EP1469197A1 - Method for controlling the ignition primary current of an spark ignition internal combustion engine - Google Patents

Abstract

The method involves measuring a current intensity (Ici) of an inductive circuit, in last tenth of a preset conduction time (tdi). Current intensity (Ifi) at the end of the time (tdi) is estimated based on the intensity (Ici). The time (tdi) is corrected for an ignition cycle during which a last intensity is measured based on the estimated intensity and a desired current intensity (Ii) at the end of the time (tdi).

Description

The present invention relates to a method for controlling the primary current ignition in an internal combustion engine with positive ignition.

In such an engine, a fuel / oxidizer mixture is ignited using a spark to cause a motor explosion. The spark is produced by a candle. The latter has two electrodes between which an electric arc is caused to achieve the spark. There is a potential difference between the electrodes large enough to be able to create an arc but a current must also flow between the electrodes to bring enough energy to the fuel / oxidizer mixture and the fire.

Conventionally, the large potential difference across the spark plug electrodes is obtained by creating a current break in a circuit comprising a primary winding, and by amplifying the resulting overvoltage in a secondary winding. A current is circulated in the primary circuit for a determined time, called dwell time and also conduction time. A length classic is around 3 to 4 milliseconds. The intensity of the current in the circuit primary gradually increases throughout the dwell time. It's important to perfectly master the value of the current at the time of the circuit break. Indeed, if this intensity is too low, the energy delivered to the candle is not sufficient to ignite the fuel / oxidizer mixture. If on the contrary this intensity is too important, thermal problems appear at the level of the coil. The flow circulating in it being too large, it heats up by Joule effect, which creates parasitic phenomena. In addition, coils are currently tending to become smaller and smaller and use smaller and smaller diameter wires. Therefore they are more susceptible to thermal problems than larger sized coils important.

When establishing the current in the primary circuit, the intensity of the current increases substantially linearly but this growth becomes faster at the end of dwell time. It is therefore advisable to perfectly master this dwell time because a small variation of it leads to a very noticeable variation in the intensity of the current in the primary circuit when the circuit breaks.

Several methods and devices are known to best control this current. We cite here for example the method and the device revealed by the document FR-2 820 465. In the preamble to this document, the problem set out above is repeated. The solution proposed in this document is to define a time window of width predetermined. An analog to digital converter (ADC) then takes measurements intensity at regular intervals. When an acquisition is made in the window predefined time, this measurement is taken into account. We then study the behavior of the curve giving the intensity of the current with respect to time in depending on the values read. From the behavior of this curve in the window predefined time, we deduce the behavior of this curve at the time of the broken electrical circuit. This process gives good results but its implementation is quite heavy because it is necessary to determine for each type of coil a set of coefficients which allows to calculate, from the acquisitions made in the predefined window, the behavior of the intensity at the time of the rupture of the primary circuit.

The object of the present invention is therefore to provide a method making it possible to control the intensity in the primary circuit at least as reliable but without requiring the setting up of a calibration as is the case for the process described in the document supra. Preferably, the implementation of the method according to the invention does not require additional cost in the ignition circuit.

To this end, it proposes a method for controlling a primary current in an ignition coil of an internal combustion engine with positive ignition, in which current is established in an inductive primary circuit for a given duration, called conduction time, and determined by calculation and / or as a function of measurements performed in the primary circuit.

• prédétermination du temps de conduction,
• réalisation d'au moins une mesure de l'intensité du courant dans le circuit primaire à une date comprise dans le dernier dixième du temps de conduction prédéterminé,
• estimation de l'intensité du courant à la fin du temps de conduction prédéterminé en fonction de la (des) mesure(s) réalisée(s),
• correction éventuelle du temps de conduction pour le cycle d'allumage au cours duquel la dernière mesure d'intensité a été réalisée en fonction de l'estimation précédente et de l'intensité de courant souhaitée en fin de temps de conduction.

According to the present invention, the conduction time is calculated according to the following steps:

• predetermination of the conduction time,
• carrying out at least one measurement of the intensity of the current in the primary circuit on a date included in the last tenth of the predetermined conduction time,
• estimation of the intensity of the current at the end of the predetermined conduction time as a function of the measurement (s) carried out,
• possible correction of the conduction time for the ignition cycle during which the last intensity measurement was carried out according to the previous estimate and the current intensity desired at the end of the conduction time.

Planning to take a measurement in the last tenth of the time conduction provides a good approximation of the current intensity during the opening of the circuit. This allows to gain precision in determining the time conduction. It is therefore unnecessary to precisely characterize the ignition coils for extrapolate their behavior just before the opening of the primary circuit because the measurement performed gives a fairly accurate indication of this behavior.

Conduction time correction can be performed for the cycle ignition during which the last intensity measurement was made but it can also be performed in a subsequent cycle.

In one embodiment of the invention, the conduction time predetermined is obtained for example from tables stored in a device for management and control of the ignition coil according to parameters such as in particular the potential difference applied to the terminals of the primary circuit.

In a preferred embodiment, estimating the current at the end of the predetermined conduction time is obtained from an extrapolation measurement linear. Such an extrapolation is easily achievable and in the present case gives very good results. To increase the precision of the process, however, we can also predict a higher order extrapolation but the gain in precision is then not very sensitive.

To avoid a "granularity" effect of the measurement and not to obtain a ignition cycle to the next cycle significant differences in correction compared to the value of the predetermined conduction time, the control method according to the invention proposes that the estimation of the current at the end of the conduction time predetermined is carried out by linear extrapolation of the measurement carried out by carrying out an average with previous measurements. In this case, an average sliding of the estimated final current intensity is for example performed.

As for the estimation of the final current, the correction of the time of conduction is preferably carried out linearly as a function of the intensity of the current final, averaged or not.

The control method according to the invention makes it possible to provide that the intensity of the desired final current is determined according to the engine speed corresponding. In this case, the predetermined conduction time, when calculated at from tables, then also depends on the speed of the corresponding engine.

La figure 1 représente schématiquement un système d'allumage pour un moteur à combustion interne à allumage commandé, et

La figure 2 est un graphe représentant les variations de l'intensité du courant dans le circuit primaire durant le temps de dwell.

Details and advantages of the present invention will emerge more clearly from the description which follows, given with reference to the appended schematic drawing in which:

FIG. 1 schematically represents an ignition system for an internal combustion engine with positive ignition, and

FIG. 2 is a graph representing the variations in the intensity of the current in the primary circuit during the dwell time.

Figure 1 schematically shows an ignition device for a internal combustion engine with positive ignition. We recognize in this figure a classic ignition coil. This coil has a primary winding 2 called also commonly "primary" and a secondary winding 4 commonly called "secondary". These two windings cooperate with each other so as to form a transformer 6 voltage booster.

The primary winding 2 is supplied by a voltage source 8 which is usually the corresponding vehicle battery. A switch 10 which presents itself here in the form of a transistor controls the power supply of the winding primary 2.

The secondary winding 4 has a common terminal with the primary winding 2. The other terminal of the secondary winding 4 is connected to a electrode of a spark plug 12, the other electrode of this spark plug being connected to the mass 14.

When a significant potential difference appears between the electrodes of spark plug 12, a spark occurs and allows, insofar as the energy at the spark is sufficient, ignite a fuel / oxidizer mixture surrounding the spark plug electrodes 12. This significant potential difference is achieved by causing an overvoltage at the terminals of the primary winding 2. In a known manner, an overvoltage occurs at the terminals of a winding having an inductance when the electrical circuit comprising this inductor is open. This overvoltage terminals of the primary winding is amplified by transformer 6 and we thus obtain conventionally a voltage of several kV at the level of the secondary winding 4 and therefore of the spark plug electrodes 12. A management and control device 16 of the ignition coil controls the opening and closing of the transistorized switch 10.

This management and control device 16 is connected to a central unit managing the engine and from which it can receive information such as for example the speed N of the corresponding engine. This command and management device 16 receives also information on the primary circuit of the ignition coil. So, he knows the potential difference V provided by the voltage source 8 and the intensity I of the current flowing in this primary circuit. An analog / digital converter 18 (or CAN) makes it possible to measure the intensity of the current I. This converter 18 actually measures a potential difference across a known resistor 20. A microcontroller integrated into the converter 18 manages the acquisitions made by the latter. So when a measurement is performed, we know precisely the date on which this measurement is performed. We can thus locate this measurement in relation to the closing of the switch 10 that is to say with respect to the start of the establishment of a current in the primary circuit.

FIG. 2 presents a curve 22 showing the evolution of the intensity of the current I in the primary circuit as a function of time t. We assume that the switch 10 closes at time t = 0 and opens at time t = td _i.

At the instant t = 0, the intensity is zero while at the instant t = td _i the intensity of the current in the primary circuit is equal to I _i .

Note that near td _i the intensity I increases more quickly (ie dl / dt increasing). This curve 22 corresponds to a current curve generally observed in the primary circuit of an ignition coil.

For the spark to be produced at the spark plug 12 after the opening of the primary circuit on the date t = td _i, it is necessary that I _i ,> = I _ref where I _ref corresponds to the minimum value allowing ignition of the fuel / oxidizer mixture.

As mentioned in the preamble, the value I _{i should} not exceed the value I _ref too much so as not to risk damaging the coil. As is known to those skilled in the art, by adapting the value td _i, which corresponds to the dwell time for the ith ignition cycle, one acts directly on the value I _i. By increasing the dwell time, or conduction time, of the ith cycle, the value of the intensity is increased | _i of the current flowing through the primary circuit at the end of this conduction time. Conversely, by decreasing the dwell time, the intensity of the current at the end of the ignition cycle is reduced.

To best adjust the value of the intensity at the end of the dwell time and obtain in the primary circuit when the switch 10 opens a current of intensity as close as possible to a _target value I, the present invention proposes to carry out a measurement of the intensity using the converter 18 at a date t _i very close to td _i . We preferably choose t _i > = 0.9 td _i .

The value td _i is for example calculated by the command and management device 16 using a table stored therein and giving for each cycle a dwell time as a function of the voltage V at the terminals of the source voltage 8.

The value of the intensity measured at the date t = t _i is Ic _i .

The line 24 then passing through the origin and through the point (t _i, Ic _i ) is then determined. The equation of this line is as follows: I = (Ic i / t i ). t

To then make an estimate of the intensity of the current at the end of the conduction time, the intersection of the line 24 with the line of equation t = td _i is calculated. We then find the point of coordinates (td _i , If _i ), where If _i is the estimated value of the intensity at the end of the conduction time. We then compare the value of If _i to the value I _{aimed i} of the intensity of the current that we wish to have when the switch 10 is opened. Of course, if If _i = I _{aimed i} then the command and management 16 will command the opening of the switch 10 on the date t = td _i . Otherwise, a new dwell time is calculated. It is calculated for example by linear approximation, which then gives the following equation: td horn i = (I aim i / If i ) .td i

We therefore apply a correction coefficient to the value of the dwell time or conduction time, previously determined by the control and management 16 as a function in particular of the voltage V across the terminals of the voltage source 8.

In theory and in practice, this process works and provides a satisfactory intensity when opening the primary circuit. We notice that counts given the shape of curve 22, in particular in the vicinity of its end (on the right on Figure 2), the actual intensity is normally slightly higher than the target intensity. This difference is very small and does not risk damaging the ignition coil.

Given the measurement uncertainties, both in terms of intensity and time, it is preferable to average the measurements made. This avoids the "granularity" effects of the measurement, effects well known to those skilled in the art. The invention therefore proposes to calculate td _{cor i} not only as a function of the measurement carried out during the ith cycle but also as a function of the measurements carried out during the (n-1) preceding cycles. The final intensity of the current is then estimated as a function of the final intensity estimated during the previous cycle and the value of the final intensity estimated during the current cycle. We then have the equation: If av i = ((n-1) If av i-1 + If i )/not where If _{moy i} is the estimated current value at the end of the ith cycle, and
If _{av i-1} is the value of the current intensity estimated at the end of the cycle during the previous cycle.

To calculate then td _{cor i} we can use the formula indicated above but we can also, alternatively, proceed as follows. First of all, a correction coefficient k _i is calculated as a function of the same correction coefficient k _i-1 calculated during the previous cycle. We then define k _i as follows: k i = k i-1 + [filter. (I aim i - If av i ) / I aim i ] where filter is a fixed coefficient stored in the command and management device 16.

The corrected conduction time is then calculated as follows: td horn i = k i . td i

The method as described above therefore makes it simple and reliable. to obtain, at the time of opening of the primary circuit, a current whose characteristics allow to have sufficient energy at the level of the corresponding candle 12 without create a thermal problem with the ignition coil.

To obtain even greater precision, the invention proposes to vary the intensity I _targeted as a function of the engine speed. Of course, by varying the value of I _{aimed at,} the value of the dwell time predetermined by the command and management device is also varied. This value, predetermined by the command and management device 16, then depends both on the voltage V across the terminals of the voltage source 8 and on the engine speed N.

Compared to known prior art methods, the method according to the invention has the advantage of being very simple while being very precise. The only calibrations to be envisaged during the implementation of this process are the establishment of tables giving the value of the intensity I _targeted as a function of the voltage prevailing at the terminals of the voltage source supplying the primary circuit and possibly also of the engine speed. It therefore suffices to produce a dwell table like that which is usually produced for any electronic ignition system.

In addition, in the embodiment providing for averaging the measurements previously performed, there are no significant variations from one correction to another. The correction made with respect to the dwell table stored in the device ordering and management is thus progressive.

The present invention is not limited to the embodiments described above by way of nonlimiting examples. It also concerns all variants of realization within the reach of the skilled person within the scope of the claims below.

So for example we could make two intensity measurements during the dwell time, including one near the end of this dwell time, to try improve the precision of the process. One can also imagine here realizing, no longer a estimation by linear approximation, but by a higher order approximation.

The description above uses to average the measurements made a arithmetic average. Other averages can also be achieved without as well depart from the scope of the present invention.

In the embodiments described above, the measurement (s) is (are) used to modify the conduction time of the ignition cycle during which the measurement is carried out. However, it is also possible to use the corrected measured value for the determination of the conduction time of the next cycle. We can then imagine for example a converter measuring the intensity of the current as close as possible to the end of the dwell time. The measurement then carried out is compared to the intensity I _targeted and the dwell time of the following cycle is calculated as a function of the measurement carried out. One can estimate that the measurement carried out gives the real value of the intensity of the current at the end of conduction time or else estimate the value at the end of conduction time from the measurement carried out by a predetermined formula.

Claims (7)

Method for controlling a primary current in an ignition coil of an internal combustion engine with positive ignition, in which the current is established in an inductive primary circuit for a given duration, called conduction time, and determined by the calculation and / or based on measurements made in the primary circuit,
characterized in that the conduction time is calculated according to the following steps: predetermination of the predetermined conduction time (td _i ), carrying out at least one measurement of the intensity (Ic _i ) of the current in the primary circuit on a date (t,) comprised in the last tenth of the predetermined conduction time (td _i ), estimation of the intensity of the current (If _i ) at the end of the predetermined conduction time (td _i ) as a function of the measurement (s) carried out, possible correction of the conduction time (td _i ) for the ignition cycle during which the last intensity measurement was made according to the previous estimate and the desired current intensity (I _{referred to i} ) at the end of conduction time. Control method according to claim 1, characterized in that the predetermined conduction time (td _i ) is obtained from tables stored in a management and control device (16) of the ignition coil as a function of parameters such as in particular the potential difference (V) applied to the terminals of the primary circuit. Control method according to either of Claims 1 and 2, characterized in that the estimation of the current (If _i ) at the end of the predetermined conduction time (td _i ) is carried out from a measurement by linear extrapolation. Control method according to one of claims 1 to 3, characterized in that the estimation of the current (If _i ) at the end of the predetermined conduction time (td _i ) is carried out by linear extrapolation of the measurement carried out by carrying out a average with previous measurements. Control method according to claim 4, characterized in that a sliding average of the intensity of the estimated final current is produced. Control method according to one of claims 1 to 5, characterized in that the correction of the conduction time is carried out linearly as a function of the intensity of the final current, averaged or not. Control method according to one of Claims 1 to 6, characterized in that the intensity of the desired final current (I _{referred to i} ) is determined as a function of the speed (N) of the corresponding motor.

Images