JP2000237541A - Exhaust gas purifying device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purifying device capable of suppressing a hydrocarbon discharge amount by controlling an air/fuel ratio of gaseous mixture to the leaner side than the theoretical air/fuel ratio and also capable of keeping good exhaust gas characteristics without increasing a NOx discharge amount in the case when hydrocarbon is released from a adsorption catalyst arranged at an exhaust system. SOLUTION: A just-under ternary catalyst 14, an underfloor ternary catalyst 15 and the adsorption catalyst 16 are arranged in an exhaust pipe 13. Ceria having an oxide storing capacity is added to these catalysts, and an adding ratio of the ceria to the ternary catalysts 14 and 15 is larger than that of the adsorption catalyst 16.

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 purifying exhaust gas of an internal combustion engine, and more particularly to an exhaust gas purifying apparatus having a three-way catalyst and an adsorption catalyst for adsorbing hydrocarbons in an exhaust system of the internal combustion engine.

[0002]

2. Description of the Related Art An adsorbent for adsorbing HC (hydrocarbon) in exhaust gas when an internal combustion engine is cold is arranged in an exhaust system together with a three-way catalyst, and the HC is adsorbed until the three-way catalyst is activated. A technique for reducing the amount of HC emission during cold start by adsorbing to a cold start has conventionally been known. The adsorbent has the property of desorbing adsorbed HC when its temperature rises, and pays attention to the fact that the amount of desorbed HC per unit time is relatively large at the beginning of desorption and then gradually decreases. By controlling the air-fuel ratio of the air-fuel mixture supplied to the engine to be leaner than the stoichiometric air-fuel ratio in accordance with the elapsed time from the start of desorption, the HC amount in the exhaust gas flowing into the three-way catalyst can be adjusted to an appropriate value (3 An exhaust gas purifying apparatus that is controlled to a value that can be oxidized by a primary catalyst) has been conventionally known (JP-A-6-81637).

[0003]

However, when a three-way catalyst is provided on the upstream side of the adsorbent, if the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio when HC is desorbed from the adsorbent, the three-way catalyst is not provided. In some cases, the NOx purification window (range of the air-fuel ratio in which the NOx purification rate is large) deviates from the NOx purification window, and the exhaust characteristics may deteriorate.

The present invention has been made in view of this point, and controls the air-fuel ratio of the air-fuel mixture to be leaner than the stoichiometric air-fuel ratio when hydrocarbons are desorbed from the adsorption catalyst disposed in the exhaust system. Accordingly, it is an object of the present invention to provide an exhaust gas purifying apparatus capable of suppressing HC emission and maintaining good exhaust characteristics without increasing NOx emission.

[0005]

According to one aspect of the present invention, there is provided a three-way catalyst provided in an exhaust system of an internal combustion engine, and a three-way catalyst provided in the exhaust system for adsorbing hydrocarbons. An exhaust purification system comprising: an adsorption catalyst; and fuel supply means for controlling an air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine at the time of desorption of hydrocarbons from the adsorption catalyst to a leaner side than a stoichiometric air-fuel ratio. Add cerium oxide to the catalyst,
The three-way catalyst is provided on the upstream side of the adsorption catalyst and is characterized by adding more cerium oxide than the adsorption catalyst.

According to this structure, cerium oxide is added to the adsorption catalyst to have an oxygen storage capacity, whereby the stored oxygen is released when the hydrocarbon is desorbed from the adsorption catalyst, and the desorbed hydrocarbon is removed. Since the effect of purifying efficiently is obtained, and the three-way catalyst provided upstream of the adsorption catalyst contains more cerium oxide than the adsorption catalyst, NOx is reduced by the water gas reaction using the cerium oxide as a catalyst. NOx even at lean air-fuel ratios
, The purification window can be expanded, and good exhaust characteristics can be maintained.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an overall configuration diagram of an internal combustion engine (hereinafter, referred to as “engine”) and an air-fuel ratio control device thereof according to an embodiment of the present invention. For example, a throttle is provided in an intake pipe 2 of a four-cylinder engine 1. A valve 3 is provided. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3, and outputs an electric signal corresponding to the opening of the throttle valve 3 to output an electronic control unit for engine control (hereinafter referred to as “ECU”) 5. To supply.

A fuel injection valve 6 is provided for each cylinder between the engine 1 and the throttle valve 3 and slightly upstream of an intake valve (not shown) of the intake pipe 2, and each injection valve is connected to a fuel pump (not shown). The ECU 5 is electrically connected to the ECU 5 and controls a valve opening time of the fuel injection valve 6 based on a signal from the ECU 5.

On the other hand, an intake pipe absolute pressure (PBA) sensor 8 is provided immediately downstream of the throttle valve 3, and the absolute pressure signal converted into an electric signal by the absolute pressure sensor 8 is supplied to the ECU 5. . Further, an intake air temperature (TA) sensor 9 is attached downstream thereof, detects the intake air temperature TA, outputs a corresponding electric signal, and supplies the electric signal to the ECU 5. Engine water temperature (T
W) The sensor 10 is composed of a thermistor or the like, detects an engine coolant temperature (cooling coolant temperature) TW, outputs a corresponding temperature signal, and supplies it to the ECU 5.

An engine speed (NE) sensor 11 is provided around a camshaft or a crankshaft (not shown) of the engine 1.
And a cylinder discrimination (CYL) sensor 12. The engine speed sensor 11 outputs a TDC signal pulse at a crank angle position before a predetermined crank angle with respect to the top dead center (TDC) at the start of the intake stroke of each cylinder of the engine 1 (every 180 degrees of crank angle in a four-cylinder engine). The cylinder discriminating sensor 12 outputs a cylinder discriminating signal pulse at a predetermined crank angle position of a specific cylinder. These signal pulses are supplied to the ECU 5.

An exhaust pipe 13 has a three-way catalyst 14 immediately below the exhaust manifold of the engine 1, a three-way catalyst 15 below the floor that is arranged slightly downstream of the three-way catalyst 14, and An adsorbent catalyst 16 that adsorbs HC (hydrocarbon) is disposed immediately adjacent to the underfloor three-way catalyst 15 and adjacent thereto.

The direct three-way catalyst 14 has a capacity of, for example, about 1.3 liters and has ceria (C
eO ₂ , cerium oxide) is added at a rate of 1500 g / cft (grams / cubic foot). 1 cft
$ 28.317 liter, 1500g / cf
t ≒ 53 g / liter. In addition, the underfloor three-way catalyst 15
Has a volume of, for example, about 1.4 liters, and ceria is added at a rate of 750 g / cft.

The adsorption catalyst 16 is disclosed, for example, in JP-A-7-961.
As shown in JP-A-83, an adsorption layer containing zeolite as a main component and having a function of adsorbing HC, a catalyst layer containing a Pd oxide and having a function of promoting oxidation of HC, and alumina particles are mainly used. And a porous barrier layer provided between the adsorption layer and the catalyst layer. The adsorption layer adsorbs HC at a temperature of about 160 ° C. or lower,
At a temperature higher than this, it has the property of desorbing the adsorbed HC. In the present embodiment, the adsorption catalyst 16 has a volume of about 1 liter, and ceria is added at a rate of 300 g / cft.

Since the desorption of HC from the adsorption layer of the adsorption catalyst 16 starts at a relatively low temperature (about 160 ° C.), it is desirable that the activation temperature of the catalyst layer be a low temperature close to the desorption start temperature. In general, when the addition ratio RCEO of ceria is increased from 0, the activation temperature of the catalyst layer decreases, and the activation temperature of the catalyst layer tends to increase at a certain addition ratio RCEO0. Show. Therefore, in the present embodiment, the addition ratio RCEO of ceria is set to 300 g / c.
ft, this addition ratio RCEO is from 100 g / cft to 500 g / cf from the relation with the desorption start temperature.
It is desirable to set in the range of t. Addition ratio RCEO
Less than 100 g / cft, or 500 g
If the ratio is more than / cft, the activation temperature of the catalyst layer increases, and the amount of HC emission during the desorption of HC from the adsorption layer increases. In the present embodiment, palladium whose activation temperature is relatively low is supported on the catalyst layer.

The adsorbing catalyst 16 is disposed downstream of the three-way catalysts 14, 15, but by adding ceria to have an oxygen storage capacity, the desorbed HC is efficiently oxidized and purified. It becomes possible. Further, in the present embodiment, the ceria addition ratio of the three-way catalysts 14 and 15 is made larger than the ceria addition ratio of the adsorption catalyst 16, so that NOx can be reduced and purified by a water gas reaction using ceria as a catalyst. Even in the air-fuel ratio, the NOx purification window is widened, and the NOx purification rate can be improved.
As a result, as described later, when the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio during the desorption of HC, the three-way catalyst 14,
By the effect of No. 15, NOx emission can be suppressed, and good exhaust characteristics can be obtained.

The adsorption catalyst 16 is provided with a temperature sensor 19 for detecting the temperature TADS, and a detection signal is supplied to the ECU 5. The proportional air-fuel ratio sensor 17 (hereinafter referred to as “LAF sensor 1”)
7 ”), and the LAF sensor 17
Outputs an electric signal substantially proportional to the oxygen concentration (air-fuel ratio) in the exhaust gas and supplies it to the ECU 5.

At the downstream position of the three-way catalyst 14 immediately below and the downstream position of the adsorption catalyst 16, binary oxygen concentration sensors (hereinafter referred to as "O2 sensors") 18 and 19 are mounted, respectively. The signal is supplied to the ECU 5. The O2 sensors 18 and 19 have the characteristic that the output changes abruptly before and after the stoichiometric air-fuel ratio, and the output becomes higher on the rich side and lower on the lean side than the stoichiometric air-fuel ratio.

The ECU 5 has a function of shaping input signal waveforms from various sensors, correcting a voltage level to a predetermined level, converting an analog signal value to a digital signal value, and the like, and a central processing circuit ( The CPU 5b includes a storage unit 5c for storing various calculation programs executed by the CPU 5b, calculation results, and the like, an output circuit 5d for supplying a drive signal to the fuel injection valve 6, and the like.

The CPU 5b determines various engine operating states based on the various engine parameter signals described above, and according to the determined engine operating states,
Based on the following equation (1), a fuel injection time TOUT of the fuel injection valve 6 synchronized with the TDC signal pulse is calculated. TOUT = TiM × KCMD × KLAF × K1 + K2 (1) where TiM is a basic fuel amount, specifically, the fuel injection valve 6
Is determined by searching a Ti map set according to the engine speed NE and the intake pipe absolute pressure PBA. The Ti map indicates the engine speed NE.
In the operating state corresponding to the intake pipe absolute pressure PBA, the air-fuel ratio of the air-fuel mixture supplied to the engine is set to be substantially equal to the stoichiometric air-fuel ratio. That is, the basic fuel amount TiM has a value substantially proportional to the intake air amount (weight flow rate) of the engine 1.

KCMD is a target air-fuel ratio coefficient, which is set according to engine operating parameters such as the engine speed NE, the throttle valve opening θTH, and the engine coolant temperature TW.
The target air-fuel ratio coefficient KCMD is proportional to the reciprocal of the air-fuel ratio A / F, that is, the fuel-air ratio F / A.
Take. The target air-fuel ratio coefficient KCMD is set to a lean predetermined value KCMDHB for leaning the air-fuel ratio when HC is desorbed from the adsorption catalyst 16 as described later.

The target air-fuel ratio coefficient KCMD is determined by the O2 sensor 1
It is corrected by adding an offset correction term KCMDOFFSET calculated according to the output of 8 or 19. This is to suppress the effects of variations in the characteristics of the exhaust system of the engine and the LAF sensor and aging.
During warm-up of the engine (during adsorption of HC by the adsorption catalyst 16)
Is the offset correction term K using the output of the O2 sensor 18.
CMDOFFSET is calculated, and H
After the start of the desorption of C, the offset correction term KCMDOFFSET is calculated using the output of the O2 sensor 19.

KLAF is an air-fuel ratio correction calculated by PID control such that the detected equivalent ratio KACT calculated from the detection value of the LAF sensor 16 matches the target equivalent ratio KCMD when the condition for executing the feedback control is satisfied. It is a coefficient. K1 and K2 are other correction coefficients and correction variables calculated according to various engine parameter signals, respectively, and are predetermined values that can optimize various characteristics such as a fuel consumption characteristic and an engine acceleration characteristic according to an engine operating state. Is determined. The CPU 5b controls the fuel injection valve 6 based on the fuel injection time TOUT and the fuel injection timing determined as described above.
Is supplied to the fuel injection valve 6 via the output circuit 5d.

FIG. 2 is a flowchart of a desorption determination process for determining whether or not the adsorption catalyst 16 is desorbing. This process is executed by the CPU 5b every time a TDC signal pulse is generated. In step S11, it is determined whether or not the adsorption catalyst temperature TADS is higher than a predetermined temperature TDSR (for example, 140 to 180 ° C.). If TADS ≦ TDSR, it is determined that desorption is not being performed, and desorption is being performed. Is set to "0" (step S1).
5), end this processing.

The adsorption catalyst temperature TADS is equal to a predetermined temperature TDSR.
Is exceeded, the basic fuel amount Ti is calculated by the following equation (2).
An integrated value ITiM of M (formula (1)) (reset to “0” when the engine 1 is started) is calculated (step S1).
2). Itim = Itim + TiM (2) Here, itim on the right side is the previously calculated value, and TiM is the current basic fuel amount. Since the integrated value TiM is substantially proportional to the exhaust gas amount from the start of desorption, it can be used as a parameter indicating the amount of HC desorbed from the adsorption catalyst 16.

In step S13, it is determined whether or not the integrated value ITiM has become larger than a predetermined value ITiMHB.
It is determined that HC is being desorbed while TiM ≦ ITiMHB, and the desorption flag FDSR is set to “1” (step S14). Thereafter, when it becomes ITiM> ITiMHB, it is determined that the desorption is completed, the process proceeds from step S13 to step S15, and the desorption flag FDSR is reset to “0”.

FIG. 3 is a flowchart of a process for calculating the target air-fuel ratio coefficient KCMD. This process is executed by the CPU 5b every time a TDC signal pulse is generated. Step S2
At 1, it is determined whether or not the desorption flag FDSR is “1”. If FSDR = 0 and it is determined that the desorption is not being performed, the target air-fuel ratio coefficient K is determined in accordance with the engine operating state.
The CMD is set (step S24). The target air-fuel ratio coefficient KCMD is basically set according to the engine speed NE and the absolute pressure PBA in the intake pipe. When the engine is operating under a high load or the engine water temperature TW is low, the target air-fuel ratio coefficient KCMD is set according to the engine load or the engine water temperature TW. Modifications are made.

When FDSR = 1 and it is determined that the engine is being removed, the intake pipe absolute pressure PB as a parameter indicating the engine speed NE and the engine load is determined as shown in FIG.
Search KCMDHB table set according to A,
A lean predetermined value KCMDHB is calculated. KCMDHB
The table shows the low load side set value KCMDHBL applied when the intake pipe absolute pressure PBA is equal to or higher than the low load side predetermined value PBAL (eg, 260 mmHg) and the high load side predetermined value PBAH (eg, 710 mmHg). The applied high load side set value KCMDHBH is set according to the engine speed NE, and the set value KCMDHBH is set.
L and KCMDHBH are both set to decrease as the engine speed NE increases. In FIG. 4, the predetermined rotational speeds NE1 and NE2 are set to, for example, 1000 rpm and 5000 rpm, respectively. When the intake pipe absolute pressure PBA is between the low load side predetermined value PBAL and the high load side predetermined value PBAH, a lean predetermined value KCMDHB is calculated by interpolation. Therefore, the lean predetermined value KCMDHB is set so as to decrease as the intake pipe absolute pressure PBA increases, that is, as the engine load increases. In a succeeding step S23, the target air-fuel ratio coefficient KCMD is set to the predetermined leaning value KCMDHB calculated in the step S22, and this processing is ended.

According to the processing of FIG. 3, when it is determined that the vehicle is being desorbed, the target air-fuel ratio coefficient KCMD is set to the lean predetermined value KC.
MDHB is set so that the degree of leaning of the air-fuel ratio is increased as the intake pipe absolute pressure PBA increases, that is, as the engine load increases.
The amount of HC emission is commensurate with the oxidation capacity of the adsorption catalyst 16,
It is possible to maintain good exhaust characteristics after the engine is started.

FIG. 5 is a flowchart of a process for calculating the air-fuel ratio correction coefficient KLAF. This process is executed by the CPU 5b every time a TDC signal pulse is generated. Step S31
Then, it is determined whether or not the LAF feedback control condition for executing the feedback control according to the output of the LAF sensor 16 is satisfied. The LAF feedback control condition is satisfied on condition that the LAF sensor 16 is activated and the fuel supply cutoff operation or the throttle fully open operation is not executed. If the answer to step S31 is negative (NO), the air-fuel ratio correction coefficient KLAF is set to "1.
"0" (step S32), and the process ends.

When the LAF feedback control condition is satisfied, the deviation DKAF (k) (= KCMD (k) -KACT) between the detected equivalent ratio KACT obtained by converting the output of the LAF sensor 16 into an equivalent ratio and the target air-fuel ratio coefficient KCMD. (K))
Is calculated (step S33), the deviation DKAF (k) and the control gains KP, KI, and KD are applied to the following equation to calculate the proportional term KLAFP (k), the integral term KLAFI (k), and the derivative term KLAFD (k). Is calculated (step S34).
Here, (k) and (k-1) are added to indicate the current value and the previous value, respectively.

KLAFP (k) = DKAF (k) × KP KLAFI (k) = DKAF (k) × KI + KLAF
(K-1) KLAFD (k) = (DKAF (k) -DKAF (k-
1)) × KD Then, the proportional term KLAFP, the integral term KLAFI, and the derivative term KLAFD are added, and the air-fuel ratio correction coefficient KLAF (= K
LAFP + KLAFI + KLAFD) is calculated (step S35), a limit process is performed so that the calculated value of the air-fuel ratio correction coefficient KLAF falls within the range of the predetermined upper and lower limit values (step S36), and the process ends. By this processing, the detection equivalent ratio KACT based on the LAF sensor output is
Feedback control is executed so as to match the target air-fuel ratio coefficient KCMD.

In this embodiment, FIG.
Steps 3 and 5 correspond to the air-fuel ratio control means. Note that the present invention is not limited to the above-described embodiment, and various modifications are possible. For example, the ceria addition ratio of the three-way catalysts 14 and 15 is 1500 g / cft and 750 g /
It is not limited to cft, but is set within a range of 1000 to 2000 g / cft for the three-way catalyst 14 immediately below, and 550 to 1000 g / cft for the three-way catalyst 15 under the floor.
It is desirable to set within the range. The ceria addition ratio was adjusted to the lower limit (1000 g / cft, 550 g / cf
If it is smaller than t), the oxygen storage capacity becomes insufficient, while the ceria addition ratio is increased to these upper limits (2000 g / c
(ft, 1000 g / cft), there is a problem that the activation temperature rises although the purification rate is not improved.

As shown in FIG. 6, the underfloor three-way catalyst 15 comprises an upstream portion 15a and a downstream portion 15b having substantially the same volume (for example, 0.7 liter).
Was added to the downstream part 15b without adding ceria.
Ceria may be added at a ratio of g / cft. In this case, the ceria addition ratio of the downstream portion 15b is 10
It is desirable to set within the range of 00 to 2000 g / cft.

In the above-described embodiment, the start of desorption is determined by determining whether or not a predetermined temperature TDSR has been exceeded, using the adsorption catalyst degree TADS detected by the sensor. Absolute pressure PB in the intake pipe
By integrating the temperature rise integral term DCTC set according to A, the temperature of the HC adsorption catalyst may be estimated, and the estimated temperature value CTADS may be used instead of the detected temperature TADS.

Further, instead of detecting the temperature of the adsorption catalyst 16, the exhaust gas temperature on the upstream side of the adsorption catalyst 16 is detected, and the desorption is determined based on whether or not the detected exhaust gas temperature exceeds a predetermined exhaust temperature. May be determined.
Alternatively, the engine water temperature TW
May be determined to start from a point in time when a predetermined period set according to the time has elapsed. Further, the O2 sensor 19 need not be used. In that case, the offset correction term KCMDOFFSET is always calculated based on the output of the O2 sensor 18.

[0036]

As described above in detail, according to the present invention, by adding cerium oxide to an adsorption catalyst to have an oxygen storage capacity, it is possible to efficiently purify hydrocarbons when desorbing hydrocarbons. The obtained three-way catalyst provided on the upstream side of the adsorption catalyst contains more cerium oxide than the adsorption catalyst. Therefore, NOx can be reduced and purified by a water gas reaction using the cerium oxide as a catalyst. In addition, the window for NOx purification is widened even at a lean air-fuel ratio, and good exhaust characteristics can be maintained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an internal combustion engine and an air-fuel ratio control device thereof according to an embodiment of the present invention.

FIG. 2 is a flowchart of a process for determining whether or not HC is being desorbed from an adsorption catalyst.

FIG. 3 is a flowchart of a process for calculating a target air-fuel ratio coefficient (KCMD).

FIG. 4 is a diagram showing a table used in the processing of FIG. 3;

FIG. 5 is a flowchart of a process for calculating an air-fuel ratio correction coefficient (KLAF).

FIG. 6 is a diagram for explaining a modified example of the three-way catalyst under the floor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Internal combustion engine 5 Electronic control unit (air-fuel ratio control means) 6 Fuel injection valve 13 Exhaust pipe (exhaust system) 14 Direct three-way catalyst 15 Under-floor three-way catalyst 16 Adsorption catalyst

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) F01N 3/24 F01N 3/24 R 3/28 301 3/28 301E F02D 41/04 305 F02D 41/04 305Z F-term (reference) 3G091 AA23 AA28 AB03 AB10 BA03 BA14 BA15 BA19 BA32 BA33 BA39 CB02 DB04 DB05 DB06 DB10 DB13 DC01 EA00 EA01 EA06 EA07 EA15 EA16 EA17 EA18 EA30 EA31 EA34 FA02 FA04 FA07 GB04 GB10 FB10 GB04 GB10Y HA03 HA08 HA12 HA18 HA36 HA37 HA39 HA42 3G301 HA01 HA06 JA25 JA26 JB09 LB02 MA01 MA11 ND01 NE01 NE06 NE14 NE15 PA01A PA01B PA07A PA07B PA10A PA10B PA11A PA11B PD02A PD02B PD09A PD09B PD11A PD11 PE01A12 PDB12A AB02 AB05 BA03X BA11X BA19X BA31X BA41X CC32 CC46 DA01 DA02 DA03 DA08 DA13 EA04

Claims (1)

[Claims] 1. A three-way catalyst provided in an exhaust system of an internal combustion engine, an adsorption catalyst provided in the exhaust system for adsorbing hydrocarbons, and a catalyst for desorbing hydrocarbons from the adsorption catalyst. An air-fuel ratio control means for controlling an air-fuel ratio of a supplied air-fuel mixture to a lean side from a stoichiometric air-fuel ratio, wherein cerium oxide is added to the adsorption catalyst; An exhaust gas purification device for an internal combustion engine, which is provided on an upstream side and is added with cerium oxide in a larger amount than the adsorption catalyst.