Method for determining the air/fuel ratio in an internal combustion engine combustion chamber

A method for determining air/fuel ratio in an internal combustion engine combustion chamber having at least two ignition plugs spaced apart from one another by a defined distance, at least one said ignition plug being operable as an ion-current sensor, said method comprising:

• (a) in successive operating cycles of said engine, operating a first ignition plug as an ion-current sensor to detect a time of arrival (t1) at said first ignition plug of a flame front formed at an ignition time (t0) at said second ignition plug spaced by said defined distance from said first ignition plug;
• (b) determining the flame front speed as a function of the detected time of arrival (t1), the ignition time (t0), and said defined distance, and
• (c) determining said air/fuel ratio as a function of said flame front speed and dependent on a determined engine speed and/or engine load using a predetermined map showing air/fuel ratio as a function of flame front speed for different values of engine speed and/or engine load.