JPH1162559A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To rationalize a supply of hydrocarbon to a catalyzer so as not to cause poisoning due to adsorption of this hydrocarbon being overabundant in the NOx catalyzer. SOLUTION: A fundamental hydrocarbon supply is calculated from a map or the like according to catalytic temperature and engine speed (step 102), and a hydrocarbon adsorptive quantity of a NOx catalyzer detected by a hydrocarbon adsorptive quantity detector is read out (step 103). This hydrocarbon adsorptive quantity is compared with the preset poisoning limit value (step 104), and if this HC adsorptive quantity exceeds the poisoning limit value, a target HC supply is set to zero (step 107), stopping supply of hydrocarbon to the NOx catalyzer. On the other hand, when the HC adsorptive quantity is less than the poisoning limit value, a compensation factor is calculated according to the HC adsorptive quantity at the point of time (step 105), and this compensation factor is multiplied to the fundamental HC supply, finding the target HC supply (step 106). Then, an amount of fuel (HC) equivalent to this target HC supply is fed to the NOx catalyzer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine for purifying nitrogen oxides contained in exhaust gas of an internal combustion engine.

[0002]

2. Description of the Related Art In order to purify nitrogen oxides (NOx) in exhaust gas discharged from an internal combustion engine in which fuel is burned under an excessive amount of oxygen such as a diesel engine, a NO.
NOx for reducing and purifying NOx by installing a x-catalyst and supplying hydrocarbon (HC) such as fuel as a reducing agent to the NOx catalyst
Catalyst systems have been developed. In this NOx catalyst system, as disclosed in Japanese Patent Application Laid-Open No. Hei 9-14437, in order to reduce fuel consumption deterioration due to excessive supply of fuel to the catalyst, the HC adsorption ratio of the catalyst and the HC desorption rate from the catalyst have recently been reduced. It has been proposed to control the amount of HC supplied to the catalyst according to the conditions.

[0003]

SUMMARY OF THE INVENTION The present inventors have recently
When the poisoning phenomenon of the NOx catalyst in the NOx catalyst system was investigated, when a large amount of the supplied HC was adsorbed on the catalyst, the catalyst was poisoned by the HC and became difficult to recover to the original state. A phenomenon that the performance is significantly reduced was discovered. Such poisoning of the catalyst starts to occur when the amount of HC adsorbed by the catalyst exceeds a certain value (hereinafter, referred to as “HC poisoning”).
The amount of HC adsorption at this time is called "poisoning limit value").
When the catalyst is poisoned, under normal diesel engine operating conditions with lower exhaust temperatures compared to gasoline engines,
It has been found that it becomes difficult for HC to be desorbed from the catalyst, and that recovery of the catalyst becomes difficult.

[0004] However, in the method of controlling the amount of HC supplied in the above-mentioned publication, although the HC adsorption ratio of the catalyst is detected, the control is performed mainly with priority given to the improvement of the fuel consumption and the improvement of the NOx purification rate.
In some cases, the amount of HC adsorbed by the catalyst exceeds the poisoning limit value, and poisoning that cannot be recovered to the original state occurs, and the NOx purification performance is reduced.

The present invention has been made in view of such circumstances, and accordingly, an object of the present invention is to optimize the amount of HC supplied to a catalyst so that the catalyst is not poisoned by excessive adsorption of HC. Can have high NO over a long period of time
An object of the present invention is to provide an exhaust gas purification device for an internal combustion engine that can maintain x purification performance.

[0006]

According to the first aspect of the present invention, there is provided an exhaust gas purifying apparatus for an internal combustion engine, the amount of hydrocarbons to be supplied to a catalyst based on an operating state of the internal combustion engine. (Hereinafter, referred to as "basic hydrocarbon supply amount") is calculated by the basic hydrocarbon supply amount calculating means, and the amount of hydrocarbon adsorbed on the catalyst is detected by the hydrocarbon adsorption amount detecting means. The target hydrocarbon supply amount is determined by correcting the basic hydrocarbon supply amount by the correction unit so as not to exceed the set value, and the hydrocarbon supply unit is controlled by the control unit in accordance with the target hydrocarbon supply amount,
An amount of hydrocarbon corresponding to the target hydrocarbon supply amount is supplied to the catalyst. In this manner, the amount of hydrocarbon supplied to the catalyst can be appropriately limited so that the amount of hydrocarbon adsorbed by the catalyst does not become excessive, and poisoning of the catalyst due to excessive adsorption of hydrocarbons occurs more than before. Therefore, high NOx purification performance can be maintained for a long period of time.

In this case, it is preferable that the set value for limiting the amount of hydrocarbon adsorbed by the catalyst is set to a poisoning limit value of the catalyst or a value lower than the limit value. In this way, the amount of hydrocarbon supplied to the catalyst can be optimized so that the amount of hydrocarbon adsorbed by the catalyst does not exceed the poisoning limit value, and poisoning of the catalyst due to excessive adsorption of hydrocarbons can be prevented. It can be reliably prevented.

According to a third aspect of the present invention, a correction coefficient is set according to the amount of hydrocarbon adsorbed by the catalyst, and the correction coefficient is multiplied by the basic hydrocarbon supply amount to obtain a target hydrocarbon supply amount. Is also good. By doing so, the correction amount of the hydrocarbon supply amount can be appropriately set in accordance with the hydrocarbon adsorption amount of the catalyst by relatively simple arithmetic processing, and the decrease in NOx purification performance can be minimized. Poisoning of the catalyst can be effectively prevented.

Further, when the amount of adsorbed hydrocarbons of the catalyst exceeds the poisoning limit value of the catalyst, the supply of hydrocarbons by the hydrocarbon supply means is reduced by the amount of adsorbed hydrocarbons. Stopping may be performed until the value becomes equal to or less than the value. By doing so, it is possible to reliably prevent the amount of hydrocarbon adsorbed by the catalyst from exceeding the poisoning limit value, and it is possible to more reliably prevent poisoning of the catalyst.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. First, the configuration of the entire system will be described with reference to FIG. A NOx catalyst 13 that reduces and purifies NOx in exhaust gas is provided in the exhaust pipe 12 (exhaust passage) of a diesel engine 11 that is an internal combustion engine. The NOx catalyst 13 is obtained by supporting platinum as an active metal on a kind of porous zeolite.
NOx in the exhaust gas inside the x catalyst 13 is converted into hydrocarbon (HC)
And purified. An exhaust temperature sensor 14 for detecting the temperature of exhaust gas flowing into the NOx catalyst 13 is installed at the inlet of the NOx catalyst 13 to evaluate the catalyst temperature.

The NOx catalyst 13 in the exhaust pipe 12
Upstream of the fuel, such as light oil as HC for the reducing agent
An HC supply nozzle 15 (hydrocarbon supply means) for supplying to the Ox catalyst 13 is provided. This HC supply nozzle 15
Is supplied with fuel pumped by an injection pump 16 from a fuel tank (not shown). A drive circuit 17 for driving the injection pump 16 is controlled by a target HC supply amount signal from an engine control circuit 18 (control means).

In order to avoid poisoning of the NOx catalyst 13 due to excessive HC adsorption, an HC adsorption amount detecting device 19 (hydrocarbon adsorption amount detecting means) for detecting the HC adsorption amount of the NOx catalyst 13 is provided. The detection principle of the HC adsorption amount is estimated from the history of the past engine operating conditions.
For example, the amount of change in the weight of No. 3 may be measured, the energy may be externally applied to the NOx catalyst 13, and the NOx catalyst 13 may be estimated from its response.

Here, when estimating the HC adsorption amount of the NOx catalyst 13 from the history of past engine operating conditions, the estimation is performed as follows.

(1) The HC inflow amount ΔHCin flowing into the NOx catalyst 13 within a predetermined time Δt is calculated by the following equation. ΔHCin = ΔHCex + ΔHCfd Here, ΔHCex is the amount of HC discharged from the diesel engine 11 within the predetermined time Δt, and ΔHCfd is the amount of HC added from the HC supply nozzle 15 within the predetermined time Δt.

(2) The HC consumption amount ΔHCrec consumed by reacting in the NOx catalyst 13 within a predetermined time Δt is calculated by the following equation. ΔHCrec = ΔHCin × Krep where Krep is the HC purification rate of the NOx catalyst 13,
It is calculated from a map in which the catalyst temperature (temperature of exhaust gas flowing into the NOx catalyst 13), engine operating conditions, and the like are used as parameters.

(3) The HC adsorbed amount ΔHCcad adsorbed by the NOx catalyst 13 within a predetermined time Δt is converted into the HC inflow amount ΔHCin
To the value obtained by subtracting the HC consumption ΔHCrec from the
Multiply by ₁ and find. ΔHCcad = (ΔHCin−ΔHCrec) × f ₁

Here, the adsorption coefficient f ₁ is calculated by the following equation. f ₁ = f ₁₁ × f ₁₂ × f ₁₃ × f ₁₄ f ₁₁ : coefficient obtained from the latest HC adsorption amount HCadso f ₁₂ : coefficient obtained from the concentration of HC flowing into the NOx catalyst 13 f ₁₃ : obtained from the catalyst temperature factor f _14: the coefficients of these coefficient f ₁₁ ~f ₁₄ obtained from the exhaust flow, either takes a value in the range of 0 to 1, is determined by a map or the like.

(4) The HC desorption amount ΔHCdiso desorbed from the NOx catalyst 13 within a predetermined time Δt is calculated by the following equation. ΔHCdiso = ΔHCcad × f ₂

Here, f ₂ is a desorption coefficient, which is calculated by the following equation. f ₂ = f ₂₁ × f ₂₂ × f ₂₃ × f ₂₄ f ₂₁ : coefficient obtained from the latest HC adsorption amount HCadso f ₂₂ : coefficient obtained from the concentration of HC flowing into the NOx catalyst 13 f ₂₃ : obtained from the catalyst temperature Coefficient f ₂₄ : Coefficient obtained from exhaust gas flow rate These coefficients f _{21 to} f ₂₄ each take a value in the range of 0 to 1 and are obtained by a map or the like.

(5) NOx catalyst 13 at time t ₀
Of HC adsorption amount HCadso a (t ₀₎ is calculated by the following equation.

[0021]

(Equation 1)

By the way, light oil flowing into the NOx catalyst 13 contains many kinds of HCs having different chemical structures (carbon numbers), and the boiling points of the HCs are also different. Even if the low-boiling HC is adsorbed on the NOx catalyst 13, it is volatilized and desorbed from the NOx catalyst 13 in a relatively short time by the exhaust heat, so that the low-boiling HC affects the poisoning of the NOx catalyst 13. do not do. Therefore, the above-mentioned HC inflow amount ΔHCin, HC
Regarding the consumption amount ΔHCrec, the HC adsorption amount ΔHCcad, the HC desorption amount ΔHCdiso, and the HC adsorption amount HCadso at the present time t ₀ , it is possible to consider poisoning by excluding low-boiling-point HC which does not affect poisoning, thereby causing poisoning. It is preferable to calculate only for HC having a relatively high boiling point.

However, instead of such a distinction based on the boiling point of HC, the evaluation may be made based on the adsorbed amounts of all HCs including HCs having a low boiling point. Is set in consideration of the existence of HC having a low boiling point, the intended object of the present invention can be sufficiently achieved.

As described above, the HC adsorption amount detecting device 19
The information of the HC adsorption amount HCadso at the present time t ₀ detected in
It is sent to the engine control circuit 18. The engine control circuit 18 is mainly configured by a microcomputer, and each cylinder of the diesel engine 11 is based on outputs of various sensors for detecting an engine operating state, such as an exhaust temperature sensor 14 and an engine speed sensor (not shown). Control the amount of fuel injected to the engine.

Further, the engine control circuit 18 has a ROM
The target HC supply amount is calculated as follows by executing the target HC supply amount calculation program of FIG. 2 stored in the (storage medium) at predetermined time intervals or at predetermined crank angle intervals. First, in step 101, the outputs of various sensors for detecting the operating state of the engine, such as the exhaust gas temperature sensor 14 and the engine speed sensor (not shown), are read.
Then, the basic HC supply amount map of FIG. 3 is searched using the exhaust temperature (catalyst temperature) and the engine speed as parameters, and the basic HC supply amount according to the exhaust temperature (catalyst temperature) and the engine speed at that time. Ask for HCbase. The basic HC supply amount map of FIG. 3 is set in consideration of the NOx purification characteristics of the NOx catalyst 13 that the NOx purification rate of the NOx catalyst 13 is high only in the catalyst activation temperature range (for example, 200 to 350 ° C.). The processing in step 102 serves as a basic hydrocarbon supply amount calculation means referred to in the claims.

[0026] After calculating the basic HC supply amount HCbase, the process proceeds to step 103, reads the information of the HC adsorption amount HCadso of current t ₀ detected by the HC adsorption amount detection device 19. still,
In the system configuration example of FIG. 1, the HC adsorption amount detection device 19 and the engine control circuit 18 are separately provided. However, the function of the HC adsorption amount detection device 19 is incorporated in the engine control circuit 18, and the current time t HC adsorption amount HC of ₀
adso may be calculated by the calculation method described above.

Thereafter, at step 104, the HC adsorption amount H
It is determined whether Cadso exceeds a preset poisoning limit value. Here, the poisoning limit value is the HC value
Is the limit HC adsorption amount at which the NOx catalyst 13 becomes difficult to recover to the original state when adsorbed on the NOx catalyst 13. According to the experimental results of the present inventors, 4 mg of HC per 1 g of the catalyst is It has been confirmed that the catalyst becomes difficult to recover to its original state when adsorbed. in this case,
4 mg of HC per gram of catalyst is the poisoning limit.

If the HC adsorption amount HCadso exceeds the poisoning limit value, the routine proceeds to step 107, where the target HC supply amount HCt is set to zero, and the supply of HC from the HC supply nozzle 15 to the NOx catalyst 13 is stopped. I do. In this case, the supply of HC to the NOx catalyst 13 is stopped until the HC adsorption amount HCadso becomes equal to or less than the poisoning limit value. As a result, the HC adsorption amount HCadso is prevented from continuing to increase beyond the poisoning limit value, and the NOx purification ability of the NOx catalyst 13 is maintained.

On the other hand, if the HC adsorption amount HCadso is equal to or less than the poisoning limit value, the routine proceeds to step 105, where the HC adsorption amount HCadso
The correction coefficient map shown in FIG. 4 using adso as a parameter is searched, and a correction coefficient K corresponding to the HC adsorption amount HCadso at that time is searched.
Find hc. This correction coefficient Khc is based on the HC adsorption amount HCadso
Correction coefficient Khc = 1 when the HC adsorption amount HCadso ≦ 0, and the correction coefficient Khc = 0 when the HC adsorption amount HCadso ≧ poisoning limit value. When the HC adsorption amount HCadso is in the range of 0 to the poisoning limit value, the correction coefficient Khc decreases linearly from 1 as the HC adsorption amount HCadso increases.

The change of the correction coefficient Khc according to the HC adsorption amount HCadso is not limited to a linear (linear) curve, but may be a curve such as a quadratic curve, or a step-like (step-like) curve.
May be changed.

After setting the correction coefficient Khc, the routine proceeds to step 106, where the basic HC supply amount HCbase is multiplied by the correction coefficient Khc to correct the basic HC supply amount HCbase, and the target HC supply amount Hbase is set.
Find Ct. The engine control circuit 18 calculates the target HC
A signal of the supply amount HCt is output to the drive circuit 17 to control the injection pump 16, and the amount of fuel corresponding to the target HC supply amount HCt from the HC supply nozzle 15 is added to the exhaust gas. The processing of steps 104 to 107 described above plays a role as correction means in the claims.

The inventor of the present invention described the target HC of FIG.
The HC supply amount is converted to the HC adsorption amount H by the supply amount calculation program.
Since a test for evaluating the effect of the correction in accordance with Cadso was performed, the test result will be described with reference to FIGS. In this evaluation test, the load was controlled on a bench using a 4.2-liter direct-injection diesel engine so that the exhaust gas temperature changed in the pattern shown in FIG. 5, and the space velocity of 42,000 (liter / H) was applied to the platinum zeolite catalyst.
The test for circulation in r) was performed for 20 cycles. According to this test, when the HC supply amount is corrected according to the HC adsorption amount of the catalyst (corresponding to the present embodiment) and when the HC supply amount is not corrected (corresponding to the related art), 235 ° C. (NOx purification rate peaks) When the NOx purification rate was measured at the temperature shown in FIG. 6, the result as shown in FIG. 6 was obtained.

As is apparent from the test results, when the HC supply amount is corrected in accordance with the HC adsorption amount of the catalyst, the performance equivalent to the initial NOx purification characteristic can be maintained, and the decrease in the NOx purification rate is not observed. However, when no correction was made, the NOx purification rate was significantly reduced as compared with the initial stage.

That is, in the present embodiment, the amount of HC supplied to the NOx catalyst 13 is corrected according to the amount of HC adsorption, so that the amount of HC adsorption of the NOx catalyst 13 does not exceed the poisoning limit value. The amount of HC supplied to the NOx catalyst 13 can be optimized, and the NOx catalyst 1
No. 3 poisoning can be reliably prevented, and high NOx purification performance can be maintained over a long period of time.

In this embodiment, the HC adsorption amount HCadso
When the value exceeds the poisoning limit value, the target HC supply amount HCt is set to zero and the supply of HC to the NOx catalyst 13 is stopped, but the range in which the supply of HC is stopped is set to a value lower than the poisoning limit value. The supply of HC may be stopped when the HC adsorption amount HCadso approaches the poisoning limit value. Even in this case, substantially the same effect as that of the present embodiment can be obtained by appropriately changing the map of the correction coefficient Khc.

In this embodiment, the HC supply nozzle 15 is provided in the exhaust pipe 12 as a means for supplying HC to the NOx catalyst 13. However, instead of this, the HC injection nozzle 15 is provided after the fuel is injected from the fuel injection nozzle to the engine. During the expansion stroke, a small amount of fuel is injected from the fuel injection nozzle by post-injection,
You may make it supply to the catalyst 13.

In this embodiment, fuel (light oil) is used as HC supplied to the NOx catalyst 13. However, liquid HC such as kerosene or gaseous HC such as propane may be used.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an entire exhaust gas purification system according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a processing flow of a target HC supply amount calculation program;

FIG. 3 is a diagram conceptually showing a basic HC supply amount map;

FIG. 4 is a diagram conceptually showing a correction coefficient map;

FIG. 5 is a diagram showing an exhaust temperature change pattern of an evaluation test.

FIG. 6 shows NOx at 235 ° C. when the HC supply amount is corrected according to the HC adsorption amount of the catalyst (corresponding to the present embodiment) and when the HC supply amount is not corrected (corresponding to the related art).
Diagram showing the results of measuring the purification rate

[Explanation of symbols]

11: Diesel engine (internal combustion engine), 12: Exhaust pipe (exhaust passage), 13: NOx catalyst, 15: HC supply nozzle (hydrocarbon supply means), 18: Engine control circuit (control means, calculation of basic hydrocarbon supply amount) Means, correction means), 1
9 ... HC adsorption amount detection device (hydrocarbon adsorption amount detection means).

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naohisa Oyama 14 Iwatani, Shimowakaku-cho, Nishio-shi, Aichi Japan Inside the Japan Automotive Parts Research Institute (72) Inventor Shinya Hirota 1-Toyota-cho, Toyota-shi, Toyota-shi, Aichi Inside (72) Inventor Iwa ▲ saki ▼ Eiji 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Toshiaki Tanaka 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation

Claims (4)

[Claims] 1. A catalyst installed in an exhaust passage of an internal combustion engine for reducing and purifying nitrogen oxides; a hydrocarbon supply unit for supplying hydrocarbons as a nitrogen oxide reducing agent to the catalyst; Basic hydrocarbon supply amount calculating means for calculating the amount of hydrocarbons to be supplied to the catalyst (hereinafter referred to as "basic hydrocarbon supply amount") based on the operating state; and detecting the amount of hydrocarbon adsorbed on the catalyst. Hydrocarbon adsorption amount detection means, correction means for correcting the basic hydrocarbon supply amount so that the hydrocarbon adsorption amount does not exceed a set value to obtain a target hydrocarbon supply amount, And control means for controlling the hydrocarbon supply means. 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein the set value is set to a poisoning limit value of the catalyst or a value less than the poisoning limit value. 3. The correction means sets a correction coefficient according to the hydrocarbon adsorption amount, and calculates a target hydrocarbon supply amount by multiplying the correction coefficient by the basic hydrocarbon supply amount. An exhaust purification device for an internal combustion engine according to claim 1 or 2. 4. The correction means, when the hydrocarbon adsorption amount exceeds the poisoning limit value of the catalyst, adjusts the supply of hydrocarbons by the hydrocarbon supply means to the catalyst poisoning limit value. The exhaust gas purifying apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the apparatus is stopped until the following conditions are satisfied.