EP1049862B1 - Device for estimating richness in an injection system for an internal combustion engine - Google Patents

Description

The invention relates to fuel injection systems. in the combustion chambers of a combustion engine internal, including a spark ignition engine ; it particularly concerns devices allowing to estimate the air / fuel ratio admitted in combustion chambers usable in such systems.

• un capteur fournissant un signal de sortie à variation sensiblement linéaire avec la richesse, placé à un point de confluence des échappements des n chambres, et
• des moyens de calcul pour :
  • mémoriser un modèle de comportement de l'échappement au point de confluence basé sur l'hypothèse que la richesse au point de confluence, ou le rapport air/combustible, est une somme pondérée des contributions des échappements des chambres individuelles, le coefficient de pondération étant d'autant plus faible que la combustion dans la chambre est plus ancienne, et
  • estimer, après chaque passage au point mort haut, le rapport air/combustible à partir des valeurs mesurées et du modèle.

We know in particular a device for estimating the richness of the mixture admitted into each of the n combustion chambers (n being an integer greater than 1 and generally equal to 4, 6 or 8) of an engine having injection injectors in the cylinders, comprising:

• a sensor providing an output signal of substantially linear variation with the richness, placed at a point of confluence of the exhausts of the n chambers, and
• calculation means for:
  • memorize a model of behavior of the exhaust at the confluence point based on the assumption that the richness at the confluence point, or the air / fuel ratio, is a weighted sum of the contributions of the exhausts of the individual chambers, the weighting coefficient being the lower the combustion in the chamber is, and the lower
  • estimate, after each shift to top dead center, the air / fuel ratio from the measured values and the model.

Such a device is for example described in the patent US 5,548,514 or in document EP-A-0 719 922, to which can refer.

Such a device can be used in particular in a injection system of the kind shown schematically in figure 1. The diagram shows a motor 10 at n = 4 chambers of combustion, each provided with an injector 12. The air admitted to through a filter 14 passes through a throttle body 16 before to arrive at an intake manifold 18. Exhaust gases leave the rooms by individual tubes which connect at a point of confluence to a collector exhaust 20.

• la position angulaire du papillon 16, mesurée par un capteur 22,
• la pression dans le collecteur d'admission, mesurée par un capteur 24,
• la température  de l'eau de refroidissement et/ou des gaz d'échappement, et
• le signal de sortie d'un capteur de mesure de richesse 26, placé au point de confluence.

The quantities of fuel supplied to each cylinder at injection times are set by a computer 21 on the basis of operating parameters which may in particular include:

• the angular position of the butterfly 16, measured by a sensor 22,
• the pressure in the intake manifold, measured by a sensor 24,
• the temperature  of the cooling water and / or the exhaust gases, and
• the output signal from a wealth measurement sensor 26, placed at the point of confluence.

The injection times are fixed with an advance by relative to the shift to the top dead center of each chamber of combustion, using a synchronization signal supplied by a sensor 28 placed opposite the flywheel 30 of the motor 10.

A simple model of wealth representation measured at the confluence point consists in associating, with the measurement made by the sensor 26 at several successive passes combustion chambers at top dead center, a weighting coefficient which is solely a function of the seniority of the passage in the operating cycle of the engine. The entry of the model is the wealth admitted to the combustion chamber which has just shifted to top dead center (current cylinder). Exhaust puffs to the point of confluence are combined together to represent the mixture of gases.

On the other hand, there is a dispersion of characteristics between the injectors, so that an injection of the same fixed term does not correspond to the same quantities of fuel injected into the different rooms.

In the case, for example, of four combustion chambers, a vector of coefficients C _i is assigned to the sensor with i = {1,2,3,4), C ₄ corresponding to the current cylinder and the other, smaller coefficients, corresponding to other cylinders, in reverse ignition order.

This solution is in fact not entirely satisfactory, because exhaust pipes are generally asymmetrical.

The present invention aims in particular to provide a estimation device responding better than those previously known to the requirements of practice, from the fact that it very significantly reduces the incidence of asymmetries and, in precisely the case of asymmetry, the invention improves the correction of the dispersions of injector characteristics.

To this end, the invention notably proposes a device in which the behavior model includes a particular sub-model per combustion chamber having, for the order chamber i, a Kalman filter having an mxn matrix of coefficients C _ij and a own earnings matrix K _ij , i being equal to {1, ...., n} corresponding to the room number and j corresponding to the weighting coefficient number, here from 1 to m. In other words, the invention proposes a different model for each room i, defined by a set {j} of m coefficients, m being able moreover to be equal to n.

Such a device, which eliminates the asymmetry effect has the advantage of greatly reduce the effect of dispersions of characteristics between the injectors and accordingly authorize the use of injectors with machining less precise.

The model can be represented by one or more matrices (C _ij ) ^ℓ each corresponding to an operating zone ℓ of the engine determined by one or more parameters from the load range, the temperature of the exhaust gases, the temperature of the cooling water, engine speed and pressure in the intake manifold.

The matrix chosen may also depend on the setpoint richness given by the computer and which can depend on engine operating conditions according to constraints on pollution or driving pleasure.

la figure 1, déjà mentionnée, montre schématiquement les éléments d'un moteur concernés par l'invention ;

la figure 2 est un schéma synoptique, montrant les sous-ensembles principaux d'un dispositif suivant l'invention, et fonction de ces sous-ensembles pouvant être réalisé par voie matérielle ou par voie logicielle ;

la figure 3 est un schéma fonctionnel de moyens de compensation du retard de mesure introduit par le capteur de richesse ;

la figure 3A indique des courbes de réponse type des moyens de la figure 3 ;

la figure 3B montre une courbe de réponse en phase en fonction de la fréquence ;

la figure 4 est un schéma fonctionnel de moyens d'acquisition synchrone des richesses, chambre de combustion par chambre de combustion ;

la figure 5 est un schéma de moyens de correction de richesse.

la figure 6 montre un bloc de gestion d'erreur de richesse incorporant les moyens de la fig. 5.

The above characteristics as well as others will appear better on reading the following description of a particular embodiment, given by way of non-limiting example. The description refers to the accompanying drawings, in which:

Figure 1, already mentioned, schematically shows the elements of an engine concerned by the invention;

FIG. 2 is a block diagram showing the main sub-assemblies of a device according to the invention, and function of these sub-assemblies which can be produced by hardware or by software;

FIG. 3 is a functional diagram of means for compensating for the measurement delay introduced by the richness sensor;

FIG. 3A indicates typical response curves of the means of FIG. 3;

FIG. 3B shows a phase response curve as a function of the frequency;

Figure 4 is a block diagram of synchronous wealth acquisition means, combustion chamber by combustion chamber;

FIG. 5 is a diagram of means for correcting wealth.

FIG. 6 shows a wealth error management block incorporating the means of FIG. 5.

The device according to the invention presents the constitution shown in Figure 2. Most of the functions are fulfilled by the computer 21. However some of them, including functions of filtering of fixed characteristics, can be performed in analog form by wired circuits.

The device comprises a compensator 32 intended for compensate for the delay introduced by the sensor 26. Means 34 synchronous wealth acquisition can be looked at as having a Kalman filtering observer 36 and correction means 38 outputting the air / fuel ratios admitted to the rooms during the cycle that has just passed. To affect wealth in the appropriate rooms, the correction means receive a synchronization signal constituted by the output of the sensor 28 followed by a circuit 40 of modulo n division, here equal to 4.

Synchronization must be initialized, the sensor 28 not allowing to know which combustion chamber just shifted to top dead center. This initialization can be done by various known methods.

Finally, management means 42 determine the durations opening the injectors 12 from information developed by the computer 21, constituted for example by the air flow admitted and by the required richness, and from corrections provided by the means 38.

The model allowing synchronous acquisition means 34 to determine the richness of the mixture admitted to each chamber relies on the measurements provided by the sensor single 26 located at the confluence point. It's important to have, after each shift in top dead center, a representative measure of wealth while a combustion has just shifted to neutral. Now the usual sensors, in particular because they include a pierced protective cover of the probe, introduce a measurement delay.

We already know various arrangements intended to compensate for the measurement delay. However, it is advantageous to use the compensation means shown diagrammatically in FIG. 3, which are applicable not only to synchronous acquisition means which are described below, but also to means synchronous acquisition of any other type previously known.

The adopted strategy is represented functionally in Figure 3. The signal from the probe is submitted to a high pass filtering 43 whose characteristics take into account the time constant τ of the sensor cover several tens of ms. So that the filtering is stable, the value taken into account in the high pass filter will be related to the lowest time constant among all those that can be encountered under various conditions engine operation.

The high pass filter 43 amplifies the noise which is attenuated or eliminated by a feedback loop including a low pass filter 44, an adder 46 receiving the low pass filtering output and an input signal and a subtraction 48.

We thus obtain measured wealth information and compensated which can be stored in a memory vive 50, which can possibly be organized in a register shifted.

In practice, the functions shown in Figure 3 will be implemented numerically. The output current of the sensor 26 is sampled, at a rate which can be of the order of 2 ms. Filtering as a whole can be provided to implement a form inversion function: G (s) = [1 + hood -1 (S). Low pass (s)] / [1+ low pass (s)

In this expression, the hood reversal function ^-1 (s) can be of the following form, β designating a pole: hood -1 ( s ) = τ × β × s + 1 τ s + β

High pass and low pass filters introduce earnings and are expected so that these earnings will vary depending of the frequency according to laws which can be those indicated respectively by the solid lines and in phantom in Figure 3A. Low pass filtering may just be first-rate.

The compensation being provided in digital form, on discrete values, we can limit ourselves to performing a transformation of Euler.

x(k):variable d'état

u(k):valeur mesurée

y(k):valeur de sortie

k:instant considéré (échantillonnage 2 ms par exemple) la fonction d'inversion du capot est :

   et le filtrage passe-bas devient :

We can use the usual notations:

x (k): state variable

u (k): measured value

y (k): output value

k: instant considered (2 ms sampling for example) the hood inversion function is:

and the low pass filtering becomes:

In the second formula,  denotes the filter gain low pass, designed to eliminate high frequency noise generated or amplified by high-pass inversion filtering.

At the outlet of the compensator 32, there is a card wealth that allows you to find instant riches based on the instant compensated signal.

The wealth thus measured and compensated is used as inputs for observer 36 with filtering of Kalman.

Currently, this Kalman filtering is generally performed by adopting the same Kalman gain and the same weights regardless of the room of combustion for which we want to determine the richness.

According to one aspect of the invention, an optimal anticipation gain K _ij of Kalman is determined and a set of weighting coefficients C for each of the combustion chambers.

The functional diagram of the observer can then be the one shown schematically in Figure 4. This observer can be considered to consist of n = 4 observers elementary.

Each of these elementary observers can have a relatively classic constitution. The calculation allowing for example to determine the richness of cylinder 1 corresponds to the orientation of switches 52 given in FIG. 4, the switches being in fact constituted by a program permitting the permutation of gain and coefficients for calculation.

The successive measurements y _mes (k) at the confluence point are accumulated at 54 and processed by an operator z ^-1 at 56 whose output is brought back, by a gain gain loop 58, to accumulation 54.

The data obtained at the end of the top dead centers of n = 4 successive cycles are multiplied by the weighting coefficients (C _ij ) corresponding to the cylinder i. The value y _is (k) obtained at the output 60 is representative of the richness estimated at the point of confluence. It is reintroduced into an input subtractor 62, so as to generate an error signal e (k) which is applied to the input of the Kalman filtering.

avec

The equations representative of the estimate, for a given cylinder, are then the following, with the notations used in FIG. 4 and if X (k) designates the state variable. e ( k ) = there my ( k ) - there East ( k ) E KALMAN ( k ) = G KALMAN ⊗ e ( k )

with

The weighting coefficients C _ij can be obtained experimentally by identification by means of a measurement bench using a set of probes capable of measuring the richnesses on each tubing and the richness at the point of confluence.

The richness of the current cylinder is then available at the output 64 of the accumulator 54.

For the same cylinder, several assemblies will often be provided, each having a Kalman gain K _ij and a set of weighting coefficients C _ij , each assembly being assigned to a particular engine operating zone.

le signal de richesse mesurée et compensée,au point de confluence, provenant de la mémoire 50,

des signaux indiquant la richesse estimée du cylindre courant, provenant de la sortie 64 del'observateur,

et le signal de synchronisation provenant du diviseur 40 modulo 4.

The development of the corrections can be carried out according to the functional diagram of FIG. 5. The correction means receive, as inputs:

the signal of measured and compensated richness, at the point of confluence, coming from the memory 50,

signals indicating the estimated richness of the current cylinder, coming from the output 64 of the observer,

and the synchronization signal from the divider 40 modulo 4.

The richness correction to be made to a cylinder to be determined is calculated as a product of two terms,
a term 1 + λ _g , λ _g being a percentage of general correction relating to the richness measured at the point of confluence,
a term 1 + λi, specific to the cylinder of order i in which the injection will be controlled.

The first term is developed from a signal error provided by a subtractor 66 which receives on the one hand a signal representative of the wealth setpoint (which depends on engine operating conditions) and on the other hand, the output signal from memory 50. An error management module 68 develops a corrective term, which is processed by a proportional-integral filter 70 intended to stabilize the system. We thus obtain λg.

The terms λi are each developed using a subtractor 72 which receives on the one hand the output signal 64 modulo 4, developed by a switch 5, and on the other hand a richness set signal specific to the cylinder.

This wealth setpoint signal can be the same for all cylinders. The wealth deposit could also be different depending on the cylinder.

The error signal obtained is still subject to a proportional-integral filtering 74, known as PI, to obtain a corrector term λi. A circuit 76 will allow to develop the product (1 + λi) (1 + λg) which constitutes a correction factor on the injection time of cylinder i.

PI filtering has a role in compensating for the gas path between the injection points and the confluence.

The wealth error management module 68 has in particular to make the switchings of the sensor faster by acting on the error injected into the PI 70 filter. introduced, in addition to an amplification of the error of wealth, a hysteresis only causing a tilting of the sensor that beyond stoichiometry when going towards a rich mixture, below stoichiometry when returns to a lean mixture. Beyond the tilting, the management module has a substantially proportional response.

The proportional gain factors K _p and integral K _i of the correction filters 74 are chosen as a function of the travel delay between the injectors and the richness sensor, counted in number of TDCs.
K _p will generally be less than 1 to attenuate the high frequencies.
K _i can be of the form: K i = K p x P x (2 / delay time) for a 4-cylinder engine. P is an adjustable constant to adjust the dynamics.

Finally, the management circuit 42 (FIG. 2) allows, at from an input signal 78 indicating the amount of air admitted to the cylinder and from the corrector term received from the means 36, to modify a corresponding basic injection time to the wealth setpoint to set the opening time of each of the injectors 12 and order the injector. This circuit can actually include a digital part of calculation incorporated in the computer 21 and an analog part and of power developing the pulsed supply current injectors.

The wealth management circuit can correspond to the block diagram of FIG. 6. The wealth instruction for the injector i is applied to the input 80 and multiplied by a signal 82 representative of the quantity of air admitted. The product is multiplied by the gain of the injector at 84 to obtain a base injection time Ti. In module 86, the correction signal supplied by the means in FIG. 5 is used to supply Ti (1 + λ _i ) (1 + λ _g )

Establishing the model requires determining the weighting coefficients for a given engine. This determination can be made on a test bench in temporarily equipping the engine with richness sensors at the output of each cylinder, in addition to the final sensor.

• immédiatement après lancement du moteur, richesse supérieure à la stoechiométrie permettant une fin de démarrage et un départ optimal, la richesse étant fonction de la température du liquide de refroidissement est d'autant plus important que la température est basse.
• à la fin d'une période initiale (21 secondes par exemple) calcul d'un rapport R/combustible correspondant à une "limite" pauvre et de la durée d'un palier de maintien à cette valeur, uniquement en fonction de la température du liquide de refroidissement (supposée représentative de l'état du catalyseur)
• décroissance quasi exponentielle vers la limite pauvre, pour réduire la pollution, suivie d'un palier
• à l'issue du palier, au cours duquel il y a réchauffement du catalyseur, remontée vers la stoechiométrie, suivant une loi qui peut être linéaire pour assurer un bon agrément de conduite, la pente de croissance étant calibrable.

The strategy for establishing the wealth setpoint, starting from the full start, stored in the computer 21, can be as follows.

• immediately after engine launch, richness greater than stoichiometry allowing an end of starting and an optimal departure, the richness being a function of the temperature of the coolant is all the more important as the temperature is low.
• at the end of an initial period (21 seconds for example) calculation of an R / fuel ratio corresponding to a lean "limit" and of the duration of a holding stage at this value, solely as a function of the temperature of the coolant (assumed to be representative of the condition of the catalyst)
• almost exponential decrease towards the lean limit, to reduce pollution, followed by a plateau
• at the end of the plateau, during which there is heating of the catalyst, rising to stoichiometry, according to a law which can be linear to ensure good driving pleasure, the growth slope being calibratable.

Claims (5)

1. Apparatus for estimating the richness of a mixture admitted into each of n combustion chambers (n being an integer greater than 1) of an engine having injectors for injection into the cylinders, the apparatus comprising:

   · a sensor (26) providing an output signal which varies substantially linearly with richness, the sensor being placed at a junction point between the exhausts from the n chambers; and

   · computer means for:

   · storing a model of the behavior of the exhaust at the junction point based on the assumption that the richness at the junction point , or air/fuel ratio, is a weighted sum of the contributions of the exhausts from the individual chambers, the weighting coefficients decreasing with increasing age of combustion in the chamber, and

   · after each pass through top dead center, estimating the air/fuel ratio on the basis of the measured values and of the model;

      characterized in that the behavior model includes a sub-model which is specific to each combustion chamber and comprises, for each chamber of order i, a Kalman filter having an m × n matrix of coefficients C_ij and a matrix of specific gains K_ij, where i is equal to {1, ..., n} and corresponds to the numbering of the chamber, and where j lies in the range 1 to m and corresponds to the numbering of the weighting coefficient.
2. Apparatus according to claim 1, characterized in that each sub-model is associated with a plurality of sets of matrices and gains each corresponding to operating ranges of the engine as determined by one or more parameters including load range, exhaust gas temperature, cooling water temperature, engine speed, and pressure in the admission manifold.
3. Apparatus according to claim 1 or 2, characterized in that the richness sensor comprises, in addition to a probe (26) placed at the junction point, means for compensating the response delay of the probe, said means comprising a highpass filter (42) followed by a counterfeedback loop having a lowpass filter (48), an adder (46) receiving the output from the lowpass filter and the input signal coming from the probe, and a subtracter (48) receiving the output signal from the adder and the output signal from the highpass filter, feeding the lowpass filter.
4. Apparatus according to claim 3, characterized in that the compensation means are digital, in that the highpass filter functions are of the form:

   

   while the lowpass filtering is of the form:

   

   where:

   x(k): state variable;

   u(k): measured value;

   y(k): output value;

   k: instant under consideration;

    = lowpass filter gain;

   β = filter pole.
5. A system for injecting fuel into the combustion chambers of an internal combustion engine, the system comprising:

   · apparatus according to any one of claims 1 to 4;

   · a richness error management module receiving the output signal from the richness sensor and subjecting it to proportional-integral filtering so as to form a general correction term λ_g;

   · an adjustable filter (74) allocated to each combustion chamber, receiving the difference between the output from the estimator means corresponding to said chamber and a reference specific to the chamber, so as to supply a correction factor λ_i specific to the chamber;

   · a multiplier (76) providing the product of (1+λ_g) and (1+λ_i); and

   · a management circuit controlling the injectors on the basis of a signal representing the quantity of air which is drawn and of the output from the multiplier.

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