JP2015140759A - Control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simply estimate the current amount of oxygen stored in an exhaust emission control catalyst, and control it to be an amount suitable for eliminating harmful matter.SOLUTION: With reference to an output signal from an air-fuel ratio sensor 43 installed upstream of a catalyst 41, the partial pressure of oxygen in exhaust gas upstream of the catalyst is estimated, and with reference to an output signal from an air-fuel ratio sensor 44 installed downstream of the catalyst, the partial pressure of oxygen in exhaust gas downstream of the catalyst is estimated. On the basis of a difference between both partial pressures of the oxygen, the amount of oxygen stored in the catalyst or released from the catalyst is estimated, and then a fuel injection amount is adjusted to be increased/reduced to converge the current amount of the oxygen stored in the catalyst to a target value.

Description

  The present invention relates to a control device that controls an air-fuel ratio by adjusting a fuel injection amount in an internal combustion engine.

Generally, in the exhaust passage of an internal combustion engine, harmful substances HC contained in the exhaust gas discharged from the cylinders of the internal combustion engine, CO, three-way catalyst to harmless by oxidation / reduction of NO _x is mounted. In order to efficiently purify all of HC, CO, and NO _x , it is necessary to keep the air-fuel ratio of the exhaust gas within a certain range near the stoichiometric air-fuel ratio called a window. For this purpose, air-fuel ratio sensors are arranged upstream and downstream of the catalyst, and a double feedback loop using the output signals of the air-fuel ratio sensors is constructed to feedback control the air-fuel ratio (for example, the following patent document) 1).

  In an internal combustion engine mounted on a vehicle, it is known to perform a fuel cut that temporarily stops fuel injection in accordance with the driving situation. Normally, when the accelerator pedal depression amount is 0 or less than a threshold value close to 0 and the engine speed is equal to or higher than the fuel cut permission speed, the fuel cut is started assuming that the fuel cut condition is satisfied. Then, when any fuel cut end condition is satisfied, such as when the accelerator pedal depression amount exceeds the threshold value, or the engine speed decreases to the fuel cut return speed, the fuel cut ends and the fuel injection resumes. .

A large amount of oxygen is occluded in the catalyst immediately after the end of the fuel cut. It is difficult to cause a reduction reaction in an oxygen-rich atmosphere, and there is a concern that the amount of NO _x emission increases with the end of the fuel cut. Therefore, it is customary to purge the oxygen stored in the catalyst by offsetting the target air-fuel ratio in the air-fuel ratio feedback control to a richer side than the stoichiometric air-fuel ratio immediately after the fuel cut ends (for example, (See Patent Document 2).

JP2013-002430A Japanese Patent Laid-Open No. 05-026073

The purification efficiency of HC, CO, and NO _x is affected by the amount of oxygen stored in the catalyst. As already described, when the catalyst is filled with oxygen, the reduction of NO _x hardly occurs and the amount of NO _x emission increases. On the other hand, when oxygen is deficient in the catalyst, oxidation of HC and CO hardly occurs, and the amount of emission of these HC and CO increases.

  However, the conventional air-fuel ratio feedback control does not necessarily take into account the amount of oxygen currently stored in the catalyst. Therefore, it can be said that there is room for improvement in the purification performance of harmful substances.

  The present invention, which has been made paying attention to the above problems, is intended to simply estimate the current stored oxygen amount of the catalyst and control it to an amount suitable for the purification of harmful substances.

  In the present invention, the oxygen partial pressure of the exhaust gas upstream of the catalyst is estimated with reference to the output signal of the air-fuel ratio sensor installed upstream of the exhaust gas purification catalyst mounted in the exhaust passage of the internal combustion engine. Referring to the output signal of the air-fuel ratio sensor installed upstream of the catalyst, the oxygen partial pressure of the exhaust gas downstream of the catalyst is estimated, and based on the difference between the oxygen partial pressures, the catalyst is occluded or stored in the catalyst. A control device for an internal combustion engine is configured that estimates the released oxygen amount and adjusts the fuel injection amount to increase or decrease to converge the current stored oxygen amount of the catalyst to the target value.

  In estimating the amount of oxygen stored in or released from the catalyst, the oxygen partial pressure upstream of the catalyst to be compared with the oxygen partial pressure downstream of the catalyst is the past from the time when the oxygen partial pressure downstream of the catalyst is estimated. It is preferable that it is estimated at the time of going back to. In that case, the time difference between the two estimation points is changed according to the oxygen storage capacity of the catalyst.

  According to the present invention, the current stored oxygen amount of the catalyst can be easily estimated. Then, the amount of oxygen stored in the catalyst can be controlled to an amount suitable for the purification of harmful substances.

The figure which shows schematic structure of the internal combustion engine and control apparatus in one Embodiment of this invention. Diagram illustrating the output voltage of the rear O ₂ sensor control apparatus of the embodiment refers.

  An embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an outline of an internal combustion engine for a vehicle in the present embodiment. The internal combustion engine in the present embodiment is a spark ignition type 4-stroke gasoline engine, and includes a plurality of cylinders 1 (one of which is shown in FIG. 1). In the vicinity of the intake port of each cylinder 1, an injector 11 for injecting fuel is provided. A spark plug 12 is attached to the ceiling of the combustion chamber of each cylinder 1. The spark plug 12 receives spark voltage generated by the ignition coil and causes spark discharge between the center electrode and the ground electrode. The ignition coil is integrally incorporated in a coil case together with an igniter that is a semiconductor switching element.

  The intake passage 3 for supplying intake air takes in air from the outside and guides it to the intake port of each cylinder 1. On the intake passage 3, an air cleaner 31, an electronic throttle valve 32, a surge tank 33, and an intake manifold 34 are arranged in this order from the upstream.

  The exhaust passage 4 for discharging the exhaust guides the exhaust generated by burning the fuel in the cylinder 1 from the exhaust port of each cylinder 1 to the outside. An exhaust manifold 42 and an exhaust purification three-way catalyst 41 are disposed on the exhaust passage 4.

Air-fuel ratio sensors 43 and 44 for detecting the air-fuel ratio of the exhaust gas flowing through the exhaust passage are installed upstream and downstream of the catalyst 41 in the exhaust passage 4. Each of the air-fuel ratio sensors 43 and 44 may be an O ₂ sensor having a non-linear output characteristic with respect to the air-fuel ratio of the exhaust gas, or a linear A / F sensor having an output characteristic proportional to the air-fuel ratio of the exhaust gas. There may be. In the present embodiment, an O ₂ sensor that outputs a voltage signal corresponding to the oxygen concentration in the exhaust gas is assumed for each of the upstream and downstream air-fuel ratio sensors 43 and 44 of the catalyst 41. The output characteristics of the O ₂ sensors 43 and 44 show a large and steep slope in the output change rate with respect to the air-fuel ratio in the range near the stoichiometric air-fuel ratio, and gradually approach the lower saturation value in the lean region where the air-fuel ratio is larger than that. In a rich region where the air-fuel ratio is small, a so-called Z characteristic curve is drawn that gradually approaches the high saturation value.

  An external EGR device 2 is attached to the internal combustion engine of the present embodiment. The EGR device 2 realizes a so-called high-pressure loop EGR. An EGR passage 21 that communicates the upstream side of the catalyst 41 in the exhaust passage 4 and the downstream side of the throttle valve 32 in the intake passage 3, and the EGR passage 21 The EGR cooler 22 provided and the EGR valve 23 that opens and closes the EGR passage 21 and controls the flow rate of the EGR gas flowing through the EGR passage 21 are used as elements. The inlet of the EGR passage 21 is connected to the exhaust manifold 42 in the exhaust passage 4 or a predetermined location downstream thereof. The outlet of the EGR passage 21 is connected to a predetermined location downstream of the throttle valve 32 in the intake passage 3, specifically to a surge tank 33.

  An ECU (Electronic Control Unit) 0 serving as a control device for an internal combustion engine according to the present embodiment is a microcomputer system having a processor, a memory, an input interface, an output interface, and the like.

  The input interface includes a vehicle speed signal a output from a vehicle speed sensor that detects the actual vehicle speed of the vehicle, a crank angle signal b output from an engine rotation sensor that detects the rotation angle and engine speed of the crankshaft, and depression of an accelerator pedal. The accelerator opening signal c output from a sensor that detects the amount or the opening of the throttle valve 32 as an accelerator opening (so-called required load), the intake air temperature and the intake pressure in the intake passage 3 (particularly, the surge tank 33). Detects the intake air temperature / intake pressure signal d output from the temperature / pressure sensor to be detected, the coolant temperature signal e output from the water temperature sensor to detect the coolant temperature of the internal combustion engine, and the air-fuel ratio of the exhaust gas upstream of the catalyst 41 The air-fuel ratio signal f output from the air-fuel ratio sensor 43, and the air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas downstream of the catalyst 41. The air-fuel ratio signal g outputted from the sub 44, the atmospheric pressure signal h or the like to be outputted from the atmospheric pressure sensor for detecting the atmospheric pressure is inputted.

  From the output interface, the ignition signal i for the igniter of the spark plug 12, the fuel injection signal j for the injector 11, the opening operation signal k for the throttle valve 32, and the opening operation signal l for the EGR valve 23. Etc. are output.

  The processor of the ECU 0 interprets and executes a program stored in the memory in advance, calculates operation parameters, and controls the operation of the internal combustion engine. The ECU 0 acquires various information a, b, c, d, e, f, g, h necessary for operation control of the internal combustion engine via the input interface, and requests the required fuel injection amount, fuel injection timing (once Operating parameters such as fuel injection pressure, ignition timing, required EGR amount (or EGR rate), etc. are determined. The ECU 0 applies various control signals i, j, k, and l corresponding to the operation parameters via the output interface.

  The ECU 0 according to the present embodiment performs feedback control of the air-fuel ratio of the air-fuel mixture charged in the cylinder 1 and consequently the air-fuel ratio of the exhaust gas discharged from the cylinder 1 and led to the catalyst 41. The ECU 0 first calculates the amount of fresh air charged into the cylinder 1 from the intake pressure and intake temperature, the engine speed, the required EGR rate, etc., and determines the basic injection amount TP corresponding to this.

  Next, the basic injection amount TP is corrected with a feedback correction coefficient FAF determined according to the air-fuel ratio on the upstream side and / or downstream side of the catalyst 41. In general, the feedback correction coefficient FAF is adjusted according to the deviation between the air / fuel ratio of the gas actually measured via the air / fuel ratio sensors 43 and 44 and the target air / fuel ratio (normally near the theoretical air / fuel ratio). It increases when it is lean with respect to the target air-fuel ratio, and decreases when the actually measured air-fuel ratio is rich with respect to the target air-fuel ratio.

Further, various correction coefficients K determined according to the state of the internal combustion engine, correction coefficient X (described later) according to the amount of oxygen currently stored in the catalyst 41, and the invalid injection time TAUV of the injector 11 are also taken into consideration. Thus, the final fuel injection time (energization time for the injector 11) T is calculated. The fuel injection time T is
T = TP × FAF × K × X + TAUV
It becomes. Accordingly, the signal j is input to the injector 11 for the fuel injection time T, and the injector 11 is opened to inject fuel.

The feedback control with reference to the air-fuel ratio signals f and g on the upstream side and / or downstream side of the catalyst 41 is, for example, that the cooling water temperature of the internal combustion engine is equal to or higher than a predetermined temperature, the fuel is not cut, and the power increase is not This is performed when a predetermined time elapses from the start of the internal combustion engine, all the conditions such as the O ₂ sensors 43 and 43 are active and the intake pressure is normal are all satisfied.

Hereinafter, a method for estimating the current oxygen storage amount of the catalyst 41 will be described in detail. The Nernst equation for the stabilized zirconia O ₂ sensors 43 and 44 is as follows.
E = (RT / 4F) × ln (P _S / P _R )
Here, E is the output voltage (V) of the O ₂ sensors 43 and 44, R is the molar gas constant of 8.3145 (J / mol · K), T is the sensor temperature (K), and F is the Faraday constant of 9.648 × ^{10 4 (C / mol),} P S is the oxygen partial pressure of the oxygen partial pressure of the low oxygen concentration side (exhaust-side), P _R is the high oxygen concentration side (atmosphere side).

When the Nernst equation is transformed, the following equation is obtained.
P _S = P _R × e ^{4FE / RT}
Assuming that the atmospheric oxygen partial pressure is 0.213 and the temperature of the O ₂ sensors 43 and 44 is maintained at 600 (K) by the heater,
P _S ≒ (0.213 x exhaust pressure) x e ^53.2E
It becomes.

By substituting the output voltage of the front O ₂ sensor 43 and the exhaust gas pressure at the installation location of the front O ₂ sensor 43 into the above equation, the partial pressure of oxygen contained in the exhaust gas upstream of the catalyst 41 is estimated. be able to.

In order to know the pressure of the exhaust gas at the place where the front O ₂ sensor 43 is installed, an intake pressure sensor that measures the intake pressure in the surge tank 33 is used. However, it takes a certain amount of time for the intake air in the surge tank 33 to reach the vicinity of the front O ₂ sensor 43 as exhaust after combustion. Therefore, the current intake pressure in the surge tank 33 cannot be substituted into the above equation as the exhaust pressure for the purpose of estimating the oxygen partial pressure upstream of the catalyst 41 at the present time.

  In order to estimate the oxygen partial pressure on the upstream side of the catalyst 41 at the present time, the intake pressure in the surge tank 33 measured at the time of going back to the past is imitated with the exhaust pressure at the present time. The time difference from the time of measurement of the intake pressure that is substituted into the above equation as the exhaust pressure to the present time (may be in seconds, cycle unit (unit number of fuel injections), unit number of rotations of the crankshaft, etc.) may be constant. However, it may be variable according to the engine speed and load (or intake pressure, intake air amount, accelerator opening, etc.). The flow rate of the gas sucked into the cylinder 1 from the intake passage 3 and discharged into the exhaust passage 4 increases as the engine speed increases and the engine load increases. Therefore, it is preferable that the time difference is smaller as the engine speed is higher and smaller as the engine load is higher.

Further, in the above equation, the output voltage of the rear O ₂ sensor 44, and by substituting the pressure of the exhaust gases at the installation location of the rear O ₂ sensor 44, the partial pressure of oxygen contained in the exhaust gas downstream of the catalyst 41 Can be estimated.

The exhaust gas pressure at the location where the rear O ₂ sensor 44 is installed is close to atmospheric pressure when the engine load is relatively low. Therefore, the atmospheric pressure measured via the atmospheric pressure sensor can be substituted into the above equation as the exhaust pressure.

However, when the engine load increases, the temperature of the exhaust gas at the location where the rear O ₂ sensor 44 is installed increases, and the pressure also increases. For this reason, when the engine load is equal to or higher than a predetermined value, it is more preferable to substitute an estimated exhaust pressure higher than the atmospheric pressure into the above equation. The estimated exhaust pressure is estimated from, for example, the engine speed and load, atmospheric pressure, and the like. In that case, map data defining the relationship between the engine speed, the engine load, the atmospheric pressure, and the like and the estimated exhaust pressure is stored in advance in the memory of the ECU0. The ECU 0 searches the map using the current engine speed and load, atmospheric pressure, etc. as keys, and obtains an estimated exhaust pressure at the location where the rear O ₂ sensor 44 is installed.

  The difference between the oxygen partial pressure of the exhaust gas upstream of the catalyst 41 and the oxygen partial pressure of the exhaust gas downstream of the catalyst 41 indicates the amount of oxygen stored in or released from the catalyst 41. If the latter oxygen partial pressure is lower than the former oxygen partial pressure, when the exhaust gas passes through the catalyst 41, oxygen in the exhaust gas is adsorbed by the catalyst 41 and decreases (the oxygen storage amount of the catalyst 41 increases). It turns out that. In contrast, if the latter oxygen partial pressure is higher than the former oxygen partial pressure, oxygen is released from the catalyst 41 when the exhaust gas passes through the catalyst 41 and the amount of oxygen in the exhaust gas increases (the catalyst 41 This means that the oxygen storage amount has decreased).

However, a certain amount of time is required until the exhaust gas (and oxygen contained therein) near the front O ₂ sensor 43 passes through the catalyst 41 and reaches the vicinity of the rear O ₂ sensor 44. And In other words, there is a time difference between the transition of the output voltage of the front O ₂ sensor 43 and the transition of the output voltage of the rear O ₂ sensor 44. Therefore, even if the oxygen partial pressure of the exhaust gas upstream of the catalyst 41 estimated at the present time is compared with the oxygen partial pressure of the exhaust gas downstream of the catalyst 41 also estimated at the current time, the oxygen entry / exit correctly with respect to the catalyst 41 is correctly determined. I can't figure it out.

  In determining the amount of oxygen stored in or released from the catalyst 41, the oxygen partial pressure downstream of the catalyst 41 estimated at the present time is calculated based on the oxygen upstream of the catalyst 41 estimated from the current time. It is necessary to compare with the partial pressure.

The time difference between the time when the oxygen partial pressure downstream of the catalyst 41 is estimated and the time when the oxygen partial pressure upstream of the catalyst 41 is estimated is preferably changed according to the oxygen storage capacity of the catalyst 41. The time difference between the transition of the output voltage of the front O ₂ sensor 43 and the transition of the output voltage of the rear O ₂ sensor 44 is larger as the oxygen storage capacity of the catalyst 41 is higher and smaller as the oxygen storage capacity of the catalyst 41 is lower. Become. The oxygen storage capacity of the catalyst 41 varies depending on the specifications of the catalyst 41. There are also individual differences for each catalyst 41. In addition, it is known that the oxygen storage capacity of the catalyst 41 gradually decreases due to aging.

As shown in FIG. 2, the ECU 0 of the present embodiment outputs the output signal of the rear O ₂ sensor 44 from the time T ₀ when the fuel cut is finished and the fuel injection by the injector 11 is restarted (the time when the fuel cut end condition is satisfied). The elapsed time D until time T ₁ when g switches from the air-fuel ratio lean to the air-fuel ratio rich is measured. Alternatively, the time from when the fuel cut is started and fuel injection by the injector 11 is stopped (when the fuel cut condition is satisfied) to when the output signal g of the rear O ₂ sensor 44 is switched from the air-fuel ratio rich to the air-fuel ratio lean. The time D may be measured. The ECU 0 compares the output voltage of the rear O ₂ sensor 44 with a predetermined determination value (control target value), and determines that the air-fuel ratio is lean if the former is lower than the latter, and empty if the former is higher than the latter. It is determined that the fuel ratio is rich.

  The elapsed time D suggests the current degree of oxygen storage capacity of the catalyst 41 (particularly, the degree of deterioration of the catalyst 41). It can be said that the shorter the elapsed time D, the smaller the oxygen storage capacity of the catalyst 41 (in particular, the catalyst 41 is deteriorated and the oxygen storage capacity is decreased). The time difference between the estimated time of the oxygen partial pressure downstream of the catalyst 41 and the estimated time of the oxygen partial pressure upstream of the catalyst 41 is set larger as the elapsed time D is longer and smaller as the elapsed time D is shorter. That is, as the elapsed time D is longer, an estimated value of the oxygen partial pressure upstream of the catalyst 41 at a past (older) time is used, and this is compared with the oxygen partial pressure downstream of the catalyst 41 at the present time.

  The oxygen partial pressure of the exhaust gas upstream of the catalyst 41 at the past time point is subtracted from the oxygen partial pressure of the exhaust gas downstream of the catalyst 41 at the current time point to be multiplied by the flow rate of the exhaust gas downstream of the catalyst 41 at the current time point. For example, the increase (occlusion) amount or decrease (release) amount of the oxygen storage amount of the catalyst 41 per unit time is found. The flow rate of the exhaust gas downstream of the catalyst 41 at the present time is the flow rate of the exhaust gas upstream of the catalyst 41 at a past time point, and further the flow rate of the intake air at a time point going back to the past. The flow rate of the intake air at the time can be estimated from the engine speed, the intake pressure in the surge tank 33, the intake air temperature, and the like at the time. The time difference at each time point is as described above.

  Thus, the ECU 0 according to the present embodiment repeatedly calculates the oxygen amount currently stored in the catalyst 41 by integrating the increase or decrease amount of the oxygen storage amount of the catalyst 41 per unit time. Then, the current stored oxygen amount of the catalyst 41 is compared with a target value. If the former exceeds the latter, the fuel injection amount is corrected to be increased, and if the former is less than the latter, the fuel injection amount is corrected to decrease. The coefficient X is determined. When the former exceeds the latter, the correction coefficient X increases as the deviation between the two increases. When the former falls below the latter, the correction coefficient X decreases as the deviation between the two increases.

  The target value of the current stored oxygen amount of the catalyst 41 is obtained, for example, by multiplying the maximum oxygen storage capacity of the current catalyst 41 by a certain ratio (a value of about 0.5 to 0.6) smaller than 1. . Since the method for estimating the maximum oxygen storage capacity of the catalyst 41 is known as a diagnosis (self-diagnosis) function of the catalyst 41, description thereof is omitted here.

  In this embodiment, referring to the output signal f of the air-fuel ratio sensor 43 installed upstream of the exhaust gas purification catalyst 41 mounted in the exhaust passage 4 of the internal combustion engine, the oxygen of the exhaust gas upstream of the catalyst 41 The partial pressure is estimated, and the oxygen partial pressure of the exhaust gas downstream of the catalyst 41 is estimated with reference to the output signal g of the air-fuel ratio sensor 44 installed downstream of the catalyst 41, and the difference between the two oxygen partial pressures is estimated. Based on the above, the amount of oxygen stored in or released from the catalyst 41 is estimated, and the control device 0 for the internal combustion engine that adjusts the fuel injection amount to increase or decrease to converge the current stored oxygen amount of the catalyst 41 to the target value. Configured.

According to the present embodiment, the current stored oxygen amount of the catalyst 41 can be easily estimated. The amount of oxygen stored in the catalyst 41 can be feedback controlled to an amount suitable for the purification of harmful substances. Therefore, the purification efficiency of the harmful substances HC, CO and NO _x in the catalyst 41 is further increased, and the emission is improved.

  Furthermore, it is possible to make the average of the air-fuel ratio leaner than before, which can contribute to reduction in fuel consumption and improvement in fuel efficiency.

  These effects can be realized without using an expensive linear A / F sensor, and the cost is low.

Further, in estimating the amount of oxygen stored in or released from the catalyst 41, the oxygen partial pressure upstream of the catalyst 41 to be compared with the oxygen partial pressure downstream of the catalyst 41 is the oxygen content downstream of the catalyst 41. It is assumed that the pressure is estimated at a time point that goes back to the past, and the time difference between the two time points is changed according to the oxygen storage capacity of the catalyst 41. Regardless, the purification efficiency of the harmful substances HC, CO and NO _x can be maintained stably over a long period of time.

  The present invention is not limited to the embodiment described in detail above. The specific configuration of each part can be variously modified without departing from the spirit of the present invention.

  The present invention can be applied to control of an internal combustion engine mounted on a vehicle or the like.

0 ... Control unit (ECU)
11 ... injector 4 ... fuel ratio sensor in the exhaust passage 41 ... catalyst 43 ... catalyst upstream (front O ₂ sensor)
44 ... Air-fuel ratio sensor downstream of catalyst (rear O ₂ sensor)
f: Output signal of the air-fuel ratio sensor upstream of the catalyst g: Output signal of the air-fuel ratio sensor downstream of the catalyst

Claims (2)

With reference to the output signal of the air-fuel ratio sensor installed upstream of the exhaust gas purification catalyst mounted in the exhaust passage of the internal combustion engine, the oxygen partial pressure of the exhaust gas upstream of the catalyst is estimated,
With reference to the output signal of the air-fuel ratio sensor installed downstream of the catalyst, the oxygen partial pressure of the exhaust gas downstream of the catalyst is estimated,
Based on the difference in oxygen partial pressure between the two, the amount of oxygen stored in or released from the catalyst is estimated,
A control device for an internal combustion engine that adjusts the fuel injection amount to increase or decrease to converge the current stored oxygen amount of the catalyst to a target value. In estimating the amount of oxygen stored in or released from the catalyst, the oxygen partial pressure upstream of the catalyst to be compared with the oxygen partial pressure downstream of the catalyst is the past from the time when the oxygen partial pressure downstream of the catalyst is estimated. Estimated at the time of going back to
The control device for an internal combustion engine according to claim 1, wherein the time difference between the two estimation points is changed according to the oxygen storage capacity of the catalyst.

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