FR2820465A1 - Internal combustion engine ignition coil control system, in which coil conduction time is tabulated using a reference coil whose primary has a predetermined inductance - Google Patents

Abstract

The time (tctab) in the formula for conduction time tc is tabulated for a reference coil whose primary has a predetermined inductance. The primary of the controlled coil (1) has an inductance equal to or less than that of the primary of the reference coil. The coil current increases linearly at least up to a reference value (Iref). The coefficient K is calculated from the formula K = Ifth / If where Ifth is the current at time tf predetermined in the reference winding. Method of control for an air/fuel mixture ignition coil (1,2) of an i.c. engine. The coils primary (1) is connected to an electrical source (Vbat) during a pre-calculated conduction time (tc). The conduction time is calculated by: (a) evaluating the current (If) in the primary winding (1) at a predetermined instant (tf) chosen so that the current (If) is then less than a reference current (Iref) which ensure correct ignition of the flammable mixture, and; the conduction time (tc) is calculated from the formula tc = K X tctab where tctab is a conduction time tabulated as a function of at least the voltage (Vbat), and K is a correction coefficient which is a function of the current value (If).

Description

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 The present invention relates to a method for controlling an ignition coil of an air / fuel mixture in a cylinder of an internal combustion engine, method according to which the primary of the coil is connected to a source of an electrical voltage during a precalculated conduction time to ensure the formation of a spark by a candle placed in the secondary circuit of the coil, the spark initiating the combustion of the air / fuel mixture.

 This initiation must take place at a precise instant corresponding to the passage of the cylinder piston at a predetermined "ignition advance angle", measured relative to the top dead center of this cylinder.

 The spark is only possible, at this instant, if the current flowing in the primary of the coil then reaches a value at least equal to a predetermined threshold, of the order of approximately 5 to 6 amps.

 The conduction time necessary to reach this value is a function of the value of the voltage delivered by an energy source, such as a battery, selectively supplying the primary of the coil, the values of the inductance and the resistance of the primary of the coil, and the temperature of this coil, in particular.

 This conduction time must be adjusted so as not to be too short, barely preventing the formation of the spark, or too long, barely causing unnecessary, and even harmful, heating of the coil.

 For this purpose, a closed loop regulation device for the combustion time is known, operating by re-reading the current circulating in the coil and adjusting the conduction time so that the current in the primary can reach, at the cut of this current, the intensity allowing the production of a spark. Such regulation automatically compensates for variations in the parameters mentioned above (battery voltage,

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 primary inductance, temperature, etc.) which are likely to affect this conduction time. Thus, in particular, the same regulation device could be associated with all the coils of the same batch of coils, each equipping a different motor, the regulation absorbing the manufacturing tolerances which affect in particular the inductances of the coils of the lot. Such a regulating device is therefore precise and effective, but it is also expensive.

 Other conduction time control devices are known, simpler and therefore less costly, operating in open loop, the conduction of the coil primary being controlled by a switch controlled by a "controller", in the absence of any servo. The conduction time is then only adjusted as a function of the supply voltage of the coil.

 When the coils are manufactured in batches, the inductances of the coils are affected, as indicated above, by a manufacturing tolerance which does not make it possible to know precisely the inductance of a particular coil in the batch, whereas this inductance is a essential factor in calculating the conduction time.

 For batches of ignition coils intended to equip internal combustion engines used in the automotive industry, this tolerance is taken into account by setting the conduction time of the coil to a value long enough to be assured. that the intensity of the current flowing, at the end of this conduction time, in the coil with the highest inductance of the batch, is sufficient to cause correct ignition of the air / fuel mixture introduced into the engine cylinders.

 Unfortunately, the result is that for all the other coils in the batch, this conduction time is too long and leads to passing too much current through the coil, which excessively heats it,

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 detriment of its lifespan. This phenomenon is further accentuated by the non-linearity of the current rise in the primary of recent coils, smaller and with finer winding wire, of higher resistivity. These characteristics, combined with saturation of the magnetic core of the coil, result in a gradual increase in the slope of the rise in current in the coil, and therefore in an overshooting of increased amplitude of the current threshold necessary for breaking the current, in most of the coils of the batch, to ensure the ignition of the air / fuel mixture.

 The present invention therefore aims to provide a method of controlling an ignition coil of an air / fuel mixture in a cylinder of an internal combustion engine, which does not have the drawbacks mentioned above of the devices designed to implement the ignition methods known from the prior art. More particularly, the invention aims to provide such a method which combines precision and simplicity, this last characteristic allowing its implementation using an inexpensive device.

 These objects of the invention are achieved, as well as others which will appear on reading the description which follows, with a method of controlling an ignition coil of an air / fuel mixture in a cylinder of an internal combustion engine, according to which the primary of said coil is connected to a source of an electric voltage Vbat during a precalculated conduction time te, this method being remarkable in that, to calculate said conduction time, a) we evaluates the intensity If of the current flowing in said primary, at a predetermined instant tf chosen so that said intensity If is then less than a reference intensity Iref ensuring correct ignition of said mixture, and

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 b) said conduction time te is drawn from the relation: te = K. tctab where: - tCtab is a tabulated conduction time as a function of at least said voltage Vbat, and - K a correction coefficient as a function of said evaluated intensity If.

 As will be seen below in detail, by thus correcting the tabulated conduction time, it virtually eliminates any excess of the intensity of the current required in the primary of the coil at the time of breaking of this current, and this without recourse to an expensive device for closed-loop control of the conduction time.

According to other characteristics of the present invention: the time tCtab is tabulated for a reference coil whose primary has a predetermined inductance, the primary of the coil to be controlled has an inductance equal to or less than that of the primary of said reference coil , the method can be adapted to a coil in the primary of which the current increases linearly at least up to said reference value Iref. The value of the coefficient K is then drawn from the relationship K = Ifth / If, where Ifth is the current intensity reached at the instant (Tf) in said reference coil, the method can be adapted to a coil in the primary from which the current increases non-linearly.
In this case we draw the correction coefficient K from the relation:
K = k. Vbat + k 'where k and k' are coefficients which are functions of the current intensity If evaluated at said predetermined instant tf.

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The present invention also aims to provide a device for implementing this method, comprising a) means for evaluating the intensity (If) of the current flowing in the primary of said ignition coil at said instant tf predetermined, b) means for controlling the passage of a current in said primary, said means comprising a memory containing said tabulated conduction time tCtab as a function of at least said voltage and means for calculating said correction coefficient K and said conduction time te.

la figure 3 est un graphe utile à la description d'une procédure de détermination du courant passant dans le primaire de la bobine à un instant donné, suivant la présente invention, et la figure 4 est un graphe utile à la description d'un deuxième mode de réalisation du procédé selon l'invention. Other characteristics and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which: FIG. 1 is a diagram of a device for controlling an ignition coil designed to allow the implementation of the method according to the present invention, FIG. 2 is a graph useful for the description of a first embodiment of this method,

FIG. 3 is a graph useful for the description of a procedure for determining the current flowing in the primary of the coil at a given instant, according to the present invention, and FIG. 4 is a graph useful for the description of a second embodiment of the method according to the invention.

 Referring to Figure 1 of the accompanying drawing where the device shown comprises a primary winding 1, or "primary", a coil such as that commonly used in the automotive industry to ensure the ignition of an air / fuel in each of the cylinders of an internal combustion engine propelling a vehicle. We know that such a coil further comprises a secondary winding 2, or "secondary", constituting, with the primary winding, a step-up transformer. Winding 2 is connected, on one side, to a

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 winding terminal 1 and, on the other side, to a spark plug 3 whose other terminal is grounded.

 The primary winding 1 is supplied by a voltage source 4 such as a storage battery delivering a voltage Vbat, under the control of an electric switch 5 such as a transistor whose emitter-collector circuit is placed in series with winding 1. The transistor 5 can be of any suitable type, bipolar, MOS, IGBT, etc.

 The device of FIG. 1 is completed by means 6 for controlling the transistor 5, constituted by a "controller" for example, and by means 7,8 of measuring the current passing through the winding 1 and the emitter-collector circuit of transistor 5.

 According to an embodiment of the invention given by way of illustration and not limitation only, these means 7,8 include a resistance voltage divider 7 for taking a voltage representative of the current flowing in the primary winding 1 and delivering this voltage to an analog-digital converter 8, which in turn delivers digitized voltage samples to the controller 6, at the rate of a clock which regulates the operation of this converter 8. These voltage samples are converted into current samples by the controller 6.

 It will be noted that such means 7,8 for measuring the current are currently often provided in open-loop control devices for the conduction time of a coil, not for permanently reading the current flowing in the primary of the coil. but to signal to the controller 6 a possible short circuit of the primary winding, or an excessive current in this winding. The present invention therefore does not require significant changes in the hardware architecture of known devices for open-loop control of the conduction time,

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 architecture which thus retains its cost advantages.

 It will be noted, in the circuit of the primary winding 1 of the coil, a resistor 9 which symbolizes the electrical resistance of this circuit, including that of the inductive primary winding 1.

 Reference is now made to the graphs in FIG. 2 of the appended drawing to describe a first embodiment of the method according to the invention. In this figure we have shown the temporal evolution of the intensity of the current in the primary winding 1, since the conduction of this winding by the transistor 5, and this for three coils A, B, C of the same batch of coils whose nominal inductance of the primary is affected by a tolerance. This is generally the case in the industry for components manufactured in large numbers, particularly when this manufacture must be carried out at low cost, as is the case in the automobile industry.

 We have thus represented in A, C, B respectively the graphs of the rise of the current I in the primary of the coil A, of higher inductance of the batch, in the primary of the coil C, of lower inductance of the batch, and in the primary of a coil B, of intermediate inductance. These graphs illustrate a linear growth of the current over time. This type of growth is commonly encountered for conventional coils, which are quite bulky because they are produced with electric wire of fairly large diameter. The electrical resistance of windings made with such wires can, in practice, be neglected. Except at high temperatures, they do not disturb the linearity of the growth of the current in the coil.

 Also shown in Figure 2, the "reference" current Iref sufficient to ensure the production of a correct ignition spark in the spark plug, at the time of

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la coupure du courant dans l'enroulement primaire 1. Pour la bobine"de référence"de plus forte inductance du lot (graphe A) ce courant est atteint au bout d'un temps de conduction tctab qui, suivant l'invention, est tabulé dans une mémoire du contrôleur 6, en fonction de la tension Vbat et, éventuellement, de la température de la bobine et du régime du moteur.

breaking the current in the primary winding 1. For the "reference" coil with the highest inductance in the batch (graph A), this current is reached after a conduction time tctab which, according to the invention, is tabulated in a memory of the controller 6, as a function of the voltage Vbat and, optionally, of the temperature of the coil and of the engine speed.

 The controller 6 also includes calculation means duly programmed to execute the calculations indicated in the rest of this description.

 For coils B and C, this reference current Iref is reached earlier. For the coil B of intermediate inductance, it is therefore at the end of a conduction time that the invention aims to cut the current in the coil, so as not to unnecessarily heat the latter, which would be the case if the conduction time was extended to the tabulated value tCtab.

 To do this, the intensity If which passes through the primary of the coil B is evaluated or measured, according to the invention, at a predetermined instant tf. We choose tf so as to be sure that tf is before the instant te of the power cut.

 To fix ideas, the choice of tf is guided by the current values known for Iref and te. If being of the order of 5 to 6 amps approximately and tc of the order of 2 to 3.5 ms. We can then choose for tf a time between 1.5 and 2 ms, for example.

 On figure 2 it appears that: tC / tCtab = Iredh = Ifth / If where IB is the intensity of the current reached in the coil B at the end of the tabulated conduction time tctab and Ifth, the intensity of the current reached, at l 'instant tf, in the primary inductance coil A with the highest inductance, for which the conduction time has been tabulated in a memory of the controller 6, as we have seen above.

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This controller therefore knows the time tctab necessary to reach the predetermined current intensity Iref in the coil with the highest inductance, and therefore also the "theoretical" intensity Ifth of the current reached in this coil at the end of the time tf. Knowing Ifth and If (by the measurement delivered by the converter 8), the controller can be programmed to calculate the correction coefficient K = Ifth / If to be applied to the tabulated conduction time tCtab to derive the conduction time te, that is: te = K. tctab with K 1.

 By cutting off at the end of time te the conduction in the primary 1 of the coil (1,2), it is thus ensured that the intensity reached by the current in this primary is substantially equal to Iref, i.e. the intensity (of l '' order of 5 to 6 Amperes (5.25 Amperes for example) which it is not necessary to exceed to ensure the production of an ignition spark in the spark plug 3, at the cut of the current.

 The evaluation of the current If poses a problem because it is not possible, with the converter 8 of the device of FIG. 1, to ensure the production of a measurement of this current at a precise instant, determined in advance . In fact, the production of measurement samples by the converter is clocked by a clock internal to the converter, which cannot be synchronized at this time.

 We will describe, in conjunction with the graph of Figure 3, the solution provided by the present invention to this problem, in the general case of a coil in which the rise of the primary current is not necessarily linear.

 Assuming that the converter clocking period is 1 ms, for example, according to the invention, a time window f, with a width of

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 a few hundred us for example, this window being centered on the instant tf defined in connection with FIG. 2.

 Each measurement sample delivered to the controller 6 can be timed and dated by the latter, relative to the start of the conduction. The controller can then determine whether a sample is produced during the time window f, or outside this window.

 During successive ignition cycles, the controller then selects several measurement samples taken during window f of the cycle, then calculates an average value of the measurement, from around thirty samples for example. Due to the random sampling of samples, this average value can be assimilated to the value If of the current in the center of the window, ie at time tf.

 It will be noted that the noise affecting this value If is eliminated by the averaging executed by the controller 6.

Also, by producing a measurement value at a predetermined instant tf, not synchronized with the clocking of the converter 8, the "apparent resolution" of this converter is increased.

 Strictly speaking, the measurement samples taken by the controller 6 to calculate the current intensity If at the instant tf, must be taken for the same value of the voltage Vbat of the battery 4, at the time of the measurements. In practice, the battery voltage very often varies over the course of its successive charges and discharges. A correction of the samples taken into account is therefore necessary to take account of this variation.

 Firstly, according to the invention, only the measurement samples taken are validated while the changes in the battery voltage are confined within a limited range of variation. The correction then applied to these validated samples can, according to the invention, take one or the other of two forms:

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soit celle d'une correction des intensités de courant saisies, fonction linéaire de Vbat, par exemple, soit celle d'un décalage de la fenêtre temporelle f, suivant une fonction du même type.

either that of a correction of the entered current intensities, linear function of Vbat, for example, or that of an offset of the time window f, according to a function of the same type.

 The calculation of the conduction time tc as described above in conjunction with FIG. 2, assumes that the current rise characteristics of the coils of the batch in question are rectilinear. This assumption is not correct with certain recent coils because, in particular, of the nature of the magnetic materials used to carry out their core and of the smoothness of the wire used to carry out less bulky coils, which increases the resistance of the windings of these coils and therefore the influence of this resistance (accentuated by that of possible temperature variations) on the current rise characteristic.

bobine (voir les caractéristiques des bobines B'ou C') une correction du temps de conduction tctab plus forte que dans le cas des bobines"linéaires"de la figure 2, est nécessaire pour éviter un échauffement excessif de la bobine. Cette correction doit donc tenir compte de la non-linéarité de la caractéristique de montée en courant de la bobine. FIG. 4 shows the typical characteristics of the primary current rise of such coils A ′, B ′, C ′ of high, medium and low inductance respectively, of the same batch of coils, the coil AI playing the role of reference coil. We observe that these characteristics have a rectilinear part followed by an elbow which raises the current in the coil above that which would result from a linear growth, at the end of the conduction period, and this all the more so as the inductance of the primary of the coil is small compared to that of the reference coil A ′. For such

coil (see the characteristics of the coils B'or C ') a stronger correction of the conduction time tctab than in the case of the "linear" coils of FIG. 2, is necessary to avoid excessive heating of the coil. This correction must therefore take into account the non-linearity of the current rise characteristic of the coil.

 According to the invention, for a coil with a non-linear characteristic, the conduction time te

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 sought can be linked to the tabulated conduction time tCtab by the relation: tc = (Al. If + BJ. tCtab (1) where Al and Bi are linear functions of the voltage Vbat, in a limited voltage range, the relation being all the more reliable as this range is narrow (12-15 volts for example).

Al = a. Vbat+b et Bi = c. Vbatd la relation (1) s'écrit, après regroupement des termes en Vbat : te = ((a. If + c). Vbat + (b. If + d)). tctab soit : te = K. tctab, avec :
K = k. Vbat + kl k = a. If + b kl= b. If + d les coefficients a, b, c, d pouvant être établis par des opérations d'étalonnage. If therefore we have:

Al = a. Vbat + b and Bi = c. Vbatd the relation (1) is written, after regrouping of the terms in Vbat: te = ((a. If + c). Vbat + (b. If + d)). tctab or: te = K. tctab, with:
K = k. Vbat + kl k = a. If + b kl = b. If + d the coefficients a, b, c, d can be established by calibration operations.

 Thus by the calculation of the coefficients k, kl and by the evaluation of the value of If at the instant tf (as described above in connection with FIG. 3), the controller 6 derives from a correction of the value of tCtab, the value of the suitable conduction time tc, for the particular coil of the batch which equips the device according to the invention. The controller therefore controls the primary conduction time of this coil.

 The coefficients k and k being a function of the value of If, the controller 6 must carry out the calculations necessary for updating these coefficients each time the value of If varies compared to that taken into account in the preceding calculation or, in practice, with a periodicity of a few seconds to a few tens of seconds.

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It now appears that the invention makes it possible to achieve the set goal, namely to regulate with precision the conduction time of the primary of an ignition coil to a value ensuring the production of a correct ignition spark, without excessive heating of the coil, and this using a conduction time control device of the "open loop" type, therefore less costly. This result is obtained by the "learning" process of the value of the conduction time to be used described above.

 This process ensures the proper functioning of all the ignition coils of a batch of such coils equipping a batch of internal combustion engines propelling different vehicles, whatever the tolerance that must be accepted on the inductances of the lot coils.

 Advantageously, the controller 6 is designed to memorize the correction coefficient resulting from this learning. Thus, when the engine is restarted, the operation of the ignition coil control device can resume starting from the correction coefficient memorized at the time of the previous stopping of this engine. The adaptation of this correction to the current operating conditions of the device according to the invention is then only a function of the temperature variation of the coil between the previous stop of the motor and its current restart.

 Of course, the invention is not limited to the embodiments described and shown which have been given only by way of example. Thus the learning process described above could be based on a value of the tabulated conduction time for the primary coil with the lowest inductance of a batch of coils, rather than that relating to the coil primary of higher inductance of this batch. The correction coefficient K to apply to the tabulated value would then always be greater than or equal to 1.

Claims (10)

CLAIMS 1. Method for controlling an ignition coil (1, 2) of an air / fuel mixture in a cylinder of an internal combustion engine, according to which the primary (1) of said coil (1) is connected , 2) to a source (4) of an electrical voltage (Vbat) during a precalculated conduction time (te), characterized in that, to calculate said conduction time, a) the intensity (If) of the current flowing in said primary (1), at a predetermined instant (tf) chosen so that said intensity (If) is then less than a reference intensity (Iref) ensuring correct ignition of said mixture, and b) said time is drawn from conduction (te) of the relation: te = K. tctab WHERE: tCfab is a tabulated conduction time as a function of at least said voltage (bai), and - K a correction coefficient as a function of said evaluated intensity (If).  2. Method according to claim 1, characterized in that the time (tCtab) is tabulated for a reference coil whose primary has a predetermined inductance.  3. Method according to claim 2, characterized in that the primary (1) of the coil to be controlled has an inductance equal to or less than that of the primary of said reference coil.  4. Method according to claim 3, adapted to a coil (1,2) in the primary (1) from which the current increases linearly at least until said reference value (Iref), characterized in that one draws the value of said coefficient (K) of the relationship K = Iflh / If, where Ifth is the current intensity reached at said predetermined instant (tf) in said reference coil. <Desc / Clms Page number 15> 5. Method according to claim 3, adapted to a coil (1, 2) in the primary (1) from which the current increases non-linearly, characterized in that said correction coefficient (K) is derived from the relation: K = k. Vbat + k 'where k and k' are coefficients functions of said current intensity (If) evaluated at said predetermined instant (tf).  6. Method according to claim 5, characterized in that the coefficients (k) and (k ') are of the form: k = a. If + c k '= b. If + d where a, b, c, d are coefficients whose values are established by calibration.  7. Method according to any one of claims 1 to 6, characterized in that, for evaluating said current intensity (If) in the primary (1) of the coil (1,2) at said predetermined instant (tf) of said conduction time of said primary (1), a time window (f) of predetermined width is centered centered on said instant (tf), a measurement of the intensity of said current is sampled in said primary, the values of said timed samples are noted in said window during a plurality of successive ignitions and the average value of said samples taken at said current intensity (If) is identified.  8. Method according to claim 7, characterized in that the sampling of said measurement is corrected as a function of possible variations in the voltage (Vbat) of the voltage source (4) intervened during said sampling.  9. Method according to any one of claims 1 to 8, characterized in that, when the engine is stopped, said correction coefficient (K) is stored and <Desc / Clms Page number 16>  in that the memorized coefficient is applied on the next restart of the engine.  10. Device for implementing the method according to any one of claims 1 to 9, characterized in that it comprises: a) means (7,8) for evaluating the intensity (If) of current flowing in the primary (1) of said ignition coil (1,2) at said predetermined instant (tf), b) control means (5,6) for the passage of a current in said primary (1), said means comprising a memory containing said tabulated conduction time (tCtab) as a function of at least said voltage (Vbat) and means for calculating said correction coefficient (K) and said conduction time (tc).  He. Device according to claim 10, characterized in that said means for evaluating said current intensity (If) comprise means (8) for sampling the current passing through the coil, and means (6) for selecting those timed in a time window (f) centered on the instant (tf) during several successive ignitions, for calculating the average of said samples and for identifying said average at said current intensity (If) to be evaluated.  12. Device according to claim 11, characterized in that said sampling means comprise an analog-digital converter (8) of a voltage taken from a resistor (7) crossed by said current passing through said primary (1) of said coil (1,2).