A diesel engine for combusting a diesel-LPG mixture includes a fuel system for supplying diesel and LPG. The mixture is obtained by combining the fuel flow from a LPG tank and diesel tank. Increased pressure is applied in both fuel flows for mixing purposes. Immediately following the mixing a circulation system is present in which increase in pressure also takes place, wherein a part of the mixture is supplied to a high-pressure pump for injection to the diesel engine and a part is recirculated. In a method of supplying the diesel-LPG mixture, the part of the mixture that is supplied to the high-pressure pump is returned to a position upstream of the mixing point. The pressure in the fuel system is higher downstream of the mixing point. When the LPG supply is switched off the pressure can be gradually decreased through circulation.