JPS59101528A - Exhaust gas purging device of internal-combustion engine - Google Patents

Abstract

PURPOSE:To improve purging of exhaust gas without using an expensive device and to reduce fuel consumption, by a method wherein the air-fuel ratio of mixture is maintained at a specified value, and lean mixture is supplied during steady running. CONSTITUTION:During idling operation or accelerating operation, rich fuel-air mixture is fed into an engine cylinder, a solenoid 67 is actuated and controlled in response to the output signal of a pulse generator 71 with 1-2HZ frequency, and secondary air is intermittently supplied into an exhaust manifold 3 by a secondary air feed control valve 50. Since the air-fuel rate of mixture produced in a carbretter 2 is normally maintained at a specified value, the air-fuel ratio is varied at the 1-2HZ frequency within a range of from + or -02. to + or -1.0 based on an approximately theoretical air-fuel ratio. Besides, the mean value thereof is maintained within a window W0, and thereby a high purging efficiency may be obtained through utilization of the oxygen holding function of a ternary monolith catalyst 5. During steady operation, a solenoid 43 is energized, a valve port 36 is opened fully, lean fuel-air mixture is fed into the cylinder, and the feed of secondary air is disconnected with the solenoid 67 de-energized.

Description

[Detailed description of the invention] The invention relates to an exhaust gas exhaust system for a FF'3 combustion engine.

Three harmful components in exhaust gas) 1c, CO and NOx
A three-way catalyst is said to be a catalyst that can reduce k at the same time. The purification efficiency I (of this three-way catalyst is the first (
a) As shown in the figure, the price becomes even better when the air-fuel ratio A/F is approximately the stoichiometric air-fuel ratio. It has a narrow width of about 0.0c.

Normally, in this way, in order to simultaneously reduce the three harmful components in exhaust gas using a ternary M medium, there is a The air/fuel ratio must be maintained within this narrow window W at all times. For this purpose, a conventional gas purification system (with an air-fuel ratio that can be determined whether the air-fuel ratio is larger or smaller than the stoichiometric fuel-fuel ratio) is used. Based on the output signal of the oxygen 4 degree detector, the air-fuel ratio is controlled to match the fuel ratio within the window WI'E. There is a problem in that the manufacturing cost of the exhaust gas purification device increases because it requires an oxygen concentration detector and an expensive electronic control unit for determining the fuel/fuel ratio.However, recently, 5AEpaper No.

As described in No. 760201 or Japanese Patent Publication No. 56-4741, the function of the three-way catalyst was gradually elucidated, and it was found that the three-way catalyst had an oxygen retention function. In other words, the air-fuel ratio is lean compared to the stoichiometric air-fuel ratio!
When it is at i, the three-way catalyst removes all of the oxygen from NOx and completely reduces NOx, and retains this stolen oxygen, and when the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, it releases the retained vinegar and converts CO, HC. oxidation. Lean t(1
1 and rich l1l11, even if the reference air-fuel ratio deviates from the stoichiometric air-fuel ratio, the above-mentioned 1! ! The ability to retain two elements promotes the reduction of NOx and the oxidation of CO and I (C), resulting in a high conversion efficiency. Figure 1<b) shows the reference air-fuel ratio A when the air-fuel ratio is adjusted to the reference full-fuel ratio at the air-fuel ratio full-circumference number I Hz and the air-fuel ratio fluctuates by ±1.0.
/F window W. It shows. It can be seen from FIG. 1(a) and FIG. This means that if the air-fuel ratio is varied at regular intervals, high purification efficiency can be obtained even if the reference air-fuel ratio deviates somewhat from the stoichiometric air-fuel ratio. -10,000, air-fuel ratio fluctuation S
e. If the number is lowered, that is, if the period of air-fuel ratio fluctuation is lengthened, the three-way catalyst's 1 and seasoning ability will become saturated, and the moth-reducing ability, which is based on the oxygen retention function, will decrease. Figure 1(c) clearly shows this, as the purification efficiency of the catalyst decreases. In Fig. 1 (C), the vertical axis R indicates the total GJ conversion efficiency, and the horizontal axis F (indicates the increased excitation frequency of the fuel ratio IcXt-. Also, when the fluctuation of the air-fuel ratio +1) is decreased, the air-fuel ratio is shifted to the rich side. The width of the window becomes narrower because it disappears due to fluctuations in lean + U and ++.Therefore, in order to widen the width of the window, it is necessary to know that there is an optimal period of fluctuation of the fuel ratio and a period of fluctuation.

As mentioned above, if the period and frequency of fluctuation of the air-fuel ratio relative to the reference total fuel ratio are appropriately selected, the window becomes wider, and therefore high purification efficiency can be obtained even if the reference air-fuel ratio varies slightly relative to the stoichiometric air-fuel ratio. This means that by using a narrow fuel supply system during fluctuations in the reference air-fuel ratio, high purification efficiency can be obtained without using feedback control based on the output signal of the oxygen 1 degree detector. ing.

If a fuel injection valve is used as a non-B aviation fuel supply system, fluctuations in the standard air-fuel ratio can be made easier, but since the fuel injection device is expensive, there is a problem in that the manufacturing cost of the engine increases. Furthermore, if the air-fuel ratio supplied to the engine cylinders is varied in this way, combustion will fluctuate periodically even if the fluctuations are small, resulting in problems such as deterioration of vehicle drivability.0 Non-invention To provide an exhaust gas purification device that can secure high exhaust gas removal efficiency without changing the air-fuel ratio supplied to the engine cylinder without using it once on the detector. There is 0 Below, please refer to the attached drawings and see the details.
, reveal.

Referring to FIG. 2, 1 indicates an intake manifold, 2 indicates an f'Jf venturi carburetor mounted on the intake manifold 1, 3 indicates an exhaust manifold, and 4 indicates a catalytic converter. A three-way monolithic catalyst 5 is disposed inside the OT51 variable venturi type carburetor 2, which includes a carburetor) 1 housing 6, an intake passage 7 extending vertically within the intake passage 6, and an intake passage 7 extending vertically within the intake passage 7. A suction piston 8 that moves laterally, a needle 9 attached to the distal end surface of the suction piston 8, a spacer 10 fixed on the inner wall surface of the intake passage 7 facing the distal end surface vc of the suction piston 3, and a suction piston 8. A throttle valve 11 provided in the intake passage 7 downstream of the piston 8 and a float chamber 12
A bench area 13 is formed between the tip surface of the suction piston 8 and the spacer 10.A casing 14 with a cylindrical inner wall is attached to the carburetor housing 6. A guide sleeve 15 is mounted inside the housing 14 in the axial direction of the casing 14 . There are many balls 16 inside the guide sleeve 15! The bearing 17 which has been fitted is inserted, and the outer end of the guide sleeve 15 is closed by a blind cover 18. - A guide rod 19 is fixed to the suction piston 8, and the guide rod 19 is inserted into the bearing 17 so as to be movable in the direction of the guide rod 19. Since the suction piston 8 is thus supported by the casing 14 via the bearing 17, the suction piston 8 can move smoothly in its axial direction. The circular portion of the casing 14 is divided into a negative pressure chamber 20 and an atmospheric pressure chamber 21 by the suction piston 8, and a compression spring 22 is inserted into the negative pressure chamber 20 to constantly press the suction piston 8 toward the venturi portion 13. be done. The negative pressure chamber 20 is connected to the bench area 13 through a suction hole 23 formed in the suction piston 8, and the large pressure chamber 21 is connected to the suction piston 8 through an air hole 24 formed in the carburetor housing 6. A fuel passage 25 is formed in the carburetor housing 6 and extends in the axial direction of the needle 9 so that the needle 9 can enter therein. A fuel passage 25 upstream of the fuel jet 26 in which the jet 26 is provided is connected to the float chamber 12 via a fuel pipe 27 extending downward, and the fuel in the float chamber 12 is transferred to the fuel pipe 2.
7 into the combustion passage 25,
A cylindrical nozzle 28 is fixed to the spacer 10 and is arranged coaxially with the fuel cavity 1@25. This nozzle 28
protrudes from the inner wall surface of the spacer 10 to the pentagonal portion 13V'3, and the upper half of the tip of the nozzle 28 further protrudes from the lower half toward the piston 8. The needle 9 extends through the nozzle 28 and the metering jet 26, and the fuel passes through the nozzle 28 after being dtM by the annular gap formed between the needle 9 and the metering jet 26.
From the intake vent W! 17.

As shown in FIG. 2, a raised wall 29 that projects horizontally into the intake passage 7 is formed at the end of the spacer 10, and the flow rate is controlled between this raised wall 29 and the tip of the suction piston 8. will be carried out. When engine operation starts, air flows downward through the intake passage 717'3.
At this time, the pregnant airflow is narrowed between the suction piston 8 and the raised wall 29, so negative pressure is generated in the venturi 813, and this negative pressure flows through the suction hole 23 into the negative pressure chamber 20.
guided within. The suction piston 8 is arranged so that the pressure difference between the negative pressure chamber 20 and the atmospheric pressure chamber 21 becomes a substantially constant pressure determined by the spring force of the compression spring 22.
Referring to FIGS. 3 and 4, the tip surface of the suction piston located upstream of the needle 9 is entirely separated from the mounting surface 30 of the needle 9. A concave m31 is formed on the distal end surface of the suction piston, which protrudes toward the distal end of the needle 9, and extends in the axial direction of the intake passage 7. Upstream 11 of this cave 31! The Q end @31a has a U-shaped cross section and is equipped with a needle! m1fii3
It is located closer to the tip of the needle 9 than 0,
Only a few left! ! The J groove portion 31b extends approximately straight from the upstream end 31a to the needle attachment end surface 30. Historically, the cross-sectional shape of the suction piston tip surface located above the needle 9 a 110 is V-shaped, expanding from the concave groove 31 toward the bench area 13. Therefore, this suction piston tip surface portion is A pair of inclined wall surfaces 11332a that are inclined toward the groove 31.

M 32b.

As can be seen from FIG. 3, when the amount of intake air is small, the raised wall 29, the inclined wall portion 32a T 32 b#, and the concave upstream end 31a form an isosceles triangular intake air control constriction part K. It is formed.

By forming the intake air control throttle in this way, the lift ttt of the suction piston 8 is directed north toward the opening surface of the intake air control throttle 98. It will increase smoothly according to the size of the stomach. Furthermore, since the suction piston 8 is supported by a bearing 17, it moves with good response to changes in the desired intake air, and thus the suction piston 8 changes the desired intake air amount when the intake air quantity increases. Moves smoothly and responsively to increases. As a result, even when the amount of suction nitrogen changes rapidly, such as during power supply operation, the lift of the suction piston 8 increases the amount of suction from the nozzle 28 in proportion to the amount of air. The amount of fuel supplied will always be proportional to the amount of suction. Furthermore, as can be seen from Fig. 3, when the intake air flow is low, the intake spirit is sucked and circulated through the center of the first passage 7, and as a result, the fuel supplied from the nozzle 28 is immediately irradiated along with the intake air flow. Since the fuel is supplied into the engine cylinder, even when the desired intake air amount is small, the fuel supplied from the nozzle 28 is uniformly supplied into the engine cylinder. Therefore, even if the amount of intake air suddenly increases as during acceleration, the 1 ton of fuel supplied from the nozzle 28 is 1 ton compared to the desired intake air, as described above, and the amount of fuel supplied from the nozzle 28 is Since the fuel 'i:) is supplied into the engine cylinder at the new position, the air-fuel ratio of the air-fuel mixture supplied into the intertidal cylinder remains almost constant even if the intake air suddenly changes to +lJk. Ru. Also, the suction piston 8 is l1l
Since the engine temperature is supported by the suction piston 17, the suction piston 8 responds to changes in forced air pressure independently of the engine temperature. Can move easily. In this way, when the variable venturi type carburetor 2 shown in FIG. 2 is used, the air-fuel ratio of the air-fuel mixture supplied into the engine cylinders can be kept at a nearly constant value, for example, regardless of the engine operating condition or the engine temperature. It is possible to maintain the stoichiometric air-fuel ratio.

2, an annular air chamber 33 is formed for the 9th part of the d tenth jet 26, and an even number of air bleed holes 34 communicating with this annular part 2 chamber 33 are connected to the ij'izi jet 26. Formed on the inner peripheral wall of the hemostasis. The monkey-like moth trap 33 is connected to the intake passage 7 upstream of the raised wall 29 via an air bleed passage 342 and an air bleed pipe 35, and this air bleed passage 34 is connected to The first solenoid valve 40 is formed with a valve boat 36 that can be controlled by fl.
A valve body 41 that absorbs liquid, a movable plunger R42 connected to the valve body 41, and a solenoid 43 for completely suctioning the movable plunger 42 are assembled.
is continued for a long time by the solenoid and the ringing circuit 70. When the solenoid 43 is deenergized, the valve body 41 closes the valve boat 3.
6 is closed and the solenoid 43 is energized, the valve body 41 fully opens the valve boat 36.
, the dimensions of the air bleed engine 35 and the valve boat 36 are such that when the valve boat 36 is connected, the air-fuel mixture supplied into the engine cylinder has an air-fuel ratio of about 14.0.
The air-fuel mixture is set so that when the valve boat 36 is fully opened, the air-fuel mixture supplied into the engine cylinder becomes a 416 air-fuel mixture. Supply ii+ll 1i11I busy secondary debt supply 1ii111 wholesale 50 is I! 21 = J cut, secondary air 811: 'ti control P2 in intake manifold 1
A second [41: both valves 51 that control the operation of the valve 51] are attached. A secondary air supply control valve 50 (also has a negative pressure chamber 53 and an atmospheric pressure chamber 54 separated by a diaphragm 52), and a compression spring 55 for pressing a diaphragm is inserted into the negative IL chamber 53.Popular H- , an inner wall 5G protruding toward the inner diaphragm 52 of the chamber 5 is fixedly disposed, and the opening and closing of the hollow tube 56 is controlled by a valve body 58 fixed to the diaphragm 52. A valve boat 57 is formed.This valve boat 57 is connected to the exhaust manifold 31 via a check valve 592 and a secondary air supply hole 60 that allow flow only from the valve boat 57 to the exhaust manifold 3. Guko] 1
1 hit is concluded. Therefore, the valve boat 57A and the secondary air supply hole 60 form the entire secondary air supply passage.On the other hand, the second intensifier valve 51 has a valve chamber 62 and a valve located within the valve chamber 62. A negative pressure boat 64 having a south entrance into the P chamber 62 and connected to the exhaust manifold 11, and a valve chamber 62.
A popular boat 65 that opens inward and communicates with the atmosphere, an rEf moving plunger 66 connected to a valve body 63, a movable plunger 66f: a solenoid 67 for suction, and a negative pressure boat 64 and The opening and closing of the atmospheric boat 65 is controlled by the valve body 63v. The valve chamber 62 is connected to the negative pressure chamber 53 of the secondary air supply control valve 50 via a conduit 68, and the solenoid 67 is connected to a solenoid drive circuit 70. The solenoid drive circuit 70 includes a pulse generator 71 that generates a rectangular pulse with a frequency of 1Hz to 2Hz as shown in FIG. A first power amplification device comprising an AND gate 73 that is manually powered, a first power amplification device 74 connected to the output terminal of the AND gate 73, and a second power amplification device 75. The output terminal of the device 74 is connected to the solenoid 67, and the output terminal of the second ′-th power amplifier 75 is connected to the solenoid 43. The input terminal of the inverter 72 is connected to a negative pressure switch 80. The negative pressure switch 80 is completely equipped with a negative pressure chamber 82 and an atmospheric pressure chamber 83 separated by a diaphragm 81. Fixed contact 84 connected to input terminal 72
and a power source (not shown) fixed to the diaphragm 81.
A movable contact 85 connected to is arranged. The riverbank 82 is connected to a negative pressure boat 88 via a negative pressure delay valve sa>yohi negative pressure conduit 87. The negative pressure delay valve 86 is a check valve 8 that can flow only from the intake passage 7 into the negative pressure chamber 82.
The check valve 89 and the throttle valve 90 are arranged in parallel. Further, as shown in FIG. 2, the negative pressure boat 88 opens into the intake passage 7 upstream of the throttle valve 11 when the throttle valve 11 is in the idle link position, and when the throttle valve 11 opens, the negative pressure boat 88 opens into the intake passage 7 upstream of the throttle valve 11 when the throttle valve 11 is in the idle link position. It opens into the intake passage 7.

As shown in FIG. 2, when the throttle valve 11 is in the idle link position, the inside of the negative pressure chamber 82 of the negative pressure switch 8° is at almost atmospheric pressure, and therefore the diaphragm 81 moves to the right at this time. Therefore, the movable contact 85 and the fixed contact 84 are in a non-contact state. That's how it happened at this time. Since the noid 43 is deenergized, the valve body 41 shoulders the valve boat 36 by 1-4 degrees, and therefore, the rich mixture 2 with a desirable fuel ratio of about 14.0 is supplied into the engine cylinder. At this time, the output of the inverter 72 'Ichikawa' becomes a high level, so the output of the AND gate 73 'Ichikawa' becomes the pulse generator 7.
1 becomes a high level every time a pulse is generated, and thus the solenoid 67 is driven and controlled by the output signal of the pulse generator 67. Next, when the throttle valve 11 is opened to accelerate, a large negative pressure is applied to the negative pressure boat 88, but since the check valve 89 is closed, the negative pressure in the negative pressure chamber 82 is maintained at a small state. Thus, the movable contact 85 is maintained out of contact with the fixed contact 84. Then, some time after the acceleration operation starts, the negative pressure in the negative pressure chamber 82 becomes large, so the diaphragm 81 moves to the left, and as a result, fj
The J moving contact 85 and the fixed contact 84 come into contact. At this time, since the solenoid 43 is energized, the valve body 41
ffi is fully opened, and thus a lean air-fuel mixture is supplied into the engine cylinder. Furthermore, at this time, since the output voltage of the inverter 72 is at a low level, the output pressure of the AND gate 73 is maintained at a low level, and thus the solenoid 67 continues to be deenergized. In this way, after a while after starting acceleration operation,
That is, when the steady operating state is reached, the solenoid 43 is energized,
It can be seen that solenoid 67 is deenergized.

As described above, during idle link operation and 111 speed operation, a rich air-fuel mixture is supplied into the engine cylinder, and the solenoid 67 is driven and controlled by the output signal of the pulse generator b71. The valve body 63 completely closes the western valve boat 64 and opens the atmospheric boat 65, and the pulse generator 71
When P generates a pulse, the solenoid 67 is energized and P
The body 64 moves to the right, thereby causing the valve body 63 to open the negative pressure boat 64r and bend the atmospheric boat 65 completely fMj. Therefore, the negative pressure boat 64 and the atmospheric boat 65 are 11
(The opening and closing operations are repeated at a frequency of 2 Hz, and the secondary air 1 (monthly pressure supply 53 of the supply control valve 50
(When negative pressure is applied to the negative pressure chamber 53, which is alternately occupied by negative pressure or atmospheric pressure at a frequency from z to 2 Hz, the valve body 58 is turned off.) 57 f At this time, secondary air is sucked into the exhaust manifold 3 from the air supply hole 60 due to the negative pressure generated in the exhaust manifold 3 due to the exhaust pulsation. Therefore, as mentioned above, when the inside of the negative pressure seedling 53 reaches atmospheric pressure at a frequency of 1 r (z to 2H7) or when the valve body 58 is used, the valve body 58
f 1 ) opens with a frequency of 1z to 2klz,
In this way, secondary air is intermittently supplied into the exhaust manifold 3 at a frequency of I Hz to 2 Hz.When the secondary air is intermittently supplied into the air manifold 3, the exhaust C-stage elements in the exhaust gas in manifold 3? 4
′! The temperature will fluctuate periodically, thus causing the air-fuel ratio to fluctuate. Note that the term air-fuel ratio is used here in a slightly different meaning from its usual meaning, and this air-fuel ratio is defined as the total air B supplied into the working gas passage of the three-way catalytic converter 4; The ratio between (+the sum of the energy and the secondary energy) and the total amount of energy is 39°. It does not matter whether this 7-station oxygen is due to the intake air supplied to the intake system or the secondary air supplied to the exhaust system. Manifold 3
If the air-fuel ratio is made to fluctuate throughout the entire period by varying the flow of secondary air supplied within the window W, the average value of the fuel ratio will be the window W in Figure 1(b). If maintained within the range, the tube purification efficiency can be obtained. The implementation world 1 shown in Figure 2
The dimensions of the valve boat 57 and the secondary air supply hole 60 are such that when the valve body 58 of the diaphragm 52 opens and closes the valve boat 57 all the time, the average value of the air-fuel ratio A/F is ↓ 5 (
b) As shown in the figure, the air-fuel ratio is almost buried, and the fluctuation of the air-fuel ratio is approximately ±0.2 to ±1 compared to the stoichiometric air-fuel ratio.
．． It is set to be 0. The reason why the average value of the air-fuel ratio A/F can be maintained at almost the stoichiometric air-fuel ratio by repeating the simple opening and closing operations of the valve body 58 is because the air-fuel ratio of the air-fuel mixture formed in the carburetor 2 is maintained constant. This is because Therefore, the air-fuel ratio is 11 regardless of the engine operating condition.
(From Z to 21 (at the frequency of z, the air-fuel ratio is approximately the same as the stoichiometric air-fuel ratio, and is varied in the range of ±0.2 to ±1.0, and the average of this air-fuel ratio is 1@ as shown in Figure 1(b). Since it is maintained within the window 1V0, high purification efficiency can be obtained by using the oxygen retention function of the three-way monolith catalyst 5.
10,000, during steady operation = +1 As mentioned above, solenoid 43
Since the valve boat 36 is energized, the valve boat 36 is fully opened, and at this time (a lean air-fuel mixture is supplied into the α cylinder).
Since a lean air-fuel mixture is supplied during steady-state abuse, which is often performed, the fuel consumption rate can be improved. However, when a lean mixture is supplied like this, unburned HC, C
The amount of CO emitted is small, and therefore, if secondary air is supplied in such a case, the exothermic effect due to the oxidation reaction of unburned UC and CO cannot be expected, but rather the cooling effect due to secondary air relief becomes greater. m: This causes a problem that the temperature of the medium 5 falls below the activation temperature. Therefore, in the present invention, during steady operation, the solenoid 67 is deenergized and the valve port 57 of the secondary steam supply control valve 50 is closed. Thus, according to the present invention, the temperature of the catalyst 5 is prevented from decreasing by stopping the supply of secondary air. and an expensive 'electronic IF! for fuel ratio control! Since the low-cost carburetor can effectively purify the gas without using an I control unit, the overall manufacturing cost of the exhaust sludge converter can be greatly reduced. Since the air-fuel ratio of the air-fuel mixture supplied into the cylinder is maintained constant, combustion fluctuations do not occur, thus ensuring smooth engine operation.Also, during steady driving, the air-fuel ratio is kept constant. By supplying the three-way catalyst, the fuel consumption rate can be completely improved, and in this case, by stopping the supply of secondary air (V), the temperature of the three-way catalyst is 75 ≦ active temperature, and the temperature is prevented from decreasing below. be able to.

[Brief explanation of drawings]

Figure 1 shows the exhaust gas purification efficiency of 1%1. Figure 2 is a side sectional view of the engine intake and exhaust system.
4 is a side sectional view of the SAXI-ONE piston, and FIG. 5 is a diagram showing the air-fuel ratio movement. Carburetor, 8... Succincone piston) 9... Needle, 25... Fuel IP+ passage, 28... Nozzle, 50... Secondary desired air supply control valve, 51... Solenoid valve , 60... Secondary air supply hole, 80... Negative pressure switch, 86... Negative pressure delay valve. Patent applicant Toyota Motor Corporation Patent application agent PuP Attorney Akira Yoiki Patent attorney Kazuyuki Nishi Patent attorney Kyosuke Nakayama Patent attorney Akira Yamaguchi

Claims (1)

[Claims] In an internal combustion engine equipped with a fuel supply device capable of selectively supplying a rich mixture or a lean mixture within the engine cylinder, and a three-way catalytic converter installed in the coal exhaust passage.1. A steady-state operation detector capable of detecting the steady-state operating state of the engine is connected to the fuel supply system, and a lean mixture is supplied to the engine cylinders during steady operation, and a lean mixture is supplied to all engine cylinders during non-steady operation. A solenoid valve responsive to the steady operation detector is provided in the secondary air supply oil passage by connecting the secondary air supply passage to the secondary air supply passage upstream of the three-way fusion converter. During steady operation, the secondary air supply passage is closed by the solenoid valve, and when other than steady operation, the secondary air supply passage is closed by the tunnel.
H? , the solenoid valve is opened and closed at a constant frequency of 2 Hz. When the fecal air supply curve is periodically opened at 1, 1-], the air-fuel ratio varies from about 0.2 to ±1 from the average value.
．． The exhaust scum purifying device for an exhaust engine is characterized in that the passage area of the secondary air supply passage is constant so that the air-fuel ratio periodically fluctuates between zero and the average value of the air-fuel ratio becomes approximately the stoichiometric air-fuel ratio.