METHOD AND ARRANGEMENT FOR GENERATING IGNITION SPARKS IN AN INTERNAL COMBUSTION ENGINE

Abstract: Method and arrangement for a capacitive ignition system in an internal combustion engine. In this, the discharging and charging of an ignition capacitor (24) is controlled by a control unit (40) which actuates a first circuit-breaking element (14) in a discharging circuit and a second circuit-breaking element (30) in a charging circuit. The ignition spark which is obtained in an ignition circuit on discharging of the ignition capacitor via the primary winding of an ignition coil is certainly powerful, but of short duration. Under certain operating conditions it is difficult for such a spark to ignite reliably in particular a lean fuel/air mixture. For the purpose of prolonging the said ignition spark, the present invention is chiefly characterized in that the control unit (40), at a time following the ordinary ignition time, actuates the second circuit-breaking element (30) in such a way that this is kept conductive simultaneously with the first circuit-breaking element (14). Current is then supplied from an electrical energy source (33) and via the primary winding (10). Thereafter, the control unit (40) actuates either of the circuit-breaking elements in such a way that the current supply via the primary winding (10) is interrupted, by which means a renewed ignition voltage generation is obtained which prolongs the ordinary ignition spark.

Description: Method and arrangement for generating ignition sparks in an internal combustion engine The present invention relates to a method for gene¬ rating ignition sparks in an internal combustion engine pro¬ vided with an ignition system in which there is included at least one spark plug which obtains ignition voltage from the secondary winding of an ignition coil, which ignition system comprises at least one ignition capacitor, which cooperates with at least one discharging circuit and one charging cir¬ cuit, which discharging circuit comprises in series the pri¬ mary winding of the ignition coil and a first circuit-break¬ ing element switchable from a control unit, in which connec¬ tion, at a first time, the control unit emits a first output signal to the first circuit-breaking element, which triggers the dis-charging of the ignition capacitor via the dis¬ charging circuit, in which conπec t ion there is produced, in the secondary winding, a first ignition voltage generation correspondiπg to this. Such methods are previously generally known and applied to capacitive ignition systems in internal combustion engines. The ignition spark which is obtained in a capacitive ignition system is relatively powerful and of short duration. Under certain operating conditions, such as idle running, it is difficult for such a spark to ignite reliably the fuel/ air mixture in the engine cylinders, particularly if the mix- ture is relatively lean. For this reason, various solutions have previously been proposed for prolonging the duration of the ignition spark in a capacitive ignition system. Thus far, US 3,906,919 describes a solution in which the primary wind¬ ing of the ignition coil is divided into two separate wind- ings where the one winding is included in the discharging circuit of an ignition capacitor while the second winding obtains oscillating current from an oscillator. The duration of the ignition spark is thereby controlled by the time duriπg which the oscillating current passes through the said separate part of the primary winding. A similar solution with a divided primary inding is also described in US 3,972,315. The ignition spark is pro-' duced by a combination of inductive and capacitive ignition voltage generation. In US 4,258,296, use is made of a circuit-breaker between the primary winding of an ignition coil and an ignition capacitor in order to permit simultaneous ignition voltage generation in the ignition coil both inductively and capac i t el . However, all the said known solutions comprise a large number of components, inter alia for creating an oscillating current and for charging the capacitor and current supply via the primary winding. Furthermore, the solutions are suited for relatively large ignition coils primarily in¬ tended for inductive ignition in which connection a single ignition coil is used for all the spark plugs of the engine, at the same time as the current to these is distributed by means of a conventional ignition distributor. The present "invent ion is primarily applicable to a microprocessor-controlled capacitive ignition system for multiple cylinder engines for vehicle operation. In addition, the invention is used with advantage in an ignition system of this type which does not have mechanical ignition distri¬ bution and in which an ignition coil of relatively small size is used for each spark plug. For the purpose of achieving prolongation of the duration of the ignition spark in a s.i - ple manner in ignition systems of this type, the present in¬ vention is characterized in that, at a second time which occurs later than the first time, the control unit emits a second output signal to a second c rcuit-brea ing element located in the charging circuit, which element is connected in series to the discharging circuit, by which means the first and second circuit-brea ing elements are simultaneously kept conductive for current supply from an electrical energy source via the primary winding and the said circuit-breaking elements, and in that, at a third time, which occurs later than the second, the control unit emits a third output signal to the first and/or second circuit-breaking element for a non- conducti e state of this or these, in which connection the current supply via the primary winding is interrupted and a second i gn i t ion voltage generation takes place in the secondary winding. By means of the invention, a microprocessor-based con¬ trol unit can be simply programmed for controlling the two ci cuit-breaking elements in such a way that the method according to the invention is achieved. It is also possible to allow the microprocessor to adapt the control of the cir¬ cui -breaking elements to the operating condition of the engine, b . which means the duration of the ignition spark can be changed in dependence on changes in the operating condi¬ ion. The invention also relates to an arrangement for carrying out the method according to the invention in the ignition system of an internal combustion engine. In the ignition system, an ignition capacitor is thereby electri¬ cally connected to, on the one hand, at least one discharging circuit comprising the primary winding of the ignition coil connected in series to a first circuit-breaking element and, on the other hand, a charging circuit which comprises a second circuit-breaking element, a coil and at least one d iode. The arrangement according to the invention is characterized in that the first and second circuit-breaking elements and the primary winding are connected in series to each other in a circuit which connects a di ect-current source to earth, via which circuit direct current flows when both the first and the second circuit-breaking elements -are conductive for passage of current, and in that the ign tion capacitor is electrically connected to the first circuit- breaking element and the primary winding so that, when the first circuit-breaking element is conducting, the ignition capacitor discharges via the primary winding, which circuit- breaking elements are electrically connected to an electronic control unit which, in dependence on input signals represent¬ ing the operating condition of the engine, emits output signals to the said circuit-breaking elements for a conductive or noncoπductive state of the same. The arrangement according to the invention provides a particularly simple and inexpensive solution for producing a prolonged ignition soark. Other features characterizing the invention emerge from the attached patent claims and the following description of an exemplary embodiment of the invention. In the des¬ cription, reference is made to the figures in which Figure 1 shows an arrangement according to the invention in an ignition system, Figure 2 shows schematically the primary current during implementation of a method according to the inven¬ tion in the said arrangement, Figure 3 shows schematically the appearance of the primary voltage under the said method and Figure 4 shows schematically the corresponding secondary voltage with the method according to the invention. Figure 1 shows the parts of an ignition system which are essential for describing a method according to the inven- tioπ. , A number of spark plugs 1-4 are connected each one to its secondary winding 5-8 in a corresponding number of ignition coils. The primary windings 10-13 of the ig¬ nition coils are each connected in series to their own cir¬ cuit-breaking element 14-17, here designed as a triac. Each primary winding and triac is included in a discharging cir¬ cuit 20-23 which is connected in parallel to an ignition capacitor 24 via line 25. Similarly connected in parallel to the ignition capacitor 24 is a choke coil 26 connected in series to a diode 27 via line 28. The line 25 with the ignition capacitor 24 and all the lines 20-23, 28 connected in parallel thereto are connected, on one side, to a second circu t-brea ing element 30, for example a transistor, con¬ nected in series to a second diode 31 via line 32 and, on the other side, to a direct-current source 33, preferably a 12 V battery. The diodes 27, 31 are directed in such a way that, when the transistor 30 allows current to pass through, current can be fed from the battery 33 via the lines 28 and 32 to earth. The triacs 14-17 and the transistor 30 are controlled by a control unit 40 between a conductive state when current is allowed to flow in the circuit in question and a nonconduc- tive state when the circuit is nonconducting and current can¬ not be fed through it. The control unit 40 is preferably built around a microprocessor. The control unit 40 is supplied with input signals on the lines 41-43 in respect of the engine speed, load, temperature, fuel/air ratio etc. The engine speed is obtained from a crankshaft sensor 44 the out¬ put signal of which also provides information on the angle position of the crankshaft before the ignition in the res¬ pective cylinder. Depending on other input signals, an ini¬ tial value in respect of the ignition position is corrected so as to assume a value adapted to each operating condition of the engine. The correction values are determined by the control unit 40 by means of reading tables or the like stored in a memory unit. At the ignition time fixed by the control unit 40 for a particular cylinder, for example containing spark plug 1, an output signal is emitted to the triac 14 which then closes the discharging c rcuit 20, in which coπ- πectioπ the ignition capacitor 24 is discharged via the p i¬ mary inding 10. In Figures 2, 3 and 4 the ignition time is indicated by TO. The discharging causes a rapidly increasing flow of current according to Figure 2 via the primary winding 10, at the same time as the voltage of the ignition capacitor 20 falls in a corresponding manner according to Figure 3 from an initial level of about 400 V. At the time T1, the pri¬ mary voltage is about 0 V and at the same time the primary current has its highest positive value. It can be seen from Figures 2 and 3 that, when voltage reaches its greatest nega¬ tive value, the current passes through zero level. When the voltage again reaches zero level at a, time T2, the primary current has its greatest negative value. The rapid increase in the current flow via the primary winding as a result of the discharging of the ignition capa¬ citor results in known manner in a first voltage pulse in the secondary wind ng, in this case negative, as represented in Figure 4. The said voltage pulse has a first powerful and extremely transient - a few microseconds long - voltage peak, also called voltage spike. This cari reach absolute values around 40 kV and is thus able, even under difficult operat¬ ing conditions, to generate a spark between the spark plug electrodes. The voltage spike then becomes a pulse section with a considerably lower and only slowly falling potential before the said section is finished by a rapid return to zero level which is reached at time T2, which can occur for example 10 to 20 microseconds after TO. Without the method according to the invention, from and including time T2 the primary current according to Figure 2, the primary voltage according to Figure 3 and the secondary voltage according to Figure 4 would have followed a deca ing oscillating curve represented in each figure by a dashed line. However, by means of the invention, the transistor 30 by the control unit 40 is made conduc i e at a ti e T3 which can occur at ti es T 1 or T2 or between these. It is therefore possible for current to be fed from the battery 33 via the already previously conducting triac 14, the primary winding 10, the diode 31 and the transistor 30 to earth. The current thus flows more easily through the pri¬ mary winding 10 than through the choke coil 26, since the formation of a current flow through the latter is made difficult by its high inductance. The latter is at least ten times higher than the inductance of the primary winding. The said current supply from the battery 33 via the primary winding means that, when the voltage across the capaci- tor 24 an-d the primary winding 10 according to the curve in Figure 3 again changes character at time T2, then the primary voltage assumes a positive value essentially similar to the value of the battery voltage. This low voltage results in a rapid fall in the pri ary current to a relatively low value corresponding to the primary voltage. The secondary voltage at. the same time assumes a low positive value and the said value is able to maintain the ignition spark during a period T2 to T4 which can be several times longer than the period T1 to T2. During the period T2- T4, the secondary voltage is in fact supported by an almost constant primary voltage and a primary current slowly in¬ creasing in absolute values depending on this. The ignition spark therefore burns without difficulty during the period T2 to T4 with the help of the electrical energy for which the said secondary voltage is an expression. However, in order to prolong the burning time of the ignition spark further, it is necessary for the secondary voltage to be retained at a higher level. This is achieved by the fact that, at T4, the transistor 30 receives a signal from the control unit 40 for interrupting the current through the latter. The interruption of the primary current results in a secondary voltage again being induced in the secondary winding 5, and this secondary voltage results in prolongation of the burning time of the ignition spark. Immediately after switching to a πonconduct ive state of the transistor 30 at T4, an additional signal is fed at time T5 to the transistor 30 in order to permit new current supply from the battery 33 via the primary winding 10 and the transis¬ tor 30 to earth. A new magnetic field is formed in the igni¬ tion coil 5, 10 and, at a predetermined time T6, the control unit 40 again -emits a signal to the transistor 30 which thereby interrupts the current. The interrupted current supply via the primary winding 10 again induces a secondary voltage in the sec¬ ondary winding 5 and this is able to maintain the ignition spark further with electrical energy. In this way the burning time of the ignition spark can be prolonged for an optional period of time by means of the closure and opening of the transistor 30 in dependence on the output signals from the control unit 40. For example, the burning time can be prolonged from the burning time of about 80-100 microseconds in the case of the conventional capacitive ignition spark to the burning times of up to about 2,000 microseconds which are possible in in¬ ductive ignition systems. It is of course also possible to wait for the first pulses of the primary voltage before the control unit emits a signal to the transistor at a time T3. However, this only means that the interruption of the primary current according to T2 in Figure 2 is shifted to a later time which corresponds to the zero transition of the primary voltage which follows immediately after time T3. T is is then followed by the same method as described above with reference to Figures 2, 3 and 4. However, T3 advantageously ' 5 occurs during one of the first ten primary voltage pulses or at least within the time during which a capacitive ignition spark reliably burns, i.e. generally within 80-100 micro seconds of the time TO. 10 By varying the burning time of the ignition spark, it is also possible to vary the electrical energy transmitted via the ignition spark within wide limits. This is advan¬ tageously carried out in dependence on the operating con¬ ditions of the engine in such a way that, for example during 15 operation with a lean fuel/air mixture which can be detected by means of a conventional oxygen meter in the exhaust gas system, the control unit controls the signals to the trans stor 30 so that a predetermined prolongation of the burning time 'of t-he ignition spark is effected. 2.0 " ' The exemplary embodiment described above in no way limits the invention but can be modified in a rfumber of embodiments within the scope of the subsequent claims. Thus, the importance of the inductance ratio between the primary 25 winding and the choke coil can be eliminated by providing the wire 28 with a circuit-breaking element which, in depen¬ dence on an output signal from the control unit 40, controls the passage of current through the choke coil 26.

Claims: C LA I M S 1. Method for generating ignition sparks in an inter¬ nal combustion engine provided with an ignition system in which there is included at least one spark plug (1), which obtains ignition voltage from the secondary winding (5) of an ignition coil, which ignition system comprises at least one ignition capacitor C24), which cooperates with at least one discharging circu t (20, 25) and one charging circuit (28, 32), which discharging circuit comprises in series the primary winding (10) of the ignition coil and a first cir- cu i t-brea iπg element (14) switchable from a control unit (40), in which connection, at a first time (TO), the control unit (40) emits a first output signal to the first circuit- breaking element (14), which triggers the discharging of the ignition capacitor (24) via the discharging circuit (20, 25), in which connection there is produced, in the secondary winding (5), a, first ignition voltage generation correspond¬ ing to this, ςharacter i zed in that, at a second time (T3), which occurs later than the first time, the control unit (40) emits a second output signal to a second circuit-brea ing element (30) located in the charging circuit, which element is connected in series to the discharging circuit, by which means the first and second ci cuit-breaking elements (14 and 30) are simul¬ taneously kept conductive for current supply from an electrical energy source (33) via the primary winding (10) and the said circuit-breaking elements, and in that, at a third time (T4), which occurs later than the second, the control unit (40) emits a third output signal to the first and/or second circuit-breaking element (14, 30) for a noπconduct ive state of this or these, in which connection the current supply via the primary winding (10) is interrupted and a second ignition voltage gene¬ ration takes place in the secondary winding (5). 2. Method according to Claim 1, characterized in that only the second circuit-breaking element (30) is set in a πonconduc t i ve state at the said thi ' rd time (T4). 3. Method according to Claim 2, characterized in that, at a fourth time (T5) which occurs immediately after the third time (T4), the control unit (40) emits a fourth output signal to the second circuit-breaking element (30) for setting the latter in a conductive state, in which connection current is again supplied from the electrical energy source (33) via the primary winding (10) and said circuit-breaking elements (30) to earth. 4. Method according to Claim 1, characterized in that the control unit (40) emits output signals to the second circuit-breaking element (30) at the second time (T3) only at predetermined values of input signals supplied to the control uπ i t (40 ) . 5. Method according to Claim 4, characterized in that the said output signal at the second time (T3) is only emitted if a fuel/air mixture supplied to the engine is leaner than a predetermined value. 6. Method according to Claim 1, characterized in that the said second time (T3) occurs at least during a period of time during which an ignition spark reliably burns in depen¬ dence on the discharging of the ignition capacitor (24) via the primary winding (10). 7. Arrangement for generating, in the manner indi¬ cated in Claim 1 inrthe ignition system of an internal com¬ bustion engine, ignition sparks at at least one spark plug (1) which obtains ignition voltage from the secondary winding (5) of an ignition coil, in which ignition system at least one ignition capacitor (24) is electrically connected to, on the one hand, at least one discharging circuit (20, 25) comprising the primary winding (10) of the ignition coil con¬ nected in series to a first circuit-breaking element (14) and, on the other hand, a charg ng circuit (28, 32) which comprises a second circuit-breaking element (30), a coil (26) and at least one diode (27), characterized in that the first and second c rcuit-breaking elements (14 and 30) and the primary winding (10) are connected in series to each other in a circuit (20, 32) w ch connects a direct- current source (33) to earth, via which circuit (20, 32) direct current flows when both the first and the second circuit-breaking elements (14 and 30) are open for passage of current, and in that the ignition capacitor (24) is electrically connected to the first circuit-breaking element (14) and the primary winding (10) so that, when the first circuit-breaking - element (14) is conductive, the ignition capacitor (24) discharges via tfr-e primary winding (10), which circuit- break ng elements (14, 30) are electr cally connected to an electrical control unit (40) which, in dependence on nput signals representing the operating condition of the engine, emits output signals to the said circuit- breaking elements (14, 30) for a conductive or πoncoπduc- tive state of the same. 8. Arrangement according to Claim 7, characterized in that the first circuit-breaking element (14) is a triac while the second ci cuit-breaking element (30) is a traπsis- tor. 9. Arrangement according to Claim 7, characterized in that the inductance of the primary winding is at least ten times lower than that of the coil (26).

Inventors: JOHANSSON HANS, NYTOMT JAN, JOHANSSON, HANS, NYTOMT, JAN
Applicants: SAAB SCANIA AB, SAAB-SCANIA AKTIEBOLAG
Application year: 1987
Main classification: F02P3
IPC codes: F02P3, F02P9, F02P15