh = 0.175 x 42 = 7.35 MJ. Useful work per kWh = 3.6 MJ. Efficiency = 3.6/7.35 = 0.49, about 49%.
Common mistakes
- Thinking thick fuel can be used cold. It must be heated or it will not flow or spray properly.
- Thinking hotter lube oil is always better. Too hot makes it thin, the film breaks and parts wear.
- Mixing up flash point (safety, when vapour can catch fire) with pour point (when oil stops pouring).
- Ignoring the unit of density. Convert mass to volume with density in t/m³.