The present invention relates to a method and a device for evaluating the flow rate of air entering an internal combustion engine fitted with an electronic injection system and, more particularly, to such a method and such a device enabling this air flow rate to be accurately evaluated during transient phases of operation of the engine, such as a sudden acceleration or a sudden deceleration.

The driver of a motor vehicle propelled by an internal combustion engine varies the speed of the vehicle by acting upon an accelerator pedal which conven¬ tionally is mechanically coupled to a throttle for regulating the flow rate of air admitted into the engine. The latter then varies instantaneously whereas the guan- tity of petrol injected is modified only after some delay so as to match up with the air/fuel ratio corresponding to the stoichiometry. This results, ^'' in the transient regime, in. a deviation in this ratio relative to the stoichiometry. In modern vehicles fitted with antipollu- tion devices consisting of a "catalytic" silencer, this deviation is particularly troublesome since such a silencer is designed to operate with an engine supplied with an air/fuel mixture as strictly stoichiometric as possible. To attempt to reduce this deviation, it is necessary, on the one hand, to make available means permitting accurate knowledge of the flow rate of air admitted into the engine in the transient regime, at any instant, and on the other hand, bring it about that the quantity of petrol provided to the air/fuel mixture then follows, without delay, the variations in the flow rate of air admitted.

To instantaneously evaluate the flow rate of air entering an engine, a method is known based on the assumption according to which this flow rate is propor¬ tional to the product of a measurement of the angular opening α of the throttle (regarded as a variable load loss) times the speed of rotation of the engine (regarded as a pump whose intake pressure depends on the rate of

rotation) . This method is ra zpid since it is based on the causes of variation of the flow rate but it is also very inaccurate (in particular, the air pressure varies with altitude) and can only be implemented in bottom-of-the- range injection devices for which the performance stan¬ dards and the antipollution standards are the least demanding. Furthermore, this method is not adaptive and the results may thus be affected by the fouling of the inlet assembly of the engine, including the air filter. Means of direct measurement of the air mass flow rate are also known, such as vane or hot-wire flow meters which, associated with other sensors, enable the flow rate of air admitted into the engine to be evaluated with good accuracy. These measurement means have however the disad¬ vantage of introducing substantial time delays. With vane flow meters, these delays are due to time constants of mechanical origin and to info.rmation processing times. With hot-wire flow meters, the delays result from a necessary filtering of the signal delivered (this signal being strongly pulsed and affected by overshoot at times of transitions) as well as from information processing taime. These time constants delay the measurement relative to the actual variation in the flow rate, this delay possibly reaching several "engine" revolutions.

In order to limit these delays, the document EP-A-115 868 describes a method of controlling the fuel supply of an internal combustion engine, according to which the quantity of air entering the engine is evaluated beforehand with the aid of samples of a signal provided by an air flow meter, of a dynamic model of the entry of air into the engine and of recursive calcula¬ tions on the quantities of air calculated based on an assumption of linear variation between two samples of this quantity. This method is therefore based on calculated quantities of air which are available only after a delay. Furthermore, the assumption of linear variation generates inaccuracy in the event of sudden variations (with high gradient) , as is often the case in

the transient regime.

The present invention therefore aims to provide a method and a device enabling, in the transient regime, the flow rate of air entering an internal combustion engine to be evaluated, and which do not have the abovementioned disadvantages of the methods and devices of the prior art, and which consequently ensure rapid and accurate evaluation of this air flow rate.

The present invention also aims to provide such a method and such a device which are autoadaptive.

These aims of the invention are achieved, as well as others which will emerge in the remainder of the present description, with a method of evaluating the flow rate of air entering an internal combustion engine when the latter operates in a transient reg:Lme, according to which a measurement is made of this flow rate, and this measurement is corrected with an additive correction term. According to the invention, an estimated air flow rate is extracted from a reference model of this flow rate and the additive correction term is extracted from an error model controlled through the estJLmated air flow rate.

According to a mode of implementing the method according to the invention, the input variables of the reference model are the angle of opening of a throttle for controlling the flow rate of air admitted and the speed of the engine. Such a reference model permits a rapid estimation of the flow rate of air admitted and therefore no less rapid availability of the additive correction term provided by the error model.

According to a variant of the method according to the invention, parameters of the reference model are corrected as a function of the observed deviation, in the steady regime, between the measured and estimated air flow rates. This ensures the autoadaptivity of the method and of the device according to the invention.

In order to implement the method according to the invention, the invention provides a device comprising a sensor of flow rate of air entering the engine and means

supplied with the output signal from this sensor in order to calculate an additive correction term of the flow rate measured by the sensor in such a way as to accurately evaluate the flow rate of air entering the engine. According to the invention, the calculation means com¬ prise means for storing and .implementing a model for calculating a reference air flow rate and an error model controlled through the output from the reference air flow rate model in order to provide a correction term and means for adding this correction term to the measured flow rate, this sum representing the flow rate of air actually entering the engine in the transient reg.ime.

According to a preferred embodiment of the device according to the invention, the latter further comprises a sensor of position of a throttle for controlling the flow rate of air admitted, a sensor of speed of the engine, and means of inputting the signals delivered by these sensors into the reference model in order to calculate the est.imated air flow rate. According to a variant, the calculation means of the device according to the invention further comprise means responsive to a deviation observed, in the steady regime, between the measured and estimated air flow rates in order to backwards-correct parameters for regulating the reference model.

Other features and advantages of the method and of the device according to the invention will emerge on reading the following description and on examining the attached drawing in which: - Figure 1 represents a set of graphs useful in understanding the method according to the invention,

- Figure 2 is a block diagram of a device for implementing the method according to the invention, and

- Figure 3 is a block diagram of one part of a variant of the device according to the invention, ensu¬ ring an autoadaptation of the reference model used in this device.

Reference is made to Figure 1 of the attached drawing in which the graphs 1 and 2 represent

6 ^" respectively the change in the opening α(n) of the throttle valve of an internal combustion engine and of the speed N(n) of this engine in a transient phase of operation of this engine corresponding, by way of non- limiting example, to a sudden acceleration. The instants n-1, n, n+1, ... etc. are sampling instants of measure¬ ments made by sensors of throttle angle and of speed of the engine. In an internal combustion engine, these sampling instants may correspond to the instants of passage of the pistons to the top dead centre, as is conventional in digital devices for controlling fuel injection and/or ignition. The graph 3 represents the average quantity of air D(n) actually entering the engine. Generally in what follows, the symbol D designates an average air flow rate, filtered of disturb¬ ing fluctua ions. The graph 4 represents the flow rate of air D _m . ₈ (n) admitted into the engine, as deduced from a signal delivered by a hot-wire or hot-film air flow meter for example. From comparing the graphs 3 and 4, it appears that the air flow rate thus measured by virtue of the flow meter (graph 4) changes parallel to the true flow rate D(n) (graph 3) but with a delay due, as seen above, to the filtering and to the times for acquiring and processing the signals provided by the flow meter. In the example represented, the case has been chosen of a four-cylinder, four-stroke engine for which the sampling period (1/4 of the engine cycle) corresponds to a half- revolution of the transmission shaft (or 1/2 "engine" revolution) . As illustrated, the flow rate measurement provided by the flow meter may conventionally suffer a delay greater than 1 engine revolution. It is the aim of the present invention to substantially reduce the significance of this delay, so as to make the latter less than the sampling period for the parameters of the system, namely a 1/2 engine revolution. From a more instantaneous and more accurate knowledge thereby obtained of the flow rate of air admitted, it is then possible to regulate the quantity of petrol injected into the engine so as to conform to the stoichiometry in a

(o more constant manner than is possible with the means of the prior art. Advantageously, better adjustment to the operating requirements of "catalytic" silencers and better driving comfort are extracted therefrom. According to the invention, in order to achieve these results, use is made of a reference model providing an est.imate D„ _t (n) of the flow rate of air admitted. According a mode of implementation of the method accord¬ ing to the invention, this model takes the form: D.. _t (n) = f( (n), N(n))

As seen above in the preamble of the present description, such a model is in fact able to provide an estimate of the flow rate of air admitted. It will be noted that the graph 5 (D _#it (n)) is thus extracted from the samples of a(n) and N(n) appearing in graphs 1 and 2. This model very closely follows over time the variation in the true flow rate, by virtue in particular of the parameter α(n), changes in which are rapid.

According to an essential feature of the method according to the invention, use is made of an error model controlled through the estimated flow rate D. _8t (n) in order to extract (see graph 7) a correction term ΔD(n) which, added to the measured flow rate δ _nι . _s (n), represents the corrected flow rate D _cor (graph 6), this graph corres- ponding to the addition of graphs 4 and 7.

By virtue of the rapid estimate of the flow rate and of the error model which are used in the invention, the corrected flow rate D _COE (graph 6) is very near the true flow rate (graph 4), the delay affecting it being in particular considerably reduced (less than 1/2 engine revolution) relative to the delay of the measurement - _ι»s ( ⁿ ) with respect to the true flow rate D(n) .

The invention is thus essentially based on the use of two models, a reference model describing the behaviour of the inlet assembly of the engine, and an error model.

The reference model set out above is given merely by way of example. More, generally, this reference model may be expressed in t ^* ιe form:

δ..t(n) - f["(n), N(n), etc..)

Parameters such as the temperature and the pressure of the atmospheric air may be taken into account, as well as the angle of opening of the throttle and the speed of the engine. The model is established from bench measurements made on the engine. It will be possible to use a dynamic model of the behaviour of the inlet assembly. As a variant, this behaviour could be recorded in a table comprising for example two inputs (engine speed, throttle angle) and one output (estimated air flow rate) . Between two bench-tested operating points, the table is traversed by linear interpolation, for example.

The estimate D„ _t (n) thus obtained of the mass of air entering the engine is used to calculate the change, at any instant within the transient phases of operation of the engine, in the error distancing the measurement E _m . _« ( ⁿ ) from the flow rate of air actually admitted into the engine at this instant. This calculation is carried out with the aid of an error model, one of the possible expressions of which will be given below by way of example. The estimate of the error takes into account, on the one hand, the existence of a pure delay between a measured value and a true value, due to the acquisition system and, on the other hand, the filtering exerted by the inlet assembly on the mass of incoming air, owing to a capacitive effect developed within this assembly.

Generally, the error model can be expressed in the form: Δϋ(n) - F[ΔD(n-l), D. _8t (n), D„ _t (n-1), N(n)] which expresses the fact that the error in the quantity of air admitted at the instant n is a function of the error in the quantity of air at the instant (n-1), of the quantity of air estimated at the instant n and (n-1) and of the engine speed at the instant n. The expression for the error could possibly take account of other parameters such as the temperature of the inlet air for example.

By way of non-1JLmiting example, a usable error model can take the followinc form:

ΔD(n) =ΔD(n-l ) +ΔT_N(n) [D _βst (n) -D _βst (n-l) ] -_dtN(n) ] ΔD(n-l) , r ' T ' where T' is a time constant dependent on the characteristics of the inlet assembly (capacity, etc.) and

ΔT is the error estimation horizon, equal to an integer number of sampling periods dt, for example three periods. The final estimate of the average flow rate of air entering the engine at the instant n is obtained by taking the sum of the measured average flow rate and of the error in the average flow rate, namely: E _cor (n) ^« B _m . ₈ (n) + ΔD(n) . Reference is now made to Figure 2 of the attached drawing in which a block diagram has been represented of a device for implementing the method according to the invention. The admission of air into an engine 1 is regulated by a throttle valve 2 whose angular position is controlled by the driver of a motor vehicle, by means of an accelerator pedal 3. The engine 1 receives control signals from a calculator 4 (injection time, instant of ignition, etc.) and delivers, via appropriate sensors, measurement information relating to its operation (engine speed N, cooling water temperature ø _wmt , etc.). A flow meter 5 measures the air flow rate in the inlet assembly 6 of the engine. The calculator comprises means of acquiring and processing this measurement, which deliver the signal D _mβ8 (n) affected by a significant delay which may be several "engine" revolutions for example, as seen above. According to the invention, this delay is cor¬ rected with the aid of a reference model 7 and of an error model 8 which are recorded in appropriate memory means provided in the calculator 4. The reference model is supplied with a signal α(n) originating from a throttle position sensor 10 and with a signal N originat¬ ing from an engine speed sensor (not shown), such as an electromagnetic reluctance sensor for example. The reference model calculates an estimated air flow rate D.« _^

3 which controls the error model 8. The latter delivers a correction signal ΔD(n) which is combined with the signal

D _mβ8 (n) in an adder 11 in order to provide a signal D _cor representing the flow rate of air actually admitted into the engine. This signal supplies calculation and control means 12 which formulate in particular, from the air flow rate thus known, the quantity of petrol to be injected into each of the cylinders of the engine so as to conform to the stoichiometry of the air/fuel mixture during the transient phases of operation of the engine. Outside of these transient phases, these means 12 calculate in a conventional manner the quantity of petrol to be injected into the engine, in order to ensure the stoichiometry of the air/fuel mixture. One part of the block diagram of a variant of the device of Figure 2, enabling a characteristic of auto- adaptivity to be imparted to the latter, has been repre¬ sented in Figure 3.

It is observed in fact that the various para et- ers characterising the engine are able to change in the course of t.ime. The value of the est.imated flow rate calculated by virtue of the reference model may eventu¬ ally deviate from the true value, owing to fouling of the air filter, fouling of the valves, wear to the cams, etc.. Other phenomena may affect the validity of the reference model, a variation in atmospheric pressure due to a variation in altitude for example.

According to the invention, use is made of a matching algorithm 13 enabling the values of the parame- ters used in the reference model to be corrected in order to preserve the accuracy of the estimate. This algorithm is brought into play in steady phases of operation of the engine during which, at given throttle opening and given engine speed, the values of the measured average air flow rate D,,,., and of the est ated flow rate D, _8t are compared. The values of the various parameters a ) coming into the reference model are corrected, as a function of the deviation observed, with the aid of an algorithm of the form:

Λo a _u = a _i0 + aSa. _L where a. _t = ith parameter of the reference model, δa. _t = value of the correction, and α = weighing coefficient (α<<l) . As seen above, the value of the correction Sa.^ is a function of the measured flow rate and of the estimated flow rate, for an angle of opening of the throttle, an engine speed and, possibly, an air temperature which are given. The weighing coefficient may consist of a certain percentage of the derivative of the parameter taken into account.

It is now apparent that the present invention enables the sought-after aims to be achieved. By virtue of the use of an error model controlled through a rapid estimate of the flow rate of air admitted, it is possible to provide the fuel injection time calculator with a rapid and accurate instantaneous evaluation of the flow rate of air admitted, which permits adjustment without delay of the stoichiometry of the air/fuel mixture supplying the engine, required for driving comfort and for correct operation of the antipollution devices based upon catalysis of certain exhaust gases.