METHOD AND APPARATUS TO CONTROL A MULTI-CHARGE IGNITION SYSTEM WITH AT LEAST TWO COILS PER SPARK PLUG

A multi-charge ignition system including a spark plug control unit (13) adapted to control at least two coil stages so as to successively energise and de-energise said coil stage(s) to provide a current to a spark plug, said two stages comprising a first transformer (T1) including a first primary winding (L1) inductively coupled to a first secondary winding (L2); a second transformer (T2) including a second primary winding (L3) inductively coupled to a second secondary winding (L4); a first switch Q1 electrically connected between the low side of the first primary winding L1 and the low side/earth, a second switch Q2 having a connection to the low side of the second primary winding L3; and including a first diode D1 electrically connected between the low side of said first secondary winding L3 and ground, and a second diode D2 connected to a point between the low side of said second secondary winding L4 and ground and, including means to measure the voltage at a first connection point (203a) between the first switch and the low side of L1 and/or the voltage at a second connection (203b) point between the second switch and the low side of L3; and means to control the operation of the system dependent upon said measured voltages.