Spark duration control for a capacitor discharge ignition system

An apparatus is provided for controlling ignition in an internal combustion engine having N cylinders and a capacitor discharge ignition system. The capacitor discharge system includes an ignition capacitor which is maintained at a predetermined electrical potential by a charging circuit. N transformers are provided, each having a primary coil and a secondary coil. The primary coils include first and second terminals and the secondary coils are electrically connected in parallel with the spark gap in an associated one of the cylinders. Selector switches are connected between the ignition capacitor and an associated one of the primary first terminals. The selector switches are normally biased open and are adapted to close in response to receiving cylinder select signals. The cylinder select signals are produced in response to a desired ignition sequence and for a period of time corresponding to a desired spark duration in an associated cylinder. A modulation switch is connected between the primary coil second terminals and a source of low electrical potential. Control circuitry is provided for operating the modulation switch while a selector switch is closed such that the current flowing in an associated primary coil initially rises to a first threshold sufficient to cause a spark to arc an associated spark gap. Thereafter the current is modulated between the first threshold and a second threshold lower than the first threshold to maintain the spark. The spark is maintained in this manner until the associated selector switch is opened.