Ignition system for internal combustion engines

Abstract

An ignition system of capacitor discharge type for internal combustion engines having a DC source, an ignition transformer, a thyristor, a capacitor, a transistor and an oscillator. With all the elements in the system, the capacitor is discharged through the thyristor, transistor and primary winding of transformer when the transistor and thyristor are simultaneously rendered conductive in response to the appearance of a pulse from the oscillator. The capacitor is then charged by a counter electromotive force produced in the primary winding of the transformer in response to subsequent cut-off of the transistor.

Description

This invention relates to ignition systems for internal combustion engines and more particularly to an ignition system of the kind utilizing discharge of a capacitor for the ignition of fuel.

In conventional ignition systems for internal combustion engines employing an ignition coil, a DC source is connected to the primary winding of the ignition coil through an interrupter contact for causing flow of current through the primary winding of the ignition coil while the interrupter contact is in the closed position, and this current flow is interrupted by opening the interrupter contact in synchronism with the ignition timing of the internal combustion engines so that a high voltage induced in the secondary winding of the ignition coil as a result of the opening of the interrupter contact can be utilized for causing spark discharge in an ignition plug connected to the secondary winding of the transformer thereby igniting fuel. However, the conventional ignition system of the type above described has been defective in that the duration of the spark discharge is relatively short and the conditions of ignition tend to be adversely affected by variations of the load on the engine due to the fact that the quantity of energy supplied for the ignition is relatively small. The conventional ignition system has further been defective in that, when the electrode portion of the ignition plug is excessively fouled, a spark is difficult to occur and this results in failure of proper ignition or incomplete combustion of fuel which gives rise to undesirable contamination of atmospheric air.

An ignition system of the type utilizing discharge of a capacitor has been developed recently. This ignition system is advantageous in that the ignition is substantially free from the contamination of the electrode portion of the ignition plug due to the fact that the secondary voltage rises to a predetermined level relatively quickly. However, this ignition system is disadvantageous in that the duration of spark discharge and the energy supplied by the spark discharge have a certain limit. Another disadvantage of this ignition system resides in the fact that complex and expensive means such as an osicllator and a converter are required and the overall size and weight of the ignition system are inevitably increased.

With a view to eliminate such prior art disadvantages, it is an object of the present invention to provide a novel and improved ignition system of the capacitor discharge type for internal combustion engines which can reliably generate sparks without being substantially adversely affected by contamination of the ignition plug or load conditions of the engine.

Another object of the present invention is to provide an ignition system of the type above described in which the spark discharge lasts for an extended period of time and the energy supplied for the ignition can be increased for attaining reliable ignition of fuel.

A further object of the present invention is to provide an ignition system of the type above described which is relatively small in size, light in weight and compact in construction.

Other objects, features and advantage of the present invention will be apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of an embodiment of the present invention with a part shown by a block; and

FIG. 2 is a circuit diagram of another embodiment of the present invention with parts shown similarly by blocks.

Referring to FIG. 1 showing a preferred embodiment of the present invention, a DC source V.sub.B of, for example, 12 volts is connected to a resistor r.sub.1 through a main switch MS. This resistor r.sub.1 is grounded through an interrupter contact K which is opened and closed in synchronism with the rotation of an intenal combustion engine. The main switch MS is connected through a diode D.sub.1 to one end of a primary winding N.sub.1 of an ignition transformer TR and to one end of a secondary winding N.sub.2 of the transformer TR.

The primary and secondary windings N.sub.1 and N.sub.2 of the ignition transformer TR are relatively closely coupled to each other as in, for example, an ignition coil for an automotive vehicle. The other end of the primary winding N.sub.1 is grounded through the collector-emitter path of a transistor Q.sub.1. The other end of the secondary winding N.sub.2 is grounded through an ignition plug P as is customary in the art. The connection point or node 10 between the resistor r.sub.1 and the interrupter contact K is connected to an input terminal of an oscillator OSC which has two output terminals. The first output terminal of the oscillator OSC is connected to the base of the transistor Q.sub.1, and the second output terminal of the oscillator OSC is connected through a capacitor C.sub.2 to one end of a resistor r.sub.2 and to one of the electrodes of a diode D.sub.2. The other end of the resistor r.sub.2 is connected to the main switch MS.

A thyristor Q.sub.2 and a capacitor C.sub.1 are connected in series between ground and the node 12 between the diode D.sub.1 and the ignition transformer TR, and the thyristor Q.sub.2 is disposed in a direction opposite to the forward direction of the diode D.sub.1. The gate of the thyristor Q.sub.2 is connected to the other electrode of the diode D.sub.2. Another diode D.sub.3 is connected between the node 14 between the thyristor Q.sub.2 and the capacitor C.sub.1 and the node 16 between the primary winding N.sub.1 of the ignition transformer TR and the collector of the transistor Q.sub.1. Obviously, the forward directions of these diodes and thyristor should be determined to suit the operation of the system described below. The oscillator OSC may be of any suitable type provided that it can generate a pulse having a predetermined pulse width T.sub.1 every ignition timing, i.e., each time the contact K is urged to the open position.

The ignition system of the present invention having a structure as shown in FIG. 1 operates in a manner as described below. For convenience of description, it is assumed that the main switch MS is turned on when the interrupter contact K is in the closed position. The voltage drop across the contact K is negligible, and therefore, the oscillator OSC is in the deenergized state and the transistor Q.sub.1 remains non-conductive. Current is supplied from the DC source V.sub.B to the capacitor C.sub.1 through the route including the main switch MS, diode D.sub.1, primary winding N.sub.1 of transformer TR, diode D.sub.3 and capacitor C.sub.1, with the result that the capacitor C.sub.1 is charged up to substantially the power supply voltage level in the illustrated polarity.

Although the power supply voltage is applied at the same time from the DC source V.sub.B to the gate of the thyristor Q.sub.2 through the resistor r.sub.2 and diode D.sub.2, this voltage is inactive against the thyristor Q.sub.2 due to the fact that the potential difference across the thyristor Q.sub.2 is opposite to that in the normal direction. Further, the capacitor C.sub.2 is charged in the illustrated polarity by the current which is supplied from the DC source V.sub.B through the route including the resistor r.sub.2, capacitor C.sub.2 and oscillator OSC.

The engine starts to rotate in the state in which the capacitors C.sub.1 and C.sub.2 are charged in the manner above described. When the interrupter contact K is urged to the open position in synchronism with the rotation of the engine, a pulse having a predetermined pulse width T.sub.1 as above described appears at the first output terminal of the oscillator OSC and the transistor Q.sub.1 conducts in response to the application of the oscillator output to the base thereof. Thus, current flows from the DC source V.sub.B through the route including the diode D.sub.1, primary winding N.sub.1 of transformer TR and transistor Q.sub.1 for energizing the transformer TR. The output pulse appearing at the second output terminal of the oscillator OSC is applied at the same time to the capacitor C.sub.2 so that a control signal voltage is applied to the gate of the thyristor Q.sub.2 through the diode D.sub.2. However, the thyristor Q.sub.2 remains still non-conductive due to the fact that the potential difference there across is opposite to that in the normal direction.

Upon terminaion of the generation of the first pulse from the oscillator OSC, the transistor Q.sub.1 is rendered non-conductive thereby interrupting the flow of current through the primary winding N.sub.1 of the transformer TR. Thus, a high voltage is induced in the secondary winding N.sub.2 of the transformer TR and spark discharge occurs in the ignition plug P as in the conventional ignition system. At the same time, counter electromotive force is produced in the primary winding N.sub.1 due to a leakage inductance and current flows through the diode D.sub.3 to charge the capacitor C.sub.1 up to a level of, for example, 200 volts, thereby inverting the direction of the potential difference across the thyristor Q.sub.2. Further, the capacitor C.sub.2 is charged again in the illustrated direction.

Two points should be noted in this connection. In the first place, although a relatively high voltage of, for example, 15 kilovolts is required in order to initiate the spark discharge at the ignition plug P, the voltage required for maintaining the spark discharge after initiation of such discharge may be relatively low or of the order of, for example, 2 kilovolts. Secondly, in the arrangement in which the primary winding N.sub.1 is short-circuited unidirectionally and the DC source V.sub.B is connected there-across, the spark discharge is maintained until the DC source V.sub.B is electrically disconnected from the circuit once the spark discharge takes place. It is considered that the spark discharge can be maintained for the following reasons:

Minute oscillatory variations (of such an extent which will not cause inversion of the polarity) in the magnitude of the spark discharge current or current flowing through the secondary winding N.sub.2 are reflected in the primary current flowing through the primary winding N.sub.1 due to corresponding variations in the magnetic flux in the iron core, and such primary current coacts with the number of magnetic flux lines in the iron core again to induce a secondary voltage against thereby providing the voltage required for maintaining the spark discharge. As a result, the current supplied from the DC source V.sub.B can effectively maintain the spark discharge at the ignition plug P. Further, in such an arrangement, the input impedance of the transformer TR when locked from the side of the DC source V.sub.B is remarkably reduced upon initiation of the spark discharge at the ignition plug P resulting in an increase in the energy supplied from the DC source V.sub.B through the transformer TR to be released at the ignition plug P. Because of the fact that the discharge route for the capacitor C.sub.1 includes the primary winding N.sub.1 of the transformer TR, these elements constitute an LC oscillation circuit. However, a portion of the oscillatory electromotive force produced in the primary winding N.sub.1 is absorbed by the circuit including the thyristor Q.sub.2 and diode D.sub.3, and the capacitor C.sub.1 would not be charged in the reverse direction after the discharge of the previously stored charge. Therefore, no natural oscillation is produced by this LC combination.

With the above description in mind, the operation of the system of the present invention at the second ignition timing will now be described. At this second ignition timing, the ignition system behaves in a manner somewhat different from the behavior in the preceding operation.

This is due to the fact that the capacitor C.sub.1 has been charged up to a voltage level considerably higher than the power supply voltage V.sub.B by the charging action carried out in the last stage of the preceding operation. In response to the opening of the interrupter contact K, a second pulse appears from the oscillator OSC, and the transistor Q.sub.1 and thyristor Q.sub.2 are rendered conductive substantially simultaneously in response to the appearance of the pulse from the oscillator OSC.

As a result, the charge stored in the capacitor C.sub.1 is discharged through the primary winding N.sub.1 of the transformer TR. A steeply rising high voltage is thereby applied to the ignition plug P for initiating the spark discharge at the ignition plug P. The current from the DC source V.sub.B is additionally supplied to the ignition plug P in the manner above described so that the spark discharge can be maintained over a period of time corresponding to the duration T.sub.1 of the pulse. Further, a large quantity of energy is supplied for igniting fuel as above described. Upon completion of the discharge of capacitor C.sub.1, thyristor Q.sub.2 is rendered nonconductive. Therefore, upon disappearance of this pulse, the transistor Q.sub.1 is rendered non-conductive to interrupt the flow of current through the primary winding N.sub.1 of the transformer TR. As a result, a portion of the magnetic energy stored in the iron core of the transformer TR is subsequently released at the ignition plug P as ignition energy to cause spark discharge in a reverse direction for a short period of time. (This corresponds to the spark discharge in the conventional system described in the preface of this specification.). The remaining portion of the magnetic energy produces a counter electromotive force in the primary winding N.sub.1 and this counter electromotive force is supplied to the capacitor C.sub.1, as described previously, to be held in the capacitor C.sub.1 until the next ignition takes place.

It will be understood from the above description that the present invention provides such an advantage that the ignition can be attained by a large quantity of energy supplied in the form of electrical power from the DC source V.sub.B and maintained over a relatively long period of time corresponding to the pulse width T.sub.1 of the pulse output of the oscillator OSC, in addition to the advantage pertinent to the initiation of spark discharge utilizing the capacitor discharge.

The preferred embodiment of the present invention above described has been so arranged that the oscillator OSC generates a pulse having a pulse width T.sub.1 each time the interrupter contact K is urged to the open position. In a modification of this embodiment, the oscillator OSC may generate two or more pulses T.sub.1 having a short time interval T.sub.2 therebetween.

According to this modification, the transistor Q.sub.1 is turned on and off twice or more every ignition timing and the capacitor C.sub.1 is charged and discharged each time the transistor Q.sub.1 is turned on and off. Further, discharge in one direction due to conduction of the transistor Q.sub.1 and discharge in the opposite direction due to non-conduction of the transistor Q.sub.1 occur repeatedly at the ignition plug P as described previously.

The spark discharge can be substantially continuously carried out when the time interval T.sub.2 between the pulses is relatively short and the next supply of the primary current is started during the period of time in which the spark discharge is sustained by the release of the magnetic energy stored in the iron core after the transistor Q.sub.1 is rendered non-conductive. It is thus not necessarily required to cause discharge of the capacitor C.sub.1 in response to each of these pulses T.sub.1. In such a case, the thyristor Q.sub.2 is rendered conductive to cause discharge of the capacitor C.sub.1 in response to the first pulse T.sub.1 supplied from the oscillator OSC by suitably selecting the resistance value of the charging resistor r.sub.2 for the capacitor C.sub.2 so that the capacitor C.sub.2 can be insufficiently charged during the period of time T.sub.2 between the pulses. Therefore, the charge in the capacitor C.sub.1 is accumulated each time the primary current is interrupted.

These modifications may be suitably employed depending on the structure of the engine combustion chamber, manner of supplying fuel and other conditions for attaining the desired object.

In FIG. 2 showing another preferred embodiment of the present invention, like reference numerals are used to denote like parts appearing in FIG. 1, and therefore, any detailed description as to such elements is unnecessary. In the following description therefore, those elements which are newly added or replace the corresponding elements in FIG. 1 will be especially described.

Referring to FIG. 2, the node 10 between resistor r.sub.1 and an interrupter contact K is connected to an input terminal of a trigger pulse generator TRIG which has two output terminals. One of the output terminals of the trigger pulse generator TRIG is connected to an input terminal of a first oscillator OSC No. 1, while the other output terminal of the trigger pulse generator TRIG is connected to an input terminal of a second oscillator OSC No. 2. In response to the application of a trigger pulse from the trigger pulse generator TRIG to the first oscillator OSC No. 1, the oscillator OSC No. 1 generates a pulse T.sub.1 having a relatively small pulse width of, for example, 1 ms and this pulse is applied to the base of a transistor Q.sub.1 from the first output terminal of the oscillator OSC No. 1. In response to the application of the trigger pulse from the trigger pulse generator TRIG to the second oscillator OSC No. 2, the oscillator OSC No. 2 generates a pulse T.sub.3 having a relatively large pulse width of, for example, 2 ms. A main switch MS is connected through a diode D.sub.1 to one end of primary and secondary windings N.sub.1 and N.sub.2 of an ignition coil IGC and to the cathode of a thyristor Q.sub.2. Further, the main switch MS is grounded through a primary winding N.sub.1 ' of a transformer TR' and a transistor Q.sub.3 which are connected in series.

The base of this transistor Q.sub.3 is connected to an output terminal of the second oscillator OSC No. 2. A secondary winding N.sub.2 ' of the transformer TR' is grounded directly at one end thereof and is connected at the other end thereof to the node 14 between the thyristor Q.sub.2 and a capacitor C.sub.1 through a diode D.sub.4. A capacitor C.sub.2 is connected at one of the electrodes thereof the gate of the thyristor Q.sub.2 through a diode D.sub.2 and at the other electrode thereof to the second output terminal of the first oscillator OSC No. 1 to receive the pulse T.sub.1 therefrom. A protective resistor r.sub.3 may be disposed in parallel with the transistor Q.sub.1.

In the operation of the second embodiment of the present invention shown in FIG. 2 which will be now described, it is needless to waste words for the operation of those parts which have been described already with reference to FIG. 1. In response to the turn-on of the main switch MS, the capacitor C.sub.2 is charged in the illustrated polarity through the charging resistor r.sub.2. At the same time, the capacitor C.sub.1 is charged up to substantially the power supply voltage V.sub.B through the diode D.sub.1, primary winding N.sub.1 of ignition coil IGC and diode D.sub.3. The remaining parts of the circuit are still not in operation.

When the interrupter contact K is urged to the open position in such a situation in synchronism with the rotation of the engine, the trigger signal is applied from the trigger pulse generator TRIG to the first and second oscillators OSC No. 1 and OSC No. 2. In response to the application of the trigger pulse, the pulses of predetermined duration, that is, the pulse T.sub.2 having a relatively small pulse width and the pulse T.sub.3 having a relatively large pulse width appear at the output terminals of the first and second oscillators OSC No. 1 and OSC No. 2 respectively so that the transistor Q.sub.1, thyristor Q.sub.2 and transistor Q.sub.3 are rendered conductive almost simultaneously. As a result, energizing current flows through the primary winding N.sub.1 of the ignition coil IGC and through the primary winding N.sub.1 ' of the transformer TR' and magnetic energy is accumulated in the iron core of the ignition coil IGC and transformer TR'. However, no current flows through the thyristor Q.sub.2.

The transistor Q.sub.1 is cut off before the transistor Q.sub.3 is cut off due to the fact that the pulse width of the pulse T.sub.1 is smaller than that of the pulse T.sub.3. The cut off of the transistor Q.sub.1 results in interruption of the flow of the primary current in the ignition coil IGC and a portion of the magnetic energy accumulated in the iron core of the ignition coil IGC is released at an ignition plug P as spark producing energy. The remaining portion of the magnetic energy provides a counter electromotive force in the primary winding N.sub.1 of the ignition coil IGC and the capacitor C.sub.1 is charged thereby through the diode D.sub.3. Then, when the transistor Q.sub.3 is rendered non-conductive, the magnetic energy accumulated in the iron core of the transformer TR' connected thereto induces a voltage in the secondary winding N.sub.2 ' of the transformer TR' and this voltage is applied from the secondary winding N.sub.2 ' to the capacitor C.sub.1 through the diode D.sub.4 for charging the capacitor C.sub.1. The charge supplied to the capacitor C.sub.1 from the primary winding N.sub.1 of the ignition coil IGC is subject to slight variations depending on the condition of release of the energy at the ignition plug P. However, the charge supplied from the secondary winding N.sub.2 ' of the transformer TR' compensates for such variations so that a voltage sufficient for initiating spark discharge at the ignition plug P in the next ignition timing is built up in the capacitor C.sub.1.

When the interrupter contact K is opened at the next ignition timing and the trigger signal is applied to the first and second oscillators OSC No. 1 and OSC No. 2, the pulses T.sub.1 and T.sub.3 are applied to the respective transistors Q.sub.1 and Q.sub.3 to render these transistors conductive.

The gate control signal is applied to the gate of the thyristor Q.sub.2 almost simultaneously with the result that the charge stored in the capacitor C.sub.1 is discharged by way of the discharge route including the thyristor Q.sub.2, primary winding N.sub.1 of ignition coil IGC and transistor Q.sub.1. An abruptly rising high voltage is thereby induced in the secondary winding N.sub.2 of the ignition coil IGC to initiate spark discharge at the ignition plug P. In this case, the natural oscillation that may occur in the LC discharge route is absorbed by the circuit including the diode D.sub.3 and thyristor Q.sub.2. Thus, spark discharge with large energy is continued at the ignition plug P by the electrical power supplied from the DC source V.sub.B during the period of time in which the transistor Q.sub.1 remains conductive. Upon completion of the discharge of the capacitor C.sub.1, the thyristor Q.sub.2 is rendered nonconductive. Thereafter, the capacitor C.sub.1 is re-charged upon disappearance of the pulses T.sub.1 and T.sub.3 to prepare for the next ignition.

The second embodiment of the present invention has been described with reference to the arrangement in which one pulse T.sub.1 of predetermined duration and one pulse T.sub.3 of predetermined duration apppear from the first and second oscillators OSC No. 1 and OSC No. 2 respectively every ignition timing. In a modification of such embodiment, the first oscillator OSC No. 1 may generate two or more pulses T.sub.1 having a time interval T.sub.2 therebetween in response to the application of one trigger pulse input from the trigger pulse generator TRIG, and the second oscillator OSC No. 2 may generate a pulse T.sub.3 whose duration is larger than the total sum of these pulses T.sub.1 and time intervals T.sub.2.

Further, the relation between the time interval T.sub.2 and the pulse width of the pulse T.sub.1 in such a case may be such as is considered in the modifications of the first embodiment. Further, although the transformer TR' in FIG. 2 is shown having a secondary winding, it may have merely a primary winding wound around an iron core so that a counter electromotive force produced in the primary winding may be derived as magnetic energy for charging the capacitor C.sub.1 as shown in FIG. 1.

The embodiments of the present invention above described have been illustrated as the type provided with an interrupter contact K which is mechanically opened and closed in synchronism with the rotation of the engine. However, the system having such mechanical on-off means is merely illustrative of one form of the present invention and those skilled in the art can easily make modifications in which electromagnetic, optical or any other suitable means are employed in lieu of such mechanical on-off means for controlling the oscillators or trigger signal generator. It should be understood therefore that such modifications are also included in the scope of the present invention in addition to the preferred embodiments above described.

Claims

What is claimed is:

1. An ignition system for internal combustion engines comprising an ignition transformer means having a primary and a secondary winding, and said primary winding being grounded at one end thereof through a transistor and connected at the other end thereof to a DC source through a diode, said the other end of said primary winding being further grounded through a thyristor and a capacitor connected in series, and said secondary winding being connected at one end thereof to an ignition plug, an oscillator means for generating at least one pulse having a predetermined pulse width every predetermined ignition timing of the engine in synchronism with the rotation of the engine, means for coupling the output pulse of said oscillator means to the base of said transistor, means for coupling the output pulse of said oscillator means to the gate of said thyristor, said thyristor, said transistor and said primary winding of said transformer means providing a discharge circuit for said capacitor when both said transistor and said thyristor are rendered conductive substantially simultaneously in response to the application of the output pulse from said oscillator means, and means for charging said capacitor by a counter electromotive force produced in said primary winding of said transformer means when said transistor in the conducting state is rendered non-conductive.

2. An ignition system for internal combustion engines as claimed in claim 1, wherein said oscillator means generates at said predetermined ignition timing at least two pulses having a predetermined pulse width with a relatively short time interval therebetween.

3. An ignition system for internal combustion engines as claimed in claim 2, wherein only one of the output pulses of said oscillator means is coupled to the gate of said thyristor at said predetermined ignition timing.

4. An ignition system for internal combustion engines as claimed in claim 1, further comprising a second oscillator means and a second transformer means having a primary and a secondary winding, said primary winding of said second transformer means being grounded at one end thereof through a second transistor and connected at the other end thereof to said DC source, while said secondary winding of said second transformer means being connected at one end thereof to said capacitor through another diode and grounded directly at the other end thereof, and said second oscillator means being operative in such a manner that an output pulse, which appears substantially simultaneously with the appearance of the output pulse from said first oscillator means and disappears after the disappearance of the output pulse from said first oscillator means, is generated from said second oscillator means to be applied to said second transistor for maintaining said second transistor conductive during such period of time.

5. An ignition system for internal combustion engines as claimed in claim 2, further comprising a second oscillator means and a second transformer means having a primary and a secondary winding, said primary winding of said second transformer means being grounded at one end thereof through a second transistor and connected at the other end thereof to said DC source, while said secondary winding of said second transformer means being connected at one end thereof to said capacitor through another diode and grounded directly at the other end thereof, and said second oscillator means being operative in such a manner that an output pulse, which appears substantially simultaneously with the appearance of the first output pulse from said first oscillator means and disappears after the disappearance of the second output pulse from said first oscillator means, is generated from said second oscillator means to be applied to said second transistor for maintaining said second transistor conductive during such period of time.