The combined fuel tank developed by KUS has a valve installed at the connection between the main and auxiliary fuel tanks. In cold weather, when the main and auxiliary fuel tanks need to be filled with diesel of different grades, the valve is closed, and the oil between the main and auxiliary fuel tanks cannot flow into each other. In hot weather, high-grade oil is not needed, the valve is opened, and a connecting vessel is formed between the main and auxiliary fuel tanks. The oil added to the main tank will automatically flow to the auxiliary tank through the connecting vessel. At this time, the auxiliary tank becomes an extension tank of the main tank, achieving the purpose of increasing the cruising range, while also achieving energy saving and efficiency improvement.