What Is Fuel Oil?
Fuel oil is the heavier part of crude oil left after lighter products such as gasoline, jet fuel and diesel are taken out. It is thick, dark and usually needs heating before it can be pumped and burned. Ships, power stations and industrial boilers use it because it is cheaper per tonne than lighter fuels.
Residual oil, bunker C and number 6 fuel oil
You will see several names for the same family of products. Residual oil is the general term for heavy fuel oil from the bottom of the refinery. Bunker C and number 6 fuel oil are older North American names for heavy residual fuel. Number 2 fuel oil, by contrast, is a light distillate close to diesel and is mostly used for home heating.
Fuel oil vs diesel: what's the difference?
Diesel is a light, clean distillate made for engines in vehicles and machinery. Heavy fuel oil is much thicker, contains more sulphur and impurities, and is burned in large, slow-running ship engines and boilers that can handle it. A diesel engine in a truck cannot run on CST180.
What Is CST180 Fuel Oil (IFO 180)?
CST180 means fuel oil with a maximum kinematic viscosity of 180 centistokes (cSt) at 50°C. It is also called IFO 180 (intermediate fuel oil 180). Under the marine fuel standard ISO 8217, the matching grade is RME 180.
CST180 is usually a blend of heavy residual oil with a lighter "cutter" stock to bring the viscosity down to 180 cSt. Its thicker sister grade, CST380 or IFO 380, is more common in large ports today. CST180 is still used by smaller vessels, some power plants and industrial users, and it appears often in trade offers, which is why traders need to understand it properly.
CST180 Specifications Buyers Check
The table shows key limits for ISO 8217 grade RME 180. The 2024 edition of ISO 8217 changed some limits, so always check which edition your contract names.
| Property | RME 180 limit | Why it matters |
|---|---|---|
| Kinematic viscosity at 50°C | Max 180 mm²/s (cSt) | Heating and injection settings |
| Density at 15°C | Max 991.0 kg/m³ | Purifier performance and quantity calculation |
| Flash point | Min 60°C | Fire safety on board (SOLAS) |
| Pour point | Max 30°C | Fuel must flow in tanks and pipes |
| Sulphur | Set by law: max 0.50% globally, 0.10% in ECAs (higher only with a scrubber) | Emissions compliance |
| Water | Max 0.50% by volume | Engine damage and quantity disputes |
| Carbon residue | Max 15.00% by mass | Deposits and combustion quality |
| Ash | Max 0.070% by mass | Engine wear |
| Total sediment, aged | Max 0.10% by mass | Fuel stability and filter blocking |
| Vanadium | Max 150 mg/kg | Hot corrosion in engines |
| Aluminium + silicon (cat fines) | Max 50 mg/kg | Severe engine wear if too high |
| CCAI (ignition quality) | Max 860 | Ignition delay in the engine |
Values based on ISO 8217:2017 grade RME 180. Your contract should name the exact standard edition and test methods.
What Is Bunkering? Bunkering Meaning
Bunkering means supplying fuel to a ship for its own use. The word comes from the coal bunkers of old steamships. Today "bunker fuel" or "marine fuel" means any fuel a ship burns, from heavy fuel oil to marine gas oil and LNG.
How ship bunkering works
Most bunkering is done by a barge that comes alongside the ship (ship-to-ship), by truck at smaller ports, or from a pipeline at a terminal. The big bunkering hubs are Singapore, Rotterdam and the wider Amsterdam-Rotterdam-Antwerp area, Fujairah, Panama and Houston. LNG bunkering is growing as more ships run on gas.
The bunker delivery note (BDN)
Every delivery ends with a bunker delivery note signed by the supplier and the ship's chief engineer, showing the grade, quantity, density and sulphur content. Under MARPOL Annex VI a sealed sample is kept as well. Some ports, such as Singapore, also require mass flow meters on bunker barges to reduce quantity disputes.
Bunker Fuel Grades Compared
| Grade | What it is | Typical sulphur | Who uses it |
|---|---|---|---|
| HSFO 380 (IFO 380) | High-sulphur residual fuel, 380 cSt | Up to 3.5% | Ships fitted with scrubbers |
| CST180 (IFO 180) | Lighter residual blend, 180 cSt | High sulphur, or 0.50% when sold as compliant fuel | Smaller ships, power and industry |
| VLSFO | Very low sulphur fuel oil | Max 0.50% | Most ships without scrubbers |
| MGO / MDO | Marine gas oil or diesel oil (distillate) | Max 0.10% for ECA use | Ships inside Emission Control Areas |
| LNG | Liquefied natural gas | Near zero | Gas-powered ships |
IMO 2020 and the Sulphur Rules
Since 1 January 2020, the International Maritime Organization limits sulphur in ship fuel to 0.50% worldwide. Inside Emission Control Areas (ECAs), such as the North Sea, the Baltic, the North American and US Caribbean ECAs, and the Mediterranean ECA in force since 1 May 2025, the limit is 0.10%.
A ship can still burn high-sulphur fuel only if it has an approved exhaust gas cleaning system (a scrubber). This is the single biggest change many CST180 offers ignore. If a seller offers "CST180 with 3.5% sulphur" to a buyer whose ships have no scrubbers, the fuel cannot legally be used at sea, whatever the price.
How Bunker Fuel Prices Work
Bunker prices are quoted in US dollars per metric tonne, delivered at a named port. They follow crude oil and fuel oil benchmarks published by agencies such as Platts and Argus, and port price indices from services such as Ship & Bunker. Each port has its own price because of local supply, demand, barging costs and taxes.
For larger cargoes of fuel oil sold to traders or power plants, pricing usually works like other oil products: a benchmark average plus or minus a premium, with Incoterms such as FOB or CIF.
How Bunker Fuel Is Traded
Bunker trading looks very different from the "SCO and ICPO" procedures that circulate online. A real bunker deal usually runs like this:
- Enquiry. The ship operator or a bunker broker sends the vessel name, port, arrival date, grade (for example VLSFO or 380 cSt) and quantity.
- Quote. A bunker trader or physical supplier quotes a delivered price per tonne, valid for a short time.
- Confirmation (stem). The order is confirmed in writing with the supplier's terms and conditions.
- Delivery. The barge delivers alongside, samples are drawn and sealed, and the BDN is signed.
- Invoice and payment. The supplier invoices, often on credit terms of around 30 days for approved buyers.
Bunker trader vs physical supplier
A physical supplier owns or charters the barges and holds the fuel. A bunker trader buys from physical suppliers and resells to shipowners, often adding credit and convenience. A bunker broker connects the two for a commission. Knowing which one you are dealing with tells you who carries the risk.
Bunker Fuel and Bunkering in the US
Bunkering meaning: bunkering is supplying fuel to ships, either at a berth or by barge alongside. Bunker fuel is the fuel ships burn. Since the IMO 2020 rule, most ships outside emission control areas use very low sulfur fuel oil (VLSFO, max 0.50% sulfur), or high sulfur fuel oil (HSFO) if they have a scrubber.
Close to US and Canadian coasts, ships are inside the North American Emission Control Area, where fuel sulfur is limited to 0.10%. That is why marine gasoil (MGO) and ultra low sulfur fuel oil are common at US ports.
| Fuel | Max sulfur | Where it is used |
|---|---|---|
| HSFO (incl. CST180 / IFO180, 380) | 3.50% | Ships with scrubbers |
| VLSFO | 0.50% | Open sea, outside ECAs |
| MGO / ULSFO | 0.10% | Inside ECAs such as US coastal waters |
Major US bunkering hubs include Houston and the US Gulf Coast, New York and New Jersey, and Los Angeles and Long Beach.
What is fuel oil?
Fuel oil is the heavier part of crude oil left after lighter products are refined. Heavy grades such as CST180 are used as bunker fuel and in industry, and are sold by viscosity (centistokes) and sulfur content.
CST180 and Bunkering: Frequently Asked Questions
What is CST180 fuel oil?
CST180 is a residual fuel oil with a maximum viscosity of 180 centistokes at 50°C. It is also called IFO 180 and matches ISO 8217 grade RME 180. It is used by ships, power plants and industrial boilers.
What is bunkering?
Bunkering means supplying fuel to a ship for its own use, usually from a barge alongside the ship, by truck or from a terminal pipeline. Each delivery is recorded on a bunker delivery note.
What is bunker fuel?
Bunker fuel is any fuel a ship burns in its engines, including heavy fuel oil such as IFO 380 and CST180, very low sulphur fuel oil (VLSFO), marine gas oil and LNG.
What is the difference between fuel oil and diesel?
Diesel is a light, clean distillate for vehicles and machinery. Heavy fuel oil is the thick residual part of crude oil, with more sulphur and impurities, and is burned in large ship engines and industrial boilers.
What is bunker C or number 6 fuel oil?
Bunker C and number 6 fuel oil are North American names for heavy residual fuel oil. They are similar to the heavy grades traded today as IFO 380 or HSFO.
Is CST180 the same as IFO 180?
Yes. CST180 and IFO 180 both describe fuel oil with a maximum viscosity of 180 cSt at 50°C. Under ISO 8217 the matching grade is RME 180.
Can ships still use high-sulphur CST180 after IMO 2020?
Only ships fitted with an approved scrubber. Since 1 January 2020 ship fuel must have no more than 0.50% sulphur worldwide and 0.10% inside Emission Control Areas.
How are bunker fuel prices quoted?
Bunker fuel prices are quoted in US dollars per metric tonne, delivered at a named port. They follow published fuel oil benchmarks and vary by port because of local supply, barging costs and demand.
What documents prove a bunker delivery?
The main document is the bunker delivery note (BDN), signed by the supplier and the ship, plus sealed MARPOL samples, the supplier invoice and any independent survey or mass flow meter report.
What does bunkering mean?
Bunkering means supplying fuel to ships, at a berth or by barge. The fuel itself is called bunker fuel.
What sulfur limit applies to ships near the US coast?
Inside the North American Emission Control Area the limit is 0.10% sulfur, so ships use marine gasoil, ultra low sulfur fuel oil or scrubbers. Outside ECAs the global IMO limit is 0.50%.
Where are the main US bunkering ports?
Houston and the US Gulf Coast, New York and New Jersey, and Los Angeles and Long Beach are the largest US bunkering hubs.