Ignition system for an internal combustion engine

An ignition system for an internal combustion engine, comprising:

• at least one spark plug,
• at least one ignition coil whose secondary winding is connected or connectible to the at least one plug,
• a power transistor whose output path is in series with the primary winding of the coil between the two terminals of a direct-current voltage supply,
• a driver circuit for the transistor,
• means for detecting the potential difference between the output terminals (drain, source) of the transistor, and
• control circuit means connected to the driver circuit for switching the transistor gradually from the cut-off condition to the saturated condition in a controlled manner in dependence on the potential difference in order to cause current to flow in the primary winding of the coil,
• the control means including electronic processing means for generating a logic control signal in which, to generate a spark, there is a first change of level to start the switching of the transistor to the saturated condition and the flow of current in the primary winding of the coil, and a second change of level to cut off the transistor and trigger the spark;
• the control circuit means also including:
• a pulse generator for generating pulses at a frequency greater than the reciprocal of the minimum time interval between the first and second changes in the control logic signal,
• a comparator for supplying a control signal when the potential difference between the output terminals of the transistor falls to a predetermined threshold value during the switch from the cut-off condition to the saturated condition, and
• a selection logic circuit having first and second inputs connected to the processing means and to the pulse generator respectively, a control input connected to the input of the comparator, and an output connected to the driver circuit for the transistor, the selection logic circuit being arranged to transmit to the driver circuit:
• a) the logic control signal generated by the processing means as long as the potential difference between the output terminals of the transistor is above the threshold value, and
• b) the pulses emitted by the generator when the comparator circuit emits its control signal.