Inductive-discharge ignition device for an internal combustion engine

Abstract: The device (1) is of the type comprising for each engine spark plug (2) a transformer (3) with a secondary winding (4) to be connected to the respective spark plug (2) and a primary winding (5) connected to the output of a power stage (6) able to provide an ON-OFF signal for controlling the transformer (3). The device (1) also comprises a damping network (15) and, interposed between the network (15) and the output of the power stage (6), unidirectional connection means (20) which connect said network (15) to the output of the power stage (6) each time the signal present at said output switches from an OFF condition to an ON condition.

Description: This invention relates to a inductive-discharge ignition device for an internal combustion engine according to the first part of claim 1 and known from US-A-4 285 322, which will be discussed later. More particularly, the invention relates to a device of the type comprising, for each spark plug, a transformer with a secondary winding to be connected to a respective spark plug and a primary winding connected to the output of a power stage able to generate an ON-OFF signal for controlling the transformer. The power stage is generally provided at its output with a Darlington-connected transistor the collector of which is directly connected to a terminal of said transformer primary winding, the other primary winding terminal being connected to a direct current power source such as an accumulator battery. The operation of known devices of the aforesaid type is essentially to provide the transformer primary winding with an alternating signal by putting said power transistor alternately into a conducting and inhibited state. By this means, a high voltage is induced (especially following the inhibition of the transistor) in the transformer secondary winding of such an extent as to trigger an arc in the respective spark plug. It has also been noted that particularly during the OFF-ON switching stage, a damped oscillating signal is superposed on the voltage induced in the secondary winding, this signal being of high frequency (depending not only on the transformer parameters but also on the switching speed) and having the amplitude of its first half-wave directly proportional to the transformation ratio (turn ratio) of the transformer and to the direct current voltage with which said power transistor and primary winding are powered. Under certain engine operating conditions said oscillating signal can trigger the arc in the spark plug, resulting in considerably misplaced spark advance. To obviate said drawback it is known to provide and inductive discharge ignition system with a damping network and interposed between said network and the power stage a diode which connects the network to the output of the power stage each time the signal present at the output switches from an OFF condition to an ON condition. For instance, in Patent US-A-4 285 322 in the name of Nippon Soken, Inc is described a damping network comprising a capacitor having one end connected to base of a final transistor of the power stage and the other end connected to the output of the power stage via a Zener diode. This Kind of damping network cannot be easily applied to an already existing power stage. The object of the present application is to realize an ignition system with a damping network that is easily applicable to the power stage of the ignition system. Said object is attained according to the present invention by an inductive-discharge ignition device for an internal combustion engine, of the type comprising for each spark plug a transformer with a secondary winding to be connected to a respective spark plug and a primary winding connected to an output of a power stage; said power stage comprising at least a final power transistor having a collector that realizes said output; said power transistor being switched in order to provide an ON-OFF signal for controlling said transformer; said device further comprising a damping network and interposed between said network and the output of said power stage a diode which connects said network to the output of said power stage each time the signal present at said output switches from an OFF condition to an ON condition, characterized by the fact that said damping network comprises a first resistor, a second resistor series connected with said first resistor and a capacitor series connected with said second resistor; a joining point between said first and said second resistor is connected to the anode of the diode, the cathode of which is connected to a joining point between output of said power stage and said primary winding; said first resistor being also connected to a power source terminal. The invention will be more apparent from the description of an embodiment thereof given hereinafter by way of non-limiting example with reference to the accompanying drawing, in which: Figure 1 is an electric schematic of an ignition device constructed in accordance with the present invention; and Figure 2 shows the pattern of an electrical signal taken from a predetermined point of Figure 1. In Figure 1, the reference numeral 1 indicates overall an inductive-discharge ignition device for spark plugs 2 of an internal combustion engine (not shown). The device 1 is of the type comprising for each spark plug 2 a transformer 3 with a secondary winding 4 to be connected to the respective spark plug 2 and a primary winding 5 connected to the output of a power stage 6 of conventional type able to provide an ON-OFF signal for controlling the transformer 3. Specifically, the stage 6 has an output section consisting of an N-P-N power transistor in Darlington connection, not shown. The transistor 8 has its emitter connected to earth via a resistor 9 and its collector connected to one end of the primary winding 5. This latter has its other end connected to a terminal 10 to which, when in use, the positive pole of a direct current power source (such as an accumulator battery) is connected. One end of the winding 4 is also connected to the terminal 10 and its other end is connected to a terminal 11. When in use, this latter terminal is connected to the electrode 12 of the spark plug 2, its body 13 being connected to earth. According to the present invention the device 1 comprises a damping network 15 and unidirectional connection 20 means interposed between the network 15 and the output of the power stage 6, said means 20 connecting said network 15 to the output of the stage 6 each time the signal present at said output passes from an OFF condition to an ON condition. Specifically, the damping network 15, extending between a terminal 16 (also connected to said power source) and earth, comprises a first resistor 17, a second resistor 18 and a capacitor 19. A joining point between the resistors 17 and 18 is connected to the anode of a diode 21 (constituting said unidirectional connection means 20), the cathode of which is connected to a joining point between the output of the power stage 6 and the primary winding 5. The operation of the device 1 is described hereinafter with reference also to Figure 2, which shows the pattern of the voltage signal taken at the terminal 11, ie at the exit of the secondary winding 4 of the transformer 3. Specifically, during the time in which the transistor 8 is inhibited (OFF), the output signal of the stage 6 coincides substantially with the battery voltage at the terminal 10. During this time period the capacitor 19 charges through the resistors 17 and 18 to the value of the battery voltage present at the terminal 16. When the transistor switches from inhibited (OFF) to conducting (ON) (time t1 of Figure 2), the output signal of the power stage 6 passes from the battery voltage to almost zero (sum of the saturation voltage of the transistor 8 and the voltage drop across the resistor 9) slowly because of the presence of the capacitor 19. This is because under such conditions the diode 21 is directly biased so that the voltage across the power stage 6 decays substantially exponentially with a time constant equal approximately to the product of the capacitance of the capacitor 19 and the sum of the resistance of the resistor 18 and the internal resistance of the transistor 8, which operates in a linear zone almost until the capacitor 19 is completely discharged. The resistance of the resistor is considered negligible. It is apparent that the charge time constant, determined by the product of the capacitance of the capacitor 19 and the sum of the resistances of the resistors 17 and 18 must be such as to allow the capacitor 19 to be recharged during the non-conducting (OFF) phase of the stage 6 when the engine is at maximum r.p.m. Indicatively, the value of this time constant could be between 2 ms and 3 ms. Because of the fact that the ON-OFF switching is not sudden, the voltage Vs induced in the secondary winding 4 of the transformer 3 no longer has the said superimposed oscillating signal which is the source of the drawbacks of known devices. In this respect, from the pattern of the signal Vs shown in Figure 2a (one unit of the time scale equals 0.4 ms), it can be seen that the only substantial component of Vs is the induced voltage (of the order of about 0.6 kV). In contrast there is no longer any appreciable component of the said oscillating signal, which usually raises the value of Vs in known devices to as high as about 1.3 kV, to cause undesirable advance triggering of the arc in the spark plug. The damping network 15 does not act during the ON-OFF switching of the output signal of stage 6 because under these conditions the diode 21 is inversely biased. Consequently, this switching can take place as rapidly as possible to enable an extremely high voltage to be induced in the secondary winding 4 (up to more than 15 kV) to cause very efficient triggering of the arc in the spark plug 2. The advantages of the device of the present invention are apparent from an examination of its characteristics. Firstly, said oscillating signal is prevented from arising in the secondary winding during the ON-OFF transition of the power stage. In addition, particularly costly components are not required as the range of voltages within which they have to operate is very limited and moreover the respective nominal ratings do not have to be particularly precise. Finally, it is apparent that modifications can be made to the described device 1 provided they do not leave the scope of the claim.

Claims: An inductive-discharge ignition device (1) for an internal combustion engine, of the type comprising for each spark plug a transformer (3) with a secondary winding (4) to be connected to a respective spark plug (2) and a primary winding (5) connected to an output of a power stage (6); said power stage (6) comprising at least a final power transistor (8) having a collector that realizes said output; said power transistor (8) being switched in order to provide an ON-OFF signal for controlling said transformer (3); said device (1) further comprising a damping network (15) and interposed between said network (15) and the output of said power stage (6) a diode (21) which connects said network (15) to the output of said power stage (6) each time the signal present at said output switches from an OFF condition to an ON condition, characterized by the fact that said damping network comprises a first resistor (17), a second resistor (18) connected in series with said first resistor (17) and a capacitor (19) connected in series with said second resistor (18); a joining point between said first resistor (17) and said second resistor (18) is connected to the anode of the diode (21), the cathode of which is connected to a joining point between output of said power stage (6) and said primary winding (5); said first resistor (17) being also connected to a power source terminal (16).

Inventors: COLLA RICCARDO, DI NUNZIO VITTORIO, COLLA, RICCARDO, DI NUNZIO, VITTORIO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1989
Main classification: F02P3
IPC codes: F02P3, F02P15