JPH11223143A - Exhaust emission control method of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a ratio between an oxidation component and an oxidized component in exhaust gas without relying on a signal from an O2 sensor with high accuracy in a theoretical ratio, and remove a harmful component efficiently even when using natural gas etc., as a fuel. SOLUTION: Change in catalytic temperature at the time of changing a ratio between the oxidation component and oxidized component in exhaust gas flowing into catalyst is measured, and an air-fuel ratio is controlled in response to the ratio between the oxidation component and the oxidized component in the exhaust gas when the catalytic temperature becomes maximum. Catalyst outlet temperature becomes maximum in an air-fuel ratio in which the harmful component can be removed most efficiently in the catalyst, accurate control of air-fuel ratio can be carried out without using an O2 sensor, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification method for controlling an atmosphere of exhaust gas flowing into a catalyst for efficiently purifying harmful components in the exhaust gas of an internal combustion engine having an exhaust gas purification catalyst.

[0002]

2. Description of the Related Art Exhaust gas emitted from an automobile engine contains harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO _x ). Environmental pollution. Therefore, in order to purify these harmful components by oxidation or reduction, exhaust gas purifying catalysts such as oxidation catalysts and three-way catalysts are arranged in the exhaust passage.

[0003] In order to make the three-way catalyst function effectively, the air-fuel ratio (A / F =
Air / Fuel) needs to be controlled. Are harmful components in the exhaust in is because if the three-way catalyst CO, the air-fuel ratio range that can offset the HC and NO _x are in the case of gasoline engine, as shown in FIG. 7, it is limited to a very narrow range of near stoichiometric air-fuel ratio Because it is. Therefore, an oxygen sensor is arranged in the exhaust passage upstream of the exhaust purification catalyst, and the amount of fuel or air supplied to the engine is increased or decreased based on a signal from the oxygen sensor, so that the mixing supplied to the engine is increased. The air-fuel ratio of the air is feedback controlled.

That is, when the signal from the oxygen sensor is lower than the determination potential, the ratio of the oxidized component to the oxidized component is in a lean state in which the oxidized component is excessive. It is determined that the ratio of the components is in an excessively rich state for the components to be oxidized. Therefore, in the conventional air-fuel ratio control method, as shown in FIG. 6, first, the fuel supply amount is gradually increased from the lean state. Then, since the potential of the oxygen sensor gradually increases, the fuel supply amount is rapidly reduced to a predetermined value when the potential reaches the determination potential, and thereafter the fuel supply amount is gradually reduced. As a result, the potential of the oxygen sensor draws a mountain-shaped arc curve, gradually decreases after having reached the maximum value. Then, at the time when the potential becomes the determination potential, the fuel supply amount is rapidly increased to a predetermined value, and thereafter, the fuel supply amount is gradually increased.

[0005] By performing such control, the air-fuel ratio of the air-fuel mixture supplied to the engine can be controlled to approximately the stoichiometric air-fuel ratio. For example, the theoretical ratio can be controlled to approximately one to one, and harmful components can be efficiently removed from the catalyst.

[0006]

In the control method described above, the ratio of the oxidized component to the oxidized component in the exhaust gas is detected by the oxygen sensor. This is based on the output voltage of the oxygen sensor at the sudden change point of the oxygen concentration. Alternatively, a phenomenon in which the electric resistance value changes correspondingly is used. The oxygen concentration sudden change point corresponds to a theoretical ratio of the oxidized component to the oxidized component of 1: 1.

Therefore, in the conventional exhaust gas purification method, the air-fuel ratio of the air-fuel mixture supplied to the engine is determined by detecting the current ratio of the oxidized component to the oxidized component in the exhaust gas based on the output signal of the oxygen sensor. It is determined whether there is a shortage (rich) or an excess of oxygen (lean), and the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is controlled in accordance with the result of the determination. Feedback control is performed to control the ratio with the component to be oxidized.

However, a real oxygen sensor has a problem that a signal does not always change suddenly at a stoichiometric ratio of an oxidized component and a component to be oxidized, and the signal reliability is not sufficiently high. As described above, there are two causes of the deviation of the signal of the oxygen sensor from the actual value, that is, a static characteristic and a dynamic characteristic. <Shift due to Static Characteristics> It is known that the sudden change point of the signal of the oxygen sensor shifts depending on the composition of the exhaust gas, for example, including a large amount of hydrogen. Therefore, when an alternative fuel is used or the operating conditions of the engine change significantly, the exhaust gas composition changes greatly, and there is no guarantee that the signal sudden change point is the stoichiometric air-fuel ratio.

As an oxygen sensor, a sensor utilizing an oxygen concentration cell using zirconia is generally used. The electromotive force of this oxygen sensor greatly depends on the temperature, and the higher the temperature, the lower the electromotive force. Therefore, in a control system that does not perform temperature correction, a shift due to a temperature change occurs.
Further, it is known that the oxygen sensor itself changes over time, and the signal sudden change point may be shifted during use.

<Difference Due to Dynamic Characteristics> There is a slight delay from when the atmosphere around the oxygen sensor is actually switched to when the oxygen sensor outputs a signal. It has also been found that the delay time is different between the case where the atmosphere is switched from the rich atmosphere to the lean atmosphere and the case where the atmosphere is switched to the lean atmosphere. Therefore, when the oxygen sensor is used under various fluctuation conditions, it is difficult to determine with high accuracy the ratio between the controlled oxidizing component and the oxidized component of the exhaust gas.

[0011] When further such as natural gas (CNG) was used as the fuel, CO, can be most efficiently purifying HC and NO _x are rich slightly oxygen-deficient than the stoichiometric air-fuel ratio as shown in FIG. 8 It is known to be the side. This is because, since the methane contained in the exhaust is not easily purified in the catalyst, the theoretical ratio is because the NO _x becomes oxygen excess is no longer purified. It is also known that there is a phenomenon that the oxidation reaction of methane is suppressed by oxygen remaining in the exhaust gas. However, the degree of shifting to the rich side varies depending on the use conditions such as the performance of the catalyst, the temperature, and the flow rate of exhaust gas. Therefore, it is difficult to perform optimal control by the above-described control method.

The present invention has been made in view of such circumstances, and it is possible to control the ratio of the oxidized component to the oxidized component in the exhaust gas with high accuracy to the theoretical ratio without relying on the signal from the oxygen sensor. It is an object of the present invention to efficiently remove harmful components even when using natural gas or the like as a fuel.

[0013]

The exhaust gas purifying method for an internal combustion engine according to the present invention which solves the above-mentioned problems is characterized in that at least one of the inside of the exhaust gas purifying catalyst arranged in the exhaust path and the end of the catalyst on the exhaust outlet side. Temperature detecting means for detecting a physical quantity related to the temperature of the exhaust gas, and measuring the change in the physical quantity detected by the temperature detecting means by changing the ratio of the oxidized component and the oxidized component in the exhaust gas flowing into the catalyst, and An object is to control the ratio of the oxidizing component and the oxidized component in the exhaust gas flowing into the catalyst according to the value of the ratio between the oxidized component and the oxidized component in the exhaust gas flowing into the catalyst when the maximum value is reached.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The present inventors have conducted intensive studies on the relationship between the temperature of an exhaust gas purifying catalyst in use and the purification efficiency.
A phenomenon in which the temperature of the catalyst becomes maximum in a state where the catalyst is operating most effectively has been found. The present invention has been made based on this finding. The exhaust gas purifying catalyst promotes an oxidation-reduction reaction as a reaction. More specifically, the oxidizing gases O ₂ and NO _x and the reducing gases H ₂ , CO and HC cancel each other, thereby removing harmful components in the exhaust gas and purifying the exhaust gas.

When the offset is considered in reverse, the condition in which the reaction proceeds most actively on the catalyst makes the catalyst work most effectively. The reaction involving oxygen is an exothermic reaction, and the calorific value in the catalyst is the largest under the conditions that can remove harmful components most efficiently. Therefore, from this viewpoint, the relationship between the air-fuel ratio of the air-fuel mixture and the temperature at the catalyst outlet was investigated with the catalyst-containing gas temperature kept constant. As a result, as shown in FIGS. 9 and 10, in both the case of gasoline fuel and the case of natural gas fuel, the catalyst outlet temperature reaches the maximum value at the air-fuel ratio at which harmful components can be removed most effectively on the catalyst. It became clear.

That is, the exhaust gas purification method of the present invention has the greatest feature in that a physical quantity related to the temperature of at least one of the inside of the exhaust gas purifying catalyst and the end of the catalyst on the exhaust outlet side is used as an index. Temperature is usually used as the physical quantity. However, if the physical quantity is a physical quantity related to at least one of the temperature of the inside of the exhaust gas purification catalyst and the end of the exhaust outlet side, for example, the intake air amount of the engine, the space velocity, the vehicle speed, the catalyst Other indicators, such as inlet temperature, fuel supply, and secondary air flow, can also be used. When temperature is selected as a physical quantity, its relative value is important, not its absolute value.

As the temperature detecting means, a temperature sensor such as a thermocouple or a thermistor is generally used, but a means capable of indirectly detecting the temperature of at least one of the inside of the catalyst and the end of the catalyst on the exhaust outlet side may be used. it can. 2 ° C. in the temperature range where the catalyst is used, ie 0 ° C. to 950 ° C.
It is desirable to be able to detect a temperature difference of less than 3, but at least a device capable of detecting a temperature difference of 3 ° C. in the normal temperature range of 300 to 400 ° C. is required.

Note that it is not always necessary to detect the absolute value of the temperature, and it is only necessary to detect the relative value, that is, the temperature difference. Therefore, the aging of the temperature detecting means itself or the aging of the temperature assurance device is not an obstacle. No. When the temperature is directly detected, the position of the temperature detecting means is at a position where the maximum value of the temperature of the catalyst can be detected, that is, at least one of the inside of the catalyst and the end on the exhaust outlet side of the catalyst.

In the exhaust gas purification method of the present invention, first, the ratio of the oxidized component to the oxidized component in the exhaust gas flowing into the catalyst is changed. This can be easily performed by changing the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine. Then, the temperature detecting means detects a physical quantity related to at least one of the temperature inside the catalyst and the end of the catalyst on the exhaust outlet side at that time. This physical quantity varies with a change in the ratio of the oxidized component to the oxidized component, and has a maximum value when it is changed from rich to lean to rich to lean.

The ratio between the oxidizing component and the oxidized component in the exhaust gas flowing into the catalyst is controlled in accordance with the ratio of the oxidized component and the oxidized component in the exhaust at the maximum value. This may control the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine, or may add oxidizable components such as fuel or air to the exhaust gas upstream of the catalyst. For example, when the average air-fuel ratio of the air-fuel mixture flowing into the internal combustion engine is controlled to be constant, the value of the air-fuel ratio when it takes the maximum value may be increased or decreased to the control value.

This method is desirably performed when the warm-up is completed and the vehicle speed, load, and the like can be regarded as substantially constant. This method does not need to be performed all the time, and may be performed once, for example, after traveling 3000 km, or once every three months. As a result, a defect due to deterioration of the oxygen sensor can be corrected.

[0022]

The present invention will be described below in detail with reference to examples. (Embodiment 1) FIG. 1 shows the configuration of an exhaust gas purifying apparatus for performing the exhaust gas purifying method of the present embodiment. The exhaust purification device includes a fuel supply device 2 that supplies fuel to an engine 1 and a honeycomb-shaped monolithic catalyst 3 disposed in an exhaust passage from the engine 1.
An oxygen sensor 4 disposed upstream of the catalyst 3, a temperature sensor 5 for detecting the temperature of the outlet end face of the catalyst 3, and an air-fuel ratio control for controlling the amount of fuel supplied to the engine 1 by the fuel supply device 2. And a unit 6.

The air-fuel ratio control section 6 comprises a signal processing section 60 and a fuel control circuit 61 as shown in FIG. 2. The signal processing section 60 processes the signal of the oxygen sensor 4 and sends it to the fuel control circuit 61.
The fuel control circuit 61 controls the fuel supply amount of the fuel supply device 2 to the engine 1. The signal processing unit 60 includes an average air-fuel ratio control circuit 66, a determination potential control circuit 62, and a comparison circuit 6.
3. It is composed of a delay time control circuit 64 and a delay circuit 65.

The parameters for determining the average air-fuel ratio include the determination potential used for the oxygen sensor 4, the delay time before the start of the air-fuel ratio control, the air-fuel ratio width to be jumped at the time of control, and the like. The delay time until the start of fuel ratio control is used. Hereinafter, the content of the control at the normal time by the air-fuel ratio control unit 6 will be described. The oxygen sensor 4 is made of a zirconia solid electrolyte, and its electromotive force varies from 0.1 V as shown in FIG.
It fluctuates between 0.9V. When the electromotive force is low, the atmosphere is on the lean side with excess oxygen, and when the electromotive force is high, it indicates that the component to be oxidized is on the rich side with excess oxygen.

To determine the state of the exhaust atmosphere,
Appropriate judgment potential V between 0.3-0.6V _sSet in advance
You. This determination potential V_sIs controlled by the determination potential control circuit 62.
ing. In the comparison circuit 63, the actual oxygen sensor 4
Electromotive force and determination potential V_sAnd FIG. 3 (2).
Thus, the electromotive force of the oxygen sensor 4 is equal to the determination potential V_sLower place
If the exhaust atmosphere is lean, the electromotive force of the oxygen sensor 4 is determined.
Constant potential V_sIf higher, the exhaust atmosphere is determined to be rich
I do. However, the signal itself of the oxygen sensor 4 contains noise.
ing. Therefore, practically, the delay time control circuit 64
Based on the set time, as shown in FIG.
Extension circuit 65 is time T₁And T_TwoJust delayed the decision.

The fuel control circuit 61 controls the fuel supply device 2 based on the signal shown in FIG. 3 (3), and the fuel amount is increased or decreased as shown in FIG. 3 (4). As a result, the average air-fuel ratio is feedback-controlled to be constant, and the ratio of the oxidized component to the oxidized component in the exhaust gas is optimized, so that the purification performance by the catalyst is highly exhibited and the harmful substances in the exhaust gas can be efficiently purified. It becomes possible.

Here, it is assumed that the oxygen sensor 4 has deteriorated and the electromotive force has shifted to a lower level. In this case, the exhaust atmosphere of reality be a determined potential V _s or more from the following electromotive force determination potential V _s is on the lean side, the control precision is significantly reduced. Therefore, in the present embodiment, the temperature sensor 5 is disposed and the following control is performed. That is, in the present embodiment, the control by the oxygen sensor 4 is corrected by the signal of the temperature sensor 5. That is, the oxygen sensor 4
In this case, a disturbance is given to the control parameter (1), and the optimum average air-fuel ratio is determined based on a change in temperature due to the disturbance.

Warm-up is completed based on the cooling water temperature, etc.
At the time of steady running, the pre-programmed FIG. 4 (1)
The average air-fuel ratio fluctuates according to the mountain-like pattern shown in
The catalyst outlet temperature at that time is measured. In other words, the lean → rich delay time T ₁ is continuously changed from 490 ms to 10 ms, and the rich → lean delay time T ₂ is simultaneously changed continuously from 10 ms to 490 ms. 500 total
Set to milliseconds. Lean → Rich delay time T ₁
There average air-fuel ratio at the time of 490 ms is the most lean side, then the average air-fuel ratio when the delay time T ₁ of the lean → rich becomes richer of 10 ms and gradually becomes most rich side. After that, the lean → rich delay time T ₁ is increased by 10 again.
With continuously changing from milliseconds to 490 milliseconds, continuously changing the rich → lean delay time T ₂ from 490 milliseconds to 10 milliseconds.

As described above, the catalyst outlet temperature is continuously measured by the temperature sensor 5 while the average air-fuel ratio is reciprocally changed. The result is as shown in FIG. 4 (2), and two maximum values appear at the catalyst outlet temperature. The average value of the two air-fuel ratios ((a + b) / 2), which is the maximum value, is calculated. The reason why the average value is obtained by reciprocating measurement is to cancel the response delay of the temperature sensor 5. The obtained average value is used as an average air-fuel ratio correction amount, and the determination potential control circuit 62 is controlled so that the determination potential Vs is increased or decreased by an electromotive force of the oxygen sensor 4 corresponding to the correction amount. The calculation of the average air-fuel ratio correction amount is performed by the average air-fuel ratio correction circuit 66.

[0030] Incidentally, a constant delay time T _1, T ₂ is 30 milliseconds, be continuously reciprocally changed between the determination voltage V _s from 0.2V to 0.75 V, the catalyst outlet temperature Two similar The average value of the two air-fuel ratios ((a + b) /
2)) had the same value. That is, the delay time and the determination voltage can be equally used as the control parameters.

The above control is applied to a 4-cylinder 2000 cc gasoline engine, a 4-cylinder 2000 cc LPG engine and a 4-cylinder 2000 cc natural gas engine.
, CO, and NOx purification rates at the time of operation were measured. Also in the case where control was performed only with the oxygen sensor 4 without using the temperature sensor 5, the purification rate was measured in the same manner, and this was used as a conventional example. Table 1 shows the results.

[0032]

[Table 1] From Table 1, it can be seen that by performing control including the temperature of any fuel, the purification rate of HC is improved as compared with the conventional example. Even with effect resulting hard LPG and natural gas catalyst, it can be seen that _the NO _x purification rate not only improves the HC purification rate is improved greatly.

(Embodiment 2) In this embodiment, an example in which an oxygen sensor is not used will be described. FIG. 5 shows a configuration of an exhaust gas purification apparatus that performs the exhaust gas purification method of the present embodiment. The exhaust gas purifying apparatus includes a fuel supply device 2 for supplying fuel to the engine 1.
A honeycomb-shaped monolithic catalyst 3 disposed in an exhaust passage from the engine 1, a temperature sensor 5 for detecting the temperature of the outlet side end face of the catalyst 3, and a fuel supply amount of the fuel supply device 2 to the engine 1. An air-fuel ratio controller 6 for controlling the engine 1
And an air amount sensor 7 for detecting the amount of air supplied to the air conditioner.

In this exhaust gas purification apparatus, the air-fuel ratio control unit 6 calculates the amount of fuel to be supplied to the engine 1 based on the signal of the air amount sensor 7, and the fuel supply unit 2 supplies the fuel based on the result. Supply to engine 1. Since the fuel amount F to be supplied is proportional to the supplied air amount V, F = a · V
(A: constant). If the correction coefficient by temperature is k, then F = kaV.

Although all the driving conditions of the engine can be applied to this equation, in a gasoline engine, a method of intermittently supplying fuel in synchronization with the rotation speed is generally used.
In this case, it is necessary to convert the value of F into the supply amount per rotation. Considering the conversion error and the linearity of the signal of the air amount sensor 7 at this time, the range of the engine condition that satisfies the formula at a certain value of a can be limited, and the accuracy of F can be further improved. For example, rotational speed, if stored as map values of k _i and a _i in the throttle opening, F at any engine condition i = k _i
Ai · V. Here, a _i is a value given in advance.

[0036] Then in the running condition is regarded as substantially steady-state operation, in the condition where V _i is deemed substantially constant,
the value of k _i reciprocally changed from 0.9 to 1.1, by changing the ratio of the oxide component and the oxidizable component in the exhaust gas flowing into the catalyst, a continuous catalyst outlet temperature by the temperature sensor 5 in the same manner as in Example 1 To measure. Then, a maximum value appears at the catalyst outlet temperature in the same manner as in the first embodiment, and k at the time when the maximum value is reached is reached.
Value is obtained.

By using the obtained k value as a correction coefficient, the fuel amount F at a predetermined air amount V is calculated with high accuracy, and the average air-fuel ratio is held at a point where the harmful components in the exhaust gas are minimized. It becomes possible. Note that in order to obtain the true air amount from the signal of the air amount sensor 7, correction based on temperature and humidity is necessary, but this correction is necessarily included in the process of determining the above-described optimum correction coefficient k. I will. Therefore, in normal air-fuel ratio control, one or both of the temperature sensor and the humidity sensor may be unnecessary.

[0038]

According to the method for purifying exhaust gas of an internal combustion engine of the present invention, the ratio between the oxidized component and the oxidized component in the exhaust gas can be controlled with high accuracy to the theoretical ratio without relying on the signal from the oxygen sensor. Further, even when natural gas or the like is used as a fuel, the ratio between the oxidized component and the oxidized component in the exhaust gas can be controlled with high accuracy to the theoretical ratio, and harmful components can be efficiently removed.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a configuration of an exhaust gas purification device used in one embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of an air-fuel ratio control unit of the exhaust purification device used in one embodiment of the present invention.

FIG. 3 is a time chart showing a normal air-fuel ratio control method by the exhaust gas purification device used in one embodiment of the present invention.

FIG. 4 is a graph showing a change in catalyst outlet temperature when an average air-fuel ratio is continuously changed in one embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a configuration of an exhaust gas purification device used in a second embodiment of the present invention.

FIG. 6 is a time chart showing a conventional air-fuel ratio control method.

FIG. 7 is a graph showing a relationship between an air-fuel ratio and a purification rate in a gasoline engine.

FIG. 8 is a graph showing a relationship between an air-fuel ratio and a purification rate in a natural gas engine.

FIG. 9 is a graph showing a relationship between an air-fuel ratio and a catalyst outlet temperature in a gasoline engine.

FIG. 10 is a graph showing a relationship between an air-fuel ratio and a catalyst outlet temperature in a natural gas engine.

[Explanation of symbols]

1: Engine 2: Fuel supply device 3:
Exhaust gas purification catalyst 4: Oxygen sensor 5: Temperature sensor 6:
Air-fuel ratio control section

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Katsunori Yamada 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. (72) Inventor Yoshikazu Murakami 3-chome Hinodai, Hino-shi, Tokyo No. 1 1 Inside Hino Motors, Ltd.

Claims (1)

[Claims] 1. An exhaust purifying catalyst disposed in an exhaust path and a temperature detecting means for detecting a physical quantity related to at least one temperature of an end of an exhaust outlet side of the catalyst. The change in the physical quantity detected by the temperature detecting means by changing the ratio of the oxidizing component to the oxidized component in the sample is measured, and the oxidizing component in the exhaust gas flowing into the catalyst when the physical quantity reaches a maximum value is measured. An exhaust purification method for an internal combustion engine, comprising: controlling a ratio of an oxidizing component and an oxidizing component in exhaust gas flowing into the catalyst according to a value of a ratio of the oxidizing component.