A fuel system for an internal combustion engine comprising - a first fuel container for a first fuel, a second fuel container for a second fuel, a pump, and a first fuel injector, the fuel system being arranged to provide a communication between the first fuel container and the pump and between the pump and the first fuel injector, - wherein the fuel system further comprises a reservoir (7, 8) with a separation device (701, 702, 801, 802) adapted to divide the reservoir (7, 8) into a first volume (703, 803) and a second volume (704, 804), whereby the separation device may be moved or flexed by a pressure difference in the first and second volumes (703, 803, 704, 804) so as to change the sizes of the first and second volumes (703, 803, 704, 804), - wherein the fuel system is arranged to provide a communication between the pump and the first volume (703, 803), and to provide a communication between the second fuel container and the second volume (704, 804) and between the second volume (704, 804) and a second fuel injector, whereby the fuel system provides for a single fuel pump being used for delivering two fuels of different types to a cylinder of the engine, - characterized in that said reservoir, separation device, first volume and second volume are a primary reservoir, a primary separation device (701, 702), a primary first volume and a primary second volume, respectively, the fuel system also comprising a secondary reservoir with a secondary separation device (801, 802) adapted to divide the secondary reservoir into a secondary first volume and a secondary second volume, whereby the secondary separation device (801, 802) may be moved or flexed by a pressure difference in the secondary first and second volumes (803, 804) so as to change the sizes of the secondary first and second volumes (803, 804), wherein the fuel system is arranged to provide a communication between the pump and the secondary first volume, and to provide a communication between the second fuel container and the secondary second volume and between the secondary second volume and the second fuel injector. - a first fuel container for a first fuel, a second fuel container for a second fuel, a pump, and a first fuel injector, the fuel system being arranged to provide a communication between the first fuel container and the pump and between the pump and the first fuel injector, - wherein the fuel system further comprises a reservoir (7, 8) with a separation device (701, 702, 801, 802) adapted to divide the reservoir (7, 8) into a first volume (703, 803) and a second volume (704, 804), whereby the separation device may be moved or flexed by a pressure difference in the first and second volumes (703, 803, 704, 804) so as to change the sizes of the first and second volumes (703, 803, 704, 804), - wherein the fuel system is arranged to provide a communication between the pump and the first volume (703, 803), and to provide a communication between the second fuel container and the second volume (704, 804) and between the second volume (704, 804) and a second fuel injector, whereby the fuel system provides for a single fuel pump being used for delivering two fuels of different types to a cylinder of the engine, - characterized in that said reservoir, separation device, first volume and second volume are a primary reservoir, a primary separation device (701, 702), a primary first volume and a primary second volume, respectively, the fuel system also comprising a secondary reservoir with a secondary separation device (801, 802) adapted to divide the secondary reservoir into a secondary first volume and a secondary second volume, whereby the secondary separation device (801, 802) may be moved or flexed by a pressure difference in the secondary first and second volumes (803, 804) so as to change the sizes of the secondary first and second volumes (803, 804), wherein the fuel system is arranged to provide a communication between the pump and the secondary first volume, and to provide a communication between the second fuel container and the secondary second volume and between the secondary second volume and the second fuel injector.