Marine fuel oil (MFO) viscosity control system reference. Covers ISO 8217 grades (MDO / MGO / HFO 380 / VLSFO), injector-inlet viscosity target (typ. 10-15 cSt at engine inlet), fuel oil viscosity controller with pilot-operated 3-way steam mixing valve, and the role of the fuel homogeniser in blending residual and distillate fuels during grade change-over.
| Grade | ISO 8217 category | Viscosity at 40 °C (cSt) | Sulphur cap | Typical use |
|---|---|---|---|---|
| MGO (Marine Gas Oil) | DMA | 2.0 – 6.0 | 0.10% (ECA), 0.50% (global) | Aux engines, harbour manoeuvring, ECAs |
| MDO (Marine Diesel Oil) | DMB | 2.0 – 11.0 | 0.50% | Small vessels, medium-speed aux |
| HFO 180 | RMD 80 | 180 max at 50 °C | 3.50% (pre-2020) / 0.50% VLSFO variant | Legacy main engine |
| HFO 380 (IFO 380) | RMG 380 | 380 max at 50 °C | 3.50% (pre-2020) | Main engine, most bunker deliveries pre-2020 |
| VLSFO (Very Low Sulphur Fuel Oil) | RMG 380 or RMG 180 variant | 180 or 380 max at 50 °C | 0.50% (IMO 2020 global cap) | Post-2020 main engine, non-scrubber vessels |
| ULSFO (Ultra Low Sulphur Fuel Oil) | RMD or RMG variant | Varies | 0.10% (SECA/NECA) | North Sea, Baltic, US coast within 200 nm |
A marine two-stroke or four-stroke diesel engine's fuel injector needs the fuel to reach the injector inlet within a narrow viscosity window (typically 10-15 cSt for a main engine, 2-6 cSt for MGO on an aux). Too viscous = poor atomisation, incomplete combustion, deposit formation, high smoke. Too thin = injector leakage past the plunger, loss of lubrication in the fuel pump. Since HFO ships in the bunker tank at 380 cSt at 50 °C, the fuel is heated on the way to the injector to reduce viscosity down to the injection target — heater outlet ~130-140 °C for HFO 380.
Modern engines use an inline viscosity sensor (typically a torque-cell viscometer or a vibration-cup viscometer) reading in real time at the fuel supply pump discharge. The signal drives a PID controller that modulates a 3-way pilot-operated steam valve at the fuel-oil heater's steam supply. Rising viscosity opens the steam valve (raises heater outlet temperature); falling viscosity closes it. VAF Aage Christensen, Yokogawa and Krohne are the dominant OEMs for marine viscosity controllers.
A homogeniser mechanically shears the fuel to break asphaltene agglomerates and produce a stable suspension. Used in two situations: (1) grade change-over from HFO to VLSFO to LSFO — different fuels are miscible in bulk but stratify locally in service tanks, and the homogeniser shears them into a stable blend before injection; (2) recovery of old fuel oil sludge from centrifuge separators — the homogeniser re-suspends the sludge into the main fuel stream in small controlled dose.
The 2020-01-01 global 0.50% sulphur cap forced most operators to switch from HFO 380 to VLSFO. VLSFO viscosity behaviour is more variable than legacy HFO (different refinery blend components), and cold-flow issues (wax precipitation) are more common on VLSFO than HFO. Some vessels retained HFO by fitting an exhaust gas scrubber (IMO 2020 exemption); those vessels operate a scrubber wash-water discharge system in addition to the fuel oil viscosity plant.