The present invention relates to a method for controlling, in the ignition system of a multi-cylinder four-stroke internal combustion engine, the spark ignition at spark plugs in at least two cylinders, the pistons of which simultaneously assume a top dead centre position, in which method an ignition capacitor operates in conjunction with at least two discharging circuits and one charging circuit, the discharging circuits each comprising in series a primary winding of an ignition coil and a first circuit breaker element which can be switched over from an electric control unit, and the charging circuit cαtprising a coil in series with a second circuit breaker element which can be switched over frcm the control unit, the charging circuit being supplied with current from a direct-current source.

An engine as specified in the introduction can comprise, for example, a four-cylinder Otto-cycle engine where the electric control unit included in the ignition system replaces a conventional mechanically controlled ignition distributor. Lacking a cam shaft transmitter, it is therefore not possible to obtain information right from the start about which cylinder is the first one to be in firing position. To ensure that ignition voltage is generated in the cylinder which is in firing position, it is suggested to allow the control unit to trigger ignition at the same time in two or all cylinders as soon as the signal from a crank shaft transmitter indicates that the pistons in a pair of cylinders assume the top dead centre position. In this arrangement, the charge of the ignition capacitor is utilized for simultaneous ignition in two or more cylinders and, as a result, only half or less than half of the charge of the ignition capacitor is used for generating ignition sparks in a cylinder. if the voltage of the direct-current source drops, for example due to low environmental temperatures, special starting problems occur with such engines. Thus, a low supply voltage can result in the control unit not being able to control the charging

processes occurring during a starting process within specified periods of time.

Through German Patent Specification 2448 302, a method and an arrangement are already known for guaranteeing, during the start of an internal combustion engine, the charging of an ignition capacitor even with a decreasing battery voltage. But this does not deal with how the charging and discharging of the ignition capacitor will be arranged to obtain a more reliable ignition when starting an engine as specified in the introduction.

The present invention has the object of creating a method whereby a more reliable ignition of this type is obtained. In this object, the invention is characterized in that ^" when passing through one and the same crankshaft angle range close to the top dead centre position, the control unit emits a first signal to the circuit breaker element of one discharging circuit for triggering ignition in one of the abovementioned cylinders, a second signal to the second circuit breaker element for starting current supply to the charging circuit, a third signal to the second circuit breaker element for inte:rrupting current supply to the charging circuit, the ignition capacitor being charged with the energy stored in the coil, and a fourth signal to the circuit breaker element of the second discharging circuit for triggering ignition in the second one of the abovementioned cylinders, between which second and third signals there runs a first time period and between which first and fourth signals there runs a second time period which is longer than the said first time period.

By means of the invention the ignition voltage can thus be generated for the cylinder pair in such a manner that within a limited crankshaft angle range before the top dead centre position of the pistons, the ignition sparks in the cylinders 35 can occur at separate times. The complete charge of the ignition capacitor can thus be fully utilized at a first point of time for ignition in one of the cylinders. After being charged up again to full charge during the first period of time, the discharging of the ignition capacitor can then be fully utilized for ignition in the other cylinder at a second point of time. Compared with the solutions described above, much stronger ignition sparks are thus obtained for igniting the combustion air mixture which is present in at

least one of the cylinders of the pair of cyl inders . The consequence is a correspondingly increased possibility for a successful cold start of the engine.

In an advantageous embodiment, the voltage level of the

5 direct current source is detected and a signal corresponding to it is supplied to the control unit which, below a predetermined voltage level, controls the first time period in dependence on the said voltage level .

This makes it possible to adapt the time between discharges

10 to . the voltage level so that a longer time is allowed when a lower voltage level is available for charging the ignition capacitor than when the voltage level corresponds to a fully charged battery.

The present invention also provides an arrangement for carrying out the method according to the invention . This

15 arrangement comprises an ignition system for an Otto-cycle engine with at least two cylinders the pistons of which simultaneously assume a top dead centre position, this system compris ing an ignition element in each cylinder, at least two ignition coils each with a secondary winding which is electrically connected to an

20 ignition element , at least one ignition capacitor which is electrically connected, on the one hand, to a discharging circuit for each ignition coil , which circuit comprises the primary winding

_* of the ignition coil connected in series with a first circuit breaker element, and, on the other hand, to a charging circuit

25 which comprises a coil and a second circuit breaker e lement connected in series with each other, a direct-current source electrically connected to the charging circuit and a control unit electrical ly connected to the circuit breaker elements for controlling them.

30 The arrangement according to the invention is characterized in that the system comprises elements for detecting the direct- » current source voltage level and supplying a signal corresponding thereto to the control unit which, in conjunction with a timing unit , supplies s ignals , which are separate in time and are

35 dependent on the said voltage level , to the circuit breaker elements when one and the same crankshaft angle range close to the said top dead centre position of the pistons is passed through.

Other features characterizing the invention are apparent from the attached patent claims and the following description.of an embodiment exemplifying the invention. The description is given with reference to the attached figures, in which Figure 1 shows a block diagram of the arrangement according to the invention,

Figure 2 shows a basic circuit diagram of the arrangement according to the invention,

Figure 3 shows a flow chart for a method according to the invention and

Figure 4 exemplifies a circuit diagram for a timing unit incorporated in the invention.

Figure 1 shows how a signal is supplied from a crankshaft transmitter 5 attached to an Otto-cycle engine 1 via one of lines 6 to a microcomputer-controlled ignition system 2 controlling the ignition of the engine. It includes a control unit 3 in which a microcomputer calculates the point of time for the ignition in the respective cylinder on the basis of incoming data from the crankshaft transmitter 5, an intake pressure transmitter 7, an engine temperature transmitter 8 and any other transmitters. The ignition system 2 is of the capacitive type and also comprises a charging circuit 4, discharging circuits 9 and ignition circuits 10 for spark plugs 11 - 14 of the respective cylinders Cl, C2, C3, C4 in the Otto-cycle engine. Th said cylinders are divided into cylinder pairs Cl, C3; C2, C4, in which the pistons run parallel in the known manner but with 360 degrees of crankshaft angle (hereinafter simply called degrees) in phase difference. When the piston in one cylinder Cl in the cylinder pair Cl, C3 starts off in the compression stroke of the four-stroke cycle, the piston of the second cylinder C3 is thus in the exhaust stroke. - The pistons of one cylinder pair Cl, C3 however run with 180 degrees difference relative to the pistons of the second cylinder pair C2, C4 which means that when the pistons of one cylinder pair Cl, C3 are in the top ^' dead centre position, the pistons of the second cylinder pair C2, C4 are in the bottom dead centre position.

Figure 2 shows those parts of the ignition system which are essential for describing the present invention. Only the spark

plugs 11 and 13 of the spark plugs 11 - 14 in Figure 1 are here diagrammatically reproduced each connected to its secondary winding 15, 16 at a corresponding number of ignition coils 17, 18. The primary windings 21, 22 of the ignition coils 17, 18 are each series-coupled to its circuit breaker element 23, 24, here constructed as triacs. Each primary winding 21, 22 and triac 23, 24 constitutes a discharging circuit 25, 26 which is coupled in parallel with an ignition capacitor 20 in a line 27. A coil 28 is similarly coupled in parallel with the ignition capacitor 20; this coil being called choke hereinafter, coupled in series with a diode 29 in a line 31. Line 27 with the ignition capacitor 20 and all lines 25, 26, 31 coupled in parallel therewith are connected at one end to a second circuit breaker element 30, for example a transistor, series-connected to a second diode 32 and a resistor 33 in a line 34 and at the other end to a direct-current source 35, preferably a 12 V battery via a line 36 including an ignition key circuit breaker 37. Diodes 29, 32 are arranged in such a manner that when the transistor 30 is open to pass current, current can be fed frcm the battery 35 through the lines 31, 34 to earth. Triacs 23, 24 and transistor 30 are controlled by means of signals on lines 44, 45 and 46 frcm control unit 3. Control unit 3 is supplied with, in addition to the input signals specified in Figure 1 on lines 6, an input signal representing the voltage level of the battery 35 on a line 47. A line 48 connects control unit 3 to line 34 between transistor 30 and resistor 33 and transfers a potential corresponding to the charging current to control unit 3. In addition, control unit 3 also obtains information about the potential of ignition capacitor 20 via a line 49 with a resistor 42 and a diode 43. in principle, the arrangement according to Figure 2 operates as follows.

When the engine starts, circuit breaker 37 closes lipe 36 and battery 35 delivers direct current via charging circuit 31, 34 with choke 28, diodes 29, 32, transistor 30 and resistor 33 to earth. Thus, control unit 3 keeps triacs 23, 24 closed whilst transistor 30 is kept open for the passage of current. When the charging current and a potential corresponding thereto on line 48 have reached a predetermined level, control unit 3 interrupts the

current through transistor 30. Energy stored in choke 28 is thereby transferred to capacitor 20 which is thus charged. If control unit 3, on the basis of the input signals on lines 6, 41, subsequently supplies an output signal to, for example, the triac 23 at the ignition time determined in control unit 3, triac 23 opens and ignition capacitor 20 discharges through primary winding 21. This causes an ignition voltage to be generated in secondary coil 15 which is followed by the generation of an ignition spark at spark plug 11. The potential of ignition capacitor 20 is sensed by control unit 3 via line 49 and when it has fallen below a predetermined value, control unit 3 starts a new charging cycle by conducting, on line 46, an output signal to transistor 30 for opening the latter. At the same time, triac 23 -has closed line 25 again for current flow. In the same manner as above, control unit 3 then again manages the charging and discharging of ignition capacitor 20.

According to the present inventive method, control unit 3 controls the ignition on engine start in accordance with a starting program stored in its micro-computer which is explained in accordance with the flow chart shown in Figure 3.

The program starts with an operation step 50 in which the pulses of the output signal of crankshaft transmitter 5 are derived in a manner known per se from cylinder pair Cl, C3 and C2, C4, respectively. The said pulses relating to the respective cylinder pair recur with a spacing of 180 degrees and have a distance corresponding to, for example, 35 degrees between a first negative edge and a second positive edge. In a subsequent operational step 51, the next 180 degree pulse, for example relating to cylinder pair Cl, C3, is awaited. When the negative edge of the pulse is detected, the program follows a flow line 53 to an inquiry step 55 where it is determined whether the speed of the engine is less than or greater than, for example, 400 rpm. This is a criterion for whether the engine has left the starting process or not. If the engine speed is below said limit, the program continues to an inquiry step 56 where it is determined whether the battery voltage is less than or greater than, for example, 11 V. The said voltage limit is used as criterion for possible starting problems in cold weather.

If either the engine speed exceeds or is.equal to 400 rp or the battery voltage exceeds or is equal to 11 V according to inquiry steps 55, 56, the program proceeds via flow lines 57 and 58, respectively, to an operation step 59 where at the same time the ignition is generated in both cylinders Cl, C3. Operation step 59 is followed by an operation step 60 where it is determined whether ignition occurs in cylinder Cl or C3. This can be done by means of an ionizing current arrangement of the type which is shown in our Swedish Patent 8406457-5. From the information about the ignition in either cylinder, the control unit determines the ignition sequence of the engine which is the end product of the starting program. This forms the basis for continued control of the engine ignition by control unit 3.

However, if both the engine speed is below 400 rpm and the battery voltage is below 11 V, the starting program follows a flow line 61 to an operation step 62. There, the control unit provides an output signal to the triac in the discharging circuit 25 of one cylinder, for example Cl, where- the positive edge of the 180-degree pulse is detected. This preferably arrives approximately 15 degrees before the top dead centre position of the piston pair, whereby an ignition voltage is generated for spark plug 11 in cylinder 1.

After that, the program continues to an operation step 63 in which control unit 3 determines and waits for a predetermined time by controlling a signal for transistor 30 in such a manner that it opens and starts a new charging cycle in the manner described above with reference to Figure 2. The duration of the charging cycle (charging time) is determined by the time of a signal of control unit 3 to transistor 30 for opening it and the time at which the ignition capacitor is charged. The latter time occurs almost immediately afterwards, for example a few ms after the control unit has emitted a signal to transistor 30 for closing it. The charging times vary in dependence on the battery voltage level in such a manner that, the lower the battery voltage, the longer the charging time. The predetermined time which control unit 3 waits at operation step 63 is as long as the charging time where control unit 3 is designed in such a manner that it can determine via a signal on line 49 when ignition capacitor 20 is charged. If

control unit 3 is not capable of reading this information from the said signal, the predetermined time is determined with the help of tables stored in control unit 3 or similar. Thus, different times are selected in dependence on the battery voltage level and the time it waits at operation step 63 varies, with advantageously close matching to the charging time, between 6 ms at a battery voltage of 11 V up to 12 ms or at least less than 15 ms with a battery voltage of 5 V. At the prevailing starter motor speed, the said times correspond to between approximately 2 and 10 degrees rotation of the crankshaft.

After the said waiting time determined by control unit 3, the program continues to operation step 65 where control unit 3 opens triac 24 via a signal on line 45. This results in ignition capacitor 20 being discharged through primary winding 22. A corresponding generation of ignition voltage in secondary coil 16 results in the formation of an ignition spark at spark plug 13 in cylinder C3. From operation step 65, a flow line 66 leads back to operation step 51 where the next 180 degree pulse from the crankshaft transmitter is awaited. The said pulse represents the second cylinder pair C2, C4 and when the negative edge of the pulse is detected, the starting program follows the flow diagram back in the manner described above.

Thus, two ignition sparks separated in time can be formed in the cylinder pair concerned with a not fully charged battery by means of the method according to the invention. This is done during the time the crankshaft passes through one and the same crankshaft angle range close to the top dead centre positions for the pistons of the cylinder pair. The time between ignition sparks is utilized for charging the ignition capacitor up again after the first discharging by means of which a fully charged ignition capacitor is available in both instances of ignition.

In Figure 4, an example is shown of the case in which control unit 3 includes a timing unit 70 constructed as a combined circuit which controls the change-over of transistor 30 and thereby the charging time for ignition capacitor 20.

Line 46 reproduced in Figure 2 is connected to the base of transistor 30. As is shown in Figure 4, line 46 is also connected to the collector of a control transistor 71. Transistor 30 is

closed to current flow as long as control transistor 71 connects line 46 to earth. When control transistor 71 is not conducting, the base of transistor 30 receives a high potential via line 47, connected to battery 35, which includes a resistor 72 for matching the potential of transistors 30, 71.

Control transistor 71 is open to current flow with a high potential on its base 73 and consequently closed with a low potential at the base. The said potential is determined by a comparator 74 the output of which is connected to base 73 via line 75. Base 73 receives a high potential if the potential at the positive input 76 of comparator 74 exceeds the potential at its negative input 77. Line 49, shown in Figure 2, with resistor 42 and diode 43 and also another diode 78 leads to the positive input 76. Line 49 is connected to ignition capacitor 20 and the potential at the positive input 76 of comparator 74 thereby represents the voltage condition of ignition capacitor 20. A feedback line 79 with a resistor 81 between input 76 and the output of comparator 74 ensures a predetermined potential relation between the said input and output. The negative input 77 of comparator 74 is connected by a line 82 including a diode 83 to the output 84 of a second comparator 80. If output 84 is at low potential, input 77 of comparator 74 is also at low potential whereas, if output 84 is at high potential, input 77 is also at high potential for the most part. Line 48, shown in Figure 2, including a resistor 38 leads to the negative input 87 of comparator 80. Thus, line 48 transfers to input 87 a potential which corresponds to that which at any instance prevails in charging circuit 34 between transistor 30 and resistor 33. The potential transferred in line 48 is stabilized by _a capacitor 39 in an earth connection.

The positive input 86 of comparator 80 is supplied with a constant reference potential via a line 85 with a resistor 88 which forms a voltage divider with a resistor 41 which is connected to earth. Line 85 receives voltage fed from a voltage stabilizer 90 which obtains low-voltage direct current from the battery 35 via line 47 shown in Figure 2 and converts it to a stabilized 5V voltage which is supplied on a line 85 and a line 91. The last- mentioned line 91 supplies 5 V between two resistors 92, 93 in a

line 94 between line 49 and output 75 of comparator 74. A timing circuit 40, also connected to the negative input 77 of comparator 74, also leads to output 75. Timing circuit 40 includes three series-connected resistors 95, 96, 97 between input 77 and output 75 of coitparator 74 and a diode 98 which is connected in .parallel with the middle resistor whereby the diode allows current to flow from the output of the comparator to a capacitor 99 connected to earth between resistors 96, 97.

The timing unit 70 operates as follows. When the ignition is switched on, battery 35 delivers current via lines ^" 47, 46 and provides a high potential at the base of transistor 30 which opens and allows a current to flow through charging circuit 31, 34 shown in Figure 2. The potential in charging circuit 31, 34 between transistor 30 and resistor 33 increases successively as does the potential at input 87 of coitparator 80 via line 48. At the end, the potential at input 87 exceeds the potential at input 86 which is at a constant level via line 85 from the voltage stabilizer. Output 84 of comparator 80 thus obtains a low voltage level and the potential at input 77 of coitparator 74 also drops via line 82 and diode 83 and thereby drops below the potential at input 76. The latter is held at a fixed output level by being fed with voltage via line 91 from voltage stabilizer 90 and lines 94, 49 with resistor 92 and diode 78. Output 75 of comparator 74 goes to high potential which means, according to what is stated above, that transistor 30 closes.

When transistor 30 is closed, the electrical energy in choke 28 is transferred to ignition capacitor 20. Input 76 of comparator 74, and thus also output 75 of comparator 74, receives a high potential via line 49, which keeps transistor 30 in its closed state.

When control unit 3 causes ignition capacitor 20 to discharge, the potential at input 76 of comparator 74 thereby drops below the level at input 77. Output 75 of comparator 74 goes to low potential which means that transistor 30 opens and a new charging cycle is begun.

During the time in which ignition capacitor 20 waits to be discharged and output 75 of comparator 74 is at high potential , capacitor 99 located in timing circuit 40 is charged frcm voltage

stabilizer 90 via line 91, resistor 93 and resistor 95 and diode 98 of timing circuit 40. When output-75 of coitparator 74 goes back to low potential with the discharging of ignition capacitor 20, a discharging of capacitor 99 begins via resistors 96, -95 and an earth connection 85 in comparator 74. When the potential of capacitor 99 has dropped sufficiently, the potential at input 77 also drops through resistor 97. Thus, the potential at input 77 drops below the potential of input 76 and output 75 of comparator 74 goes back to high potential This means that transistor 30 closes followed by immediate transfer of the energy σf choke 28 to ignition capacitor 20. Thus, a predetermined drop of the potential at input 77 of comparator 74 occurs with the help of timing circuit 40 until its coitparator 74 switches over and ignition capacitor 20 is again charged. By choosing the capacitance and resistances of timing circuit 40, a required value of the maximum charging time can be determined.

The said time value can be, for example, up to 12 ms which means that the hitherto limited maximum charging time is only utilized with low battery voltages, for example down to 5 V. Below this voltage level there is no limiting of the maximum charging time. With higher battery voltage, comparator 80 switches over, which, in turn, results in comparator 74 switching over and thus the ignition capacitor being charged within the said 12 ms period.

Thus, the opening and closing of transistor 30 is controlled by the timing unit shown in Figure 4 so that the charging time approaches the said value at a maximum. This enables ignition capacitor 20 to be charged even with low battery voltages between the output signals of control unit 3 to the triacs of a cylinder pair for igniting first one and then the other cylinder at the time in which the pistons of the cylinder pair pass through one and the same crankshaft angle range close to the top centre position.

The embodiment of the invention described above is not intended to limit the latter but can be varied in a plurality of embodiments within the context of the claims which follow. Thus, for example, an ignition capacitor or similar can be considered to cover solutions including several parallel-connected ignition capacitors which functionally operate as a single capacitance.