Method for controlling a gasoline internal combustion engine, having two spark plugs for each cylinder

A method for controlling operation of a gasoline internal combustion engine,

• wherein said engine comprises:
• a crankcase having a plurality of cylinders and a cylinder head defining a combustion chamber and a combustion pre-chamber associated with each cylinder of the plurality of cylinders, and a plurality of pistons movable within the cylinders and operatively connected to an engine shaft, and
• a first spark plug and a second spark plug associated with the combustion pre-chamber and the combustion chamber of each cylinder, respectively,
• wherein said method comprises operations of:
• controlling, for each cylinder, in each cylinder operating cycle, an ignition of the first spark plug at a given engine crank angle (SA1) and an ignition of the second spark plug at a given engine crank angle (SA2), depending on an engine load and an engine rotational speed, and depending on a required value of a torque delivered by the engine,
• determining said engine crank angle (SA1) of ignition of the first spark plug and said engine crank angle (SA2) of ignition of the second spark plug on a basis of the following operations:
• empirically detecting, for said engine, a plurality of three-dimensional reference surfaces, in a diagram with three orthogonal axes x, y, z, wherein each three-dimensional reference surface corresponds to a respective engine operating point including a determined pair of values of engine rotational speed and engine load, and wherein each of said three-dimensional reference surfaces provides, on the z axis, a value of a gross indicated mean effective pressure (IMPEh) obtained at each operating cycle inside the cylinder, as a function of a value reported on the x axis of the engine crank angle (SA1) of ignition of the first spark plug, and a value reported on the y axis of the engine crank angle (SA2) of ignition of the second spark plug,
• empirically determining, for each of the engine operating points, and as a function of the engine crank angle (SA1) at which the first spark plug is ignited and the engine crank angle (SA2) at which the second spark plug is ignited, a value of the engine crank angle at which 50% of a mass of fuel is burnt (MFB50),
• on a basis of the empirically detecting and empirically determining operations, for each of the engine operating points, defining a respective IMEPh/MFB50 curve of a variation of said gross indicated mean effective pressure (IMPEh) as a function of the value of the engine crank angle at which 50% of the fuel mass is burnt (MFB50),
• calculating, for each point on said IMEPh/MFB50 curve, a ratio between a value of IMEPh at that point and a maximum value of IMEPh along said IMEPh/MFB50 curve, where said maximum value of IMEPh corresponds to a maximum point on said IMEPh/MFB50 curve or to a maximum point defined by an extension of said IMEPh/MFB50 curve obtained by extrapolation,
• taking said ratio as representative of a value of an efficiency of the engine, corresponding substantially to a ratio between a torque delivered by the engine and an optimum torque, which would be delivered for said maximum value (IMEPh opt) of IMEPh,
• on a basis of said calculating, transforming each of said three-dimensional reference surfaces, into a transformed three-dimensional surface, which, in a diagram with axes x, y, z, provides on the z axis the value of said efficiency of the engine, as a function of the value of the engine crank angle (SA1) of ignition of the first spark plug on the x axis and of the value of the engine crank angle (SA2) of ignition of the second spark plug on the y axis,
• translating each of said transformed three-dimensional surfaces, parallel to the x-axis and the y-axis, assigning:
• to the x-axis values of a differential (Delta SA1) between an optimum value (SA1 opt) of an ignition crank angle of the first spark plug for which said engine efficiency is unitary, and the value of the ignition crank angle (SA1) of the first spark plug, and
• to the y axis values of a differential (Delta SA2) between an optimum value of an engine crank angle (SA2 opt) of ignition of the second spark plug for which said engine efficiency is unitary, and the value (SA2) of the engine crank angle of ignition of the second spark plug,
• defining a single normalized reference surface which approximates said transformed and translated three-dimensional surfaces, said single normalized reference surface being used to determine, as a function of the engine operating point, a relationship between torque delivered by the engine and values of the engine crank angle of ignition (SA1) of the first spark plug and of the engine crank angle of ignition (SA2) of the second spark plug, and
• determining values of the engine crank angle (SA1) of ignition of the first spark plug and of the engine crank angle (SA2) of ignition of the second spark plug using said single normalized reference surface, so as to reach a target value of torque delivered at a given operating point of the engine.