Fuel injection system for a spark-ignition internal combustion engine and relative control method

A fuel injection system (1) for a spark-ignition internal combustion engine comprising:

• a number of cylinders (11), which receive a gas mixture comprising fresh air and wherein a plurality of respective main combustion chambers (MC) are defined;
• a number of first injectors (2), each coupled to a respective cylinder (11), into which it injects fuel under pressure during a combustion cycle;
• a number of spark plugs (13), each coupled to a respective cylinder (11) to cyclically determine the ignition of the fuel present in the main combustion chamber (MC);
• a number of combustion pre-chambers (PC), each obtained in the area of a respective spark plug (13), where the combustion of a gas mixture comprising fresh air and fuel takes place in order to increase the turbulence inside the main combustion chamber (MC) of the respective cylinder (11);
• the injection system (1) is characterized in that it comprises:
• a number of extraction ducts (17;
• 28), each originating from a respective cylinder (11) in the area of an extraction point (P) to extract the gas mixture present inside the respective main combustion chamber (MC) during a combustion cycle;
• at least one reserve (18), which receives the gas mixture from the extraction ducts (17;
• 28);
• inside the reserve (18), the gas mixture coming from the extraction ducts (17;
• 28) is mixed with the quantity of fuel needed to obtain a combustion under stoichiometric conditions inside the combustion pre-chambers (PC);
• a number of second injectors (27), each coupled to a respective combustion pre-chamber (PC), into which it injects the gas-and-fuel mixture coming from the reserve (18);
• and
• an injector system (22), which feeds the fuel to the reserve (18) and is connected, by means of a first duct (23), to a low-pressure pump (8) or, alternatively, to a high-pressure pump (4).