Dual fuel system for powering combustion engines comprising: a primary fuel tank in a form of a gasoline or diesel with a primary fuel pump, an alternative fuel tank in a form of gas, a mixed fuel tank, an alternative fuel circulation pump, a twin chamber high pressure pump, an injector rail, a return circuit for returning excess fuel from the high pressure pump and valves and solenoid valves, and a controller for controlling the operation of the system, wherein the mixed fuel tank is equipped with a mixed fuel pump and a mixed fuel level sensor, which is connected to the controller, the inlet of the twin chamber high pressure pump is connected to the mixed fuel pump with a line and its high pressure output is connected to the injector rail, the inlet of an alternative fuel circulation pump, located at the inlet of the mixed fuel tank or in the mixed fuel tank, is connected through a first line having a solenoid valve and a check valve to the outlet of the primary fuel tank in the form of gasoline or diesel, and through a second line having a solenoid valve and a check valve to the outlet of an alternative fuel tank in the form of gas, wherein a line of the return circuit connects the inlet of the mixed fuel tank with the refueling line of the alternative fuel tank in the form of gas, characterized in that, the return circuit extending from the second output of the twin chamber high pressure pump comprises a return line with a turned on pressure sensor and a temperature sensor which connects through a return line, having a solenoid and a check valve, to the inlet to the mixed fuel tank and connects through another return line, having a solenoid and a check valve, to the inlet to the refueling line of the alternative fuel tank in the form of gas. a primary fuel tank in a form of a gasoline or diesel with a primary fuel pump, an alternative fuel tank in a form of gas, a mixed fuel tank, an alternative fuel circulation pump, a twin chamber high pressure pump, an injector rail, a return circuit for returning excess fuel from the high pressure pump and valves and solenoid valves, and a controller for controlling the operation of the system, wherein the mixed fuel tank is equipped with a mixed fuel pump and a mixed fuel level sensor, which is connected to the controller, the inlet of the twin chamber high pressure pump is connected to the mixed fuel pump with a line and its high pressure output is connected to the injector rail, the inlet of an alternative fuel circulation pump, located at the inlet of the mixed fuel tank or in the mixed fuel tank, is connected through a first line having a solenoid valve and a check valve to the outlet of the primary fuel tank in the form of gasoline or diesel, and through a second line having a solenoid valve and a check valve to the outlet of an alternative fuel tank in the form of gas, wherein a line of the return circuit connects the inlet of the mixed fuel tank with the refueling line of the alternative fuel tank in the form of gas, characterized in that, the return circuit extending from the second output of the twin chamber high pressure pump comprises a return line with a turned on pressure sensor and a temperature sensor which connects through a return line, having a solenoid and a check valve, to the inlet to the mixed fuel tank and connects through another return line, having a solenoid and a check valve, to the inlet to the refueling line of the alternative fuel tank in the form of gas.