With reference to the main engine burning heavy fuel, the ship has been asked to use low-sulphur diesel oil. Problems likely to be encountered and risks involved in continuous running of the main engine on such low-sulphur fuels:

1. Reduced cylinder lubrication/cold corrosion: Low-sulphur fuel produces less sulphuric acid, so the cylinder oil's alkalinity (BN) is not consumed as much. If the engine continues to use a high-BN cylinder oil at the same feed rate, the excess alkalinity can form hard deposits (ash) on the piston crown, ring grooves, and liner, causing ring sticking, liner polishing, and increased wear. Conversely, if the feed rate is not adjusted, the liner may be over-lubricated. The cylinder oil feed rate and BN must be reduced to match the low-sulphur fuel.
2. Fuel pump/injector lubrication: Low-sulphur (and low-viscosity) diesel oil has poorer lubricating properties than heavy fuel oil. The fuel injection pump and the injector rely on the fuel for lubrication; running on low-viscosity diesel oil can cause increased wear of the fuel pump plunger and the injector needle, and possible seizure. The fuel must be kept at the correct viscosity, and the pump/injector may need attention.
3. Viscosity/temperature: Diesel oil has a much lower viscosity than heavy fuel oil and does not need heating. If the fuel system is still set for heavy fuel oil (heated), the diesel oil may be overheated, causing vapour lock, poor atomization, and pump problems. The fuel temperature must be reduced for diesel oil.
4. Change-over problems: Changing from heavy fuel oil to diesel oil (and back) requires a careful change-over procedure to avoid mixing the fuels, which can cause sludge, filter blockage, and injector problems. The change-over must be done at the correct temperature and load.
5. Fuel system leaks: Diesel oil is thinner and can leak through seals and joints that were tight for heavy fuel oil, causing fuel leaks and a fire hazard.
6. Combustion/emissions: Diesel oil burns more cleanly (less smoke, less SOx), but the engine's combustion may need adjustment (injection timing) for the different fuel.
7. Cost: Diesel oil is more expensive than heavy fuel oil, increasing the operating cost.

Risks: the main risks are increased cylinder liner/ring wear (from over-alkalinity or under-lubrication), fuel pump/injector wear and seizure, fuel system leaks, and the risk of a fire from fuel leaks. These are managed by adjusting the cylinder oil feed rate/BN, controlling the fuel temperature/viscosity, carrying out a proper change-over, and monitoring the fuel system for leaks.