Ignition device for internal combustion engine equipped with protective device
United States Patent Suda et al. 5] Apr. 1, 1975 IGNITION DEVICE FOR INTERNAL 3,357,416 12/1967 Huntzinger 123/1413 E 3,791,364 2/1974 Saita 123/148 E COMBUSTION ENGINE EQUIPPED WITH PROTECTIVE DEVICE Primary Examiner-Wendell E. Burns [75] Inventors: gi g i f gg fig igl gs ggi of Attorney, Agent, or Firm-Craig &amp; Antonelli Japan 3 [73 Assignee: Hitachi, Ltd., Tokyo, Japan ABSTRACT [22] Filed: Aug. 8, 1973 An ignition ttilevice for afn internal combustion engine provided wit a circuit or protecting the ignition de- [21] Appl&#39; 386549 vice from thermal damage due to the current flowing across the ignition coil when the engine is stopped. [30] Foreign A li ti P i it D t The protective device consists of a charge-discharge Aug 9 1972 Japan 47 79l22 circuit having a time constant larger than the ignition period. The charge-discharge circuit is reset by the 52 US. c1 123/148 E 123/148 R Pulses indicative fighttth ttmthgwhen the engine is [51] Int. Cl. F02p 3/02 Stopped the charge&#39;discharge Circuit produces an 58 Field of Search 123/148 R 148 E Put after the lapse time determined by its time stant due to the absence of the input pulse. By the out- [56] References Cited put of the circuit the current flowing across the igni UNITED STATES PATENTS tion coil is automatically interrupted.  
 3.262.438 7/1966 Holford 123/148 E 3 Claims, 2 Drawing Figures IO 20 3O 4O 5O Sim DIFZTEJIQP&#39;JEIGWP MoNosmELE SWITCHING POWTER MULTI- sw| CHING mm CIRCUIT VIBRATOR CIRCUIT TIME (INSTANT CIRCUIT PATENTEDAPR&#39; 1 ms SELZT 2 OF 2 NNN IGNITION DEVICE FOR INTERNAL COMBUSTION ENGINE EQUIPPED WITH PROTECTIVE DEVICE The present invention relates to an ignition device for an internal combustion engine, and more particularly, to an ignition device provided with a protective device for preventing the ignition coil of an ignition device in an operation interrupted state from being kept supplied with an electric current for a long time.  
  There have been two types of ignition devices. In one type of ignition device, an electric current is allowed to always flow through the primary coil of the ignition coil, which is interrupted at ignition timing of an inter nal combustion engine, when the energy stored in the primary coil is discharged at the gap of a spark-plug connected with the secondary coil.  
  In a second type of ignition device, an electric current is normally not allowed to flow through the primary coil of the ignition coil. The primary current is allowed to flow only at ignition timing, when the energy thereof is discharged at the gap of the spark-plug in the secondary coil circuit.  
  In the former type, when a voltage continues to be applied to the ignition device of an engine in the state that the engine is stopped and yet the key switch is set, an electric current continues to flow through the primary coil of the ignition device. The ignition coil would become over-heated by this current to be damaged. The heat generation by a switching circuit for cutting off the primary current is also so considerable that it would be damaged if the current continues to flow for a long time.  
  Recently, it has become required to clean the exhaust gas of the internal combustion engine. To meet this requirement, the sparking energy of the spark-plug must be large, which requires a low internal resistance of the ignition coil. However, the reduction in the internal resistance of the ignition coil results in an increase in the trouble due to the overheat of the ignition coil itself.  
  An object of the present invention is to provide an ignition device having a protective device capable of automatically cutting off the electric current flowing across the ignition coil in the state that the engine is stopped and yet a voltage is applied to the ignition circuit.  
  Another object of the present invention is to provide an ignition device capable of protecting the ignition coil by a very simple and inexpensive method.  
  The ignition device according to the present invention has a time constant circuit which detects the interruption of the engine. When the engine is stopped, a driving or switching transistor connected in series with the primary coil of the ignition device is turned off.  
  During the operation period of the engine, pulses indicative of the ignition timing of the engine are supplied to the ignition device at the intervals determined by the rotational speed of the engine. When the rotation of the engine is reduced, the pulse intervals become longer, and when the engine is stopped, the pulse intervals become infinitely long, or in other words, no pulse is supplied to the ignition device.  
  The time constant circuit according to the present invention has a predetermined time constant, and produces a constant level of voltage when the time determined by the time constant has elapsed. Consequently, when the time constant circuit having a time constant larger than the interval of the pulses is started to operate by the application of the pulse, the stopping of the engine can be detected from the output of the time constant circuit. That is, when the predetermined level of output is produced, the engine can be regarded as having stopped.  
  Then, it is sufficient to turn off the driving transistor in accordance with the output of the time constant circuit.  
  It is necessary for the time constant circuit according to the present invention that the starting time thereof is always the time at which a pulse is applied thereto, or in other words, it is always reset by the pulse. According to the present invention, this is achieved very simply by forming a charge-discharge circuit of a resistor and a capacitor in which the accumulated charge of the capacitor is discharged by the application of a pulse. When the pulse applied thereto is no longer present, the capacitor is charged to its saturation level. By detecting the output of the capacitor the driving or switching transistor is turned off. Consequently, when the engine is stopped, the ignition coil is surely cut off from the power source, and no heat production by the switching circuit occurs, either.  
  The above and other objects, features and advantages of the present invention will become more apparant from the following detailed description of the preferred embodiment of the present invention, which is made by way of example only, when taken in conjunction with the accompanying drawings, in which:  
  FIG. 1 is a block diagram of an ignition device according to the present invention; and  
  FIG. 2 is a detailed circuit diagram of the ignition device of FIG. I.  
  The circuit of FIG. 1 is to automatically interrupt the current flowing across the ignition coil when the engine is stopped.  
  In motorcars generally used, when the key is set, a voltage is applied to the ignition device. In the ignition device of the type that in the above state a high tension is produced at the secondary coil by cutting the current flowing across the primary coil, a current is kept flowing across the ignition coil when the engine is stopped. Consequently, it is necessary to automatically interrupt the current when the engine is stopped.  
  In the circuit of FIG. 1, a signal or pulses indicative of the ignition timing of the engine are produced by a signal generator 10, and supplied to a differentiating circuit 20. The signal differentiated by the differentiating circuit 20 triggers a monostable multivibrator circuit 30. The output of the monostable multivibrator 30 switches off a power switching circuit 50 through a switching circuit 40. The power switching circuit 50 which is connected in series between the primary coil of the ignition device and the power source is normally switched on to supply the primary coil with a current. Only when the output of the monostable multivibrator 30 is supplied, the power switching circuit 50 is switched off to interrupt the primary current of the ignition coil. A high tension is produced in the secondary coil of the ignition device by the change in the magnetic flux developed in the ignition coil at that time and supplied to a predetermined plug through a distributor.  
  The output of the monostable multivibrator 30 is also applied to a time constant circuit 60. The time constant circuit 60 detects the interruption of the engine by detecting the interruption for a certain time of the output of the monostable multivibrator 30, and operates the switching circuit 40 to switch off the power switching circuit 50.  
  The signal generator is a device for generating a signal or pulses indicative of the ignition timing of the engine and is ordinarily provided in the distributor. The signal generator may be a pulse generator coupled with the shaft of the engine and generating a pulse when the shaft reaches a predetermined rotational angle.  
  The monostable multivibrator 30 is not important to the primary purpose of the present invention. In principle, even if the output of the signal generator 10 is supplied directly to the switching circuit 40 to actuate the power switching circuit 50, the system operates properly. In this case, the output of the signal generator 10 is also supplied directly to the time constant circuit 60 to reset it.  
  The monostable multivibrator 30 is provided to adjust the off time and on time of the power switching circuit 50. If the rotation of the engine is accelerated, the time period during which a current is flowing across the ignition coil becomes shorter. At a high speed rotation, since a current may be interrupted before sufficient energy is accumulated in the primary coil, the spark energy may be insufficient. To reduce the difference between the spark energy at a low and a high rotation it is quite preferable to provide the monostable multivibrator 30. However, the provision of the monostable multivibrator 30 has nothing to do with the prevention of the thermal damage of the ignition coil and the power switching circuit 50.  
  FIG. 2 is a materialized circuit diagram of the system of FIG. 1. The output of the signal generator 10 in FIG. 1 is applied to the differentiating circuit through an input terminal 222. The differentiating circuit 20 is composed of a capacitor 224, a resistor 226, and a diode 228.  
  The monostable multivibrator is composed of resistors 231, 232, 233, 234 and 235, a capacitor 236, transistors 237 and 238, and a diode 239. The collector of the transistor 238 is connected electrically to the power switching circuit 50 through a diode 272 and, at the same time, to the time constant circuit 60 through a diode 274 and a resistor 276.  
  The switching circuit is composed of a resistor 242 and a transistor 244.  
  The power switching circuit 50 is composed of resistors 252 and 254, an amplifying transistor 256, and a switching transistor 258. The switching transistor 258 is connected in series with a power source terminal 290 through the primary side of the ignition coil 270 and a resistor 278. The secondary side of the ignition coil 270 is connected to a spark plug 280.  
  The time constant circuit 60 consists of a resistor 262 and a capacitor 264.  
  When the key switch of the motorcar is set, the battery voltage is applied to the terminal 290.  
  The transistor 237 of the monostable multivibrator 30 is in the on state and the transistor 238 thereof is in the off state. The base current of the transistor 244 of the switching circuit 40 which is in the on state charges the capacitor 264 through the resistor 262. The collector current of the transistor 244 flows in the base of the transistor 256 through the resistor 242 to put the transistor 256 in the on state. Consequently, the transistor 258 is put in the on state to enable a current f0 flow from the terminal 290 into the primary side of the ignition coil 270 through the resistor 278.  
  Now, if a pulse indicative of ignition timing is applied from the input terminal 222 to the differentiating circuit 20, the transistor 238 is turned to the on state to reduce the base potential of the transistor 256 to substantially the ground level through the diode 272. At the same time, the charge stored in the capacitor 264 is discharged through the resistor 276 and the diode 274. As a result, the transistor 256 is switched off, hence the transistor 258. Then, the energy stored in the primary coil of the ignition coil 270 descharges at the gap of the spark plug 280.  
  Ifa pulse is applied to the terminal 222 according to the ignition sequence of the engine, a spark is produced at the gap of the spark plug 280.  
  When the engine stops, the transistor 238 maintained its off state because no pulse is applied to the terminal 22 any more. The capacitor 264 is charged with the base current of the transistor 244 through the resistor 262. With the increase in the amount of charge of the capacitor 264, the base current of the transistor 244 decreases to eventually put the transistor 244 in the of state. At the same time, the transistor 256 turns off because no base current flows any more, and hence the transistor 258 also turns off. Consequently, the current flowing across the primary coil of the ignition coil 270 is automatically interrupted.  
  The pulse interval of the pulses representative of the ignition timing applied to the terminal 222 varies with the rotating speed of the engine. This change can be known beforehand. If the time constant determined by the resistor 262 and the capacitor 264 is set sufficiently longer than the above pulse interval, the switching circuit 40 operates only when the engine stops.  
  The diodes 272 and 274 are provided for preventing back current current. resistor 276 is to limit the current flowing from the capacitor 264 into the transistor 238. To discharge the capacitor 238 immediately after the application of a pulse to the terminal 222 to switch on the transistor 238, the time constant of the resistor 276 and the capacitor 264 is made very small.  
  The transistor 256 is provided to amplify the base current of the transistor 258. The collector of the transistor 244 may be connected directly to the base of the transistor 258 through the resistor 242. However, since the base current of the transistor 258 is required to high, it is difficult to supply the base current of the transistor 244 directly to the transistor 258. Consequently,  
 1 it is preferable to amplify the base current of the transistor 244 by the transistor 256.  
  The switching circuit 40 and the time constant circuit 60 are connected with the collector of the transistor 238 through the diodes 272 and 274, respectively. However, even if pulses of negative polarity are supplied directly to the diodes 272 and 274, the fundamental purpose of the present invention can be achieved. That is, the purpose of the present invention can be achieved by discharging the capacitor 264 of the time constant circuit 60 by the input pulse and by burning the power switching transistor 258 off.  
  The monostable multivibrator circuit 30 is provided to adjust the off time of the transistor 258.  
  The primary coil of the ignition coil 270 operates as an inductance element. Consequently, if the switching period of the transistor 258 becomes short, the current flowing into the ignition coil 270 becomes low. As the rotation of the engine becomes higher, the energy stored in the primary coil of the ignition coil 270 becomes lower. Consequently, there is the difference in the spark energy discharged at the spark plug 280 between at a high rotation and at a low rotation. If the designing factors of the ignition are set at those at a low rotation, the spark energy becomes defficient at a high rotation. To the contrary, if the designing factors are set at those at a high rotation, heat generation becomes considerable at a low rotation. Consequently, it is preferable to make the currents flowing across the primary coil of the ignition coil 270 at a high and a low rotation substantially the same.  
  In order to reduce the difference in the spark energy between at a high rotation and at a low rotation, it is preferable to decrease the off time period of the transistor at a low rotation. That is, if the off period of the transistor is prolonged at a low rotation, it becomes equivalent to the situation at a high rotation. This is effected by the monostable multivibrator circuit 30.  
  When the transistor 237 is in the on state, the transistor 258 is also in the on state, while when the transistor 238 is on state, the transistor 258 is off state.  
  The on period of the transistor 238 is determined by the discharge period of the capacitor 236. Consequently, it is sufficient to determine the resistance and the capacity such that the charge on the capacitor 236 at a high rotation is reduced, and it is increased at a low rotation.  
  This is done by increasing the current at the resistor 232 and by reducing the current flowing across the resistor 233 into the capacitor 236. That is, the time constant of the resistor 233 and the capacitor 236 is made sufficiently large. As a result, the capacitor 236 is not saturated by the pulses of the pulse interval applied to the terminal. Consequently when the pulse interval is large, the charge stored in the capacitor 236 is increased, while when the pulse interval is small the charge stored in the capacitor 236 is reduced. Thus, the time period of the on state of the transistor 238 is automatically varied depending on the pulse interval of the pulses applied to the terminal 222.  
  The amplifying transistor 256 of the power switching circuit 50 may be of the inverter type. However, in the case of the inverter type, either one of the transistors 256 and 258 is necessarily in the on state, i.e., conductive to produce heat. Consequently, the type shown in FIG. 2 is preferable. A darlington type of amplifying circuit is also very preferable similarly to the type of FIG. 2.  
  When the transistor 258 is turned on, a very high cur rent, for example a current of 3A to 4A flows across the resistor 278 and the ignition coil 270. Consequently, the heat generated is very large. Also, the collector current of the transistor 256 which supplies the base current to the transistor 258 is high. Further, also the heat generated by the resistor 252 cannot be neglected.  
  Consequently, it is desirable that the transistors 256 and 258 are simultaneously turned off when the engine is stopped.  
  When the engine is stopped, the resistance of the ignition coil 270 due to the reactance component becomes zero, so that a current flowing across the ignition coil 270 sometimes reaches as high as ground 6A. Consequently, the heat generation by the ignition coil 270 and the power switching circuit 50 furte increases. During the operation period of the engine, the current is reduced compared with that when the engine is stopped by the resistance of the ignition coil due to its reactance because the on and off states of the transistor 258 alternate.  
 What we claim is:  
  1. An ignition device for an internal combustion engine, comprising:  
 input means to which pulses indicative of the ignition timing of an internal combustion engine are applied; first switching means being switched off at an ignition time in response to the output of said input means;  
 an ignition coil comprising the primary coil and the secondary coil, said primary coil being connected in series with said first switching means between terminals for supplying a DC voltage, said secondary coil being connected with a spark plug;  
 time constant means comprising a charge-discharge circuit consisting of a capacitor and a resistor, said charge-discharge circuit having a time constant sufficiently longer than the pulse interval of said pulses, said time constant means producing an output the value of which varies with the lapse of time from the time at which an output of said input means is applied thereto and which reaches a predetermined value after the lapse of time determined by the said time constant; and  
 second switching means for turning said first switching means off when the value of the output of said time constant means exceeds a predetermined level, said second switching means comprising a transistor, the emitter and collector of said transistor being connected to one terminal of said DC source and the input terminal of said first switching means, respectively, and the base of said transistor being connected to the output of said time constant means; said resistor and capacitor of said time constant means being connected in series between the base of said second switching transistor and said DC source, and ajunction point of said resistor and capacitor being electrically connected with said input means through a first diode, the input terminal of said first switching means being electrically connected with the output of said input means through a second diode.  
  2. An ignition device according to claim 1, in which said input means comprises at least one switching transistor, the collector of which is connected to said time constant means and said first switching means through said first diode and said second diode, respectively, and when said pulse is applied to said input means, said switching transistor of said input means discharges said capacitor of said time constant means through said first diode.  
  3. An ignition device according to claim 1, in which said input means comprises a transistor monostable multivibrator, the collector of the transistor of said monostable multivibrator, which is normally in the off state and turned on by the application of an input pulse, is electrically connected with the inputs of said time constant means and said first switching means through said first diode and said second diode, respectively, and said capacitor of said time constant means is discharged through said first diode when an input pulse is applied to said input means.