JP2005155493A - Exhaust emission control device of engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of reducing the discharge level of HC and NOx by reasonably supplying oxygen corresponding to desorption of HC when providing an HC adsorptive catalyst which adsorbs HC under exhaust in a cooled condition and desorbs and purifies HC at the temperature not lower than the HC desorption starting temperature T1 in an exhaust passage of an engine. <P>SOLUTION: The temperature of an HC adsorption catalyst is detected or estimated. The air-fuel ratio is gradually shifted to the lean side from the HC desorption starting temperature T1 so as to respond to the increase of the desorption. The lean-shift quantity of the air-fuel ratio is maintained at a predetermined maximum value between the temperature T2 at which the desorption reaches a peak and the temperature T3 at which the desorption is started to decrease. The lean-shift quantity is gradually reduced so as to return to stoichiometric amount of air at the desorption completion temperature T4. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device for an engine, and more particularly to an engine equipped with an HC adsorption catalyst.

The HC adsorption catalyst has an HC adsorption function and an oxidation function, adsorbs HC in the exhaust when it is cold, such as immediately after starting the engine, and desorbs and purifies HC above the HC desorption start temperature.
In Patent Document 1, when it is determined that HC is desorbed by the HC adsorption catalyst, the lean degree of the air-fuel ratio is temporarily increased at the start of the desorption, and then a smaller lean degree corresponding to the concentration of HC being desorbed. In addition, the purification efficiency of desorbed HC is increased by controlling the air-fuel ratio.
JP 2000-2138 A (particularly FIG. 8)

  However, HC desorption at the HC adsorption catalyst starts slowly, gradually increases with temperature rise, reaches a peak, and gradually decreases with subsequent temperature rise. If the degree of chemical conversion is increased, the inside of the catalyst will be in an oxygen excess state, leading to an increase in NOx emissions, and conversely, if there is an oxygen shortage when the amount of HC desorption increases, There was a problem that the part might be discharged as it is.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to supply oxygen without excess or deficiency when desorbing HC from an HC adsorption catalyst so as to reduce the HC and NOx emission levels. To do.

  For this reason, in the present invention, the temperature of the HC adsorption catalyst is detected or estimated, and the air-fuel ratio is gradually increased from the HC desorption start temperature to the lean side corresponding to the increase in the desorption amount (desorption amount per unit time). The lean shift amount is gradually decreased so as to return to stoichiometry at the desorption completion temperature in accordance with the subsequent decrease in the desorption amount.

  According to the present invention, by performing a lean shift in accordance with the amount of HC desorbed from the HC adsorption catalyst, oxygen can be supplied into the HC adsorption catalyst without excess or deficiency, and the HC and NOx emission levels can be reduced. Reduction can be achieved.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is an engine system diagram showing an embodiment of the present invention.
An electric throttle valve 3 for controlling the amount of intake air is installed in the intake passage 2 of the engine (internal combustion engine) 1, and a fuel injection valve 6 is installed in the combustion chamber 4 of the engine 1 together with a spark plug 5. However, the fuel injection valve 6 may be installed in each intake cylinder (intake port) for each cylinder.

The fuel injection valve 6 is energized to open a solenoid by an injection pulse signal output in synchronization with engine rotation from an engine control unit (hereinafter referred to as ECU) 10 so as to inject fuel adjusted to a predetermined pressure. It has become. Therefore, the fuel injection amount is controlled by the pulse width of the injection pulse signal.
The ECU 10 includes an accelerator opening Apo detected by an accelerator pedal sensor 11, an engine speed Ne detected by a crank angle sensor 12, an intake air amount Qa detected by an air flow meter 13, and an engine detected by a water temperature sensor 14. The cooling water temperature (water temperature) Tw is input. Further, an oxygen sensor 9 is provided in the exhaust passage 7 (upstream of the three-way catalyst 8) to output a signal corresponding to the rich / lean exhaust air-fuel ratio. This signal is also input to the ECU 10. The oxygen sensor 9 has a built-in heater, and can be activated early by energizing the heater from the start and increasing the element temperature.

The opening control of the electric throttle valve 3 by the ECU 10 will be described. The ECU 10 sets a target throttle opening mainly based on the accelerator opening Apo, and controls the throttle opening according to this.
The control of the fuel injection amount of the fuel injection valve 6 by the ECU 10 will be described. In the ECU 10, the basic fuel injection amount Tp = K × Qa / Ne (K is equivalent to the stoichiometry) from the actual intake air amount Qa and the engine speed Ne. Constant) and multiplying this by the air-fuel ratio feedback correction coefficient α, the final fuel injection amount Ti is calculated as in the following equation. In practice, various correction coefficients are used in addition to the air-fuel ratio feedback correction coefficient α, but the description thereof is omitted here.

Ti = Tp × α
The air-fuel ratio feedback correction coefficient α is a correction value of the fuel injection amount by the air-fuel ratio feedback control, and is increased or decreased according to the rich / lean signal from the oxygen sensor 9 under the air-fuel ratio feedback control condition. When the air-fuel ratio feedback control condition is not satisfied, the air-fuel ratio feedback correction coefficient α is fixed to 1 to achieve open control.

When the fuel injection amount Ti is calculated, the fuel injection valve 6 is driven by an injection pulse signal having a pulse width corresponding to this Ti.
On the other hand, the exhaust gas after combustion is discharged to the exhaust passage 7. The exhaust passage 7 is provided with a three-way catalyst 8 on the relatively upstream side and an HC adsorption catalyst 9 on the downstream side.
The three-way catalyst 8 can oxidize HC and CO in the exhaust and reduce NOx when the air-fuel ratio is near stoichiometric.

  The HC adsorption catalyst 9 is obtained by adding an HC adsorbent (for example, zeolite) to a three-way catalyst, and has an HC adsorption function and a three-way catalyst function (at least an oxidation function). That is, it has a function of adsorbing HC in the exhaust when cold and desorbing and purifying HC at an HC desorption start temperature T1 (for example, 100 ° C.) or higher. In addition, since the HC adsorbent (zeolite) is inferior in heat resistance compared with a normal three-way catalyst, the three-way catalyst 8 is arranged on the upstream side (high temperature side) where early activation is possible, and the HC adsorption catalyst 9 is placed on the downstream side. It is arranged on the (low temperature side).

FIG. 5 is a time chart after the start, in which the engine speed Ne after the start, the HC emission amount (HC discharge amount at the engine outlet and the HC discharge amount at the tail pipe), the temperature of the HC adsorption catalyst (catalyst temperature). ) Shows a change in Tc. Note that T1 <T2 <T3 <T4.
After the start-up, HC is adsorbed by the HC adsorption catalyst until the catalyst temperature Tc reaches the HC desorption start temperature T1 (for example, 100 ° C.). Therefore, compared with the HC discharge amount at the engine outlet, the HC discharge amount at the tail pipe is greatly reduced, and the difference becomes the HC adsorption amount.

  The desorption of HC starts from the HC desorption start temperature T1, the desorption amount (desorption amount per unit time) reaches a substantially constant peak at the temperature T2 (for example, 200 ° C.), and the temperature T3 (for example, 300 ° C.). Then, the desorption amount starts to decrease, and at temperature T4, the desorption amount becomes zero and the desorption is completed. Therefore, if the desorbed HC is not purified, the HC discharge amount at the tail pipe increases during this period compared to the HC discharge amount at the engine outlet, and the difference becomes the desorption amount.

In the present invention, by desorbing HC by supplying oxygen into the HC adsorption catalyst without excess or deficiency by shifting the air-fuel ratio to the lean side in accordance with the amount of HC desorption from the HC adsorption catalyst. For optimum efficiency, the amount of HC emissions in the tailpipe is reduced from the solid line level in FIG. 5 to the dotted line level.
FIG. 2 is a flowchart of air-fuel ratio lean shift control after startup.

In S1, it is determined whether or not the start of the engine has been completed by monitoring whether the start switch is turned on or off. If the start is completed, the process proceeds to S2.
In S2, it is determined whether or not the water temperature Tw is a predetermined value (for example, 40 ° C.) or less. As a result, if the water temperature Tw exceeds the predetermined value and is a so-called hot restart, the air-fuel ratio lean shift amount LS = 0 is set in S3 and the control is terminated. If the water temperature Tw is equal to or lower than the predetermined value and cold, the process proceeds to S4.

In S4, the temperature (catalyst temperature) Tc of the HC adsorption catalyst is estimated as follows, for example.
(1) The engine speed Ne and the basic fuel injection amount Tp corresponding to the load are read.
(2) The exhaust temperature (or the amount of exhaust heat flowing into the HC adsorption catalyst) is calculated from Ne and Tp with reference to a map prepared in advance. Needless to say, the higher the rotation speed and the higher the load, the higher the exhaust temperature (the greater the exhaust heat amount).
(3) The catalyst temperature is calculated by subjecting the exhaust gas temperature obtained above to a first-order lag process. Alternatively, the catalyst temperature is calculated by integrating the exhaust heat amount and referring to the table based on the integrated value.

Note that the present invention is not limited to this, and various estimation methods can be used, and the exhaust temperature may be detected by a sensor. Further, the catalyst temperature (bed temperature) may be directly detected using a catalyst temperature sensor.
In S5, it is determined whether or not air-fuel ratio feedback control (λ control) is in progress. The air-fuel ratio feedback control is performed under the condition that at least the oxygen sensor is active. If the air-fuel ratio feedback control is not being performed, the air-fuel ratio lean shift amount LS = 0 is set in S6, and the process returns to S4. This is because the air-fuel ratio lean shift in the present embodiment is performed using air-fuel ratio feedback control as will be described later. If air-fuel ratio feedback control is being performed, the process proceeds to S7.

In S7, it is determined whether or not the catalyst temperature Tc is lower than the HC desorption start temperature T1 (for example, 100 ° C.). If it is lower than T1, the lean shift amount LS = 0 of the air-fuel ratio is set in S8 and the process returns to S4.
If the catalyst temperature Tc is equal to or higher than T1, the process proceeds to S9.
In S9, it is determined whether or not the catalyst temperature Tc is equal to or higher than the HC desorption start temperature T1 and lower than a temperature T2 (for example, 200 ° C.) at which the desorption amount reaches a peak. When T1 ≦ Tc <T2, the process proceeds to S10, and the air-fuel ratio lean shift amount LS is calculated by the following equation.

LS = LSmax × (Tc−T1) / (T2−T1)
Therefore, as shown in FIG. 4, the lean shift amount LS of the air-fuel ratio is predetermined from 0 in accordance with the temperature rise from the HC desorption start temperature T1 to the temperature T2 at which the desorption amount reaches a peak. The maximum value LSmax is increased at a constant slope.
When the catalyst temperature Tc is equal to or higher than T2, the process proceeds to S11.

In S11, it is determined whether or not the catalyst temperature Tc is equal to or higher than a temperature T2 at which the desorption amount reaches a peak and is lower than a temperature T3 (for example, 300 ° C.) at which the desorption amount starts to decrease. When T2 ≦ Tc <T3, the routine proceeds to S12, where the air-fuel ratio lean shift amount LS is set to the maximum value LSmax (LS = LSmax).
Therefore, as shown in FIG. 4, the air-fuel ratio lean shift amount LS is maintained at a predetermined maximum value LSmax from the temperature T2 at which the desorption amount reaches a peak to the temperature T3 at which the desorption amount starts to decrease. Will do.

If the catalyst temperature Tc is equal to or higher than T3, the process proceeds to S13.
In S13, it is determined whether or not the catalyst temperature Tc is equal to or higher than the temperature T3 at which the desorption amount starts to decrease and lower than the desorption completion temperature T4 (for example, 400 ° C.). When T3 ≦ Tc <T4, the process proceeds to S14, and the lean shift amount LS of the air-fuel ratio is calculated by the following equation.
LS = LSmax × (T4-Tc) / (T4-T3)
Therefore, as shown in FIG. 4, the lean shift amount LS of the air-fuel ratio is set to a predetermined maximum value as the temperature rises from the temperature T3 at which the desorption amount starts to decrease to the desorption completion temperature T4. From LSmax to 0, it is decreased with a constant slope.

When the catalyst temperature Tc is equal to or higher than T4, the process proceeds to S15. In S15, since warm-up is completed and the catalyst is also in a fully activated state, the control is terminated with the lean shift amount LS = 0 of the air-fuel ratio.
Next, control for shifting the air-fuel ratio to the lean side using the air-fuel ratio lean shift amount LS will be described.
In this embodiment, air-fuel ratio feedback control is performed in which the rich / lean of the air-fuel ratio is determined based on the signal from the oxygen sensor, the fuel injection amount is corrected based on the determination result, and the air-fuel ratio is feedback-controlled. The air-fuel ratio shift to the lean side is a control gain (proportional) that corrects the fuel injection amount to the decreasing side at the rich determination, compared to the control gain (proportional amount PR) that corrects the fuel injection amount to the increase side at the lean determination. This is done by setting a large value (min PL).

FIG. 3 is a flowchart of air-fuel ratio feedback control, which is repeatedly executed in time synchronization.
In S21, lean / rich is determined based on the oxygen sensor output.
In the case of lean, the process proceeds to S22, and it is determined whether or not the inversion from rich to lean (previous rich).

In the case of inversion from rich to lean, the process proceeds to S23, and the proportional amount PR toward the rich side is set to a value obtained by subtracting the lean shift amount LS from the basic proportional amount P0 as in the following equation.
PR = P0-LS
Then, the process proceeds to S24, where the air-fuel ratio feedback correction coefficient α is increased by a proportional amount PR and updated (α = α + PR).

When the lean state is continuing, the routine proceeds to S25, where the air-fuel ratio feedback correction coefficient α is increased by a minute integral I and updated (α = α + I).
In the case of rich, the process proceeds to S26, and it is determined whether or not the time of reversal from lean to rich (previous lean).
In the case of inversion from lean to rich, the process proceeds to S27, and the proportional amount PL toward the lean side is set to a value obtained by adding the lean shift amount LS to the basic proportional amount P0 as in the following equation.

PL = P0 + LS
Then, the process proceeds to S28, and the air-fuel ratio feedback correction coefficient α is updated by decreasing the proportional amount PL (α = α−PL).
When the rich state is continuing, the process proceeds to S29, and the air-fuel ratio feedback correction coefficient α is updated by decreasing by a minute integral I (α = α−I).

As a result of the above control, when the lean shift amount LS = 0, the proportion PR to the rich side and the proportion PR to the lean side are both equal to P0, so the air-fuel ratio is stoichiometrically controlled by the air-fuel ratio feedback control. Controlled.
On the other hand, when the lean shift amount LS increases, the proportional component PL = P0 + LS to the lean side becomes larger than the proportional component PR = P0-LS on the rich side, the balance is lost to the lean side, and the sky The air-fuel ratio is shifted to the lean side by the amount corresponding to the lean shift amount LS by the fuel ratio feedback control.

  As described above, the temperature of the HC adsorption catalyst is detected or estimated, and the air-fuel ratio is gradually shifted to the lean side from the HC desorption start temperature T1 to cope with the increase in the desorption amount. In accordance with the decrease, the lean shift amount is gradually decreased so as to return to the stoichiometry at the desorption completion temperature T4, so that oxygen is not excessively or deficient in the catalyst in accordance with the HC desorption amount from the HC adsorption catalyst. It can be supplied, and the emission level of HC and NOx can be reduced. Accordingly, the HC emission amount in the tail pipe is reduced from the solid line level in FIG. 5 to the dotted line level.

  In addition, from the HC desorption start temperature T1 to the temperature T2 at which the desorption amount reaches a peak, the air-fuel ratio lean shift amount LS is increased from 0 to the maximum value LSmax with a constant slope as the temperature rises. Between the temperature T2 and the temperature T3 at which the desorption amount starts to decrease, the air-fuel ratio lean shift amount LS is maintained at the maximum value LSmax, and between the temperature T3 and the desorption completion temperature T4. As the temperature rises, the lean shift amount LS of the air-fuel ratio is decreased from the maximum value LSmax to 0 with a constant slope, so that it can be controlled with relatively simple logic.

  In addition, the air-fuel ratio is shifted to the lean side by using air-fuel ratio feedback control and setting the control gain to the lean side (proportional part PL) larger than the control gain to the rich side (proportional part PR). By doing so, a desired lean air-fuel ratio can be obtained with high accuracy while performing feedback control of the air-fuel ratio.

Engine system diagram showing an embodiment of the present invention Flow chart of air-fuel ratio lean shift control after startup Flow chart of air-fuel ratio feedback control Diagram showing the relationship between catalyst temperature and lean shift Time chart of control after starting

Explanation of symbols

1 engine
2 Intake passage
6 Fuel injection valve
7 Exhaust passage
8 Three-way catalyst
9 HC adsorption catalyst
10 ECU
15 Oxygen sensor

Claims (5)

In an engine exhaust gas purification apparatus provided with an HC adsorption catalyst that adsorbs HC in exhaust gas when it is cold and desorbs and purifies HC at an HC desorption start temperature or higher in an engine exhaust passage.
Detects or estimates the temperature of the HC adsorption catalyst, gradually shifts the air-fuel ratio from the HC desorption start temperature to the lean side in response to the increase in the desorption amount, and desorbs as the desorption amount thereafter decreases An exhaust emission control device for an engine, wherein the lean shift amount is gradually decreased so as to return to stoichiometry at a completion temperature.   From the HC desorption start temperature T1 to the temperature T2 at which the desorption amount reaches a peak, the air-fuel ratio lean shift amount is increased from 0 to a predetermined maximum value with a constant slope as the temperature rises. The exhaust emission control device for an engine according to claim 1.   2. The lean shift amount of the air-fuel ratio is maintained at a predetermined maximum value from a temperature T2 at which the desorption amount reaches a peak to a temperature T3 at which the desorption amount starts to decrease. Item 2. An exhaust emission control device for an engine according to Item 2.   Decreasing the air-fuel ratio lean shift amount from a predetermined maximum value to 0 with a constant slope between the temperature T3 at which the desorption amount starts decreasing and the desorption completion temperature T4 as the temperature increases. The engine exhaust gas purification apparatus according to any one of claims 1 to 3. Air-fuel ratio feedback control means for determining rich / lean of the air-fuel ratio based on a signal from an oxygen sensor provided in the exhaust passage, correcting the fuel injection amount based on the determination result, and feedback-controlling the air-fuel ratio With
The shift of the air-fuel ratio to the lean side is performed by setting a control gain that corrects the fuel injection amount to the decreasing side at the time of the rich determination is set larger than the control gain that corrects the fuel injection amount to the increase side at the time of the lean determination. The exhaust emission control device for an engine according to any one of claims 1 to 4, wherein the exhaust emission control device is an engine.

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