JP2022114661A - Temperature controller of exhaust emission control catalyst - Google Patents

Abstract

To provide a temperature controller of an exhaust emission control catalyst capable of suppressing poisoning of the exhaust emission control catalyst to improve purification performance for exhaust gas.SOLUTION: A CPU 34 of an electronic control unit 30 comprises: a catalyst temperature acquisition unit 34a that acquires a catalyst temperature of an exhaust emission control catalyst 17a into which exhaust gas having passed through an exhaust emission control catalyst 16a provided in an exhaust passage of an internal combustion engine flows; and a catalyst temperature control unit 34d that on the basis of a predetermined temperature range of the exhaust emission control catalyst 17a in which an amount of poisoning of the exhaust emission control catalyst 17a is greater than that on a low temperature side and a high temperature side of the temperature range, increases or decreases the catalyst temperature so that the catalyst temperature is out of the predetermined temperature range when the catalyst temperature is within the predetermined temperature range.SELECTED DRAWING: Figure 7

Description

The present invention relates to a temperature control device for an exhaust purification catalyst.

Conventionally, when it is determined that the catalyst has been poisoned, it is known to regenerate the poisoning by correcting the air-fuel ratio to be lean (see, for example, Patent Document 1).

JP-A-2002-4920

However, the technique described in Patent Document 1 relates to a technique for regenerating the poisoned catalyst after the catalyst has been poisoned, and it is assumed that the phenomenon that the catalyst is poisoned and the purification performance is lowered is avoided. not. Therefore, there is room for improvement from the viewpoint of avoiding the poisoning phenomenon itself.

Further, the exhaust passage of the internal combustion engine is provided with a front-stage exhaust purification catalyst provided on the upstream side of the exhaust passage, and a rear-stage exhaust purification catalyst provided on the downstream side of the exhaust passage from the front-stage exhaust purification catalyst. However, the technique described in the above patent document does not consider the difference in poisoning characteristics between the front-stage exhaust purification catalyst and the rear-stage exhaust purification catalyst.

In the upper exhaust purification catalyst, when the air-fuel ratio of the inflowing exhaust gas is controlled so that fuel-rich and fuel-lean alternately, HC or C adsorbed to the exhaust purification catalyst becomes fuel-lean exhaust gas. Oxidized when flowing. Therefore, the upper-stage exhaust purification catalyst is in an environment in which poisoning is relatively difficult to occur.

On the other hand, since the exhaust gas that has passed through the upper-stage exhaust purification catalyst flows into the latter-stage exhaust purification catalyst, most of the oxygen contained in the exhaust gas when the fuel-lean exhaust gas flows through the exhaust passage is in the upper-stage exhaust purification catalyst. It will be consumed by the exhaust purification catalyst. Therefore, even when fuel-lean exhaust gas flows into the latter exhaust purification catalyst, there is a tendency that the poisoning does not recover and the poisoning accumulates. Therefore, it is desirable to suppress the poisoning and improve the purification performance of the exhaust gas, particularly for the latter exhaust purification catalyst.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a temperature control device for an exhaust purification catalyst capable of improving exhaust gas purification performance by suppressing poisoning of the exhaust purification catalyst.

The gist of the present disclosure is as follows.

(1) a catalyst temperature acquisition unit that acquires the catalyst temperature of a second exhaust purification catalyst into which exhaust gas that has passed through a first exhaust purification catalyst provided in an exhaust passage of an internal combustion engine flows;
The catalyst temperature is adjusted based on a predetermined temperature range of the second exhaust purification catalyst in which the amount of poisoning of the second exhaust purification catalyst is greater than that on the low temperature side and the high temperature side of the temperature range. a catalyst temperature control unit that raises or lowers the temperature of the catalyst so that the temperature of the catalyst is outside the predetermined temperature range when the temperature is within the predetermined temperature range;
A temperature control device for an exhaust purification catalyst, comprising:

Advantageous Effects of Invention According to the present invention, it is possible to provide an exhaust purification catalyst temperature control device capable of improving exhaust gas purification performance by suppressing poisoning of the exhaust purification catalyst.

1 is a schematic diagram showing the configuration of an internal combustion engine; FIG. FIG. 4 is a characteristic diagram showing the relationship between the catalyst temperature of an exhaust purification catalyst and the THC concentration in the exhaust gas discharged from the catalytic converter, with respect to the upstream catalytic converter. FIG. 4 is a characteristic diagram showing the relationship between the catalyst temperature of an exhaust gas purification catalyst and the THC concentration in the exhaust gas discharged from the catalytic converter, with respect to the downstream side catalytic converter. FIG. 3 is a characteristic diagram showing the relationship between the steam carbon ratio in the exhaust gas and the amount of carbon poisoning of the catalyst; FIG. 4 is a schematic diagram showing the relationship between the amount of C deposited on an exhaust purification catalyst of a downstream catalytic converter and the exhaust gas purification rate of the catalytic converter; FIG. 2 is a schematic diagram showing in detail the configuration around the upstream catalytic converter and the downstream catalytic converter; 4 is a schematic diagram showing functional blocks of a CPU of the electronic control unit; FIG. FIG. 3 is a characteristic diagram showing how the amount of deposited C, the amount of inflow, the amount of oxidized C, and the amount of desorbed C in formula (1) change according to temperature; FIG. 5 is a characteristic diagram showing how an air-fuel ratio detected by an air-fuel ratio sensor provided upstream of a downstream catalytic converter changes over time; FIG. 5 is a schematic diagram showing a map for determining which of temperature increase/decrease control and poisoning regeneration control of the exhaust purification catalyst is to be performed based on the catalyst temperature and the amount of deposited C of the exhaust purification catalyst. FIG. 4 is a schematic diagram showing a map for determining which of temperature increase control and temperature decrease control is to be performed based on the catalyst temperature of the exhaust purification catalyst and the time-series temperature gradient. 4 is a flow chart showing processing performed by a CPU of the electronic control unit in each predetermined control cycle in the first embodiment; 5 is a timing chart showing how the catalyst temperature of the exhaust purification catalyst, the temperature increase/temperature decrease coefficient, and the deposited C amount of the exhaust purification catalyst change when the process according to the first embodiment is performed. 4 is a timing chart showing how the catalyst temperature of the exhaust purification catalyst and the amount of deposited C on the exhaust purification catalyst change when the process according to the first embodiment is not performed; From the top, the indicated value of the air-fuel ratio of the exhaust gas, the measured value of the air-fuel ratio of the exhaust gas, the air-fuel ratio of the exhaust gas discharged from the upstream catalytic converter, the catalyst temperature of the exhaust purification catalyst of the downstream catalytic converter, The air-fuel ratio of the exhaust gas emitted from the upstream catalytic converter, the concentration of THC emitted from the downstream catalytic converter, the concentration of CO emitted from the downstream catalytic converter, and the concentration of CO emitted from the downstream catalytic converter 4 is a timing chart showing how the concentration of NOx changes in time series. 9 is a flowchart showing processing performed by the CPU of the electronic control unit in each predetermined control cycle in the second embodiment; 4 is a timing chart showing how the catalyst temperature of the exhaust purification catalyst and the temperature increase/temperature decrease coefficient change when the process according to the present embodiment is performed. 10 is a flowchart showing processing performed by the CPU of the electronic control unit in each predetermined control cycle in the third embodiment; FIG. 10 is a flowchart showing processing performed by a CPU of an electronic control unit in each predetermined control cycle in the fourth embodiment; FIG. 10 is a flow chart showing processing performed by the CPU of the electronic control unit in each predetermined control cycle in the fifth embodiment. FIG. 21 is a schematic diagram showing a map applied when the catalyst is degraded in step S14b of FIG. 20;

Several embodiments according to the present invention will be described below with reference to the drawings. These descriptions, however, are intended as illustrations of preferred embodiments of the invention only, and are not intended to limit the invention to such specific embodiments.

(First embodiment)
FIG. 1 shows an overall view of an internal combustion engine. Referring to FIG. 1, 1 is an engine main body, 2 is a combustion chamber of each cylinder, 3 is a spark plug arranged in the combustion chamber 2 of each cylinder, and 4 is a fuel for supplying fuel, for example, gasoline to each cylinder. A fuel injection valve, 5 is a surge tank, 6 is an intake branch pipe, and 7 is an exhaust manifold. The surge tank 5 is connected through an intake duct 8 to the outlet of a compressor 9a of an exhaust turbocharger 9, and the inlet of the compressor 9a is connected through an intake air amount detector 10 to an air cleaner 11. A throttle valve 12 driven by an actuator 13 is arranged in the air intake duct 8 , and an intercooler 14 is arranged around the air intake duct 8 for cooling intake air flowing through the air intake duct 8 .

On the other hand, the exhaust manifold 7 is connected to the inlet of an exhaust turbine 9b of the exhaust turbocharger 9, and the outlet of the exhaust turbine 9b is connected via an exhaust pipe 15a to a catalytic converter 16 containing an exhaust purification catalyst 16a. The outlet of the catalytic converter 16 is connected via an exhaust pipe 15b to a catalytic converter 17 containing an exhaust purification catalyst 17a. In this embodiment, the exhaust purification catalyst 16a is one aspect of the first exhaust purification catalyst provided in the exhaust passage of the internal combustion engine, and the exhaust purification catalyst 17a is the second catalyst into which the exhaust gas that has passed through the first exhaust purification catalyst flows. It is one aspect of an exhaust purification catalyst, and both consist of a three-way catalyst. The exhaust manifold 7 and the surge tank 5 are connected to each other through an exhaust gas recirculation (hereinafter referred to as EGR) passage 18, in which an EGR control valve 19 is arranged. Each fuel injection valve 4 is connected to a fuel distribution pipe 20 , which is connected to a fuel tank 22 via a fuel pump 21 . As shown in FIG. 1, the surge tank 5 is provided with a pressure sensor 23 and a temperature sensor 24 for detecting the pressure and temperature in the surge tank 5, respectively. Further, in the exhaust pipe 15a, an air-fuel ratio sensor 25 for detecting the air-fuel ratio of the exhaust gas flowing into the catalytic converter 16, and an HC concentration sensor 26 for detecting the HC concentration in the exhaust gas or A CO concentration sensor 26 is arranged for detecting the CO concentration of the . An air-fuel ratio sensor 28 for detecting the air-fuel ratio of the exhaust gas flowing into the catalytic converter 17 is arranged in the exhaust pipe 15b. Furthermore, a temperature sensor 27a for detecting the temperature of the exhaust purification catalyst 16a is arranged on the exhaust purification catalyst 16a, and a temperature sensor 27b for detecting the temperature of the exhaust purification catalyst 17a is arranged on the exhaust purification catalyst 17a. It is Although FIG. 1 shows a configuration example in which the internal combustion engine includes the exhaust turbocharger 9 , the internal combustion engine does not have to include the exhaust turbocharger 9 .

The electronic control unit 30 comprises a digital computer, ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input port 35 and output port 36 connected together by a bi-directional bus 31. Equipped with Input port 35 receives output signals from intake air amount detector 10, pressure sensor 23, temperature sensor 24, air-fuel ratio sensors 25 and 28, HC concentration sensor 26 or CO concentration sensor 26, and temperature sensors 27a and 27b. input through the AD converter 37. A load sensor 41 is connected to the accelerator pedal 40 to generate an output voltage proportional to the amount of depression of the accelerator pedal 40 . Furthermore, the input port 35 is connected to a crank angle sensor 42 which generates an output pulse each time the crankshaft rotates, for example, by 30°. The engine speed is calculated in the CPU 34 based on the output signal of the crank angle sensor 42 . On the other hand, the output port 36 is connected to the spark plug 3 , the fuel injection valve 4 , the throttle valve drive actuator 13 , the EGR control valve 19 and the fuel pump 21 through the corresponding drive circuit 38 . In addition, the output port 36 is connected to a temperature raising device 50 and a temperature lowering device 52, which will be described later.

The exhaust purification catalysts 16a and 17b, which are three-way catalysts, oxidize HC to water and carbon dioxide, oxidize CO to carbon dioxide, and reduce NOx to nitrogen and oxygen, respectively. . Therefore, the exhaust purification catalyst 16a and the exhaust purification catalyst 17b have noble metals such as Pt (platinum), Pd (palladium), and Rh (rhodium) as catalyst components. Hereinafter, these noble metals are collectively referred to as PGM (Platinum Group Metal).

FIG. 2 shows the relationship between the catalyst temperature (horizontal axis) of the exhaust purification catalyst 16a and the THC (total hydrocarbon) concentration (vertical axis) in the exhaust gas discharged from the catalytic converter 16 on the upstream side of the catalytic converter 16. It is a characteristic diagram. FIG. 3 is a characteristic diagram showing the relationship between the catalyst temperature (horizontal axis) of the exhaust purification catalyst 17a and the THC concentration (vertical axis) in the exhaust gas discharged from the catalytic converter 17 on the downstream side of the catalytic converter 17. is.

In FIGS. 2 and 3, the □ mark indicates the case where the flow rate (Ga) of the exhaust gas is 10 [g/s], the white □ mark indicates the case with EGR, and the solid □ mark indicates the case without EGR. is shown for the case of When EGR is present, the EGR control valve 19 is opened to recirculate the exhaust gas through the EGR passage 18. When EGR is not present, the EGR control valve 19 is closed to allow the exhaust gas to pass through the EGR passage 18. This corresponds to the case without recirculation. In addition, the ◇ mark indicates the case where the flow rate (Ga) of the exhaust gas is 20 [g/s], the white ◇ mark indicates the case with EGR, and the solid ◇ mark indicates the case without EGR. there is In addition, the ◯ mark indicates the case where the flow rate (Ga) of the exhaust gas is 30 [g/s], the white ◯ mark indicates the case with EGR, and the solid ◯ mark indicates the case without EGR. there is

As shown in FIG. 2, with respect to the upstream catalytic converter 16, the higher the catalyst temperature, the higher the reaction rate, so the THC concentration in the exhaust gas is reduced and THC emissions are suppressed. On the other hand, as shown in FIG. 3, in the downstream catalytic converter 17, the THC concentration in the exhaust gas tends to increase as the catalyst temperature rises, and the THC concentration tends to decrease as the catalyst temperature further rises. The inventors of the present invention have obtained new findings that the THC concentration increases and the purification rate decreases in the temperature range. As shown in FIG. 3, it can be seen that the THC concentration tends to increase particularly in the temperature range of 500 to 600°C.

Thus, the characteristics of the THC concentration with respect to the catalyst temperature differ between the upstream catalytic converter 16 and the downstream catalytic converter 17 . In the downstream side catalytic converter 17, the exhaust purification performance is lowered in the temperature range of 500 to 600° C. because the exhaust purification catalyst 17a of the downstream side catalytic converter 17 is coated with HC or C in the exhaust gas. The C poisoning that occurs is considered to be the cause, and will be described in detail below. Although FIGS. 2 and 3 show the temperature dependence of the THC purification performance, the temperature dependence of the NOx purification performance caused by C poisoning also exhibits the same characteristics as in FIGS. .

HC contained in the exhaust gas is converted to water (H ₂ O) and CO ₂ by steam reforming on the catalyst and rendered harmless. C poisoning occurs when water becomes insufficient in the process of steam reforming, and only C (carbon) is left behind and precipitates on the PGM, and the PGM is covered with C and deposited. C poisoning is more likely to occur as the exhaust gas discharged from the engine body 1 becomes richer in fuel, because the amount of unburned components in the exhaust gas increases. FIG. 4 is a characteristic diagram showing the relationship between the steam carbon ratio in the exhaust gas (horizontal axis) and the amount of C poisoning of the catalyst (amount of C deposited on the catalyst, vertical axis). The steam carbon ratio on the horizontal axis is the ratio of steam (water vapor) to carbon (carbon) in the exhaust gas. As shown in FIG. 4, the smaller the amount of water (steam) and the larger the amount of carbon, the greater the amount of C poisoning and the greater the poisoning effect. When C poisoning occurs, the frequency of collision between the oxidizing gas and the reducing gas against the PGM is reduced, so the activation state of the catalyst is lowered, and the exhaust purification performance is lowered.

In this embodiment, the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 16 is controlled so that the fuel-rich and fuel-lean states are alternately repeated. Therefore, HC or C adsorbed on the exhaust purification catalyst 16a of the upstream catalytic converter 16 is oxidized when the fuel-lean exhaust gas flows, and is discharged as water and _CO2 . Therefore, the upstream catalytic converter 16, into which fuel-rich exhaust gas and fuel-lean exhaust gas alternately flow, is in an environment in which carbon poisoning is relatively difficult to occur, and THC emissions to the downstream side can be suppressed. .

On the other hand, exhaust gas that has passed through the upstream catalytic converter 16 flows into the downstream catalytic converter 17 . When fuel-lean exhaust gas flows into the upstream catalytic converter 16, most of the inflowing oxygen flows into the upstream catalytic converter 16 due to the oxygen storage capacity of the upstream catalytic converter 16 and the high reactivity of oxygen. 16, only NOx flows into the catalytic converter 17 on the downstream side. Therefore, even when fuel-lean exhaust gas flows in, NOx contained in the fuel-lean exhaust gas does not recover from the carbon poisoning, so the poisoning is accumulated. In this way, the downstream catalytic converter 17, into which almost no oxygen flows, is in an environment where carbon poisoning is likely to occur.

Since the C poisoning starts from the steam reforming reaction, a temperature high enough to cause the steam reforming is required to accelerate the C poisoning. According to FIG. 3, when the catalyst temperature reaches 500 ° C. or higher, the THC concentration increases rapidly, so steam reforming occurs at a high temperature of about 500 ° C., and carbon poisoning is promoted, resulting in exhaust gas purification performance. is considered to be declining.

In general, the higher the temperature, the higher the reaction rate. Therefore, the higher the temperature, the less likely all adsorption phenomena, including carbon poisoning, will occur. In other words, the higher the temperature, the easier the substances adsorbed on the catalyst are oxidized and the easier they are to desorb from the catalyst. Therefore, when the temperature rises above a certain level, desorption of C deposited on the catalyst becomes dominant, and C poisoning is recovered. According to FIG. 3, when the catalyst bed temperature is about 650° C. or higher, the THC concentration sharply decreases, indicating that desorption becomes dominant.

Even if the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 16 is controlled to be stoichiometric, the actual air-fuel ratio shifts to the fuel-rich side or the fuel-lean side. occurs. Therefore, the present embodiment is not limited to controlling the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 16 alternately between rich and lean.

FIG. 5 is a schematic diagram showing the relationship between the amount of C deposited on the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side (horizontal axis) and the exhaust gas purification rate of the catalytic converter 17 (vertical axis). As shown in FIG. 5, the exhaust gas purification rate of the catalytic converter 17 decreases as the amount of C accumulated in the catalytic converter 17 increases, and becomes 0 when the amount of accumulated C exceeds the threshold TH. Similar characteristics can be obtained by replacing the horizontal axis of FIG. 5 with the C coverage of PGM.

As described above, the catalyst temperature of the catalytic converter 17 on the downstream side is in the poisoning temperature range of 500 to 650° C., and the exhaust purification catalyst 17a is easily poisoned (fuel-rich, or low-concentration oxidant containing only NOx). If the exhaust gas (lean) continues, the amount of C deposited on the exhaust purification catalyst 17a increases, resulting in a decrease in the purification rate. Since the purification reaction (adsorption reaction) in the catalyst occurs on the PGM, the purification rate will not recover unless poisoning regeneration control such as fuel cut or sufficiently deep fuel lean control is executed.

On the other hand, if oxygen is supplied to the catalytic converter 17 on the downstream side by poisoning regeneration control, NOx may be discharged from the catalytic converter 17 on the downstream side. Therefore, it is preferable not to perform poisoning regeneration control as much as possible. By doing so, it is preferable to avoid the catalyst temperature from entering the poisoning temperature range.

Therefore, in the first embodiment, the catalyst temperature of the downstream catalytic converter 17 is controlled so as not to reach the poisoning temperature range of 500 to 650.degree. Further, the amount of carbon poisoning of the catalytic converter 17 on the downstream side is estimated, and poisoning recovery control is performed when the amount of carbon poisoning exceeds a predetermined value.

FIG. 6 is a schematic diagram showing in detail the configuration around the catalytic converter 16 on the upstream side and the catalytic converter 17 on the downstream side. As shown in FIG. 6, a temperature raising device 50 for raising the temperature of the exhaust gas is arranged in the exhaust pipe 15a on the upstream side of the catalytic converter 16 on the upstream side. The temperature raising device 50 is composed of, for example, an electric heater, and heats the exhaust gas passing through the exhaust pipe 15a with electric power supplied from a vehicle battery or the like to raise the temperature of the exhaust gas.

A temperature lowering device 52 for lowering the temperature of the exhaust gas is arranged in the exhaust pipe 15b between the catalytic converter 16 on the upstream side and the catalytic converter 17 on the downstream side. As an example, the temperature lowering device 52 includes a bypass flow path 52a branched from the exhaust pipe 15b, a heat exchanger 52b provided in the bypass flow path 52a, and a portion where the bypass flow path 52a branches from the exhaust pipe 15b, and a flow control valve 52c for controlling the flow rate of the exhaust gas flowing through the bypass passage 52a. The temperature lowering device 52 causes part or all of the exhaust gas flowing through the exhaust pipe 15b to flow through the bypass flow path 52a, and the heat exchanger 52b exchanges heat between the exhaust gas and the outside air, thereby lowering the temperature of the exhaust gas. Let Instead of the flow control valve 52c, a valve (for example, a three-way valve) that controls both the flow rate of the exhaust gas flowing through the exhaust pipe 15b and the flow rate of the exhaust gas flowing through the bypass passage 52a may be provided. Also, the temperature lowering device 52 may include a heat storage unit that stores the heat of the exhaust gas instead of the heat exchanger 52b.

The temperature raising device 50 and the temperature lowering device 52 control the temperature of the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side so as not to reach the poisoning temperature range of 500 to 650.degree.

The catalyst temperature can also be raised by retarding the fuel injection timing of the fuel injection valve 4 so that the fuel is burned on the catalyst side, or by increasing the fuel injection amount. Additionally, catalyst temperature can be controlled by varying the EGR rate.

FIG. 7 is a schematic diagram showing functional blocks of the CPU 34 of the electronic control unit 30 for realizing the above processing. The CPU 34 is one aspect of a temperature control device for an exhaust purification catalyst, and includes a catalyst temperature acquisition unit 34a, a catalyst temperature determination unit 34b, a deposited C amount estimation unit 34c, a catalyst temperature control unit 34d, and a poisoning regeneration control unit. 34e and a catalyst deterioration degree determination unit 34f. Each of these units of the CPU 34 is, for example, a functional module implemented by a computer program that runs on the CPU 34 . In other words, the functional block of the CPU 34 is composed of the CPU 34 and a program (software) for causing it to function. Moreover, the program may be recorded in the ROM 32 provided in the electronic control unit 30 or in a recording medium connected from the outside. Alternatively, each of these units included in the CPU 34 may be a dedicated arithmetic circuit provided in the CPU 34 .

The catalyst temperature acquisition unit 34a of the CPU 34 acquires the catalyst temperature (catalyst bed temperature) of the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side. The catalyst temperature acquisition unit 34a acquires the catalyst temperature from the detection value of the temperature sensor 27b. Further, the catalyst temperature acquisition unit 34a may acquire the catalyst temperature by estimating the catalyst temperature from the heat balance according to the operating state of the internal combustion engine.

The catalyst temperature determination unit 34b of the CPU 34 determines whether or not the catalyst temperature of the exhaust purification catalyst 17a of the downstream catalytic converter 17 acquired by the catalyst temperature acquisition unit 34a belongs to the poisoning temperature range.

The deposited C amount estimating section 34c of the CPU 34 estimates the amount of C deposited on the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side, that is, the amount of C poisoning. More specifically, the deposited C amount estimator 34c calculates the deposited C amount based on the following equation (1).
Deposited C amount = Inflow C amount - Oxidized C amount - Desorbed C amount (1)

In equation (1), the inflow C amount is the amount of C generated by steam reforming, and is obtained by multiplying the concentration of HC discharged from the upstream catalytic converter 16, the flow rate of the exhaust gas, and the reaction rate. . Further, the amount of oxidized C is the amount of C that is oxidized by reacting with oxygen or NOx among the amount of C generated by steam reforming. , and the reaction rate. The reaction rate for the amount of inflow C and the amount of oxidized C can be obtained from the following equation (2) representing an Arrhenius type reaction. In equation (2), A is a collision frequency factor, E is activation energy, and T is temperature. As the temperature T, the value acquired by the catalyst temperature acquiring section 34a can be used.
Reaction rate ∝A・exp(-E/T) (2)

In equation (1), the amount of desorbed C is the amount of desorbed carbon adsorbed on the exhaust purification catalyst 17a, and is obtained from the Langmuir adsorption isotherm represented by the following equation (3). In equation (3), V is the poisoning amount (accumulated C amount), _Vmax is the poisoning capacity, K is the adsorption-desorption parallel constant, and p is the partial pressure of the poisoning substance. Also, the adsorption-desorption parallel constant K is a function of temperature and can be obtained from equation (2) representing an Arrhenius-type reaction.
V=(Vmax_・p・K)/(1+p・K) (3)

By considering the partial pressure p of the poisoning substance as the amount of desorbed C and modifying the equation (3), the following equation (4) is obtained.
Amount of desorbed C = (1/K) V/(V _max -V) (4)

FIG. 8 is a characteristic diagram showing how the amount of deposited C, the amount of inflow, the amount of oxidized C, and the amount of desorbed C in equation (1) change according to temperature. In FIG. 8, the characteristic C1 indicated by the thick solid line indicates the amount of deposited C, the characteristic C2 indicated by the dashed line indicates the amount of inflow C, and the characteristic C3 indicated by the one-dot chain line indicates the amount of outflow C, which is the sum of the amount of oxidized C and the amount of desorbed C. ing. Since the inflow C amount (characteristic C2) is caused by steam reforming, it starts to increase from about 500° C. as the reaction of steam reforming starts, and then increases as the temperature rises. The outflow C amount (characteristic C3) tends to rise later than the inflow C because the oxidized C amount becomes 0 and only the desorbed C amount is present in a fuel-rich environment. When the temperature dependence of the amount of deposited C is obtained from the characteristics C2 and C3 of FIG. C. and a temperature of 650.degree. Therefore, the amount of deposited C increases in the poisoning temperature range.

In summary, if Vt is the amount of instantaneous change in the poisoning amount of C at time _t , that is, the amount of minute change in the amount of deposited C, the estimation formula for Vt can be _expressed by the following formula (5). In equation (5), α, β, and γ are unit conversion factors or reaction rate constants, and Ga is the exhaust gas flow rate [g/s]. Further, A1 is _a collision frequency factor related to the amount of _C inflow, E1 is the activation energy related to the amount of C inflow, _A2 is the collision frequency factor related to the amount of _C oxide, and E2 is the activation energy related to the amount of C oxide. [THC] is the concentration of HC discharged from the catalytic converter 16 on the upstream side, and [oxidant] is the concentration of the oxidant flowing into the catalytic converter 17 on the downstream side. A value detected by the intake air amount detector 10 can be used as the flow rate Ga of the exhaust gas. [THC] and [oxidant] are obtained from a map that defines the relationship between the operating state of the internal combustion engine and their concentrations, or are estimated from the air-fuel ratio of the exhaust gas. The values of α, β, γ, A ₁ , A ₂ , E ₁ , E ₂ , V _max , K, etc. are such that the difference between the estimated value of the deposited C amount based on, for example, formula (5) and the measured value is the minimum. It is found by fitting The same applies to the collision frequency factor A and the activation energy E regarding the adsorption-desorption parallel constant K. Note that the fitting may be done separately for each of the three terms on the right hand side.
V _t =α·Ga·(β·[THC]·A ₁ ·exp (−E ₁ /T)−γ·[oxidizing agent]·A ₂ ·exp (−E ₂ /T))−(1/K )・(V _t−1 /V _max −V _t−1 )
... (5)
The catalyst temperature determining unit 34b calculates the amount of deposited C in the exhaust purification catalyst 17a by integrating the amount of minute change in the amount of deposited C expressed by the formula (5).

The accumulated C amount estimator 34c simply estimates the accumulated C amount from the difference between the poisoning amount and the poisoning recovery amount based on the air-fuel ratio of the exhaust gas flowing into the catalytic converter 17 on the downstream side. Here, the poisoning amount and the poisoning recovery amount can be expressed by the following equations (6) and (7). Further, the total accumulated amount of C in the exhaust purification catalyst, that is, the amount of C poisoning, is the difference between the amount of poisoning in Equation (6) and the amount of poisoning in Equation (7), and is calculated from Equation (8) below. be.

Poisoning amount = Σ ((AF _{stoici −AF Rr} )/AF _trg )·Ga·0.23
... (6)
Amount of recovery from poisoning=Σ((AF _{Rr −AF stoici} )/AF _trg )·Ga·0.23
... (7)
Accumulated C amount = Σ (poisoning amount - poisoning recovery amount) (8)

In equations (6) and (7), AF _Rr is the air-fuel ratio detected by the air-fuel ratio sensor 28 provided upstream of the downstream catalytic converter 17, AF _stoici is the theoretical air-fuel ratio, and AF _trg is the target air-fuel ratio. , Ga is the flow rate of the exhaust gas flowing into the catalytic converter 17 on the downstream side. Also, in equations (6) and (7), 0.23 represents the weight ratio of oxygen in the air.

FIG. 9 is a characteristic diagram showing how the air-fuel ratio detected by the air-fuel ratio sensor 28 provided upstream of the catalytic converter 17 on the downstream side changes over time. As shown in FIG. 9, the characteristic C4 of the detected value of the air-fuel ratio sensor fluctuates up and down around the stoichiometric air-fuel ratio (AF _stoici ). In FIG. 9, the poisoning amount in Equation (6) corresponds to the area A1 of the region surrounded between the characteristic C4 and the stoichiometric air-fuel ratio AF _stoici on the fuel-rich side of the stoichiometric air-fuel ratio AF _stoici . In FIG. 9, the poisoning recovery amount in equation (7) corresponds to the area A2 of the region surrounded between the characteristic C4 and the stoichiometric air-fuel ratio AF _stoici on the fuel-lean side of the stoichiometric air-fuel ratio AF _stoici .

In the example shown in FIG. 9, between times t1 and t2, the area A1 is larger than the area A2, and the air-fuel ratio of the exhaust gas is on the fuel-rich side. Therefore, between times t1 and t2, poisoning progresses due to fuel-rich exhaust gas, and the amount of deposited C increases. As described above, the deposited C amount estimator 34c can easily calculate the deposited C amount from the equation (8).

The catalyst temperature control unit 34d of the CPU 34 controls the temperature range of the exhaust purification catalyst 17a, which is a predetermined temperature range of the exhaust purification catalyst 17a and in which the amount of poisoning of the exhaust purification catalyst 17a increases more than the low temperature side and the high temperature side of the temperature range. poison temperature range), when the catalyst temperature of the exhaust purification catalyst 17a is within a predetermined temperature range, the catalyst temperature is increased or decreased so that the catalyst temperature is outside the predetermined temperature range. At this time, by controlling the temperature raising device 50 or the temperature lowering device 52, the catalyst temperature of the exhaust purification catalyst 17a is controlled. The catalyst temperature control unit 34d controls the temperature raising device 50 to raise the catalyst temperature, and controls the temperature lowering device 52 to lower the catalyst temperature. When the temperature of the catalyst is to be increased, the catalyst temperature control section 34d heats the exhaust gas with the temperature raising device 50, thereby increasing the catalyst temperature of the exhaust purification catalyst 17a. Further, when the catalyst temperature is lowered, the catalyst temperature control unit 34d opens the flow rate control valve 52c of the temperature lowering device 52 and causes the exhaust gas to flow through the bypass flow path 52a, thereby reducing the heat of the exhaust gas in the heat exchanger 52b. radiates heat to the outside air or the like, and the temperature-lowered exhaust gas is sent to the catalytic converter 17, thereby lowering the catalyst temperature of the exhaust purification catalyst 17a.

Further, the catalyst temperature control unit 34d may increase the temperature of the catalyst by controlling the fuel injection timing of the fuel injection valve 4 to the retarded side so that the fuel is burned on the catalyst side. Further, the catalyst temperature control section 34d may change the catalyst temperature by controlling the EGR rate.

The poisoning regeneration control unit 34e of the CPU 34 performs poisoning regeneration control when the accumulated amount of C on the exhaust purification catalyst 17a exceeds a predetermined value. The poisoning regeneration control unit 34e performs control to regenerate the exhaust purification catalyst 17a from poisoning by performing fuel cut or sufficiently deep fuel lean control. When the fuel cut is performed, oxygen is supplied to the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side, so the poisoning regeneration can be reliably performed. When sufficiently deep fuel lean control is performed for poisoning regeneration, the air-fuel ratio of the exhaust gas is controlled to, for example, about 15 to 16, and HC and C adsorbed on the exhaust purification catalyst 16a of the catalytic converter 16 on the upstream side. The exhaust gas, which is fuel-lean even after the is oxidized, is supplied to the catalytic converter 17 on the downstream side.

Further, the poisoning regeneration control unit 34e performs the poisoning regeneration control when the air-fuel ratio (fuel rich) at which the poisoning of the exhaust purification catalyst 17a of the downstream catalytic converter 17 increases continues for a predetermined time or longer. good.

A catalyst deterioration degree determination unit 34f of the CPU 34 determines the degree of deterioration of the exhaust purification catalyst 16a of the catalytic converter 16 on the upstream side. Note that the catalyst deterioration degree determination unit 34f is a component related to a fifth embodiment, which will be described later, so the processing performed by the catalyst deterioration degree determination unit 34f will be described in the fifth embodiment.

FIG. 10 is a schematic diagram showing a map for determining which of the temperature increase/decrease control and the poisoning regeneration control of the exhaust purification catalyst 17a should be performed based on the catalyst temperature and the deposited C amount of the exhaust purification catalyst 17a. is. As shown in FIG. 10, when the temperature of the exhaust purification catalyst 17a is within the poisoning temperature range and the amount of accumulated C is equal to or less than the threshold TH1, the temperature of the exhaust purification catalyst 17a is controlled to increase or decrease by the catalyst temperature control unit 34d. will be Further, when the temperature of the exhaust purification catalyst 17a is within the poisoning temperature range and the amount of deposited C exceeds the threshold TH1, the poisoning regeneration control unit 34e controls the exhaust purification catalyst 17a without performing temperature increase control or temperature decrease control. poisoning regeneration control is performed. When the temperature of the exhaust purification catalyst 17a is not within the poisoning temperature range, none of the temperature increase control, temperature decrease control, and poisoning regeneration control is performed.

When the temperature of the exhaust purification catalyst 17a is in the poisoning temperature range and the amount of deposited C is equal to or less than the threshold TH1, whether temperature increase control or temperature decrease control is performed depends on the catalyst temperature of the exhaust purification catalyst 17a and the exhaust purification catalyst 17a. determined by the time-series temperature gradient of 17a. FIG. 11 is a schematic diagram showing a map for determining whether to perform temperature increase control or temperature decrease control based on the catalyst temperature of the exhaust purification catalyst 17a and the time-series temperature gradient of the exhaust purification catalyst 17a. In the map shown in FIG. 11, the temperature rising region (1) and the temperature falling region (2) are divided by the boundary line L1, and the catalyst temperature of the exhaust purification catalyst 17a and the temperature gradient of the exhaust purification catalyst 17a in time series. belongs to the temperature increase region (1), the temperature increase control of the exhaust purification catalyst 17a is performed by the catalyst temperature control section 34d. When the point determined by the catalyst temperature of the exhaust purification catalyst 17a and the time-series temperature gradient of the exhaust purification catalyst 17a belongs to the temperature drop region (2), the catalyst temperature control unit 34d performs temperature drop control of the exhaust purification catalyst 17a.

According to the map of FIG. 11, basically, when the catalyst temperature is higher than 575°C with the boundary of 575°C, which is the temperature in the middle of the poisoning temperature range, temperature increase control is performed, and the catalyst temperature reaches 575°C. Temperature increase control is performed in the following cases, but when the catalyst temperature is near 575°C, if the temperature gradient is greater than 0 (when the catalyst temperature tends to increase), even if the catalyst temperature is 575°C or less, Temperature control is performed, and when the temperature gradient is less than 0 (when the catalyst temperature tends to decrease), temperature drop control is performed even if the catalyst temperature is higher than 575°C. For example, even if the catalyst temperature is 575° C. or less, if the temperature gradient is greater than 0, the catalyst temperature tends to increase, and it is easier to increase the catalyst temperature by temperature increase control than by temperature decrease control. In some cases, it can be controlled within the poison temperature range. Similarly, even when the catalyst temperature is higher than 575° C., when the temperature gradient is smaller than 0, the catalyst temperature tends to decrease, and it is easier to control the catalyst temperature by controlling the temperature decrease than by increasing the temperature. In some cases, the temperature can be controlled within the poisoning temperature range. Through such control, either temperature increase control or temperature decrease control, which is more efficient for controlling the catalyst temperature outside the poisoning temperature range, is selected in accordance with the time-series temperature gradient of the catalyst temperature and the exhaust purification catalyst 17a. Therefore, the catalyst temperature can be quickly controlled outside the poisoning temperature range.

The setting of the temperature rising region (1) and the temperature falling region (2) in the map of FIG. 11 is an example. It may be separated by L2 or L3.

Further, when the temperature raising device 50 is composed of an electric heater, power consumption may increase in order to raise the temperature of the exhaust purification catalyst 17a of the downstream catalytic converter 17 to 650° C. or higher. On the other hand, temperature lowering by the temperature lowering device 52 does not consume electric power, so temperature lowering control is more reasonable in terms of energy. Therefore, from the viewpoint of energy consumption, there are cases where it is preferable to positively utilize temperature decrease control rather than temperature increase control. Note that if the temperature decrease control is more actively used than the temperature increase control, the temperature decrease area (2) should be wider than the temperature increase area (1) in the map of FIG.

FIG. 12 is a flow chart showing processing performed by the CPU 34 of the electronic control unit 30 in each predetermined control cycle in the first embodiment. First, the catalyst temperature determination unit 34b determines whether or not the temperature T of the exhaust purification catalyst 17a acquired by the catalyst temperature acquisition unit 34a satisfies 500° C.<T<650° C. (step S10). If 500° C.<T<650° C., it is determined whether or not the amount of deposited C estimated by the deposited C amount estimation unit 34c is equal to or less than a predetermined value (step S12). On the other hand, if not 500° C.<T<650° C., the processing in this control cycle ends (End).

If the deposited C amount is equal to or less than the predetermined value in step S12, the catalyst temperature control unit 34d determines the temperature increase/temperature decrease coefficient based on the map of FIG. 11 (step S14). When the point determined by the temperature of the exhaust purification catalyst 17a and the time-series temperature gradient of the exhaust purification catalyst 17a belongs to the temperature increase region (1) in the map of FIG. When "1" is determined and the point determined by the temperature of the exhaust purification catalyst 17a and the time-series temperature gradient of the exhaust purification catalyst 17a belongs to the temperature decrease region (2), the temperature increase/temperature decrease coefficient is determined to be "2".

Next, it is determined whether or not the temperature increase/temperature decrease coefficient is "1" (step S16). to raise the temperature of the exhaust purification catalyst 17a (step S18).

If the temperature rise/drop coefficient is not "1" in step S16, it is determined whether or not the temperature rise/drop coefficient is "2" (step S20). , the temperature of the exhaust purification catalyst 17a is lowered by the catalyst temperature control unit 34d performing temperature lowering control (step S22).

After steps S18 and S22, the processing in this control cycle ends. Further, when the temperature increase/temperature decrease coefficient is not "2" in step S20, the point determined by the temperature of the exhaust purification catalyst 17a and the time-series temperature gradient of the exhaust purification catalyst 17a is the temperature increase region (1 ) and the temperature decrease region (2), the process in this control cycle ends without performing temperature increase control or temperature decrease control.

Further, when the deposited C amount is not equal to or less than the predetermined value in step S12, the poisoning regeneration control unit 34e performs poisoning regeneration control of the exhaust purification catalyst 17a (step S24). After step S24, the processing in this control cycle ends.

FIG. 13 is a timing chart showing how the catalyst temperature of the exhaust purification catalyst 17a, the temperature increase/decrease coefficient, and the deposited C amount of the exhaust purification catalyst 17a change when the process according to the present embodiment is performed. As shown in FIG. 13, when the catalyst temperature rises from time t0 and exceeds 500° C. at time t11, the temperature increase/temperature decrease coefficient " 1”. As a result, temperature increase control is performed, so the catalyst temperature exceeds 650° C. at time 12 . Therefore, since the catalyst temperature is out of the poisoning temperature range of 500 to 650° C., an increase in the deposited C amount is suppressed.

Thereafter, when the catalyst temperature drops below 650° C. at time t13, the temperature rise/fall coefficient is set to "1" from the map of FIG. 11 based on the catalyst temperature and the temperature gradient. As a result, temperature increase control is performed again, so the catalyst temperature exceeds 650° C. at time 14 . Therefore, since the catalyst temperature is out of the poisoning temperature range of 500 to 650° C., an increase in the deposited C amount is suppressed.

Then, as shown in FIG. 13, by suppressing an increase in the amount of deposited C, the interval until the amount of deposited C reaches the recovery threshold value (predetermined value in step S12 in FIG. 12) for performing poisoning regeneration control. (interval) becomes longer.

As described above, according to the present embodiment, it is possible to prevent the catalyst temperature from reaching the poisoning temperature range of 500 to 650° C., thereby suppressing an increase in the deposited C amount. Therefore, the interval until poisoning regeneration control is performed becomes longer, and the frequency of poisoning regeneration control decreases. Furthermore, the longer the interval until poisoning regeneration control is performed, the higher the probability that external factors such as a fuel cut or a sudden temperature rise in the catalyst due to acceleration will occur during that interval. less frequent. When oxygen is supplied to the downstream catalytic converter 17 by poisoning regeneration control, NOx may be discharged from the downstream catalytic converter 17, but as the number of times of poisoning regeneration control decreases, the downstream catalytic converter NOx emissions from 17 are suppressed.

In FIG. 13, the temperature increase/temperature decrease coefficient is set to "1" when the catalyst temperature exceeds 500° C. at time t11. ) is L2 or L3.

Further, FIG. 13 shows a case where the temperature increase/temperature decrease coefficient is set to "1" from the map of FIG. 11, exemplifying a case where the catalyst temperature rises above the poisoning temperature range. However, when the temperature increase/temperature decrease coefficient is set to "2" from the map of FIG. 11, the catalyst temperature is decreased below the poisoning temperature range. For example, when the catalyst temperature decreases at time t13 and becomes lower than 650° C. at time t13, temperature decrease control is performed if the temperature tends to decrease significantly.

On the other hand, FIG. 14 is a timing chart showing how the catalyst temperature of the exhaust purification catalyst 17a and the deposited C amount of the exhaust purification catalyst 17a change when the process according to the present embodiment is not performed. As shown in FIG. 14, even if the catalyst temperature rises from time t0 and exceeds 500° C. at time t21, if the process according to the present embodiment is not performed, temperature increase control or temperature decrease control is performed. do not have. Therefore, the time for the catalyst temperature to reach the poisoning temperature range of 500 to 650° C. becomes longer, and the amount of deposited C increases. Therefore, unless there is an external factor such as a sudden temperature rise of the catalyst due to fuel cut or acceleration, the amount of accumulated C accumulates, and the amount of accumulated C reaches the recovery threshold for performing poisoning regeneration control earlier than in FIG. Arrives and poison regeneration control is performed. As a result, the interval until poisoning regeneration control is performed is shortened, and the frequency of poisoning regeneration control is increased.

FIG. 15 shows, from top to bottom, the indicated value of the air-fuel ratio (A/F) of the exhaust gas discharged from the engine body 1, the measured value of the air-fuel ratio of the exhaust gas discharged from the engine body 1, and the upstream catalytic converter. The air-fuel ratio of the exhaust gas discharged from 16, the catalyst temperature of the exhaust purification catalyst 17a of the downstream catalytic converter 17, the concentration of THC discharged from the upstream catalytic converter 16, the concentration of THC discharged from the downstream catalytic converter 17 4 is a timing chart showing how the concentration of THC, the concentration of CO emitted from a catalytic converter 17 on the downstream side, and the concentration of NOx emitted from a catalytic converter 17 on the downstream side change in time series. The catalyst temperature of the exhaust purification catalyst 17a of the downstream side catalytic converter 17, the concentration of THC discharged from the downstream side catalytic converter 17, the concentration of CO discharged from the downstream side catalytic converter 17, and the concentration of CO discharged from the downstream side catalytic converter 17 For each concentration of exhausted NOx, the characteristic C5 indicates the characteristic when the exhaust purification catalyst 17a of the downstream catalytic converter 17 is not poisoned (initial state), and the characteristic C6 indicates the characteristic when the exhaust purification catalyst 17a is not poisoned. shows the characteristics in a poisoned state, and the characteristic C7 shows the characteristics in a state where the poisoning of the exhaust purification catalyst 17a is released by fuel cut. Similarly, for each of the air-fuel ratio of the exhaust gas discharged from the upstream catalytic converter 16 and the concentration of THC discharged from the upstream catalytic converter 16, the characteristic C5 is the exhaust purification catalyst 16a of the upstream catalytic converter 16. shows the characteristics in a non-poisoned state (initial state), the characteristic C6 shows the characteristics in the state where the exhaust purification catalyst 16a is poisoned, and the characteristic C7 shows the characteristics when the exhaust purification catalyst 16a is poisoned. Shows the characteristics when the poison is released by fuel cut.

In FIG. 15, before time t3, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is controlled so that the fuel rich and the fuel lean are alternately repeated. After time t3, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is controlled to be rich in fuel. According to the characteristics C5 and C6 of the concentration of THC discharged from the downstream catalytic converter 17, poisoning progresses and the concentration of THC increases if the air-fuel ratio is controlled to be rich in fuel. I know. In particular, according to the characteristic C6, when the exhaust purification catalyst 17a is poisoned, the THC concentration increases immediately after the air-fuel ratio is made rich. Further, according to the characteristic C5, it can be seen that even when the exhaust purification catalyst 17a is not poisoned, the THC concentration gradually increases when the air-fuel ratio is made fuel-rich. Similarly, regarding the characteristics of the concentrations of CO and NOx discharged from the catalytic converter 17 on the downstream side, in the characteristic C6 in the poisoned state, when the air-fuel ratio is made fuel rich, CO and NOx are emitted. It can be seen that the amount is increasing. On the other hand, according to the THC concentration characteristics C5 and C6 discharged from the upstream catalytic converter 16, when the fuel becomes rich after time t3, the THC concentration increases, but the THC concentration changes according to the poisoning state. The difference is relatively small compared to the exhaust purification catalyst 17a on the downstream side. Therefore, it can be seen that it is effective to suppress the poisoning of the exhaust purification catalyst 17a on the downstream side in order to improve the exhaust gas purification performance.

Further, with respect to the concentrations of THC, CO, and NOx discharged from the downstream catalytic converter 17, the characteristic C7 is obtained after the catalyst temperature of the exhaust purification catalyst 17a is raised to about 600° C. as the poisoning regeneration control and the fuel is cut. It shows the characteristics of It can be seen that such poisoning regeneration control restores the exhaust purification performance to a level equivalent to the characteristic C5 in the initial state.

As described above, according to the first embodiment, the amount of deposited C in the downstream catalytic converter 17 is estimated, and the amount of deposited C is equal to or less than a predetermined value, and the catalyst temperature of the downstream catalytic converter 17 is poisoned. If it is within the temperature range, temperature increase control or temperature decrease control is performed so that the catalyst temperature is out of the poisoning temperature range. As a result, the amount of accumulated C on the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side is suppressed. Moreover, since the interval until the poisoning regeneration control is performed becomes longer, the frequency of performing the poisoning regeneration control can be reduced. Further, when the amount of C poisoning exceeds a predetermined value, the poisoning regeneration control is executed, so the exhaust purification performance of the catalytic converter 17 on the downstream side can be recovered.

(Second embodiment)
The second embodiment simplifies the processing of the first embodiment. The temperature of the exhaust purification catalyst 17a is increased so that the catalyst temperature T does not become 500°C<T<650°C. In the second embodiment, when the catalyst temperature is within the poisoning temperature range, only temperature increase control is performed, and temperature decrease control is not performed. Therefore, in the second embodiment, the cooling device 52 shown in FIG. 6 may not be provided.

FIG. 16 is a flow chart showing processing performed by the CPU 34 of the electronic control unit 30 in each predetermined control cycle in the second embodiment. First, the catalyst temperature determination unit 34b determines whether or not the catalyst temperature T of the exhaust purification catalyst 17a is T>650° C. (step S30). If T>650° C., the catalyst temperature T of the exhaust purification catalyst 17a is higher than the poisoning temperature range, so the process in this control cycle ends (End).

If T≦650° C. in step S30, the catalyst temperature determination unit 34b determines whether T<500° C. (step S32). If T<500° C., the catalyst temperature T of the exhaust purification catalyst 17a is lower than the poisoning temperature range, so the process in this control cycle ends.

In step S32, if T≧500° C., the temperature increase coefficient is set to “1”, and the catalyst temperature control unit 34d performs temperature increase control to increase the temperature of the exhaust purification catalyst 17a (step S34). After step S34, the processing in this control cycle ends. In the second embodiment, since temperature drop control is not performed, no temperature drop coefficient is set.

FIG. 17 is a timing chart showing how the catalyst temperature of the exhaust purification catalyst 17a and the temperature increase/temperature decrease coefficient change when the process according to the present embodiment is performed. As shown in FIG. 17, when the catalyst temperature rises from time t0 and exceeds 500° C. at time t31, the temperature increase coefficient is set to "1". As a result, temperature increase control is performed, so the catalyst temperature exceeds 650° C. at time t32. Therefore, since the catalyst temperature is out of the poisoning temperature range of 500 to 650° C., an increase in the deposited C amount is suppressed.

After that, when the catalyst temperature drops below 650° C. at time t33, the temperature increase coefficient is set to "1". As a result, temperature increase control is performed, and the catalyst temperature exceeds 650° C. at time 34 . Therefore, since the catalyst temperature is out of the poisoning temperature range of 500 to 650° C., an increase in the deposited C amount is suppressed.

As described above, according to the second embodiment, when the catalyst temperature of the catalytic converter 17 on the downstream side is in the poisoning temperature range, the temperature increase control is performed so that the catalyst temperature becomes higher than the poisoning temperature range. done. As a result, the amount of deposited C on the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side can be suppressed.

(Third embodiment)
3rd Embodiment performs temperature-increase control or temperature-decrease control, without considering the amount of deposition C with respect to 1st Embodiment.

FIG. 18 is a flow chart showing processing performed by the CPU 34 of the electronic control unit 30 in each predetermined control cycle in the third embodiment. The flowchart shown in FIG. 18 differs from the flowchart of FIG. 12 in that steps S12 and S24 are removed from the flowchart of FIG. 12 described in the first embodiment. Therefore, in the following description, the points different from the first embodiment will be mainly described.

In the process of FIG. 18, if 500° C.<T<650° C. in step S10, the catalyst temperature control unit 34d determines the temperature increase/decrease coefficient based on the map of FIG. 11 (step S14). The processing after step S14 is the same as in FIG. If not 500° C.<T<650° C., the process in this control cycle ends (End).

In the third embodiment as well, when the catalyst temperature of the downstream catalytic converter 17 is within the poisoning temperature range, temperature increase control or temperature decrease control is performed so that the catalyst temperature is out of the poisoning temperature range. As a result, the amount of deposited C on the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side can be suppressed.

(Fourth embodiment)
As described in the first embodiment, catalyst poisoning progresses as the steam carbon ratio decreases. In the fourth embodiment, in the poisoning temperature range, the amount of EGR is increased so that the poisoning does not progress, and the steam carbon ratio is improved by reducing the CO concentration and increasing the water content, so that the downstream side catalytic converter 17, the amount of accumulated C on the exhaust purification catalyst 17a is suppressed.

FIG. 19 is a flow chart showing processing performed by the CPU 34 of the electronic control unit 30 in each predetermined control cycle in the fourth embodiment. The flowchart shown in FIG. 19 differs from the flowchart of FIG. 18 in that a process for increasing EGR (step S40) is added to the flowchart of FIG. 18 described in the third embodiment. Therefore, in the following description, the points different from the third embodiment will be mainly described.

In the process of FIG. 19, if 500° C.<T<650° C. in step S10, the EGR is increased (step S40). When the amount of EGR is increased, the unburned components in the exhaust gas are burned in the engine body 1, so the HC concentration and the CO concentration in the exhaust gas decrease. In addition, combustion of HC increases moisture (H ₂ O) in the exhaust gas. In this way, by increasing the amount of EGR and re-burning, the amount of CO in the exhaust gas is reduced and the water content is increased, thereby promoting steam reforming. The increase in EGR is realized by increasing the amount of exhaust gas flowing through the exhaust gas recirculation passage 18 or by increasing the amount of internal EGR. Since the steam carbon ratio is improved by increasing the amount of EGR, the amount of accumulated C is suppressed, so that the reduction of the purification rate is suppressed.

As described above, according to the fourth embodiment, when the catalyst temperature of the catalytic converter 17 on the downstream side is in the poisoning temperature range, the EGR is increased, so the steam carbon ratio increases, and the deposited C amount increases. Since it is suppressed, the reduction in purification rate is suppressed.

(Fifth embodiment)
In the fifth embodiment, temperature increase control and temperature decrease control of the exhaust purification catalyst 17a are performed in consideration of the degree of deterioration of the exhaust purification catalyst 17a of the catalytic converter 17 on the downstream side.

When the catalyst deteriorates, the purification rate tends to decrease on the low temperature side. Therefore, if the catalyst temperature of the exhaust purification catalyst 17a is controlled to be lower than the poisoning temperature range while the exhaust purification catalyst 17a is degraded, the purification rate may decrease. Therefore, in the fifth embodiment, when the exhaust purification catalyst 17a is deteriorated, the temperature increase control is more actively utilized than the temperature decrease control.

FIG. 20 is a flow chart showing processing performed by the CPU 34 of the electronic control unit 30 in each predetermined control cycle in the fifth embodiment. The flowchart shown in FIG. 20 is different from the flowchart of FIG. 18 described in the third embodiment, with the addition of processing for determining the degree of deterioration of the exhaust purification catalyst 16a of the upstream catalytic converter 16 (step S42). , Step S14 for determining the temperature increase/decrease coefficient is divided into Step S14a and Step S14b for determining the temperature increase/temperature decrease coefficient according to the degree of deterioration of the exhaust purification catalyst 17a.

In step S42, the catalyst deterioration degree determination section 34f determines the degree of deterioration of the exhaust purification catalyst 16a of the catalytic converter 16 on the upstream side. The catalyst deterioration degree determination unit 34f determines the degree of exhaust purification catalyst 16a of the upstream catalytic converter 16 based on the detection values of the air-fuel ratio sensor 25 and the air-fuel ratio sensor 28 provided upstream and downstream of the upstream catalytic converter 16. The oxygen storage capacity (Cmax) is measured, and the degree of deterioration of the exhaust purification catalyst 16a is determined from the measurement result. Specifically, the catalyst deterioration degree determination unit 34f determines whether or not Cmax of the exhaust purification catalyst 16a exceeds a predetermined value. judge.

Since the method for measuring Cmax is publicly known, an overview will be given here. When measuring Cmax, by oscillating the target air-fuel ratio around stoichiometric, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 16a is alternately changed between the fuel-lean side and the fuel-rich side. At this time, the oxygen storage amount of the catalyst calculated by the following equation is integrated during the period from when the air-fuel ratio upstream of the catalyst changes to fuel-lean until the air-fuel ratio downstream of the catalyst changes to fuel-lean. Further, the oxygen desorption amount of the catalyst calculated by the following equation is integrated during the period from when the air-fuel ratio upstream of the catalyst changes to fuel-rich until the air-fuel ratio downstream of the catalyst changes to fuel-rich.
Oxygen storage amount or oxygen desorption amount = coefficient x (current air-fuel ratio - stoichiometric) x fuel injection amount
The catalyst deterioration degree determination unit 34f calculates the oxygen storage amount and the oxygen desorption amount a plurality of times, and calculates the average of these values as Cmax.

The reason why the catalyst deterioration degree determination unit 34f determines the degree of deterioration of the exhaust purification catalyst 16a of the upstream catalytic converter 16 is generally that the upstream exhaust purification catalyst 16a is higher than the downstream exhaust purification catalyst 17a. If it is determined that the exhaust purification catalyst 16a on the upstream side has deteriorated, it can be estimated that the exhaust purification catalyst 17a on the downstream side has deteriorated at a level lower than that on the upstream side. be. If the deterioration of the downstream side exhaust purification catalyst 17a can be determined directly, it is not necessary to determine the deterioration of the upstream side exhaust purification catalyst 16a.

In step S42, if Cmax exceeds the predetermined value, the catalyst is not degraded, so the catalyst temperature control unit 34d sets the temperature increase/decrease coefficient is determined (step S14a). The map shown in FIG. 11 is applied when the catalyst has not deteriorated.

On the other hand, if Cmax is equal to or less than the predetermined value in step S42, the catalyst is degraded. is determined (step S14b).

FIG. 21 is a schematic diagram showing a map applied when the catalyst is degraded in step S14b of FIG. In the map shown in FIG. 21, as compared with the map shown in FIG. ) is wider than the temperature drop region (2). As a result, the point determined by the temperature of the exhaust purification catalyst 17a and the time-series temperature gradient of the exhaust purification catalyst 17a is more likely to belong to the temperature increase region (1) than to the temperature decrease region (2). More frequent control.

In the flowchart of FIG. 20, in step S42, it is determined whether or not the catalyst has deteriorated based on a predetermined value, and the map for the case where the catalyst has deteriorated and the map for the case where the catalyst has not deteriorated are switched. However, the map may be switched in multiple stages according to the deterioration degree (Cmax value) of the catalyst.

If the catalyst is deteriorated, when the catalyst temperature is set to the low temperature side of 500° C. or less by performing temperature drop control, there is a tendency that the purification performance of the exhaust gas is lowered due to the deterioration of the catalyst. On the other hand, when the catalyst is degraded, on the high temperature side where the catalyst temperature is 650° C. or higher, the purification performance does not decrease as much as on the low temperature side.

According to the fifth embodiment, when the catalyst is degraded, the temperature increase control is performed more frequently than the temperature decrease control, and the temperature increase control is mainly performed. Decrease is suppressed.

34 CPUs
34a Catalyst temperature acquisition unit 34b Catalyst temperature determination unit 34c Deposited C amount estimation unit 34d Catalyst temperature control unit 34e Poisoning regeneration control unit 34f Catalyst deterioration degree determination unit

Claims (1)

a catalyst temperature acquisition unit that acquires the catalyst temperature of a second exhaust purification catalyst into which exhaust gas that has passed through a first exhaust purification catalyst provided in an exhaust passage of an internal combustion engine flows;
The catalyst temperature is adjusted based on a predetermined temperature range of the second exhaust purification catalyst in which the amount of poisoning of the second exhaust purification catalyst is greater than that on the low temperature side and the high temperature side of the temperature range. a catalyst temperature control unit that raises or lowers the temperature of the catalyst so that the temperature of the catalyst is outside the predetermined temperature range when the temperature is within the predetermined temperature range;
A temperature control device for an exhaust purification catalyst, comprising:

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