1. Field of the Invention
This invention relates to a spark coil control device for an internal combustion engine equipped with electronic ignition advance.
2. Description of the Prior Art
In any inductive coil system, spark generation is provided by breaking a high current in the primary of a high voltage coil, thereby inducing a surge of current to the secondary tied into the spark plugs. The energy stored in a coil has the form 1/2 LI.sup.2 where L is the coefficient of self-induction and I.sup.2 is the square of the intensity of the current circulating in the primary immediately before being open circuited. In conventional spark coils where the rupture is provided by contact points, the cyclical relationship of the contact point closure is maintained at a constant 66% of the period, and spark generation under all circumstances results from a compromise between a coil being heated by the Joule effect when the motor is at idle and a sufficient quantity of stored energy to provide spark at higher rpms.
In an electronic ignition system for an internal combustion engine with programmed spark cartography, it is advisable to set aside space in the memory to make it possible to engender an angle for initial coil conduction which varies in accordance with the speed of rotation of the motor, thereby eliminating that compromise. It is also possible to use the value for the speed of rotation to take into account the lower battery power during the starting phase.
In most known ignition systems involving an electronic calculator, a counter under a load which is the 180.degree. complement of the angle of ignition advance is decremented for each degree of rotation of the crankshaft and controls the zero resetting of a bridge which commands the circulation of electrical current in the primary of the coil. The high logical state of the bridge is commanded by the coincidence output from a logic comparator which is connected, on the one hand, to the outputs of the counter and, on the other hand, to the memory providing the value of the angle for initiating coil conduction. The drawback of such a system is that it is incapable of inducing coil conduction when the value of the conduction angle is greater than the 180.degree. complement to the angle of advance.
The major drawback stems from the quality of the decrementaiton signal. Indeed, it is difficult to devise a mechanical method for large-scale production of targets and of primary elements providing resolution of a crankshaft degree. Targets are generally manufactured with a number of teeth of less than 50, and interpolation is used to initiate 360 impulses for each revolution of the crankshaft. The interpolation systems have sufficient precision when the motor is operating at a steady speed but when it is operating at variable speeds, in particular when it is being started in intense cold, the number of impulses restored per half revolution generally differs substantially from 180, which induces an error in the value of the spark advance angle equal to the differences between the value restored over a half revolution and the theoretical number. One solution consists in bringing about the release of a spark when a reference mark connected with the flywheel passes by a primary element on the crankshaft of the flywheel so long as the speed of rotation of the motor is below a certain threshold, but this solution does not permit taking maximum advantage of the flexibility offered by the electronic generation of a spark advance law.