JPH03199640A - Exhaust gas purifying device for engine - Google Patents

Abstract

PURPOSE:To increase the purifying rate of HC, CO, and NOx by the Partabeit effect by periodically changing the air-fuel ratio of supply mixture from rich to lean, when an engine is in a low-temperature condition. CONSTITUTION:When it is detected that an engine 1 is in a low-temperature condition by means of a temperature sensor 12, an air-fuel ratio control means 8 controls an injector 3 to periodically change the air-fuel ratio of mixture from rich to lean. Thereby the oxygen concentration in the exhaust gas being detected by an O2 sensor 11 also changes periodically, and by the Partabeit effect, the purifying rate of HC, CO, and NOx by means of catalytic converter rhodium in an exhaust gas purifying device 7 can be increased. Thus, only by adding a software capable of periodically changing the air-fuel ratio, the purifying rate of exhaust gas can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an exhaust gas purification device for an engine.

[Conventional technology 1] A ternary technology that has the function of oxidizing and purifying HC (hydrocarbons) and CO (-carbon oxides) in exhaust gas, and the function of reducing and purifying NOx (nitrogen oxides). Engine exhaust gas purification devices using catalysts have been known for some time. In such an exhaust gas purification device, one three-way catalyst brick can produce H
Since C, CO, and NOx can be purified at the same time, there is no need to separately provide a plink for an oxidation catalyst and a brick for a reduction catalyst, which makes the exhaust gas purification device more compact, and the catalyst splitter installation of,
This has the advantage of improving the workability of removal.

However, in general, the oxidation effect of a three-way catalyst becomes stronger when the oxygen concentration in the exhaust gas is high, and the directional reduction effect becomes stronger when the oxygen concentration is low. For this reason, in the conventional exhaust gas purification device using a three-way catalyst, there was a problem in that it was difficult to increase the purification rate of both IIC, CO, and NOx.

Further, in general, in a three-way catalyst, sufficient catalytic activity cannot be obtained unless the temperature is raised to a certain level or higher (for example, 150° C. or higher). For this reason, when the engine is in a cold state, the catalyst activity is low and the emission performance is poor.

By the way, three-way catalysts generally have the property of combining with oxygen when the oxygen concentration in the exhaust gas is high, and releasing this oxygen when the oxygen concentration is low. Focusing on these characteristics, the air-fuel ratio of the air-fuel mixture (the ratio A/F of intake air to fuel, and the same applies below) is calculated as the stoichiometric air-fuel ratio (
While setting the A/F = 14゜7) to the rich side, supply an appropriate amount of secondary air to the exhaust circuit upstream of the three-way catalyst for 1 to 2 hours.
An exhaust gas purification device has been proposed that uses the so-called perturbation effect to increase the activity of the three-way catalyst by supplying it periodically in a pulsed manner to improve the exhaust gas purification rate (for example, (Refer to Japanese Patent Publication No. 59-96425) In such an exhaust gas purification device, a state in which the oxygen concentration is high and a state in which the oxygen concentration is low occur in the exhaust gas at regular intervals, and the three-way catalyst is activated in the following process. activity is increased, and the purification rate of exhaust gas is increased (vertabate effect).

■When secondary air is supplied to the exhaust gas, the oxygen concentration in the exhaust gas that comes into contact with the three-way catalyst increases, causing HC
and CO are sufficiently oxidized, and their purification efficiency is increased. At this time, since the oxygen concentration is high, the reduction of NOx should normally be hindered by oxygen, but since the three-way catalyst combines with oxygen, oxygen does not interfere with the reduction of NOx during this time. Therefore, when supplying secondary air, HC and CO
It is possible to increase both the purification rate of NOx and NOx.

If the oxygen concentration continues to be high, the three-way catalyst's ability to combine with oxygen (hereinafter referred to as oxygen retention) will reach saturation, and oxygen will begin to interfere with NOx reduction. Become. However, since the supply of secondary air is stopped before the oxygen retention effect of the three-way catalyst reaches a saturated state, the reduction of NOx is not obstructed.

(2) When the supply of secondary air is stopped, the oxygen concentration in the exhaust gas that contacts the three-way catalyst becomes low, NOx is sufficiently reduced, and the NOx purification rate is increased. At this time, since the oxygen concentration is low, oxidation of HC and CO should be suppressed, but since the oxygen that has been bonded to the three-way catalyst is released, this oxygen oxidizes HC and CO. and their purification rates will not decrease. Therefore, 2
When the next air supply is stopped, it is possible to increase the purification rate of both HC, CO, and NOx. Note that if the oxygen concentration continues to be even lower, all of the oxygen bonded to the three-way catalyst will be released, so oxidation of Hc and co will be suppressed. However, since the supply of secondary air is started before all the oxygen is released from the three-way catalyst, HC and c
The oxidation of o is not suppressed.

Such a process is repeated to obtain HC and c.

Both the NOx and NOx purification rates have been improved.

[Problem to be Solved by the Invention 1] However, in the exhaust gas purification device disclosed in JP-A-59-96425, the air-fuel ratio of the air-fuel mixture is always set to the Lynch side of the stoichiometric air-fuel ratio. There is a problem that performance deteriorates.

In addition, since the oxygen retention effect of the three-way catalyst is relatively weak, the Partabait effect cannot be sufficiently obtained, and the exhaust gas temperature decreases due to the secondary air, so the exhaust gas purification rate cannot be sufficiently maintained, especially when the engine is cold. The problem is that it cannot be increased.

Furthermore, since it is necessary to provide a secondary air supply mechanism for supplying secondary air in a pulsed manner, there is a problem that the structure of the exhaust gas purification device becomes complicated.

The present invention has been made in view of the above-mentioned conventional problems, and is intended to improve the purification rate of both HC, Co, and NOx, especially when the engine is cold, in an engine equipped with an exhaust gas purification device using a three-way catalyst. It is an object of the present invention to provide an engine exhaust gas purification device with a simple structure that can improve the performance of the engine.

[Means for Solving the Problems 1] In order to achieve the above object, the present invention provides a three-way catalyst containing cerium oxide in the exhaust system, and a temperature sensor for detecting whether or not the engine is in a low temperature state; An engine characterized by being provided with air-fuel ratio control means for periodically changing the air-fuel ratio of the air-fuel mixture supplied to the engine from rich to lean when a temperature sensor detects that the engine is in a low temperature state. provides exhaust gas purification equipment.

[Operations and Effects of the Invention] According to the present invention, the air-fuel ratio of the air-fuel mixture is periodically changed by the air-fuel ratio control means, so that the oxygen concentration in the exhaust gas is changed periodically, and the barbating effect causes the oxygen concentration in the exhaust gas to change periodically. , the purification rates of HC, CO, and NOx are all increased. Therefore, the activity of the three-way catalyst is sufficiently increased even when the engine is cold, and the purification rate of exhaust gas is increased.

Here, the air-fuel ratio control means does not need to be specially constituted by hardware; for example, in an engine equipped with a fuel injection device, the control unit for controlling fuel injection may be configured to periodically change the air-fuel ratio. It can be implemented with a simple configuration just by adding software.

In addition, cerium oxide is added to the three-way catalyst, but cerium oxide easily changes into an oxide such as the general 2CemOn (m and n are integers) depending on the surrounding oxygen concentration. Therefore, it has a particularly strong oxygen retention effect. Therefore, the perturbation effect can be enhanced, and the purification rate of exhaust gas can be further increased.

[Example] Examples of the present invention will be specifically described below.

As shown in FIG. 1, the engine 1 injects fuel from an injector 3 into intake air supplied from an intake passage 2 to create a mixture, and ignites and burns this mixture using an ignition system 4 (spark plug). , the combustion gas is discharged into the exhaust passage 5.

The intake passage 2 is provided with an air meter 6 for detecting the amount of intake air, while the exhaust passage 5 is provided with an exhaust gas purification device 7 using a three-way catalyst. The above three-way catalyst is a monolithic carrier catalyst, which is not shown in the figure, or has a catalyst coat layer formed on the surface of a catalyst carrier made of a material such as cordierite, and this catalyst coat layer has an oxygen retention effect. Cerium oxide (which strongly promotes the perturbation effect)
Ceo 2) as the main component, and platinum and rhodium as catalyst components at a blending ratio of 5:1, containing a total of 1.6y/Q.

When the engine is warmed up, normal air-fuel ratio feedback control is carried out, and when the engine is cold, the air-fuel ratio of the air-fuel mixture is enriched in order to increase the activity (low-temperature activity) of the three-way catalyst using the barbating effect. A control unit 8 is provided to periodically change the lean state. This control section 8
is composed of a microcomputer, and includes an air intake amount Q1 detected by an air meter 6, an engine rotation speed N1 detected by a rotation speed sensor (not shown) provided for the ignition system 4 (distributor), and an exhaust passage. 5
The air-fuel ratio is controlled using the 02 sensor detection value Vo2 detected by the 02 sensor 11 installed in the exhaust passage 5, the exhaust gas temperature t (catalyst inlet) detected by the temperature sensor 12 installed in the exhaust passage 5, etc. It is now possible to do this.

Hereinafter, a method of controlling the air-fuel ratio will be explained according to the flowcharts shown in FIGS. 3(a) to 3(C).

In step S1, various control information such as intake air amount Q1, engine rotation speed N, and exhaust gas temperature are read.

In step S2, the basic pulse width Tp of the injector 3 is calculated using the following equation.

Tp=-Q/N In the above equation, K is a proportionality constant used to calculate the basic pulse width for a single intake amount.

In step S3, a correction amount C for performing various corrections such as battery voltage correction and invalid injection time correction is calculated.

In step S4, a feedback sensor to perform feedback control of the operating state of the engine l or the air-fuel ratio;
That is, a comparison is made to see if it is within the normal operating range.

As a result of this comparison, if the operating state of engine 1 is in the feedback zone (YES), the foot control routine of steps S5 to S15 is executed, whereas if it is not in the feedback zone (N
O, for example, at high load), there is no need to perform feed pack control, so steps 35 to S15 are skipped and steps 532 to S3, which will be explained later, are performed.
3 is executed.

The feedback control routine from step S5 to step S15 will be explained below.

In step S5, the 02 sensor detection value ■0□ (voltage) is read.

In step S6, it is compared whether the operating state of the engine 1 is within the normal feedback zone. Here, if the exhaust gas temperature t1 entering the exhaust gas purification device 7, that is, the catalyst temperature, exceeds 300°C, the engine 1 is in a warm-up state, so normal air-fuel ratio feedback control is performed, and the exhaust gas temperature t is When the temperature is below 300'C, the engine 1 is in a cold state, so the air-fuel ratio is periodically changed from rich to lean in order to increase the activity of the three-way catalyst.

As a result of the comparison in step S6, if it is not within the normal feedback zone (No), that is, t≦300°
If C, a cycle control routine (steps S17 to S31) to be described later is executed, and the air-fuel ratio is periodically changed at a constant cycle.

On the other hand, as a result of the comparison in step S6, if it is within the feedback zone (YES), that is, t>300'
If c, in step S7, O, sensor detection value Vo2
is the value VR corresponding to the stoichiometric air-fuel ratio (A/F-14,7)
(for example, 400 mV). Note that the 02 sensor detection value Vo is approximately 100 OmV when the air-fuel ratio is extremely rich, and approximately OmV when it is extremely lean, and the stoichiometric air-fuel ratio (A/F = 1
4.7), the richer the detection value (voltage), the higher the detected value (voltage).

As a result of the comparison in step S7, if Vo, )V, then (
YES), the air-fuel ratio is rich, so in order to adjust it to a lean direction, steps S8 to S are performed.
ll is executed.

In step S8, it is compared whether the air-fuel ratio flag FL is l. This air-fuel ratio flag F is set to 1 if the air-fuel ratio was lean last time, and is set to 0 if it is rich.

As a result of the comparison in step S8, if FL=1 (YE
S), in step S9, the feedback control term DFB for air-fuel ratio control is decremented by the proportional amount ΔP in a stepwise manner. In other words, since the last time it was 100% and this time it is rich, the adjustment in the phosphorus direction will start from this time. There is a risk that so-called overshoot may occur, in which the amount of liquid becomes too rich.

Therefore, in order to prevent overshoot in the rich direction, the fuel injection amount is reduced in steps.

Subsequently, in step 510, the air-fuel ratio 7 lag FL is set to 0 (rich).

Thereafter, in step S32, the final pulse width T of the injector 3 is calculated using the following equation, and in step S33, this T is output, and fuel injection is performed according to this, after which the process returns to step Sl.

On the other hand, if the comparison result in step S8 is FLf-1, that is, FL-0 (No), step S11 is executed, and the feedback control term D0 is set to a predetermined integral Δ. is decremented by ■, the fuel injection amount is gradually reduced, and the air-fuel ratio is adjusted toward lean.

Thereafter, steps S32 and S33 are executed, and then the process returns to step Sl, but since the operations in these steps are the same as described above, their explanation will be omitted.

As a result of the comparison in step S7, if vO1≦vll (NO), the air-fuel ratio is lean.
15 is executed.

In step 512, it is compared whether the air-fuel ratio flag FL is l. As a result of the comparison, if FL≠1, then (N
O), in step S14, the feedback control term DF8
is incremented in steps by a proportional amount ΔP.

In other words, since the previous time was rich and this time is lean, the adjustment toward the rich direction will start from this time, but since the adjustment toward the lean direction has been made until the previous time, In order to prevent open neat, the fuel injection amount is increased in steps.

Then, in step S15, the air-fuel ratio flag is set to FL or l (lean). After that, after steps S32 and S33 are executed, the process returns to step Sl, but since the operations in these steps are as described above,
The explanation will be omitted.

As a result of the comparison in step 512, if FL-1 (Y
ES), step S13 is executed, the feedback control term D□ is incremented by a predetermined integral ΔI,
The fuel injection amount is gradually increased and the air-fuel ratio is adjusted toward richness. Thereafter, after steps 332 to S33 are executed, the process returns to step Sl, but since the operations in these steps are the same as described above, their explanation will be omitted.

Hereinafter, a cycle control routine (step S17 to step 531) for periodically changing the air-fuel ratio in order to increase the activity of the three-way catalyst when the engine I is cold will be explained.

In step 517, it is compared whether the 02 sensor detection value Vo2 is higher than the value VR corresponding to the stoichiometric air-fuel ratio (A/F=14.7).

As a result of the comparison in step S17, if VO□>VR (YES), that is, if the current air-fuel ratio is rich, then in step 51g, it is further determined whether the air-fuel ratio filler knob FL is l. It is compared and the previous rich/lean is determined.

As a result of the comparison in step S18, if Ft=1, then (
YES), since the previous time was lean and this time is rich, timer T is set in step S19.

is reset to 0.

In this embodiment, when the engine 1 is cold, the air-fuel ratio is adjusted at a period S (for example, 1 second) with the stoichiometric air-fuel ratio (A/F = 14.7) as shown in FIG. It is made to vary periodically from rich to lean in a sawtooth pattern. That is, from the time when the air-fuel ratio changes from lean to rich, the air-fuel ratio is adjusted toward richness at a rate corresponding to the integral Δ■ for a certain period of time T, and after T, the air-fuel ratio is adjusted to be equal to ΔI. When the air-fuel ratio changes from rich to lean, the air-fuel ratio is further adjusted lean by T at a rate corresponding to ΔI, and after T, the air-fuel ratio is adjusted to ΔI. The air-fuel ratio is periodically changed in a sawtooth pattern by repeating the operation of adjusting toward the rich direction. Here, the period S is approximately four times as long as T5.

And timer T. is a timer that counts the time from the time when the state changes to rich or lean until the present time in order to determine whether a certain period of time T has elapsed.

Next, in step 520, the feed batter control term D F
B is incremented by ΔI, the fuel injection amount is increased, and the air-fuel ratio is adjusted in the rich direction.

Subsequently, in step S21, the air-fuel ratio flag FL is set to O (rich).

Thereafter, after steps 332 to S33 are executed, the process returns to step Sl, but since the operations in these steps are the same as described above, their explanation will be omitted.

In addition, since the 7 lag F was set to O in step 521, from next time onwards, such a routine (step S1
8→Step 519-Step S20→Step 521
), instead, the routine of step S18→step S22→step 523-step 520-step S21 is executed (until timer TD times up).

On the other hand, as a result of the comparison in step 518, if FL≠1 (No), the air-fuel ratio continues to be rich, so in step S22, it is determined whether the timer TD has exceeded the set value or not. In other words, a comparison is made to determine whether or not the time has expired.

If the comparison result in step S22 is To2T, then (
No), since the timer TD has not timed up yet, the timer TD is incremented by α in step S23, and then the steps S20→S21→
Step S32 → step S33 is executed, and the process returns to step Sl.

That is, the air-fuel ratio continues to be adjusted rich at a constant rate corresponding to ΔI until the timer TD times up.

On the other hand, if the comparison result in step S22 is To>T (YES), since the timer TD has already timed up, the feedback control term DB is
F is decremented by ΔI.

After that, step S32 → step S33 is executed, and then the process returns to step s1. However, this step s24 is executed until the air-fuel ratio changes from rich to lean, and the air-fuel ratio moves in the lean direction at a constant rate corresponding to Δ■. continues to be adjusted. Note that when the air-fuel ratio changes to positive, steps 525 to 532, which will be described later, are executed.

On the other hand, as a result of the comparison in step 317, if Vo2≦■R (No), that is, if the current air-fuel ratio is lean, then in step S25, the air-fuel ratio flag FL is set to 1.
The previous rich/lean condition is determined.

As a result of the comparison in step S25, if %F, ≠ 1, (N
o) Since the previous time was rich and the current time is lean, timer T is set in step S26.

is reset to O.

Next, in step S27, the feedback control term DFB
is decremented by ΔI, the fuel injection amount is reduced,
The air-fuel ratio is adjusted toward lean.

Subsequently, in step S28, the air-fuel ratio flag FL is set to 1 (lean).

Thereafter, after steps 332 to S33 are executed, the process returns to step S1, but since the operations in these steps are as described above, their explanation will be omitted. Note that since the 7 lag FL was set to 1 in step S28, from the next time, step 525-+step S2
9→Step s31→Step S27→Step 328
The routine will be executed (until timer T times up).

On the other hand, if the comparison result in step S25 is FL"'l (YES), the air-fuel ratio continues to be lean, so in step S29, it is determined whether or not the timer TD exceeds the set value T. In other words, it is compared whether or not the time has expired.

As a result of the comparison in step S29, if To≦T, then (
NO), since timer TD has not timed up yet, timer T is reset in step S31. is incremented by α, and the steps 527-528→
After step S32→step S33 is executed, the process returns to step s1.

That is, timer T. The air-fuel ratio continues to be adjusted in the lean direction at a constant rate corresponding to ΔI until the time-up occurs.

On the other hand, if the comparison result in step S29 is TD>T (YES), the timer T. has already timed up, so in step 530 the feedback control term Dl
IF is incremented by Δ■. After this, step S32 → step S33 is executed, and then the process returns to step St. However, until the air-fuel ratio changes from lean to rich,
This step S30 is executed, and the air-fuel ratio continues to be adjusted in the rich direction at a constant rate corresponding to ΔI. Note that when the air-fuel ratio turns rich, steps S18 to S18 described above are performed.
Step S24 will be executed.

Note that in the above control method, the air-fuel ratio is periodically changed in a sawtooth shape, but it may be changed periodically in a rectangular shape.

When such an air-fuel ratio control method is performed, when the engine is cold, the activity of the three-way catalyst is increased due to the vatabate effect as described above. The light-off temperature (low-temperature activation) for HC, CO, and NOx when the above-described air-fuel ratio control is performed using a three-way catalyst on which a catalyst coat layer containing cerium oxide according to the present invention is formed, i.e. Table 1 shows the results of measuring the temperature at which 50% of each exhaust gas component was purified. Note that the test conditions are as follows.

(Test conditions) Base air-fuel ratio A/F=14.7 Fluctuation period IHz S, V, 60000H-' Catalyst Pt/Rh=5/1 layer CeO.

Aging condition: 900'Cx50H (in air)
The light-off temperature was measured by setting the intake air-fuel ratio to rich and supplying secondary air in a rectangular pulse (lHz) to the exhaust passage upstream of an exhaust gas purification device using a conventional three-way catalyst. The results are also shown as comparative examples. In the comparative example, the base air-fuel ratio is A/F = 14.7, the 02 concentration when the secondary air is supplied is 1.1%, and the 02 concentration when the secondary air supply is stopped is 0.6%. be.

As is clear from Table 1, the low-temperature activity of Honjitsu against C○ is slightly lower than that of the comparative example, but the low-temperature activity against NOx is significantly increased, and the low-temperature activity against HC is also generally high. It is being

As described above, according to the present invention, the barterbate effect can be enhanced with a simple configuration without causing a decrease in fuel efficiency.
In particular, the purification rate of exhaust gas can be increased when the engine is cold.

[Brief explanation of drawings]

Figure 1) shows the countries in which the system of the engine equipped with the exhaust gas purification device according to the present invention is constructed. FIG. 2 is a diagram showing periodic changes in the air-fuel ratio when the engine is cold. FIGS. 3(a) to 3(c) are flowcharts showing a control method for air-fuel ratio control. 1... Engine, 2... Intake passage, 3... Injector, 4... Ignition system, 5... Exhaust passage, 6...
Air flow meter, 7...Exhaust gas purification device, 8.Control unit, jl...02-t:n''j, 12...Temperature sensor.

Claims (1)

[Claims] (1) A three-way catalyst containing cerium oxide is provided in the exhaust system, and a temperature sensor is provided to detect whether the engine is in a low temperature state, and when the temperature sensor detects that the engine is in a low temperature state, 1. An exhaust gas purification device for an engine, comprising an air-fuel ratio control means for periodically changing the air-fuel ratio of an air-fuel mixture supplied to the engine from rich to lean.