Method for controlling fuel injectors of a spark ignition engine

Abstract

The invention relates to a method for controlling fuel injectors (64) of a spark-ignition engine (1), said spark-ignition engine having several cylinders (11), at least one spark plug (32) opening into each cylinder and at least one fuel injector located in each cylinder or in a fresh air intake duct into each cylinder, said control method comprising steps of measuring a breakdown voltage across the terminals of each spark plug and of controlling the fuel injectors according to the measured breakdown voltages.According to the invention, the piloting method includes the following steps: - calculation, for each cylinder, of a thermodynamic parameter as a function of the breakdown voltage measured at the terminals of the spark plug of said cylinder, - comparison of the calculated thermodynamic parameter with a reference value, and provides, in the piloting step, for injecting into each cylinder a quantity of fuel which depends on the result of said comparison.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates generally to the control of combustion quality in a spark-ignition engine and therefore its energy efficiency.

[0002] It relates more specifically to a method for controlling fuel injectors of a spark-ignition engine, said spark-ignition engine having several cylinders, at least one spark plug opening into each cylinder and at least one fuel injector opening into each cylinder or into a fresh air intake duct in each cylinder, the method comprising the steps of: measurement of a breakdown voltage across the terminals of each spark plug, and control of the fuel injectors according to the measured breakdown voltages.

[0003] The invention finds a particularly advantageous application in the management of fuel injection in spark-ignition engines of motor vehicles. It aims in particular to reduce the quantity of hydrocarbons, carbon monoxide, or nitrogen oxides in the combustion gases.

[0004] It also relates to a spark-ignition engine, comprising several cylinders, at least one spark plug opening into each cylinder, at least one fuel injector opening into each cylinder or into a fresh air intake duct in each cylinder, and a fuel injector control unit adapted to implement a process as described above. STATE OF THE ART

[0005] The air-fuel mixture ratio in the intake of a spark-ignition engine is an important parameter for combustion quality. It is defined as the ratio of the mass of fuel to the mass of air injected into the engine during each cycle. To ensure good combustion quality, it is known to regulate the overall fuel flow into the engine based on a setpoint related to the air-fuel mixture ratio in the exhaust gases. This is typically achieved by measuring the current mixture ratio, for example, using an oxygen sensor placed in the engine's exhaust system, and adjusting the overall fuel flow ratio accordingly so that the current mixture ratio reaches the setpoint value.

[0006] In this context, US patent 2018 / 0187620 A1 describes a method for controlling internal combustion engines, in which the air-fuel mixture is modified by determining pressure values ​​inside a cylinder and the breakdown voltage across a spark plug. These values ​​allow the spark plug electrode gap to be determined. Various combustion parameters, such as the air-fuel mixture, are then adjusted according to the determined electrode gap.

[0007] US document 2004 / 084025 A1 describes a closed-loop balancing method for the air / fuel ratio of each cylinder which includes: calculating an average synchronization coefficient, calculating a synchronization coefficient error, integrating the minimum synchronization coefficient error for best torque, calculating a gross fuel compensation coefficient, recalibrating the gross fuel compensation coefficient, updating a prediction lookup table based on the current engine operating conditions, and calculating a final fueling command.

[0008] The plaintiff noted that even using this method, pollutant emissions remained higher than expected. PRESENTATION OF THE INVENTION

[0009] The present invention therefore proposes a solution to further reduce these emissions.

[0010] It therefore proposes a method for controlling the injectors of a spark-ignition engine as defined in the introduction, in which, after the measurement step, the following steps are planned: calculation, for each cylinder, of the effective mass of air admitted into each cylinder at the time of ignition of a mixture of fuel and fresh air in the cylinder by the spark plug as a function of the breakdown voltage measured at the terminals of the spark plug of said cylinder, comparison of the calculated effective mass of air admitted into each cylinder at the time of ignition of a mixture of fuel and fresh air in the cylinder by the spark plug with a reference value, and at the piloting stage, injection into each cylinder of a quantity of fuel which depends on the result of said comparison.

[0011] In practice, we observe a dispersion of cylinder filling from cylinder to cylinder, for example due to manufacturing variations in engine parts and the geometry of the air distributor.

[0012] These differences in the volume of air admitted into each cylinder lead to mixture problems in each cylinder. A single conventional oxygen sensor can only correct the average mixture across all cylinders. Therefore, residual pollutants remain due to this variation in air intake.

[0013] The invention then proposes to adjust the quantity of fuel cylinder by cylinder according to the result of the comparison with the reference value, which makes it possible to rectify, for each cylinder, the value of the richness of the mixture, and therefore to improve the quality of the combustion.

[0014] Other advantageous and non-limiting features of the process according to the invention, taken individually or in all technically possible combinations, are as follows: The thermodynamic parameter is the effective mass of air admitted into each cylinder at the moment of ignition of a mixture of fuel and fresh air in the cylinder by the spark plug; the effective mass of air admitted into each cylinder is calculated as a function of an effective volume of air admitted into the cylinder at the moment of ignition of the mixture, which effective volume of air is calculated as a function of the breakdown voltage measured at the terminals of the spark plug opening into the cylinder; the effective volume of air admitted into each cylinder is calculated as a function of the pressure in the cylinder at the moment of ignition of said mixture, which pressure is calculated as a function of the breakdown voltage measured at the terminals of the spark plug opening into the cylinder; the comparison consists of calculating the ratio between said effective mass of air admitted into each cylinder and a reference mass;In the piloting stage, the following sub-steps are planned: determination of a value relating to the average richness of the air-fuel mixtures exiting the cylinders, calculation of an initial setpoint relating to the quantity of fuel to be injected into the cylinders as a function of said value, calculation, for each cylinder, of a corrected setpoint by modifying the initial setpoint according to the result of the comparison; the measurement of the breakdown voltage is carried out during the ignition of the mixture in each cylinder or continuously; the effective mass of air admitted into each cylinder is measured by a flow meter.

[0015] The invention also proposes a spark-ignition engine as defined in the introduction, which includes a control unit for at least one fuel injector that is adapted to implement a control method as defined previously.

[0016] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways as long as they are not incompatible or mutually exclusive. DETAILED DESCRIPTION OF THE INVENTION

[0017] The description that follows, with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0018] Regarding the attached drawings: [ Fig. 1 ] is a schematic view of a spark-ignition engine according to the invention; [ Fig. 2 [ ] represents the diagram of an ignition circuit associated with a cylinder of the engine of the figure 1 ; Fig. 3 ] is a curve representing the time evolution of a voltage between the electrodes of a spark plug used in the engine of the figure 1 ; Fig. 4 ] is a nomogram illustrating the correspondence between the spark plug breakdown voltage of the figure 3 and the pressure in the combustion chamber of the engine figure 1 .

[0019] On the figure 1 We have represented a spark-ignition engine which includes an engine block 10 equipped with a crankshaft and several pistons (not shown) housed in cylinders 11. The number of cylinders is, in the illustrated example, equal to four but it could alternatively be greater or less (for example equal to three).

[0020] Upstream of the cylinders 11, the spark-ignition engine 1 includes an intake line 20 which draws fresh air from the atmosphere and leads into an air distributor 25 arranged to distribute the air to each of the four cylinders 11 of the engine block 10. This intake line 20 includes, in the direction of the fresh air flow, an air filter 21 which filters the fresh air drawn from the atmosphere, a compressor 22 which compresses the fresh air filtered by the air filter 21, a main air cooler 23 which cools this compressed fresh air, and an intake valve 24 which allows the flow of fresh air leading into the air distributor 25 to be regulated.

[0021] It should be noted here that the air distributor delimits a volume of air called the "plenum".

[0022] The spark-ignition engine 1 also includes a fuel injection line 60 into the cylinders 11. This injection line 60 includes an injection pump 62 arranged to draw fuel from a reservoir 61 in order to bring it under pressure into a distribution rail 63 which opens into the cylinders 11 via four injectors 64.

[0023] At the outlet of the cylinders 11, the spark-ignition engine 1 has an exhaust line 80 which extends from an exhaust manifold 81 into which flow the gases which have been previously burned in the cylinders 11. This exhaust line includes, in addition to the exhaust manifold, a turbine 82, and a catalytic converter 83.

[0024] The spark-ignition engine may also include one or more exhaust gas recirculation (EGR) lines, originating in the exhaust line and terminating in the intake line. As shown in the figure 1 It includes a low-pressure recirculation line, which originates downstream of the turbine and terminates upstream of the compressor. This line is equipped with a filter, an air cooler, and a flue gas flow control valve.

[0025] As shown by figure 1 To control the various components of the spark-ignition engine 1, a computer 100 is provided, comprising a processor (CPU), a memory, and an input and output interface.

[0026] Thanks to its interfaces, the 100 calculator is adapted to receive input signals from various sensors relating in particular to the operation of the engine.

[0027] Among these sensors, an oxygen sensor 88 is planned, positioned between the turbine 82 and the catalytic converter 83 of the exhaust line 80, which determines the oxygen content of the combustion gases. This content is linked to the average richness of the air-fuel mixtures exiting all the cylinders.

[0028] Among the sensors, there is also an air sensor 26 which opens into the intake line, between the intake valve 24 and the air distributor 25, which allows the pressure and temperature of the air arriving in the air distributor 25 to be determined.

[0029] A flow meter 27 is also provided on the intake line 20, which allows the mass flow rate of fresh air taken from the atmosphere by the intake line 20 and filtered by the air filter 21 to be determined.

[0030] Thanks to its interfaces, the computer can control the fuel injectors, so as to introduce the desired amount of fuel at each cycle.

[0031] Thanks to its memory, the computer stores a computer application, consisting of computer programs including instructions whose execution by the processor allows the computer to implement the process described below.

[0032] At this point, it is worth recalling that the ignition of the air-fuel mixture in any one of the cylinders 11 of the engine block 10 is controlled by the computer 100, so that the engine 1 completes its operating cycle in four strokes: intake, compression, combustion, and then exhaust. The moment of ignition is then defined as the instant preceding the explosion (i.e., the combustion) of the mixture, that is to say, the instant from which the propagation of the flame in the cylinder begins.

[0033] For this purpose, the spark-ignition engine 1 has, for each of the cylinders 11, an independent ignition circuit. Such an ignition circuit is shown in the figure 2 .

[0034] All ignition circuits are powered by a single battery. Each ignition circuit includes a spark plug 32 that opens into the corresponding cylinder 11 to generate a spark, an ignition coil 31 that sends a short, intense current to the spark plug 32, and a connection circuit that connects the ignition coil 31 to the battery. This connection circuit includes a resistor r between one battery terminal and a corresponding terminal of the ignition coil 31, and a capacitor C between the other battery terminal and the corresponding terminal of the ignition coil 31. A relay, controlled by the engine control unit (ECU), is connected in parallel with the capacitor to generate the spark.

[0035] Preferably, each ignition circuit includes a voltage sensor to determine the voltage v2 across the spark plug terminals 32. This voltage sensor is connected to the computer 100 and can be configured to acquire the voltage between the electrodes of the spark plug 32 only at the time of ignition or continuously.

[0036] The voltage sensor can be of any type. For example, an ionization sensor connected to the low-voltage side of the ignition coil's secondary winding can be used to measure the spark plug ionization current immediately after the spark is extinguished. In this example, the ignition coil 31 could be a pencil-type coil with a connector to which the ionization sensor is attached.

[0037] In order to adjust the richness of the air-fuel mixture inside each cylinder 11 when the engine 1 is running, the computer 100 proceeds in the following way.

[0038] It should be noted at the outset that richness is defined here as the ratio between, on the one hand, the effective ratio of the mass of fuel and the mass of air injected into the engine, and on the other hand, the stoichiometric ratio of these masses.

[0039] Typically, when the driver of the motor vehicle turns on the ignition, the computer initializes and then commands the starter and fuel injectors 64 to start the engine.

[0040] When the engine is started, fresh air taken from the atmosphere through the intake line 20 is filtered by the air filter 21, compressed by the compressor 22, cooled by the main air cooler 23, and then burned in the cylinders 11.

[0041] Upon exiting cylinders 11, the burnt gases are expanded in turbine 32, treated and filtered in catalytic converter 83, then expanded again in exhaust silencer 87 before being released into the atmosphere.

[0042] When the control unit 100 commands the ignition of the air-fuel mixture in a cylinder 11, the corresponding ignition circuit is activated so that the corresponding ignition coil 31 injects a strong current into the spark plug 32. figure 3 This illustrates the temporal evolution of the voltage between the electrodes of spark plug 32 during ignition. It can be observed that the voltage across spark plug 32 increases during the charging of the secondary winding of the ignition coil 31 (phase A) until it reaches a plateau voltage known as the breakdown voltage (or dielectric strength). An electric arc is then established between the electrodes of spark plug 32, thus initiating the combustion of the air-fuel mixture in cylinder 11 during an ET phase, until the ignition is complete (phase E).

[0043] When this voltage reaches the breakdown voltage, the calculator 100 records the value U dielec measured by the voltage sensor.

[0044] According to the invention, the U dielec value will allow the calculation of a thermodynamic parameter of the air-fuel mixture in cylinder 11, which, compared to a reference value, will be used to adjust the richness of the mixture in cylinder 11 to a value corresponding to a stoichiometric mixture.

[0045] According to an advantageous configuration, the effective mass of air admitted into cylinder 11 is considered as the thermodynamic parameter. Indeed, this configuration allows us to directly calculate the quantity of fuel to be injected into the cylinder to adjust its richness, due to the definition of the latter.

[0046] In other words, the invention proposes to calculate the effective mass of air admitted into cylinder 11 as a function of the measured breakdown voltage in order to best adjust the mixture, cylinder by cylinder (the breakdown voltage is in fact specific to each cylinder).

[0047] To calculate this effective air mass, according to an advantageous configuration of the invention, the calculator 100 begins by calculating, from the dielectric value U and using a law known as Paschen's law, the pressure of the mixture inside cylinder 11 at the moment of ignition. Paschen's law is described by the following formula: U di é lec = B . P AVA . d ln A . P AVA . d ln 1 + 1 γ

[0048] where P AVA is the pressure in cylinder 11 at the time of ignition, d is the inter-electrode distance of the spark plug, A and B are characteristic coefficients of the electrodes and the medium inside cylinder 11 and y is the adiabatic index of the medium inside cylinder 11.

[0049] Typically, A is equal to 15 Torr -1< .cm -1< , B is equal to 365 V.Torr -1< .cm -1< , and γ is equal to 1.4. In this configuration, the inter-electrode distance d is considered to be known from the technical documentation of the spark plug 32.

[0050] We understand from this mathematical formula that the breakdown voltage is a function of the pressure of the mixture.

[0051] There figure 4 presents a nomogram allowing the pressure in the corresponding cylinder 11 to be deduced from the measured breakdown voltage value.

[0052] Then, calculator 100 is programmed to apply the following relationships, based on the assumption of adiabatic compression and the ideal gas law: P adm . V adm γ = P AVA . V AVA γ or: V adm = P AVA . V AVA γ P adm γ where P adm represents the pressure in the air distributor 25, P AVA represents the pressure in cylinder 11 at the time of ignition, previously determined, V AVA represents the volume of cylinder 11 at the time of ignition, and V adm represents the volume of air admitted from the air distributor 25.

[0053] The pressure P adm in the air distributor 25 is known from the signal coming from the air sensor 26 positioned at the level of the air distributor 25. The volume V AVA is known from the geometry of the engine 1.

[0054] Therefore, the calculator can calculate the effective mass m using the following relationship: m = P adm . V adm r . T adm where r represents the specific constant of the gas in composition in the medium inside cylinder 11 and T adm represents the temperature of the air distributor 25.

[0055] Typically, r is equal to 287 J kg -1 < K -1 < . The temperature T adm is known from the signal coming from the air sensor 26 positioned at the air distributor 25.

[0056] Once the effective mass is determined, the control unit 100 compares it to a reference value. This reference value depends on the engine's operating point. It can be read by the control unit from a map that will be stored in its memory and will assign a reference value to each operating point. This operating point could, for example, be defined by an engine speed value and an engine load value 1 (the load corresponding to the torque required by a driver in the case of a motor vehicle engine).

[0057] In the case where the thermodynamic parameter is the effective admitted air mass, the value m of this mass is compared by the calculator 100 with a reference mass m ref. This comparison can be performed by calculating the ratio: α = m m ref .

[0058] In the illustrated embodiment, the reference mass m ref is determined on the basis of the signal from the flow meter 27 placed on the intake line 20.

[0059] In another embodiment, the calculator 100 could determine the reference mass m ref on the basis of a model involving the speed and load of the engine 1.

[0060] Based on the result of comparing the thermodynamic parameter with the reference value, the calculator 100 calculates an adjusted value of the quantity of fuel to be injected into cylinder 11 in order to obtain a stoichiometric air-fuel mixture and thus ensure complete combustion.

[0061] For this, according to an advantageous configuration, the control of injectors 64 takes place in the following way.

[0062] During engine operation, the flow of fresh air admitted into the cylinders is adjusted according to the driver's input when pressing the accelerator pedal, corresponding to a torque request.

[0063] The oxygen sensor 88 positioned upstream of the catalytic converter 83 allows the current average richness of the air-fuel mixture admitted into all cylinders 11 to be adjusted in a closed loop to a setpoint value, for example equal to 1, by adjusting the overall flow of fuel from the tank 61 and injected into the engine block 10. In practice, the adjustment is made according to the error signal between the current richness and the setpoint richness.

[0064] More specifically, the engine control unit (ECU) receives a signal from the oxygen sensor 88, based on which it determines a target fuel-air mixture ratio. Then, the ECU 100 calculates an initial target to control the injectors. This initial target is, for example, the amount of fuel to inject into the cylinders, taking into account the oxygen content of the exhaust gases, the engine speed, the load required by the driver, etc. This initial target is the same for all injectors.

[0065] However, given the shape of the air distributor 25, the amount of air admitted into each cylinder is not necessarily the same. Consequently, the air-fuel ratio will not be the same in the cylinders even if the same amount of fuel were injected into them.

[0066] The idea is then to compensate for this variation in richness, cylinder by cylinder, thanks to the ratio α which has been calculated for each cylinder (and which therefore differs from one cylinder to another).

[0067] To achieve this, the control unit 100 calculates, for each cylinder 11, a corrected setpoint based on the initial setpoint and this ratio α (for example, by multiplying these two values). The control unit 100 then transmits the corrected setpoint to the injectors 64, which allows the flow rate of each injector 64 to be adjusted accordingly.

[0068] The present invention is in no way limited to the embodiment described and represented, but a person skilled in the art will be able to make any variation in accordance with the invention.

[0069] In the described embodiment, the electrode gap of the spark plug 32 is assumed to be known and constant (see the first equation). Alternatively, this gap can be determined, for example, by applying the method described in US patent 2018 / 0187620 A1. This patent instructs the use of a pressure sensor to measure the pressure inside cylinder 11. This allows, in addition to measuring the breakdown voltage of the spark plug 32, the determination of the electrode gap of the spark plug 32.

[0070] In the described embodiment, the thermodynamic parameter compared to a reference value to calculate the ratio α is the mass of air admitted into the cylinder. Alternatively, another thermodynamic parameter could be used. For example, it could be the effective volume of air admitted into cylinder 11. In this case, the effective volume of air admitted into cylinder 11 is compared to a reference volume to calculate a corrected value for the amount of fuel to be injected into cylinder 11 to correct the air-fuel mixture richness.

[0071] It should be noted that the engine may be a variable valve timing engine, that is to say that the timing (in English VVT: variable valve timing ), the opening time and / or the lifting (in English VVL: variable valve lift ) the intake and exhaust valves of the cylinders 11 of the engine block 10 can vary in a manner controlled by the computer 100.

[0072] In this case, the current position of the variable distribution, that is to say, the position of the valves or their degree of opening, influences the operating point of the engine and therefore the reference value used for comparison.

Claims

1. Method for controlling fuel injectors (64) of a spark ignition engine (1), said spark ignition engine (1) comprising several cylinders (11), at least one spark plug (32) opening into each cylinder (11) and at least one fuel injector (64) opening into each cylinder (11) or into a fresh air intake pipe in each cylinder (11), the method comprising the steps of: - measurement of a breakdown voltage at the terminals of each spark plug (32), and - control of the fuel injectors (64) according to the measured breakdown voltages, characterised in that, after the measurement step, there are provided steps of: - calculation, for each cylinder (11), of the effective mass of air admitted into each cylinder (11) at the time of ignition of a mixture of fuel and fresh air in the cylinder (11) by the spark plug (32) as a function of the breakdown voltage measured at the terminals of the spark plug of said cylinder (11), - comparison of the calculated effective air mass admitted into each cylinder (11) at the time of ignition of a mixture of fuel and fresh air in the cylinder (11) by the spark plug (32) with a reference value, and in that, in the control step, it is provided to inject into each cylinder (11) an amount of fuel that depends on the result of said comparison.

2. Method for controlling according to claim 1, wherein said effective air mass admitted into each cylinder (11) is calculated as a function of an effective air volume admitted into the cylinder (11) at the time of ignition of said mixture; this effective air volume is calculated as a function of the breakdown voltage measured at the terminals of the spark plug (32) opening into the cylinder (11).

3. Method for controlling according to claim 2, wherein the effective volume of air admitted into each cylinder (11) is calculated as a function of the pressure in the cylinder (11) at the time of ignition of said mixture; this pressure is calculated as a function of the breakdown voltage measured at the terminals of the spark plug (32) opening into the cylinder (11).

4. Method for controlling according to one of claims 1 to 3, wherein the comparison consists in calculating the ratio between said effective air mass admitted into each cylinder (11) and a reference mass.

5. Method for controlling according to any one of the preceding claims, wherein, in the control step, there are provided substeps of: - determination of a value relating to the average richness of the air-fuel mixtures leaving the cylinders (11), - calculation of an initial setpoint relating to the quantity of fuel to be injected into the cylinders (11) as a function of said value, - calculation, for each cylinder (11), of a corrected setpoint by modifying the initial setpoint according to the result of the comparison.

6. Method for controlling according to any one of the preceding claims, wherein the measurement of the breakdown voltage is performed when igniting the mixture of each cylinder (11) or continuously.

7. Method for controlling according to any one of the preceding claims, wherein the measurement of the breakdown voltage is performed by an ionisation sensor.

8. Method for controlling according to any one of claims 1 and 2 to 7 as dependent on claim 1, wherein the reference value is a reference mass (mref) measured by a flowmeter (27).

9. A spark ignition engine (1) comprising several cylinders (11), at least one spark plug (32) opening into each cylinder (11) and at least one fuel injector (64) opening into each cylinder (11) or into a fresh air intake pipe in each cylinder (11), characterised in that it comprises a control unit for at least one fuel injector (64) which is adapted to implement a method for controlling according to any one of the preceding claims.

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