A METHOD FOR IMPROVING THE STARTING ABILITY OF AN INTERNAL COMBUSTION ENGINE DURING AN ENGINE START

Abstract: The invention relates to a method for improving the starting ability of an internal combustion engine during difficult engine starting conditions. Since lower than normal engine starting speeds are the result of external circumstances, it is essential that the spark plugs are free from deposits and in their best condi­ tion, so as not to hinder an engine start. The method and arrangement solves this problem by burning-­ off any deposits present on the spark plugs (11-14) during the engine start attempt, when difficult engine starting conditions are detected. During the engine start attempt a comparison circuit (5) ascertains whether or not one or more detected engine parameters have a lower value than in normal engine starting conditions. The comparison circuit (5) sends a signal to the control unit (3) controlling the ignition system (2), so that the control unit (3) will initiate the generation of a plurality of sparks instead of a single spark at ignition time points. The resultant spark shower then burns-off any deposits that are present.

Description: The invention relates to a method for improving the starting ability of a four-stroke internal combustion engine, according to the preamble of Claim 1 (as disclosed in US-A-4 442 821). Field of invention It is known to maintain the spark plugs of an internal combustion engine free from deposits by repeated generation of sparks between the spark plug electrodes when the engine is running. Thus, US-A-4 341 195 teaches an ignition system in which, under certain engine conditions and when running of the engine has become established, a spark discharge is generated continuously across the plug with the aid of a specific ignition circuit. The number of discharges generated is inversely proportional to the speed of the engine and proportional to the engine load. Further, US-A- 4 024 469 teaches an arrangement in which the plug gap is measured by means of a measuring system which is connected to an ignition system and which applies a high alternating voltage across the plug, so as to burn-off deposits present thereon. It is also known, from 7P-A-55-112 870, to make a running engine run better by avoiding sooty spark plugs caused by misfiring at low temperatures. This is achieved by firing spark showers not only around the compression dead-centre but also during the expansion stroke and during the intake stroke. Further, DE-C-26 45 226 describes an ignition system with which a thin-walled precombustion chamber is heated, by repeatedly effecting an electrical discharge across the spark plugs. It is also known to improve the cold starting ability of an engine by generating a sequence of sparks at the end of each compression stroke during the starting sequence in order to make sure that the air-fuel mixture is property ignited. A solution of this wind is disclosed in US-A-4 442 821 and in EP-A-0 142 478. For the purpose of facilitating an engine start in cold and moist conditions, arrangements have also been proposed for heating the spark plugs with the aid of a direct current; cf. for instance US-A- 3 589 348. This prior art represents complicated solutions which require the provision of numerable ancillary devices and components additional to the conventional ignition system, while still not being capable of burning-off carbon deposits effectively, particularly in difficult engine start conditions. Object of the invention The object of the invention is to control selectively the ignition system of an internal combustion engine, preferably a multi-cylinder Otto engine, during the course of an engine start, so that a multiple of sparks are generated for effectively burning-off any deposits present in the spark-plug gap. This object is achieved with the method according to the invention having the characteristic features set forth in the characterizing clause of Claim 1. The inventive methode enables the control unit of the ignition system to operate in two functional positions. In one of these positions, or states, the control unit sends single control pulses to the charging and discharging circuits, so that in the case of a normal engine start a single spark is obtained at respective ignition times. When the control unit is in its second position, or state, the control unit sends a plurality of control signals to the charging and discharging circuits, so as to produce a plurality of sparks in close succession at respective ignition times when difficult engine start conditions prevail. When starting, for instance, the engine of an automotive vehicle, the method enables the spark plugs to be brought to their best condition, and ensures that ignition is obtained in all cylinders also when the starting conditions are very difficult. In the case of a normal engine start at an engine temperature of at least +15°C, the starting motor will turn the engine at a speed of about 400 rpm. Normally no starting problems occur in the case of such engine starts. When an attempt is made to start an engine at temperatures around -30°C, however, this engine speed is as low as 60-80 rpm. At engine starting speeds as low as this, it is essential that the spark-plug gap is in its best condition, since deposits or coatings on one or more spark plugs may render it impossible to start the engine. During the actual engine cold starting process a coating of moisture is liable to be precipitated onto respective spark plugs. This moisture coating may result, for instance, from an excessively rich fuel-air mixture or because ignition has failed to take place in the cylinder. In the case of conventional spark plugs, the isolators will then become coated with moisture, rendering the isolators electrically conductive. This prevents normal sparking between the spark plug electrodes, since the current creeps from the central electrodes across the isolator to the earthed part of the spark plug. The inventive generation of repeated control signals from the conventional ignition system results in the production of a shower of sparks both at the regular ignition time of respective cylinders and at each exhaust stroke, each spark having the same energy content as a normal or ordinary ignition spark. In the case of a capacitive ignition system having an ignition voltage of up to 40000 volts, the power development afforded by the spark shower is of such long duration that any deposits present on or adjacent to the spark plug electrodes are effectively burned away. Preferably, at least 5-6 sparks are generated on each spark plug at each moment of ignition in the cylinder concerned. By generating sparks also at the exhaust stroke further cleaning is obtained while also heating the spark plugs in preparation for the next ignition sequence. As soon as the difficult engine starting condition is indicated to have ceased, the generation of spark showers is discontinued, and instead only a single spark is provided at the regular ignition time. This reduces spark plug wear. The present invention also relates to an arrangement for carrying out the inventive method. The arrangement is based on an ignition system of the kind previously described, for instance, in the Swedish Patent Specification 437 286, corresponding to US Patent Specification 4, 637, 368, and also of the kind set forth in the preamble of Claim 13. The logic comparator, which is necessary to the invention and which detects difficult engine-start conditions during an engine start and which controls the function of the control unit, may have the form of a separate logic circuit. In the case of ignition systems of the kind disclosed in SE-C-437 286, however, the comparator instead advantageously comprises a comparison module which is proprammed into a microcomputer-based control unit. Such programming obviates the need to provide additional components, since one of the engine parameters sensed or detected by the control unit forms the basis of the comparison. Other characteristic features of the invention are disclosed in the following claims and are also made apparent in the following description of an exemplifying embodiment of the invention. The description is made with reference to the accompanying drawings, in which Figure 1 is a block schematic illustrating an inventive arrangement used in conjunction with an internal combustion engine; Figure 2 is a circuit diagram of the engine ignition system; and Figure 3 is a flow sheet which illustrates an inventive method for controlling the ignition system. Fig. 1 illustrates schematically a four-cylinder Otto-engine 1 and a crankshaft sensor or transducer 7 fitted thereto, said sensor being connected to a microcomputer-controlled ignition system 2 by means of a line 8. The system includes a control unit 3 in which a microcomputer or microprocessor calculates the ignition timing for respective engine cylinders on the basis of input signals from the crankshaft sensor 7, an inlet pressure sensor 7″, an engine temperature sensor 7′ and optionally other sensors or transducers. The ignition system 2 further includes a comparison circuit 5 which is connected to data lines 8,8′,8″. The comparison circuit 5 detects the values in question from the sensors 7-7″ and sends a signal on a line 6 to the control unit 3 when the value or values are beneath a pre-determined value. The ignition system 2 is, advantageously, a capacitive system and also includes a charging circuit 4. In addition hereto, the ignition system 2 incorporates discharging circuits 9 and ignition circuits 10 connected with the spark plugs 11- 14 of respective cylinders C1, C2, C3, C4. The cylinders are divided into pairs C1, C3; C2, C4, in which pistons run parallel with one another in a known manner but at a phase difference of 360 crankshaft-angle degrees (hereinafter referred to solely as degrees). When the piston in one cylinder C1 of the cylinder pair C1, C3 carries out its compression stroke, the piston of the other cylinder C3 performs an exhaust stroke. The pistons of the one cylinder pair C1, C3, on the other hand, run with a 180-degrees difference in relation to the pistons of the other cylinder pair C2, C4, whereby when the pistons of one cylinder pair C1, C3 occupy their upper top dead-centre position the pistons of the other cylinder pair C2, C4 occupy their bottom dead-centre positions. Fig. 2 illustrates a circuit diagram for an exemplifying embodiment of the ignition system 2 shown in Fig. 1. Of the spark plugs 11-14 shown in Fig. 1, only the plugs 11 and 13 are shown in Fig. 2, and then only schematically, each of said plugs being connected to a respective secondary winding 15, 16 of a corresponding number of ignition coils, 17, 18. Each of the primary windings 21, 22 of the ignition coils 17, 18 is connected in series with a respective switching device 23, 24 in this case in the form of triacs. Each primary winding 21, 22 and triac 23, 24 forms a discharging circuit 25, 26 which is connected in parallel with an ignition capacitor 20 incorporated in a line 27. The discharging circuit 4 used for the ignition capacitor 20 has the form of a choke 28 connected in series with a diode 29 in a line 31, which is connected in parallel with the ignition capacitor 20. The line 27 with the ignition capacitor 20 and all lines 25, 26, 31 connected in parallel therewith are connected at their respective ends to a second switching device 30, e.g. a transistor incorporated in an earthing line 34. The transistor is connected in series with a second diode 32 and a resistor 33. The other ends of respective lines 25, 26, 27, 31 are connected to a direct current source 35, preferably a 12 V battery, via a line 36 which incorporates a switch 37 included in an ignition lock. The diodes 29, 32 are turned so that when the transistor 30 is open for the passage of current, current can be supplied from the battery 35 through the lines 31, 34 to earth. The triacs 23, 24 in the discharge circuits 9 and the transistor 30 in the charging circuit 4 are controlled by signals arriving on lines 44, 45 and 46 respectively from the control unit 3. In addition to the input signal on the lines 8, 8′, 8″, shown in Fig. 1, the control unit 3 is also supplied on a line 47 with an input signal relating to the voltage level of the battery 35. A line 48 connects the control unit 3 with the line 34 between the transistor 30 and the resistor 33, and transfers to the control unit 3 a potential which corresponds to the charging current of the ignition capacitor 20. The control unit 3 is also supplied with data relating to the potential of the ignition capacitor 20, via a line 49 incorporating a resistor 42 and a diode 43. In the case of the inventive arrangement illustrated in Fig. 2, the control unit 3 obtains on the line 6 a signal from the comparison circuit 5 in those instances when the comparison circuit 5 has detected a difficult engine-start condition. The comparison circuit 5 senses the engine parameters concerned from the sensors 7, 7′, 7″ on the lines 8, 8′, 8″ and compares the prevailing values with pre-determined values which represent the engine parameter values relevant for conditions under which the engine will readily start. When the engine starting speed constitutes the basis upon which difficult engine-start conditions are indicated, detection is commenced immediately after activation of the starting motor. Only two speed pulses from the crankshaft sensor 7 are required to enable the comparison circuit 5 to be able to detect a difficult engine start, and in such cases to send a signal on the line 6 to the control unit 3. When the engine temperature and battery voltage are used as a basis for indicating difficult engine-start conditions, the comparison circuit 5 may be constructed so as to send a signal on the line 6 to the control unit 3 before the starting motor is activated. In principle, the arrangement illustrated in Figs. 1 and 2 operates in the following manner. When starting the engine 1 under both difficult and not-difficult engine start conditions, the switch 37 closes the line 36 and the battery 35 is connected to earth, via the lines 31, 34 with the choke 28, the diodes 29, 32, the transistor 30 and the resistor 33. The control unit 3 therewith holds the triacs 23, 24 closed, whereas the transistor 30 is held open for the passage of current therethrough. When the charging current and a corresponding potential on the line 48 have reached a pre-determined value, the control unit 3 interrupts the current through the transistor 30. Energy stored in the choke 28 is therewith transferred to the capacitor 20, which is therewith charged. When the control unit 3 sends an output signal, e.g. to the triac 23 in response to the input signals on the lines 8, 8′, 8″ at the ignition time point determined in the control unit 3 on the basis of the input signals, the triac 23 is opened and the ignition capacitor 20 is discharged through the primary winding 21. In this way there is generated in the secondary coil 15 an ignition voltage which produces an ignition spark on the spark plug 11 at the determined ignition time. The potential of the ignition capacitor 20 is detected by the control unit 3 via the line 49, and when the detected value lies beneath a predetermined value, the control unit 3 will initiate a new charging circle, by sending an output signal on the line 46 to the transistor 30, causing the transistor to open. The triac 23 has, at the same time, reclosed the line 25,preventing current from flowing therethrough. Consequently, recharging of the ignition capacitor 20 will commence immediately upon termination of the discharge, so as to recharge the capacitor 20 quickly for the next ignition process. In the case of an 11 V battery voltage, this charging period is up to 6 ms, whereas in the case of a 5 V voltage charging of the capacitor will take up to 12 ms, or at least less than 15 ms. At occurring starting speeds, these times correspond to between about 2 and 10 degrees of crankshaft rotation. Should the control unit 3 receive a signal on the line 6 from the comparison circuit 5 indicating that a difficult engine start condition prevails, the control unit 3 will send a plurality of output signals on the line 44 to the triac 23 so that the regular ignition spark generated across the spark plug 11 at said given ignition time is followed by a multiple of sparks in closed succession. If the ignition sequence is not determined, the control unit 3 begins to send control signals to both triacs 23, 24, whereby sparks are applied across both spark plugs 11, 13 in both cylinders C1, C3 when the pistons are located at a distance of 8-12°, preferably 10°, from the top dead centre position, which corresponds to a normal ignition setting in the compression stroke. In this way, the compression stroke is not counteracted by the application of the sparks. The control unit 3 is then able to apply the sparks to the pistons till said pistons are located up to about 60° after the top dead centre position. This obviates the risk of igniting the fuel-air mixture drawn into the cylinder upon commencement of the suction stroke. By controlling the transmission of control signals from the control unit to the triacs 23, 24 in accordance with a time sequence in which the control unit waits 12-15 ms after sending the first control signal, the ignition capacitor 20 will be charged to a maximum down to 5 V battery voltage. This time control will enable a spark shower containing at least 5-6 sparks to be applied with maximum energy within said crankshaft angle range down to 5 volts battery voltage and at occurrent engine starting speeds. The control unit 3 may also detect the potential of the ignition capacitor 20 via the line 49, and when the potential detected is sufficient the next control signal to the triacs 23, 24 can be delivered immediately. This enables a spark shower containing more than 5-6 sparks to be applied within said crankshaft angle range. Due to the rapid build-up of an electric charge in the ignition capacitor 20 of the ignition system 2, and to the rapid discharge process and also to the ability of the capacitive ignition system to produce ignition voltages of up to 40000 volts, any deposits on the spark plug electrodes will be effectively burned-off by said spark shower during the engine start. In this regard, those deposits which are often found in the vicinity of the electrodes, and particularly the deposits formed on the isolator of the centre electrode, are also effectively burned away, in addition to deposits present on the spark plug electrodes themselves. In accordance with a preferred embodiment of the inventive method, when starting an engine the control unit 3 is controlled in accordance with a start program stored in the microcomputer of the control unit, in response to the detected engine speed, as illustrated in the flow sheet shown in Fig. 3. The program begins when the ignition system is activated as a result of the application of a voltage via a conventional ignition lock, having an operation stage 50 in which pulses in the crankshaft sensor 7 output signal are applied to the cylinder pairs C1, C3 and C2, C4 respectively in a known manner. The pulses belonging to respective cylinder pairs are produced with a spacing of 180 degrees. A check is made in a following operation stage 51 to establish whether or not running of the engine is established, i.e. that the engine start sequence has been terminated. When the exemplified ignition system 2 does not incorporate a cam shaft sensor, the ignition sequence will not be given directly by the signals from the crankshaft sensor 7, these signals solely indicating those pistons of the cylinder pairs C1, C3 and C2, C4 which occupy their top dead-centre position. Detection of which cylinder is in its compression stroke can be effected by sensing the ionizing current across the spark plugs and determining the ignition sequence therefrom. This can be carried out with an ionizing current measuring arrangement of the kind described in detail in our Swedish Patent Specification 442 345. Thus, a check is carried out in the operation stage 51 to ascertain whether the ignition sequence is a determined sequence, so as to indicate whether the engine start sequence has been completed or not. If the ignition sequence is determined in the operation stage 51, the program will step forward to the operation stage 62, which results in the generation of a single spark on the cylinder concerned, whereafter the start program is left or abandoned. When the check carried out in the operation stage 51 shows that the ignition sequence is not a determined sequence it is assumed that the engine start sequence still prevails and the start program steps to the operation stage 53. Operation stage 53 is effective in determining whether or not a difficult engine start condition prevails. If the engine speed lies beneath 160 rpm, i.e. about 40% of a normal engine start speed, this indicates that a difficult engine start condition exists. The program then steps immediately, via the flow line 63, to those operation stages described hereinafter. If, however, the speed is above 160 rpm, the program steps to operation stage 55. The operation stage 55 is effective in making a further check to ascertain whether a difficult engine start condition exists. If the engine has continued to run beneath a normal starting speed of about 420 rpm for more than 2 (two) seconds, this indicates that a difficult engine start condition prevails. The program then steps forward, via the flow line 63. If the engine speed lies above 420 rpm and the attempt to start the engine has not been longer than 2 (two) seconds, the program steps to the operation stage 56. The operation stage 56 is effective in controlling the control unit 3 in a manner such as to generate a single spark immediately the piston of respective cylinders is located in its top dead-centre position. The program then returns to program stage 51. The flow line 63 extending from operation stages 53 and 55 also extends to a program stage 57 which introduces a sequence of measures when a difficult engine start condition is indicated. The operation stage 57 detects, or establishes, whether the engine speed is above 850 rpm. If the engine speed is beneath 850 rpm, the program is stepped to operation stage 58, which means that a plurality of sparks will be initiated with starts from said ignition position, whereafter the program returns to the operation stage 57. Consequently, under difficult engine start conditions, a plurality of sparks are delivered to each cylinder immediately the piston in question occupies its top dead-centre position, until the engine speed is, preferably, twice the starting speed, corresponding to a normal engine starting speed. In the case of the illustrated embodiment, this value of twice the normal starting speed is about 850 rpm. When the operation stage 57 establishes that the engine speed exceeds 850 rpm, the program moves to a safety sequence which ascertains whether or not the engine is running smoothly. The safety sequence begins with the operation stage 59, which determines whether or not it has been possible to determine the ignition sequence. If the ignition sequence is not determined, which means that an engine start sequence still prevails, the program moves to the operation stage 60, in which it is ascertained whether the engine speed is greater than 420 rpm or not. If the engine speed is above 420 rpm, the program moves to the operation stage 61, so as to generate a single spark adjacent each top dead-centre position and subsequently returns to the operation stage 59. If it is established in the operation stage 60 that the engine speed is below 420 rpm, the program returns to the operation stage 58, so as to generate a plurality of sparks. The engine start program is therefore not left until the engine ignition sequence is determined and the engine runs smoothly. The start sequence is not considered to be terminated until the operation stage 59 establishes that the ignition sequence is determined, and the program steps to the operation stage 62. In this case, a single spark is generated on respective cylinders in their ignition positions, whereafter the engine start program is left. The aforedescribed inventive embodiment does not limit the scope of the invention, since several modifications can be made and other embodiments are possible within the scope of the following claims. For example, the reference to an ignition capacitor and the like is intended to cover solutions which use several parallel-connected ignition capacitors which operate functionally as a single capacitance. It will be also understood that engine parameters other than engine speeds can be used, either directly or indirectly, for establishing the presence of difficult engine starting conditions. The operation stage 53 can be constructed to compare both the engine temperature and the battery voltage with pre-determined values for normal engine starts, in order to indicate that a difficult engine start condition exists. Alternatively, the operation stage 55 can be constructed to ascertain that the ignition sequence has not been established within a given length of time. In ignition systems with a determined ignition sequence, it is possible to ascertain in the operation stages 51 or 59 whether or not the ignition system has operated actively over a given minimum time period. By means of this time check it is possible to establish whether the engine is running smoothly and subsequently that the engine start sequence has been terminated.

Claims: 1. A method for improving the starting ability of a four-stroke internal combustion engine during the course of starting the engine, wherein the ignition system (2) of the engine (1) includes at least one charging circuit (4), and wherein the charge stored in the charging circuit is discharged therefrom through at least one ignition coil so as to generate, in difficult engine start conditions, ignition sparks in the form of a spark shower across the spark plugs (11-14) of the engine (1) in response to signals from an electric control unit (3) to which signals representing at least one engine parameter are supplied, said engine parameter signal being compared with a reference value in order to determine whether or not a difficult engine start condition exists, characterized in that upon detection of a difficult engine start condition the control unit (3) commences to produce a multiple of control signal in close succession, such as to cause the charging circuit (4) to discharge across the spark plugs (11-14) in all of those cylinders whose pistons occupy a position in which the crankshaft is located between a position 10° before a top dead-centre position and 60° after the top dead-centre position, irrespective of whether respective pistons are in their compression or exhaust stroke, thereby to produce a spark shower across the plug gap at each exhaust stroke, so as to burn-off deposits on the spark plug electrodes, and that, upon detection that a difficult engine start condition passes to a non-difficult engine start condition the control system ceases to initiate the generation of a plurality of sparks and, instead, initiates the generation of solely single sparks at each ignition time point of respective cylinders. 2. A method according to Claim 1, characterized in that the detected engine parameter is engine speed; and in that a difficult engine start condition is detected when the engine speed is lower than a given engine speed during the engine start sequence. 3. A method according to Claim 1, characterized in that the detected engine parameter is the engine temperature; and in that a difficult engine condition is detected when the temperature during the engine start sequence is lower than a given value, and/or in that the detected engine parameter is the engine battery voltage, and in that a difficult engine start condition is indicated when the voltage during the engine start sequence is lower than a predetermined value. 4. A method according to Claim 1 applied in a multi-cylinder four-stroke engine, characterized in that when a difficult engine start condition is detected the control unit produces control signals which in close succession sequentially initiate the generation of a plurality of sparks in each cylinder. 5. A method according to Claim 1, applied in an engine equipped with a capacitive ignition system which is provided with one ignition coil for each spark plug, characterized in that when a difficult engine start condition is detected the control system produces control signals for initiating the generation of sparks at a frequency in the order of 200 Hz. 6. A method according to Claim 2, characterized in that the difficult engine start condition is indicated when the engine starting speed lies beneath 40% of the starting speed for the normal non-difficult engine starting condition. 7. A method according to Claim 2, characterized in that the difficult engine start condition is indicated when, during the engine start sequence, the engine speed is lower than the starting speed for the normal, non-difficult engine start condition over a given minimum time period. 8. A method according to Claim 1, characterized in that the difficult engine start condition passes to a non-difficult engine start condition when the engine speed is twice the engine speed corresponding to a normal engine starting speed. 9. A method according to Claim 1, characterized in that the difficult engine start condition passes to a non-difficult engine start condition when the engine temperature and/or the battery voltage exceeds a treshold value for engine temperature and battery voltage respectively.

Inventors: GILLBRAND, PER, JOHANSSON, HANS, NYTOMT, JAN
Applicants: SAAB-SCANIA AKTIEBOLAG
Application year: 1988
Main classification: F02P15
IPC codes: F02B77, F02P17, F02P3, F02P15, F02B75, F02B1