This invention relates to an ignition system for an internal combustion engine, and more particularly relates to a capacitive type ignition system.
A multi spark ignition system which generates a series of ignition sparks within a demanded firing duration is well known in the art.
An typical conventional ignition system is shown in FIG. 17. The conventional ignition system comprises a charging circuit (100), an ignition capacitor (101), an ignition transformer (102), a spark plug (103) and a discharging circuit (104). This discharging circuit (104) further includes a transistor (107) and an oscillator (106).
Furthermore, a first returning circuit (105) which comprises a resistor (108) and a diode (109) is connected to a primary winding of the ignition transformer (102) so as to absorb a high voltage generated on the primary winding when the discharging circuit (104) turns off.
In this ignition system, an ignition spark may be generated at the certain moment when the transistor (107) turns on. Further, the transistor (107) turns on and off repeatedly with a cycle determined by the oscillator (106). As a result, the conventional ignition system can generate a series of ignition sparks with the cycle determined by the oscillator (106) within a demanded firing duration of the internal combustion engine.
Now, a process for generating the ignition spark is explained in detail.
When the transistor (107) turns on, the capacitive energy charged in the ignition capacitor (101) will be discharged through the primary winding of the ignition transformer (102). At this time, one part of the capacitive energy will be stored in the ignition transformer (102) as a magnetic energy. At the same time, the other part of the capacitive energy charged in the ignition capacitor (101) will be transmitted to the spark plug (103) through a secondary winding of the ignition transformer (102), and then, the ignition spark will be generated at the spark plug (103).
After generating the ignition spark, the transistor (107) turns off. When the transistor (107) turns off, the magnetic energy stored in the ignition transformer (102) circulates the first returning circuit (109) and primary winding of the ignition transformer (102) as an electric current, and is consumed by the resistor (108) partially.
While the magnetic energy stored in the ignition transformer (102) circulates the first returning circuit (109) and primary winding of the ignition transformer (102), the magnetic energy is converted into heat by the resistor (108). At this time, if a resistance of the resistor (108) is established in small value, the magnetic energy stored in the ignition transformer (102) may be discharged mainly through secondary winding of the ignition transformer (102), because the resistor (108) does not consume the magnetic energy so much. The discharged energy though the secondary winding is going to generate the ignition spark. Accordingly, if the resistance of the resistor (108) is established as a small value, a period for holding a single spark could be elongated after the transistor (107) turns off.
After the magnetic energy stored in the ignition transformer (102) is reduced in such level where the ignition spark can not be maintained, the energy discharged through the secondary winding is disappeared, then the magnetic energy remained in the ignition transformer (102) is consumed by the resistor (108) only. However, if the resistance of the resistor (108) is established in small value, the electric current flows through the first returning circuit (109) and the primary winding for a while.
Meanwhile, the transistor (107) should be turned on after the electric current through the first returning circuit (109) and primary winding completely disappears, i.e. after the magnetic energy stored in the ignition transformer (102) disappears completely, in order to generate the uniformed ignition sparks because of the non-symmetric wave form of the A.C. voltage applied to the ignition transformer (102) from the ignition capacitor (101). Otherwise, whenever the transistor (107) turns on, an exciting current of the ignition transformer is increased gradually, and thus the period for maintain the single ignition spark will be reduced. In fact, if the transistor (107) is turned on independently from the current through the first returning circuit (109) and primary winding, the ignition transformer (102) may be saturated magnetically, thus the ignition spark should stop generating.
Accordingly, if the resistance of the resistor (108) should be established in small value, the cycle of the oscillator (106) must be selected long sufficiently in order to completely disappear the current through the first returning circuit (109) and primary winding.
As described above, if the resistance of the resistor (108) is established in small value, the period for maintaining the single ignition spark can be elongated but a interval of time between two independent ignition sparks must be elongated.
Contrary, if the resistance of the resistor (108) is established as a large value, the current through the first returning circuit (105) and primary winding disappears immediately, because the resistor (108) consumes the magnetic energy. Accordingly, if the resistance of the resistor (108) is selected as a large value, the interval of time between the two independent ignition sparks can be reduced. However, the period for maintaining the ignition spark must be reduced, because the energy discharged through the secondary winding is also reduced.
Thus, the conventional ignition system can not obtain a series of ignition spark having the elongated maintain time as well as the reduced interval of time between the two independent sparks at the same time.