JP5608614B2 - Engine EGR flow rate detection device - Google Patents

Description

  The present invention relates to an EGR flow rate detection device for an engine provided with a differential pressure detection means before and after an EGR valve.

  Regulations on automobile fuel consumption and exhaust gas are becoming more and more strict in the future, and in particular, fuel efficiency is gaining more interest due to rising gasoline prices and global warming issues. Under such circumstances, development of fuel efficiency improvement technology is being carried out around the world, and one of such technologies is the EGR system. The EGR system is a system that recirculates exhaust gas once burned to the intake passage again. In order to prevent the amount of fresh air from decreasing even if EGR gas (exhaust gas) is recirculated to the intake passage, it is necessary to open the throttle valve, so that pumping loss (intake resistance) is reduced and fuel efficiency is improved. In addition, exhaust gas also has an effect that the combustion temperature is lowered by introducing EGR gas into the combustion chamber, and NOx generated at a high temperature can be suppressed.

  In general, the recirculation destination of EGR gas in the EGR system is downstream of a throttle valve where negative pressure is generated. In an engine equipped with a supercharger, in the supercharging region, the pressure downstream of the throttle is positive and it is difficult for EGR gas to enter, but EGR is not applied to the supercharging region. However, recently, in order to further improve fuel efficiency, it is necessary to apply EGR even in the supercharging region, and a system for returning EGR gas to the upstream side of the compressor has been considered.

  EGR gas is effective for improving fuel efficiency and reducing NOx, but if EGR is applied too much, combustion may become unstable, and conversely, fuel efficiency and exhaust performance may be deteriorated. First, when applying EGR, it is necessary to be able to accurately detect the EGR flow rate.

  As a technique for detecting the EGR flow rate, there is “Patent Document 1”.

Japanese Patent No. 3888024

  In Patent Document 1, the differential pressure before and after the EGR valve and the EGR valve opening are sampled during the intake stroke, and the EGR flow rate is obtained from the average value thereof. However, since the time of the intake stroke depends on the engine speed, a large number of samplings can be obtained at low speeds, but the number of samplings is insufficient at high speeds, and it is difficult to obtain an accurate EGR flow rate. In addition, the fluid flowing in the intake passage and the exhaust passage pulsates and includes a pulsation frequency component that depends on the engine speed, so that the pulsation frequency component is less likely to be removed as the rotation speed is lower. In order to remove the pulsation frequency component, it is conceivable that the time constant of the averaging process is set to be large in advance. However, since the response at the time of transient operation is delayed, a large error occurs in the EGR rate, causing deterioration of combustion. There is a possibility.

  Further, when the engine load is high even at the same rotation speed, the exhaust pressure becomes high, and the pulsation becomes large.

  Further, when the throttle opening is large, the intake air pulsation of the engine is easily transmitted, and thus the pulsation becomes large.

  Thus, the time constant is not set to one, but it is more effective to switch to the optimal time constant depending on the operating conditions of the engine. However, Patent Document 1 discloses that the differential pressure before and after the EGR valve and the EGR valve open. Although there is a description that the degree is sampled and averaged, it is not mentioned that the averaging method is switched depending on the operating conditions.

  The present invention has been made in view of the problems to be solved, and an object of the present invention is to provide an EGR flow rate detection device that accurately detects an EGR flow rate flowing in an EGR pipe regardless of operating conditions. It is to provide.

  To achieve the above object, an EGR flow rate detection device according to the present invention includes an EGR pipe that recirculates EGR gas (exhaust gas) from an exhaust passage of an engine to an intake passage, and an EGR valve that adjusts an EGR flow rate in the EGR pipe. A differential pressure detecting means for detecting a differential pressure before and after the EGR valve, an EGR flow rate calculating means for converting a differential pressure detected by the differential pressure detecting means into an EGR flow rate, and a differential pressure detected by the differential pressure detecting means And a fluctuation suppression means for suppressing fluctuations in the waveform of the EGR flow rate calculated by the EGR flow rate calculation means, and an operation condition detection means for detecting the operating condition of the engine, and the fluctuation suppression means according to the operating condition of the engine Changing the characteristics of

  The EGR flow rate detection device according to the present invention preferably detects a differential pressure from the absolute pressure before and after the EGR valve.

  In the EGR flow rate detection device according to the present invention, preferably, the operating condition is an engine speed, and the time constant of the fluctuation suppressing means is increased as the engine speed is lower.

  In the EGR flow rate detection device according to the present invention, preferably, the operating condition is an engine load, and the time constant of the fluctuation suppressing means is increased as the engine load is higher.

  In the EGR flow rate detection device according to the present invention, preferably, the operating condition is an EGR valve opening, and when the EGR valve opening changes by a predetermined value or more, the time constant of the fluctuation suppressing means is reduced.

  In the EGR flow rate detection device according to the present invention, preferably, the operating condition is a throttle valve opening, and when the throttle valve opening is larger than a predetermined value, the time constant of the fluctuation suppressing means is increased.

  According to the EGR flow rate detection device of the present invention, the EGR flow rate flowing through the EGR pipe can be accurately detected regardless of the operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic block diagram which shows one Example of the EGR flow volume apparatus which concerns on this invention with the vehicle-mounted engine to which it is applied. The figure used for description of a differential pressure sensor. The block diagram with which it uses for description of EGR flow volume calculation. The figure used for description of the sampling error at the time of a large pulsation. The figure used for description of the sampling error at the time of a small pulsation. The flowchart which shows an example of the process sequence performed when setting the time constant of a fluctuation | variation suppression means. The figure with which it uses for description of the pulsation frequency component for every rotation speed. The figure used for description of the frequency characteristic by the time constant difference of a fluctuation suppression means. The figure used for description of the responsiveness comparison by the time constant difference of a fluctuation suppression means. The block diagram which is provided for description of the time constant calculation for steady operation.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram showing an embodiment of an EGR flow rate detection device according to the present invention together with an example of an in-vehicle engine to which the EGR flow rate detection device is applied.

  In FIG. 1, an engine 10 to which the EGR flow rate detection device of this embodiment is applied is a spark ignition type multi-cylinder engine having, for example, four cylinders (# 1, # 2, # 3, # 4), The cylinder 11 includes a cylinder head 11a and a cylinder block 11b, and a piston 15 slidably inserted into each cylinder of the cylinder 11. The piston 15 is connected to a crankshaft (not shown) via a connecting rod 14. Z). A combustion chamber 17 having a ceiling with a predetermined shape is defined above the piston 15, and an ignition plug 35 to which a high-voltage ignition signal is supplied from an ignition coil 34 is erected in the combustion chamber 17 of each cylinder. Has been.

  Air necessary for fuel combustion passes from the air cleaner 19 through the intake passage 20 including the compressor 41, the throttle valve 25, the intercooler 18, the collector 27, the intake manifold 28, the intake port 29, and the like, and the intake port 29 which is the downstream end. The air is sucked into the combustion chambers 17 of the respective cylinders via an intake valve 21 disposed at the end and driven to open and close by an intake camshaft 23. A fuel injection valve 30 that injects fuel toward the intake port 29 is provided for each cylinder in the intake manifold 28 that is the downstream portion of the intake passage 20.

  The air flow sensor 50 downstream of the air cleaner 19 increases the value of the current flowing through the hot wire (heating resistor) arranged in the air flow to be measured as the amount of intake air (mass flow rate) increases, and conversely sucks The bridge circuit is configured to decrease as the amount of air decreases, and is extracted as a voltage signal from the flowing heating resistance current and input to an ECU (engine control unit) 100 described later.

  The air-fuel mixture of the air sucked into the combustion chamber 17 and the fuel injected from the fuel injection valve 30 is combusted by spark ignition by the spark plug 35 connected to the ignition coil 34, and the exhaust gas is burned into the combustion chamber 17. Then, the exhaust gas is discharged to the outside atmosphere through an exhaust passage 40 including an exhaust port, an exhaust manifold, an exhaust pipe, and the like through an exhaust valve 22 that is opened and closed by an exhaust camshaft 24.

  A turbine 42 is provided in the exhaust passage 40, and the turbine 42 is connected to the compressor 41 with the same shaft. When the pressure of the gas discharged from the engine exceeds a predetermined value, supercharging starts and combustion of the compressed air occurs. It is sent into the chamber 17. The compressed hot air is cooled by the intercooler 18. When the supercharging pressure exceeds a predetermined value, the waste gate valve 44 and the recirculation valve 43 are controlled so as to prevent further supercharging.

Further, a three-way catalyst 60 for purifying exhaust gas is provided in the exhaust passage 40, and a linear air-fuel ratio sensor 51 having an output characteristic linear with respect to the pre-catalyst air-fuel ratio is arranged upstream of the three-way catalyst 60. An O ₂ sensor 52 that outputs a switching signal for identifying whether the post-catalyst air-fuel ratio is richer or leaner than stoichiometric (theoretical air-fuel ratio) is disposed downstream of the three-way catalyst 60. .

  Further, fuel in the fuel tank 75 is supplied to the fuel injection valve 30 provided for each cylinder after being regulated to a predetermined fuel pressure by a fuel supply mechanism including a fuel pump 76, a fuel pressure regulator 77, and the like. The valve 30 is driven to open by a fuel injection pulse signal supplied from the ECU 100 and having a duty (= pulse width = corresponding to the valve opening time) corresponding to the operation state at that time, and the valve 30 is driven according to the valve opening time. An amount of fuel is injected toward the intake port 29.

  On the other hand, in order to perform various controls of the engine 10, that is, fuel injection control (air-fuel ratio control) by the fuel injection valve 30, ignition timing control by the spark plug 35, etc., an ECU 100 incorporating a microcomputer is provided. Yes.

An EGR pipe 61 is connected downstream of the three-way catalyst 60 in the exhaust passage 40, and the EGR gas flowing from the exhaust passage 40 into the EGR pipe 61 is cooled by the EGR cooler 62 and then passes through the EGR valve 64. It mixes with air at the EGR junction 66. An EGR temperature sensor 63 that measures the EGR gas temperature is disposed upstream of the EGR valve 64. An upstream pressure passage 67 and a downstream pressure passage 68 are connected to the differential pressure sensor 65 before and after the EGR valve 64. Here, an example having the absolute pressure sensors 73 and 74, the AD conversion unit 70, the differential pressure calculation unit 71, the DA conversion unit 72, and the temperature sensor 69 shown in FIG. 2 will be described as an example of the differential pressure sensor 65. Input signals to the differential pressure sensor 65, the upstream pressure P _u EGR valve 64, an EGR valve 64 downstream pressure P _d, to convert the respective input signal at an absolute pressure sensor 73 and 74 to a voltage, the voltage value After AD conversion by the AD conversion unit 70, the differential pressure calculation unit 71 calculates the differential pressure. At this time, since the pressure changes depending on the temperature, a more accurate differential pressure can be obtained by mounting the temperature sensor 69 in the differential pressure sensor 65 and correcting the temperatures of P _u and P _d based on the information of the temperature sensor 69. . After the differential pressure calculation, the DA conversion unit 72 performs DA conversion and then outputs to the ECU 100.

The EGR flow rate calculation method will be described with reference to the block diagram shown in FIG. The ECU 100 samples the differential pressure output from the differential pressure sensor 65, the EGR valve opening degree output from the EGR valve 64, and the EGR gas temperature output from the EGR temperature sensor 63, respectively. Thereafter, in block 151, the differential pressure is obtained according to the output characteristics of the differential pressure sensor 65, and in block 152, the differential pressure is converted into the flow rate of EGR gas passing through the EGR valve 64. There is a relationship of the formula (A) between the pressure around the EGR valve 64 and the flow velocity. Here, _Pu is the EGR valve upstream pressure, _Pd is the EGR valve downstream pressure, ρ is the EGR gas density, and v is the EGR gas flow rate. When this equation (A) is converted, equation (B) is obtained, and the EGR gas flow velocity v is obtained. Here, P _dif is a differential pressure, and P _dif = P _u −P _d .

  In block 153, the EGR valve opening area is calculated from the EGR valve opening. In block 154, the EGR gas density is calculated based on the EGR gas temperature obtained from the EGR temperature sensor 63. In block 155, the EGR flow rate is calculated by the following formula (C) from the EGR gas flow velocity, EGR valve opening area, and EGR gas density calculated in blocks 152 to 154. Here, Qegr is the EGR gas flow rate, A is the EGR valve opening area, v is the EGR gas flow velocity, and ρ is the EGR gas density.

  Here, LPF (Low Pass Filter) which is a fluctuation suppressing means applies either one or both of the differential pressure after block 151 and the EGR flow after block 155 in FIG.

  Normally, the intake pressure and exhaust pressure of the engine pulsate. FIG. 4 shows problems that may occur when sampling is performed during a large pulsation. Since sampling is performed at a predetermined periodic interval, when sampling is performed as shown in FIG. 4, the true average value is Vave, whereas the average value after sampling is Vsam, and a sampling error (= Vave−Vsam). ) Will occur greatly. FIG. 5 shows a case where the pulsation waveform is sampled after being effectively attenuated by the fluctuation suppressing means. When the pulsation is attenuated, the sampling error becomes small even if the same sampling as in FIG. 4 is performed. Therefore, it can be seen that reducing the pulsation by the fluctuation suppressing means with respect to the differential pressure before and after the EGR valve or the calculated EGR flow rate is indispensable for reducing the sampling error.

  In order to reduce the pulsation, the time constant of the fluctuation suppressing means may be increased. However, if this is done, the response at the time of transition will be delayed, and the EGR rate may change suddenly to make the combustion unstable.

  Therefore, in the present invention, the characteristics of the fluctuation suppressing means for reducing the pulsation are switched between the steady state and the transient state, and also changed according to the engine conditions such as the engine speed, the engine load, and the throttle valve opening even in the steady state. It is characterized by.

  Below, the setting method of the time constant of the fluctuation | variation suppression means of this invention is demonstrated using the flowchart of FIG.

  FIG. 6 is a flowchart relating to setting of the time constant of the fluctuation suppressing means when the engine 10 is in the EGR operation region.

  In step 111, the EGR valve opening degree input from the EGR valve 64 is calculated. Step 112 determines whether the operating state of the engine is in steady operation or transient operation. If the amount of change in the value of the EGR valve opening at step 111 has changed by a predetermined value or more, it is determined that the engine is in a transient operation, and the routine proceeds to step 117.

In step 117, since transient operation is performed, in order to prioritize responsiveness, a transient operation time constant τ _k having a small time constant of the fluctuation suppressing means is set. If it is determined in step 112 that steady operation is being performed, the process proceeds to step 113.

  In step 113, the engine speed is calculated. The waveforms of the engine intake pressure and the exhaust pressure always change because the pulsation frequency component depending on the engine speed is included. FIG. 7 illustrates the pulsation frequency component for each rotation speed. FIG. 7 shows, as an example, pulsation frequency components included in a pressure waveform every 1000 r / min from 1000 r / min to 4000 r / min in a four-cylinder engine. As can be seen from FIG. 7, the lower the engine speed, the more pulsation frequency components tend to be included. FIG. 8 shows an example of the frequency response due to the difference in time constant of the fluctuation suppressing means. The horizontal axis represents frequency, and the vertical axis represents Gain [abs]. The smaller this Gain, the greater the effect of reducing the pulsation frequency component. It can be seen that when the time constant is small, the reduction effect of the pulsation frequency component is small in most frequency regions, and conversely, when the time constant is large, there is a high reduction effect in almost all frequency regions. That is, it can be said that the pulsation reduction effect is higher when the time constant of the fluctuation suppressing means is larger. However, if the time constant is too large, there is a drawback that the response time is delayed. FIG. 9 illustrates how the response time varies depending on the time constant when the signal changes stepwise when time t = t0. The time constant refers to the time until the response of 63.2% of the signal when the signal changes stepwise. When the time constant is small, the response time is (t1-t0), which is the fastest, and when the time constant is large, the response time is (t3-t0), which is the slowest.

  As described above, since the pulsation frequency component included in the waveform differs depending on the rotational speed, it is desirable to change the time constant of the fluctuation suppressing means depending on the rotational speed. Specifically, the pulsation frequency component can be effectively reduced by setting the time constant to be large at low rotation and to be small at high rotation.

  In step 114, the engine load is calculated. Even at the same rotation speed, the higher the engine load, the higher the exhaust pressure, so the pulsation increases. Therefore, the pulsation frequency component can be effectively reduced by setting the time constant so that the time constant increases as the engine load increases and the time constant decreases as the engine load decreases.

  In step 115, the throttle valve opening is calculated. When the throttle valve opening is open, the pulsation of the engine is easily transmitted, so the pulsation increases. In particular, under conditions where the throttle valve 25 is fully open or close to full open, the pressure waveform greatly fluctuates due to the large pulsation of the engine 10. Therefore, under the condition that the throttle valve 25 is opened by a predetermined opening or more, the pulsation frequency component can be effectively reduced by setting a large time constant of the fluctuation suppressing means.

In step 116, a time constant τ _t for steady operation is set based on the results of steps 113 to 115. FIG. 10 shows a time constant τ _t calculation block for steady operation.

In block 156, a basic time constant τ ₀ that is the basis of the τ _t calculation is determined from a three-dimensional map composed of the engine speed and the engine load. The three-dimensional map is set so that the horizontal axis is the engine speed, and the time constant of the fluctuation suppressing means is smaller as the engine speed is higher, and the time constant of the fluctuation suppressing means is larger as the engine speed is lower. The vertical axis represents the engine load, which is set such that the time constant of the fluctuation suppressing means is smaller as the load is lower, and the time constant of the fluctuation suppressing means is larger as the load is higher.

In block 157, the value of C _th which is a correction coefficient to be multiplied by τ ₀ is changed depending on the size of the throttle valve opening. Specifically, when the throttle valve 25 is opened more than a predetermined opening degree (θ ₀ ), the pulsation of the engine 10 increases, so that the correction coefficient is changed so that C _th > 1 and the time constant increases. Conversely, if the throttle valve 25 is smaller than θ _0, C _th = 1.

In block 158, τ _t is calculated. τ _t is obtained by multiplying τ ₀ calculated in block 156 by C _th calculated in block 157.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, in the above-described embodiment, an example in which the EGR gas recirculation destination is the upstream of the compressor has been described. However, the EGR gas recirculation destination may be downstream of the compressor, and even in an engine not equipped with a supercharger, The same effect can be obtained even with an injection-type engine, and it can also be applied to an engine mounted on a hybrid vehicle.

DESCRIPTION OF SYMBOLS 10 Engine 11a Cylinder head 11b Cylinder block 14 Connecting rod 15 Piston 17 Combustion chamber 18 Intercooler 19 Air cleaner 20 Intake passage 21 Intake valve 22 Exhaust valve 23 Intake cam shaft 24 Exhaust cam shaft 25 Throttle valve 27 Collector 28 Intake manifold 29 Intake port 34 Ignition Coil 35 Spark plug 40 Exhaust passage 41 Compressor 42 Turbine 43 Recirculation valve 44 Waste gate valve 51 Linear air-fuel ratio sensor 52 O ₂ sensor 60 Three-way catalyst 61 EGR pipe 62 EGR cooler 63 EGR temperature sensor 64 EGR valve 65 Differential pressure sensor 66 EGR junction 67 Upstream pressure passage 68 Downstream pressure passage 69 Temperature sensor 70 AD converter 71 Differential pressure calculator 72 DA converter 73 Absolute pressure sensor (EGR valve upstream
74 Absolute pressure sensor (downstream of EGR valve)
75 Fuel tank 76 Fuel pump 77 Fuel pressure regulator 100 ECU (Engine control unit)

Claims (6)

An EGR pipe that recirculates exhaust gas from the exhaust passage of the engine to the intake passage, an EGR valve that adjusts an EGR flow rate in the EGR pipe, a differential pressure detection means that detects a differential pressure before and after the EGR valve, and the differential pressure EGR flow rate calculation means for converting the differential pressure detected by the detection means into an EGR flow rate, and fluctuation suppression that suppresses fluctuations in the waveform of the differential pressure detected by the differential pressure detection means or the waveform of the EGR flow rate calculated by the EGR flow rate calculation means An EGR flow rate detection device comprising: means; and an operating condition detecting means for detecting an engine load and an engine speed as an operating condition of the engine ,
An EGR flow rate detecting device characterized in that a characteristic of the fluctuation suppressing means is changed according to the engine load and the engine speed .   Having a map with the engine load and engine speed as axes;
  2. The EGR flow rate detection device according to claim 1, wherein the characteristics of the fluctuation suppressing means are changed based on the map. The higher the engine load, the larger the time constant of the fluctuation suppressing means,
The lower the pre-SL engine speed, EGR flow rate detecting device according to claim 1 or 2, characterized in that to increase the time constant of the fluctuation suppressing means.   The EGR flow rate detection device according to claim 1 or 2, wherein the differential pressure detection means detects a differential pressure from absolute pressures before and after the EGR valve. Wherein is included further EGR valve opening degree operating conditions, when the EGR valve opening degree has changed more than a predetermined value, according to claim 1 or 2, characterized in that for reducing the time constant of the fluctuation suppressing means EGR flow detection device. The throttle valve opening is further included in the operating condition , and when the throttle valve opening is larger than a predetermined value, the time constant of the fluctuation suppressing means is increased. EGR flow detection device.