A system for supplying an internal combustion engine, comprising: a first tank containing fuel usable for realising a diesel cycle; a first conduit for fuel supply emerging from the first tank and ending in a distribution chamber which supplies electro-injectors associated to cylinders of the engine, means (P 1, P 2) being comprised in the first conduit for pressurised supply of the fuel to the distribution chamber, and being comprised regulating means of a fuel flow through the first conduit; a second conduit for fuel recirculation originating from a pressure limiter associated to the distribution chamber and ending into the first tank, the exhaust channels of the electro-injectors being connected to the second conduit; a first control board to which at least sensors 11, 12, 13 are connected, for detecting the temperature of the fuel in the first conduit, the fuel pressure in the chamber and the speed of the drive shaft of the engine, the first control board being for controlling and managing at least the electro-injectors and the regulating means and pressure limiter, and first electrical means destined to pre-heat, on start-up of the engine and for a predetermined time, the portion of the cylinders affected by the electro-injectors; wherein it comprises: a second tank containing fuel in the liquid state; a vaporiser, supplied by the second tank, for supplying the fuel, in a gaseous state, to electrically-controlled gas injectors, the gas injectors opening into the aspirating manifold of the cylinders of the engine; a tension pulse generator with the connection actuated in phase relation with the unpowering of the heating means actuated by the first control board; second electrical means, arranged in the cylinders of the engine at the related cylinder head and destined, if supplied by the generator, to create electric arcs; a second control board to which the pressure sensor (12,13) of the distribution chamber and the sensor of the number of revolutions of the engine are connected, which second control board is destined to intervene, in suitable phase relation, on the regulating means and pressure limiter so as to deactivate the electro-injectors and control and manage the vaporiser, gas injectors, tension pulse generator, and second electrical means, for realising supply to the cylinders of the engine with the fuel in gaseous state, such as to define an Otto cycle. a first tank containing fuel usable for realising a diesel cycle; a first conduit for fuel supply emerging from the first tank and ending in a distribution chamber which supplies electro-injectors associated to cylinders of the engine, means (P 1, P 2) being comprised in the first conduit for pressurised supply of the fuel to the distribution chamber, and being comprised regulating means of a fuel flow through the first conduit; a second conduit for fuel recirculation originating from a pressure limiter associated to the distribution chamber and ending into the first tank, the exhaust channels of the electro-injectors being connected to the second conduit; a first control board to which at least sensors 11, 12, 13 are connected, for detecting the temperature of the fuel in the first conduit, the fuel pressure in the chamber and the speed of the drive shaft of the engine, the first control board being for controlling and managing at least the electro-injectors and the regulating means and pressure limiter, and first electrical means destined to pre-heat, on start-up of the engine and for a predetermined time, the portion of the cylinders affected by the electro-injectors; a second tank containing fuel in the liquid state; a vaporiser, supplied by the second tank, for supplying the fuel, in a gaseous state, to electrically-controlled gas injectors, the gas injectors opening into the aspirating manifold of the cylinders of the engine; a tension pulse generator with the connection actuated in phase relation with the unpowering of the heating means actuated by the first control board; second electrical means, arranged in the cylinders of the engine at the related cylinder head and destined, if supplied by the generator, to create electric arcs; a second control board to which the pressure sensor (12,13) of the distribution chamber and the sensor of the number of revolutions of the engine are connected, which second control board is destined to intervene, in suitable phase relation, on the regulating means and pressure limiter so as to deactivate the electro-injectors and control and manage the vaporiser, gas injectors, tension pulse generator, and second electrical means, for realising supply to the cylinders of the engine with the fuel in gaseous state, such as to define an Otto cycle.