The fuel system can be changed over so that the engine can be run on diesel fuel. This is normally done if work is to be carried out on the main engine fuel pumps, or before going for a refit. It can be also changed over if there was a fault with the fuel oil heating plant.

It used to be normal practise to change to diesel fuel for manoeuvring purposes. Modern methods of fuel oil recirculation to keep the fuel hot at the injectors has now made this practise redundant

The change over valve can be operated automatically from a switch in the engine control room or by the handwheel on top of the valve.