EP2250360A1 - System for controlling a combustion engine with exhaust gas recirculation - Google Patents

Abstract

The invention relates to a system for controlling an internal combustion engine comprising a combustion chamber (1), an inlet duct (7), an exhaust duct (9) and an EGR system with an exhaust gas recirculation duct (13) and an EGR valve (14). According to the invention, the system also comprises an exhaust back-pressure valve (15) in the exhaust duct (9) downstream of the exhaust gas recirculation duct (13), means (16) of estimating the EGR ratio, and a sensor (17) for measuring the exhaust pressure; the EGR valve (14) is controlled in a first closed-loop control system on the basis of the difference between the estimated EGR ratio value delivered by said estimating means (16) and a predetermined EGR ratio set point value, and the back-pressure valve (15) is controlled in a second closed-loop control system on the basis of the difference between the exhaust pressure value measured by the measurement sensor (17) and a predetermined exhaust pressure set point value. Advantage: control and achievement of a high EGR ratio.

Description

 SYSTEM FOR CONTROLLING A THERMAL ENGINE WITH GAS RECIRCULATION

EXHAUST

The present invention generally relates to combustion engines internal combustion for a motor vehicle, using an exhaust gas recirculation system, known as EGR system (EGR is the English initials set for "Exhaust Gas Recirculation" ).

Internal combustion engines are generally classified into two main families: Gasoline engines and Diesel engines. Figure 1 schematically illustrates the components common to both types of engines.

An internal combustion engine thus conventionally comprises one or more cylinders each forming a combustion chamber 1. In each cylinder, a piston 2 slides in a reciprocating rectilinear motion in a cycle which will be described in the following. This movement is then converted into a continuous rotational movement via a connecting rod 3 connecting the piston 2 to the crankshaft 4. Each cylinder is further closed by a cylinder head 5 equipped with two types of valves: the valves 6 of intake connecting the intake duct 7 to the combustion chamber 1, and allowing the combustion chamber to be fed with an air / fuel mixture (in the case of a petrol engine with indirect injection), or with air (in the case of a diesel engine or gasoline direct injection), and a second exhaust valve 8, connecting the combustion chamber 1 to the exhaust duct 9, so as to allow the evacuation of exhausted flue gas to the exhaust. The positioning of the valves is controlled by a camshaft (not shown) connected to the crankshaft 4.

In indirect injection gasoline engines used on motor vehicles, a combustion cycle is classically decomposed according to the following four times, each time corresponding to a quarter of a complete rotation of the crankshaft 4:

- first time (admission): Lowering the piston 2 and opening the intake valve 6 to allow a homogeneous air / fuel mixture, obtained by a carburettor or an indirect injection system (not shown), to enter the combustion chamber 1.

second time (compression): closing of the inlet valve 6 and raising of the piston 2 so as to compress the mixture in the combustion chamber;

- third time (combustion, expansion): Generation of a spark by a spark plug 10, and combustion of the mixture in the combustion chamber 1 which causes the descent of the piston 2;

fourth step (exhaust): Opening of the exhaust valve 8 to allow the evacuation of the burnt gases.

The control of the spark plug 10 is performed by an electronic control unit 11 which also receives information corresponding to the angular position of the crankshaft 4 via an angle sensor 12. The decomposition of a combustion cycle for a conventional diesel engine (direct injection) is quite similar to that described above, except the first and third time. Indeed, during the first time, only air is admitted into the combustion chamber 1. In addition, the operation of the diesel engine is based on the auto-ignition of diesel. It is thus not necessary to provide a spark plug to cause combustion. In a different way, during the third stage (injection - combustion - expansion), an injector 10 'makes it possible to introduce into the combustion chamber fuel that mixes with the compressed air. Self-ignition is obtained as a result of the heating of the air under the effect of compression. Just as in the case of the spark plug 10 of a gasoline engine, the injector 10 'is also controlled by an electronic control unit 11 in relation to the angular position of the crankshaft 4.

In addition to the two major families of internal combustion engines previously described, the so-called HCCI engines (English initials for "Homogeneous Charge Compression Ignition") are also known, exhibiting characteristics derived from both diesel engines of the same type. Conventional (ie self-ignition combustion), and indirect injection engines (ie, achieving a homogeneous mixture of air and fuel). In a combustion cycle for an HCCI engine, the third time corresponds only to the combustion of the homogeneous mixture compressed strongly enough to obtain the point of auto-ignition. Since there is no direct trigger for combustion (spark plug or injector type), combustion control for this type of HCCI engine is more difficult to achieve.

Furthermore, it is known to equip an internal combustion engine with gasoline and especially diesel of conventional type of an exhaust gas recirculation system, or EGR system, with the aim of reducing the production of pollutants whose oxides of nitrogen, and thus meet the requirements of anti-pollution standards. The production of nitrogen oxides is essentially related to the presence of oxygen and to high combustion temperatures. Diesel engines are therefore particularly concerned by this problem because they operate at combustion temperatures higher than those encountered in gasoline engines.

The amount of nitrogen oxides produced can be reduced by mixing the gas admitted by the engine with an inert gas which will slow down the rate of combustion and absorb the calories, which will result in a decrease in the temperature of the engine. combustion. The principle of an EGR system is to take a part of the exhaust gases, including inert gases, to recirculate in the intake duct. An EGR system is thus conventionally constituted of a gas recirculation duct interposed between the exhaust duct and the inlet duct, of a so-called EGR valve, allowing, under the control of the engine control electronic module, to adjust the flow of the burned gas that will be redirected to the intake duct. The EGR system may also include a heat exchanger for cooling recirculated flue gas, thereby avoiding an undesirable increase in particle production. The HCCI engines described above also include an EGR system, but for different reasons. Indeed, an HCCI engine has extremely low emissions of nitrogen oxides. However, the EGR system will be used here permanently to slow down the ignition conditions of the air / fuel mixture.

Many control systems have already been proposed to attempt an optimum adjustment of various engine parameters so as to reduce pollutant emissions, while optimizing the consumption and performance of the engine. On the one hand, no system allows one to obtain a sufficiently high EGR rate (typically of the order of 50%), and secondly, to control this rate very precisely, this control being all the more crucial in the case of an HCCI engine for the reasons explained above.

The present invention aims to overcome these disadvantages. More specifically, the present invention relates to a control system of an internal combustion engine for a motor vehicle, comprising a combustion chamber, an intake duct to allow a gas mixture containing air to enter the combustion chamber. the combustion chamber, an exhaust duct for allowing the escape of burnt gases from the combustion chamber, and an exhaust gas recirculation system comprising an exhaust gas recirculation duct interposed between the exhaust duct; exhaust and the intake duct, and an EGR valve adapted to adjust the flow of exhaust gas which will be redirected to the intake duct via the recirculation duct, characterized in that it further comprises a valve exhaust back-pressure in the exhaust duct downstream of the exhaust gas recirculation duct, able to regulate the flow of the non-redirected exhaust gases; rs the recirculation duct, means for estimating the EGR rate, and a sensor for measuring the exhaust pressure, in that the operation of said EGR valve is slaved in a first closed-loop control system as a function of the difference between the estimated value of the EGR rate issued by the said means estimation method and a set value of the predetermined EGR rate, and in that the operation of said backpressure valve is controlled in a second closed-loop control system as a function of the difference between the pressure value of exhaust measured by the measuring sensor and a set value of the predetermined exhaust pressure.

The use of an exhaust back-pressure valve will allow to obtain a much higher EGR rate than in known systems. In addition, controlling the operation of the EGR valve and the backpressure valve by two separate closed-loop control systems, one, realizing the regulation from a measurement of the exhaust pressure the other from an estimate of the EGR rate makes it possible to obtain more quickly a stability in the operation of the engine.

The system may further comprise a heat exchanger, adapted to cool the recirculating exhaust gas in the exhaust gas recirculation duct, and an intake temperature sensor, the operation of said heat exchanger being enslaved in a third closed-loop control system according to the difference between the intake temperature value measured by the temperature sensor and a set value of the predetermined intake temperature. Alternatively or in combination, the system may also further comprise a valve or an intake butterfly in the intake duct upstream of the exhaust gas recirculation duct, able to regulate the flow of air entering the duct. intake duct, and a sensor for measuring the inlet pressure, the operation of said inlet valve or butterfly being controlled in a fourth closed-loop control system as a function of the difference between the pressure value of the intake measured by the measuring sensor and a set value of the predetermined inlet pressure.

A complete system according to a preferred embodiment of the invention may therefore comprise up to four separate closed-loop control systems each acting on a particular member of the engine (valve or heat exchanger) according to a single observation parameter (temperature, pressure or EGR rate depending on the case).

The present invention and the advantages it provides will be better understood from the following description of a preferred non-limiting embodiment of a control system of a heat engine, with reference to the appended figures, in which:

- Figure 1 already described schematically shows the conventional components of an internal combustion engine;

FIG. 2 diagrammatically illustrates a possible embodiment of a control system for an internal combustion engine according to the invention;

FIG. 3 diagrammatically represents the general principle of servocontrol of the different control loops present for the control system of FIG. 2; FIG. 4 details a closed loop of proportional-integral type;

FIG. 5 illustrates simulation results obtained for the motor control system of FIG. 2.

Referring to Figure 2, the components of a control system for an internal combustion engine according to a preferred embodiment of the invention, particularly suitable for an engine type HCCI, will now be described. For the sake of clarity, the elements common to Figure 1 have the same references. Thus we find in this figure the combustion chamber 1 of one of the cylinders of the engine, inside which can slide the piston 2 connected to the crankshaft (not shown) via the rod 3. This chamber of combustion can be put in relation on the one hand, with the intake duct 7, and on the other hand, with the exhaust duct 9, according to the respective positions of the intake valve 6 and the valve of Exhaust 8. The system further comprises an EGR system conventionally comprising an exhaust gas recirculation duct 13 interposed between the exhaust duct 9 and the duct intake 7, and an EGR valve 14 for adjusting the flow of gas recirculation.

According to the invention, the control system also comprises a counter pressure valve 15 located, as shown in Figure 2, in the exhaust duct 9 downstream of the inlet of the recirculation duct 13. the operation on the one hand of the EGR valve 14, and on the other hand the backpressure valve 15, is controlled in two separate closed-loop control systems, one being controlled by an estimate of the EGR rate, and the other by a measurement of the exhaust pressure. For the operation of the first regulation system acting on the EGR valve, the control system comprises means 16 for estimating the EGR rate. For the operation of the second control system acting on the back pressure valve 15, the control system comprises a sensor 17 for measuring the exhaust pressure. Furthermore, in the preferred embodiment shown here, the EGR system further comprises a heat exchanger 18 whose role is to cool the exhaust gas recirculation. In this case, it is advantageous to control the operation of this exchanger by a third closed-loop control system whose servocontrol is performed from a measurement of the inlet temperature delivered by a sensor 19 of measurement. the temperature of admission.

Finally, still in the context of the preferred embodiment, the control system advantageously comprises an intake valve 20 located in the intake duct 7 upstream of the outlet of the recirculation duct 13, this intake valve being suitable to control the flow of fresh air that will be mixed with the recirculating gases. The operation of the intake valve is advantageously also controlled by a fourth closed-loop control system whose servocontrol is performed from a measurement of the intake pressure delivered by a sensor 21 for measuring the pressure of the pressure. 'admission. The sensors 19 and 21 and the means 16 for estimating the EGR rate are preferably placed at an intake manifold 22 within which the gaseous mixture will be homogenized. Similarly, the sensor 17 for measuring the exhaust pressure is preferably placed at a reserve 23. For other embodiments that do not have an intake manifold 22 or a reserve 23 at the outlet, the Sensors or estimator will be placed directly at the intake and / or exhaust ducts.

The principle of control by separate closed-loop control systems is summarized schematically in FIG. 3. Each closed-loop control system comprises an actuator (the exhaust back-pressure valve 15, the EGR valve 14, the exhaust valve 15 respectively). heat exchanger 18 and the inlet valve 20)), all four actuators having been schematized here in the form of an actuator block B ₂ . For each system, a variable will be observed (measured or estimated) by an appropriate sensor or estimating means. The set of four sensors / estimators, consisting respectively of the exhaust pressure measurement sensor 17, the EGR rate estimation means 16, the intake temperature measurement sensor 19 and the measurement sensor 21 of the inlet pressure, is schematized here in the form of a sensor block Bi. The observed values (respectively denoted PE_ _ΓΠ , TEGR_m _> TA m and PA_ _ΓΠ ) ^are each compared to a setpoint value (respectively denoted PE_C, TEGR_C, TA C and PA_C).

The difference between the observed variable and the setpoint is then corrected by a corrector. The four correctors needed here have been grouped in a correction block B ₃ . Thus, each actuator will receive from its associated corrector a command that will be a function of the measured value and the set value, and act accordingly on the engine.

The set of control systems (including at least the first system for the control of the EGR valve and the second system for the control of the exhaust valve) will act under the control of a timing member C which samples and rhythm the flow of instructions coming out of the correctors.

The corrector of each closed-loop control system may be, by way of non-limiting example, of the proportional-integral (P1) or proportional-integral-derivative (PID) type.

An ideal PID corrector is the sum of three terms, expressed according to the following Laplacian relation:

C {p) = K + ^ + K _d - p in which P

- the first term represents the proportional action - the second term represents the integral action - -

P

the third term represents the derivative action K _d - p

The discrete representation of this corrector with a sampling frequency Te can be expressed according to the following relation: C {z) = K _p + K _r Te - ¹ - + ^ ( ¹ - _z - ¹ ) in which z ^"1 is the delay operator.

A corrector of type Pl contains only the first two terms of the preceding relation.

FIG. 4 illustrates a particularly advantageous embodiment of a discrete type P1 corrector which can be used for each closed loop of the regulation systems described above. The corrector contains the following elements:

a first amplifier Ai which amplifies the difference c -m between the set value c and the measured value. a second amplifier A ₂ of gain Kp which represents the gain of the proportional action.

a third amplifier A ₃ of gain K i which represents the gain of the integral action.

a multiplier A ₄ which makes it possible to use the period Te sampling in the calculation of the discrete integral, and

an operator A ₅ with a delay z ^"1 which represents in discrete form the value previously calculated, which is involved in the calculation of the integral.

In addition, the corrector as shown in Figure 4 advantageously comprises a saturation block A _Ô which allows to integrate the limits of the controls of the actuator considered. Typically,

If M ≥ u max then u = u max and if u ≤ u mi • n then u = u mi • n

Thus the command u, corresponding to the correction, is between the minimum value u _m i _n and the maximum value u _max . Finally, the corrector preferably comprises a second saturation block A ₇ , similar in principle to the block A 1, and which saturates the action of the integral. Indeed, in the application considered, the actuators commands must be saturated. If an instruction unrealizable by the system is requested, it may be that the corrector imposes a command higher than the limits of the actuator. Then, the command applied is the limit command. However, the integral action continues to integrate and imposes a command more and more strong. If, suddenly, the setpoint resumes a value achievable by the system, the command issued by the corrector before its saturation is so strong that it can take quite a long time before decreasing. In this case, the command applied remains the limit command and the system does not react. This runaway phenomenon is known as the Anglo-Saxon windup. The second block A ₇ is accordingly an "anti windup" device.

As indicated above, it is the timing device C that will sample and rhythm the flow of instructions coming out of the correctors. The timing can be obtained:

- regardless of the position of the motor. In this case, the sampling period is fixed.

- depending on one or more positions of the crankshaft. The sampling period is then set by the rotational speed of the motor. It is also possible to alternate the two types of timing as a function, for example, of the engine speed. Thus, when the engine is running at low speed, the calculations to be made are fewer. Synchronous sampling with the motor can therefore be used. For the strongest regimes, it is preferred to perform the calculations at a fixed rate, chosen so as to save the calculation time.

In addition to timing the correctors' calculations, this same timing device will also make it possible to manage the priorities between the different (at least two) closed loop control systems and define in the case where the correctors are dependent on each other, to do these calculations in the most judicious order. Also, for each instruction on the variables delivered by a control unit not shown, commands for the actuators are advantageously developed in a predefined order.

FIG. 5 illustrates the simulation results obtained for a control system comprising the four closed-loop control systems described above. More precisely :

the first diagram starting from the top of the figure illustrates in dashed lines the time evolution of the setpoint value PA C associated with the closed-loop control system controlling the operation of the intake valve or butterfly, and in solid line temporal evolution of the measured value P _A _ _m ;

the second diagram illustrates the corresponding change in the opening rate of the intake valve or butterfly,

the third diagram illustrates in dashed lines the time evolution of the setpoint value PE C associated with the closed-loop control system controlling the operation of the exhaust back-pressure valve, and in full line the time evolution of the measured value PE_ _m ;

the fourth diagram illustrates the corresponding change in the opening rate of the backpressure valve; the fifth diagram illustrates in dashed lines the time evolution of the setpoint value _EGR rate _c associated with the closed-loop control system controlling the operation of the EGR valve, and in full line the time evolution of the measured value _EGR rate _m ; the sixth diagram illustrates the corresponding evolution of the opening rate of the EGR valve;

the seventh diagram illustrates in dashed lines the temporal evolution of the setpoint value TA C associated with the closed-loop control system controlling the operation of the heat exchanger, and the solid line with the time evolution of the measured value T _A _ _m ;

the eighth diagram illustrates the corresponding evolution of the control of the exchanger

It can be seen from these diagrams that the enslavement of each action makes it possible to reach the setpoint without static errors. In addition, the stability is controlled. As with all slave systems, there is a trade-off between speed and stability. The controls adapt well to the various operating points of the engine. Finally, the system makes it possible to obtain an EGR rate of high value and perfectly controlled.

Other simulations have shown that a good control of a high EGR rate could be obtained by the sole use of the two separate control systems controlling the operation of the EGR valve and the back pressure valve.

The control system according to the invention is used for an on-board real-time control. It constantly acts on the engine parameters so that it provides optimal performance at all times. Thus, the system is free to act properly to stabilize the engine.

The use of such a control system is particularly indicated for conventional diesel engines or for HCCI engines.

Claims

1. Control system of an internal combustion engine for a motor vehicle, comprising a combustion chamber (1), an intake duct (7) to allow a gaseous mixture containing air to enter the chamber combustion system, an exhaust duct (9) for allowing the escape of burnt gases from the combustion chamber (1), and an exhaust gas recirculation system comprising a duct (13) for recirculating exhaust gases. an exhaust interposed between the exhaust duct (9) and the intake duct (7), and an EGR valve (14) adapted to regulate the flow of exhaust gas which will be redirected to the intake duct via the duct (13) recirculation, characterized in that it further comprises a valve (15) against exhaust pressure in the exhaust duct (9) downstream of the duct (13) for recirculation of gas exhaust, able to regulate the flow of non-redirected exhaust gases s the recirculation duct, means (16) for estimating the EGR rate, and a sensor (17) for measuring the exhaust pressure, in that the operation of said EGR valve (14) is slaved in a first closed-loop control system as a function of the difference between the estimated value of EGR rate delivered by said estimating means (16) and a set value of the predetermined EGR rate, and in that the operation of said valve (15) is controlled in a second closed-loop control system as a function of the difference between the exhaust pressure value measured by the measuring sensor (17) and a set value of the pressure of the pressure sensor. predetermined escape. 2. Control system according to claim 1, characterized in that it further comprises a heat exchanger (18) adapted to cool the exhaust gas recirculation in the conduit (13) of exhaust gas recirculation, and a temperature sensor (19), the operating said heat exchanger (18) being controlled in a third closed-loop control system as a function of the difference between the intake temperature value measured by the temperature sensor (19) and a set temperature value of d predetermined admission. 3. Control system according to any one of the preceding claims, characterized in that it further comprises a valve or a throttle (20) for admission into the intake duct (20) upstream of the duct (13). recirculating exhaust gas, adapted to regulate the flow of air entering the intake duct, and a sensor (21) for measuring the intake pressure, the operation of said valve or throttle valve (20). the inlet being controlled in a fourth closed-loop control system as a function of the difference between the intake pressure value measured by the measuring sensor (21) and a set value of the predetermined intake pressure. 4. Control system according to any one of the preceding claims, characterized in that each of the closed-loop control systems comprises a proportional-integral-derivative type corrector. 5. Control system according to any one of claims 1 to 4, characterized in that each closed-loop control system comprises a proportional-integral type corrector. 6. Control system according to claim 5, characterized in that the corrector comprises a first saturation block (A _Ô ) to limit the correction between a minimum value (u _m i _n ) and a maximum value (Umax) - 7. Control system according to claim 6, characterized in that the corrector comprises an anti windup device (A ₇ ). 8. Control system according to any one of claims 5 to 7, characterized in that it comprises a timing member (C) capable of timing the calculations of each corrector. 9. Control system according to claim 8, characterized in that the timing member is capable of generating a fixed sampling period independent of the position of the motor, and / or a sampling period fixed by the speed of rotation. of the motor. 10. Control system according to claim 8, characterized in that said timing member (C) is also able to manage priorities between at least two closed-loop control systems.