1. Fuel heating and viscosity control: HFO is highly viscous at ambient temperature and must be heated (to ~135-155 deg C at the injector for 380cSt) to reduce viscosity to the injection pump/injector limit (~12-14 cSt at injection). A fuel oil heater with automatic viscosity/ temperature control must be fitted so the fuel is atomised correctly.
2. Better filtration/purification: HFO contains more water, ash, solids and asphaltenes; centrifugal separators (purifier/clarifier) and fine duplex filters must be fitted and operated at the correct temperature and throughput to remove contaminants before the injectors.
3. Injection equipment and combustion adjustments: HFO has poorer ignition quality (increased ignition delay) and higher tendency to form carbon; the fuel injectors (with finer atomisation), injection timing (VIT) and possibly an increased compression ratio or higher boost are set/optimised; the fuel pump seals need to withstand high temperature and viscosity; combustion chamber design/ring pack is adapted for the higher ash and the corrosive products, e.g. cylinder oil of higher BN is used.
4. Modifications to handle corrosive/ash products: need for increased cylinder lubrication (higher BN oil), and attention to exhaust valve, turbocharger and scavenge cleaning because of ash and the acidic condensate; exhaust gas system may need insulation and material suitable for cold corrosion; possibly a change in the fuel system materials, fuel pump drive and valve cooling to avoid coking.

1. Viscosity (at 50 deg C, cSt): governs heating requirements, pumpability and atomization - must be heated for storage/transfer and for injection; too high viscosity gives poor atomization and carbon build-up; too low gives poor lubrication of pump.
2. Density (kg/m3): affects separation/centrifugation (higher density fuel is harder to separate from water), the capacity/discharge and the stored weight; high density fuels need more care in purifying.
3. Sulphur content (%): affects corrosivity - leads to sulphuric acid formation (cold corrosion) that needs appropriate BN cylinder oil, SOx emission control (Annex VI), and higher TBS for scrubber compliance.
4. Flash point: safety - fuel must be stored/handled above or below the flash point correctly, for fire safety and for the electrical/steam heating (must not be heated above flash).
5. Ash content (%): abrasive/corrosive catalytically-fined particles (aluminium/silicon) cause severe liner, ring and injector wear; the ash reduces combustion quality; requires longer on-board separation and upgraded components.
6. Pour point: minimum temperature at which the oil flows - must be kept above pour point for storage and transfer, so the fuel must be stored heated above pour point to avoid solidification, defining the heating of tanks/lines.

(Also acceptable: carbon residue, water content, micro-carbon and sodium/vandadium - but the above six are the key storage/burning properties.)