JPH06288235A - Exhaust emission control device of ceramic exhaust pipe for exhaust emission control and engine - Google Patents

Abstract

PURPOSE:To Provide an exhaust emission control device capable of efficiently purifying exhaust gas of an engine by way of using catalytic converter rhodium even when the engine is optionally actuated at an air-fuel ratio which comes to be at the best point of fuel consumption, at an air fuel ratio which comes to be at the best point of motive power performance and others. CONSTITUTION:A ceramic exhaust pipe 1 consisting of oxygen ion conductive solid electrolyte ceramics, an electrode formed on an inner and outer periphery and a porous electrode protective layer and capable of pumping oxygen in the reverse direction of an electric current by ion conduction is installed on an exhaust system of an engine. In accordance with outputs of air fuel ratio sensors 9, 10 to detect gas density in exhaust gas in the upstream and the downstream of the exhaust pipe 1, the electric current flowing in the ceramics is feedback controlled by a control device 16, and the oxygen density in exhaust gas of the engine is controlled with the range to purify it with catalytic converter rhodium. In accordance with output of a temperature sensor 14 to detect temperature of the exhaust pipe 1, a heater 19 is controlled, and temperature of the pipe 1 is controlled in a temperature range of high oxygen pumping efficiency.

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 ceramic exhaust pipe suitable for use in engine exhaust gas purification, and an engine exhaust gas purification device.

[0002]

2. Description of the Related Art FIG. 8 shows an exhaust gas purifying apparatus using a three-way catalyst used in an internal combustion engine of an automobile or the like. In the figure, 6 is an internal combustion engine, 22 is an intake pipe of the internal combustion engine 6, and 8 is a three-way catalyst installed in the exhaust pipe 7. 21
Is a throttle valve installed inside the intake pipe 22, and the intake air amount is adjusted by this throttle valve 21. Reference numeral 23 is a fuel injection valve for supplying fuel to the internal combustion engine 6, and the fuel injection valve 23 is installed in the intake pipe 22.

Reference numeral 20 is an air flow sensor for measuring the amount of air intake of the internal combustion engine 6, and this air flow sensor 20 is installed in an intake pipe 22. 24 is an internal combustion engine 6
Is a rotation speed sensor for detecting the rotation speed of the internal combustion engine 6, 25 is a water temperature sensor for detecting the temperature of the cooling water of the internal combustion engine 6, and these sensors 24, 25 are installed in the internal combustion engine 6. An oxygen sensor 26 detects the oxygen concentration in the exhaust gas, and the oxygen sensor 26 is installed in the exhaust pipe 7. The outputs of these sensors 20, 24-26 are supplied to the control device 27.

The controller 27 includes the above-mentioned sensors 20, 2
Based on the outputs of 4 to 26, the fuel injection amount adapted to the operating state of the internal combustion engine 6 is calculated, an open / close signal is sent to the fuel injection valve 23, and fuel is supplied. The output of the internal combustion engine 6 is adjusted by adjusting the intake air amount to the internal combustion engine 6 by the throttle valve 21. Further, based on the oxygen concentration in the exhaust gas detected by the oxygen sensor 26, the control device 27 performs feedback control so that the fuel supply amount to the internal combustion engine 6 becomes the stoichiometric air-fuel ratio, and combines with the three-way catalyst 8. To purify the exhaust gas.

The three-way catalyst 8 includes carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO) contained in the exhaust gas.
X) is carbon dioxide (CO2), water (H2O), nitrogen (N
In 2), it is a catalyst that is treated by redox reaction and purified at the same time. As shown in FIG. 9, a high purification rate is exhibited in a narrow range called a window near the stoichiometric air-fuel ratio.

[0006]

The conventional exhaust gas purifying apparatus is constructed as described above, and controls the amount of fuel supplied to the internal combustion engine 6 within the window range and purifies the exhaust gas by the three-way catalyst 8. However, there is a problem in that the internal combustion engine cannot be used without deteriorating the exhaust gas purifying property at the air-fuel ratio, which is the best point of fuel efficiency and the best point of power performance. A problem when operating the internal combustion engine 6 at a lean air-fuel ratio, which is the best point of fuel economy, is an increase in nitrogen oxides in the exhaust gas. Further, a problem when operating the internal combustion engine 6 at the rich air-fuel ratio, which is the best point of power performance, is an increase in the concentration of carbon monoxide and hydrocarbons in the exhaust gas.

When the engine is operated regardless of the air-fuel ratio,
Various concentrations of the exhaust gas of the engine change depending on the air-fuel ratio, and the exhaust gas cannot be purified by the three-way catalyst that works efficiently in the window range. The cause is that the oxygen concentration in the exhaust gas is high at a lean air-fuel ratio, so it is difficult to reduce nitrogen oxides and return them to nitrogen gas and oxygen. In addition, since there is almost no oxygen in the exhaust gas at the rich air-fuel ratio, it is not possible to oxidize carbon monoxide or hydrocarbons in the exhaust gas into carbon dioxide and water.

Therefore, conventionally, when treating exhaust gas of an engine with a rich air-fuel ratio, secondary air is introduced into the exhaust gas and an oxidation catalyst is used to treat carbon monoxide and hydrocarbons in the exhaust gas. However, this method has a problem in that secondary air introducing means must be newly provided, and it is difficult to control mechanically according to the operating state of the engine. Also, this method does not provide a solution to the problem with a lean air-fuel ratio with excess air.

The present invention has been made in order to solve such problems, and even if the engine is arbitrarily operated with an air-fuel ratio which is the best point of fuel economy or an air-fuel ratio which is the best point of power performance, An object of the present invention is to provide an exhaust gas purifying ceramic exhaust pipe capable of efficiently purifying engine exhaust gas by utilizing a three-way catalyst, and an engine exhaust gas purifying device.

[0010]

An exhaust gas purifying ceramic exhaust pipe according to a first aspect of the present invention is attached to an exhaust system of an engine, and is an oxygen ion conductive solid electrolyte ceramic, which functions as an oxygen pump. A first electrode and a second electrode respectively formed on the inner circumference and the outer circumference of the solid electrolyte ceramic; and a porous electrode protective layer formed on the inner circumference of the solid electrolyte ceramic so as to cover at least the first electrode. Be prepared.

An exhaust gas purifying apparatus for an engine according to a second aspect of the present invention is an oxygen ion conductive solid electrolyte ceramic which is attached to an exhaust system of an engine and functions as an oxygen pump, and inner and outer circumferences of the solid electrolyte ceramic. An exhaust gas purifying ceramic exhaust pipe having a first electrode and a second electrode formed respectively on the first and second electrodes, and a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover at least the first electrode; First and second gas concentration detecting means for detecting the oxygen concentration in the exhaust gas upstream and downstream of the ceramic exhaust pipe, and the ceramic exhaust pipe according to the outputs of the first and second gas concentration detecting means. And a control means for controlling the current flowing through the solid electrolyte ceramic.

According to a third aspect of the present invention, there is provided an exhaust gas purifying apparatus for an engine, which is attached to an exhaust system of the engine and which functions as an oxygen pump. The oxygen ion conductive solid electrolyte ceramics, and the inner and outer circumferences of the solid electrolyte ceramics. An exhaust gas purifying ceramic exhaust pipe having a first electrode and a second electrode formed respectively on the first and second electrodes, and a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover at least the first electrode; First and second gas concentration detecting means for detecting oxygen concentration in exhaust gas upstream and downstream of the ceramic exhaust pipe, and first temperature detection for detecting temperature of exhaust gas at least upstream of the ceramic exhaust pipe. Means, second temperature detecting means for detecting the temperature of the ceramic exhaust pipe, and outputs of the first and second gas concentration detecting means. Flip and controls the current flowing in the solid electrolyte ceramic of the ceramic exhaust pipes,
And a control means for controlling the temperature of the ceramic exhaust pipe based on the outputs of the first and second temperature detection means.

An exhaust gas purifying apparatus for an engine according to a fourth aspect of the present invention is provided with a three-way catalyst which is provided downstream of the exhaust gas purifying ceramic exhaust pipe or is formed integrally therewith. .

[0014]

According to the first aspect of the present invention, when an electric current flows through the solid electrolyte ceramics, oxygen is pumped by ionic conduction in the opposite direction to the electric current. Even if the engine is operated arbitrarily with the air-fuel ratio (lean air-fuel ratio) where the best point is, or the air-fuel ratio (rich air-fuel ratio) where the power performance is the best point, the oxygen concentration in the exhaust gas is purified with a three-way catalyst. The window range can be made possible, and the exhaust gas can be efficiently purified by utilizing the three-way catalyst.

According to the second aspect of the invention, the oxygen concentration of the exhaust gas downstream of the ceramic exhaust pipe is feedback-controlled to a predetermined value, and the air-fuel ratio and power performance at which fuel consumption is the best are the best. Even if the engine is arbitrarily operated with the air-fuel ratio, etc., the oxygen concentration in the exhaust gas can be set within the window range where it can be purified by the three-way catalyst, and the three-way catalyst can be used to efficiently purify the exhaust gas. Is possible.

According to the third aspect of the invention, the oxygen concentration of the exhaust gas downstream of the ceramic exhaust pipe is feedback-controlled to a predetermined value, and the air-fuel ratio and the power performance at which the fuel efficiency is the best are the best points. Even if the engine is arbitrarily operated with the air-fuel ratio, etc., the oxygen concentration in the exhaust gas can be set within the window range where it can be purified by the three-way catalyst, and the three-way catalyst can be used to efficiently purify the exhaust gas. Is possible. Further, since the temperature of the exhaust gas at least upstream of the ceramic exhaust pipe and the temperature of the ceramic exhaust pipe are detected, and the temperature of the ceramic exhaust pipe is controlled based on the detection output,
It is possible to control the temperature range with high efficiency as an oxygen pump.

According to the fourth aspect of the invention, since the three-way catalyst is provided downstream of the exhaust gas purifying ceramic exhaust pipe or integrally formed therewith, the exhaust gas of the engine is ternary. It is possible to control the gas concentration range that can be purified by the catalyst.

[0018]

【Example】

Example 1. An embodiment of an exhaust gas purifying ceramic exhaust pipe and an exhaust gas purifying apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing an exhaust gas purification apparatus of this example. In FIG. 1, parts corresponding to those in FIG. 8 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the figure, 1 is a ceramic exhaust pipe for purifying exhaust gas disposed in the middle of the exhaust pipe 7, 16 is a ceramic exhaust pipe control device, 17 is an air introduction port for introducing the atmosphere into the ceramic exhaust pipe 1, and 18 Is an air outlet for releasing oxygen pumped out by the ceramic exhaust pipe 1 to the atmosphere, 19 is a heater for heating the ceramic exhaust pipe 1, and 14 is a temperature sensor for detecting the temperature of the ceramic exhaust pipe 1. The heating operation of the heater 19 is controlled by the controller 16, and the output of the temperature sensor 14 is used as the temperature information of the ceramic exhaust pipe 1 to control the controller 16.
Supply to.

Reference numeral 9 denotes an air-fuel ratio sensor arranged in the exhaust pipe 7 between the internal combustion engine 6 and the ceramic exhaust pipe 1. The output of the air-fuel ratio sensor 9 is supplied to the air-fuel ratio sensor control unit 11 to control the same. The output of the device 11 is sent to the control device 16 by the exhaust pipe 1
Exhaust gas before being treated by (hereinafter, "exhaust gas before treatment")
That is) gas concentration information. Reference numeral 13 denotes a temperature sensor arranged in the exhaust pipe 7 between the internal combustion engine 6 and the ceramic exhaust pipe 1, and supplies the output of the temperature sensor 13 to the control device 16 as the temperature information of the untreated exhaust gas.

Reference numeral 10 denotes an air-fuel ratio sensor arranged in the exhaust pipe 7 between the ceramic exhaust pipe 1 and the three-way catalyst 8. The output of the air-fuel ratio sensor 10 is supplied to the air-fuel ratio sensor control device 12, The output of the control device 12 is supplied to the control device 16 as gas concentration information of the exhaust gas after being processed by the exhaust pipe 1 (hereinafter, referred to as “processed exhaust gas”). Reference numeral 15 is a temperature sensor arranged in the exhaust pipe 7 between the ceramic exhaust pipe 1 and the three-way catalyst 8. The output of the temperature sensor 15 is supplied to the control device 16 as the temperature information of the processed exhaust gas.

The control device 16 controls the heating operation of the heater 19 based on the various information supplied as described above so that the temperature of the ceramic exhaust pipe 1 works efficiently as an oxygen pump, and the pre-treatment exhaust gas is discharged. The current flowing through the ceramic exhaust pipe 1 is controlled according to the gas concentration of the gas, and oxygen is pumped into the pre-treatment exhaust gas by the oxygen pump action described later.
It is pumped out and treated so that the exhaust gas becomes a window state after treatment. The treated exhaust gas treated in the window state is purified by the redox reaction by the three-way catalyst 8,
Carbon monoxide, hydrocarbons and nitrogen oxides contained in the exhaust gas after treatment are treated into carbon dioxide, water and nitrogen.

FIG. 2 is a sectional structural view showing the basic structure of the ceramic exhaust pipe 1. In the figure, reference numeral 5 is a tubular oxygen ion conductive solid electrolyte ceramic which constitutes the ceramic exhaust pipe 1 as a whole. This ceramics 5 is mixed with zirconia (ZrO 2) which is an oxygen ion conductive solid electrolyte.
2) is composed mainly of partially stabilized zirconia (PSZ) partially stabilized with 7% yttria (Y2O3).

Reference numeral 2 is a cylindrical atmosphere-side electrode formed on a part of the outer periphery of the ceramic 5. Reference numeral 3 is a cylindrical exhaust gas-side electrode that is formed on the inner circumference of the ceramic 5 so as to face the atmosphere-side electrode 2 described above and is in direct contact with the exhaust gas of the engine.
These electrodes 2 and 3 are made of porous platinum (Pt). Reference numeral 4 denotes a porous electrode protection layer formed inside the ceramic 5 so as to cover the electrode 3. This protective layer 4
Is composed of a porous ceramic containing alumina (Al2O3) as a main component. The ceramic exhaust pipe 1 is formed by integrally firing the above-described ceramics 5, electrodes 2, 3 and protective layer 4 into a tubular shape as shown in the figure.

Next, the oxygen pump action of the ceramic exhaust pipe 1 will be described. As shown in FIG. 3, by applying a voltage from the outside between the electrodes 2 and 3 at a high temperature, the ceramics 5 will pass through the pores of the ceramics 5 and
Oxygen ions can be moved by ionic conduction. This phenomenon is called oxygen pumping action, and the current flowing through the ceramic 5 is equal to the total amount of charges of oxygen ions transported. Here, when the oxygen partial pressure on the oxygen pumping side is Pa, the oxygen partial pressure on the oxygen pumping side is Pc, and the free energies due to these are Ga and Gc, the difference in free energy due to the partial pressure ratio is (1 ) It becomes like a formula. In equation (1), T is the absolute temperature (K),
R is a gas constant (8.31 J / mol, K), and Ln is a natural logarithm.

Ga-Gc = R × T × Ln (Pa / Pc) (1)

When this difference is converted into electric energy,
From the Nernst equation, the relation of equation (2) is established. (2)
In the equation, E is the electromotive force (V) due to the oxygen concentration cell action, and F is the Faraday constant (9.65 × 10 4 C /
mol).

4E · F = R · T · Ln (Pa / Pc) (2)

Further, according to Faraday's law, if the number of moles of oxygen moving per unit time is m, then equation (3) is obtained. In the equation (3), Ip is the pump current (A).

M = Ip / 4F (3)

In the ceramic 5, since there is an internal resistance r (Ω) of a portion which functions as an oxygen pump, E does not directly become the terminal voltage. Applied power (Vp × Ip) is the sum of free energy increase and internal resistance consumption energy (r × Ip2) as the number of moles moves per unit time.
Therefore, equation (4) is obtained from equations (1) and (2). In the equation (4), Vp is a pump applied voltage.

(Ip / 4F) R · T · Ln (Pa / Pc) + r · Ip 2 = Vp · Ip (4)

By rewriting the equation (4), the equation (5) is obtained.

Vp = r · Ip + (R · T / 4F) · Ln (Pa / Pc) = r · Ip + E = r · 4F · m + E (5)

From the equation (5), according to the applied voltage Vp,
It can be seen that ionic conduction causes oxygen ions to be pumped in the opposite direction to the current.

Example 2. FIG. 4 shows a ceramic exhaust pipe 1 for purifying exhaust gas, which is formed in a star shape. With such a shape, the exhaust gas control efficiency can be improved by increasing the effective area of the oxygen pump portion and the exhaust gas contact area without increasing the exhaust gas exhaust resistance. Note that, in FIG. 4, portions corresponding to those in FIG. 2 are denoted by the same reference numerals.

Example 3. FIG. 5 shows a ceramic exhaust pipe 1 for purifying exhaust gas formed in a corrugated tubular shape. With such a shape, the exhaust gas control efficiency can be improved by increasing the effective area of the oxygen pump portion and the exhaust gas contact area without increasing the exhaust gas discharge resistance, as in the example of FIG. 4.
Note that, in FIG. 5, portions corresponding to those in FIG. 2 are denoted by the same reference numerals.

Example 4. FIG. 6 shows a ceramic exhaust pipe 1 for purifying exhaust gas in a coil shape. With such a shape, the effective area of the oxygen pump portion and the exhaust gas contact area can be increased and the exhaust gas control efficiency can be improved, as in the example of FIGS. 4 and 5.

Example 5. FIG. 7 shows an example having a basic structure in which the ceramic exhaust pipe 1 for purifying exhaust gas and the three-way catalyst are integrated. In this example, a three-way catalyst pellet 28 for purifying exhaust gas is filled in the ceramic exhaust pipe 1 shown in FIG. 2 from the middle of the exhaust gas passage side to the downstream side where the exhaust gas flows in the air exhaust direction. By using the ceramic exhaust pipe 1 in which the three-way catalyst is integrated in this way, the three-way catalyst 8 (see FIG. 1) can be omitted from the exhaust gas purification device. Note that, in FIG. 7, portions corresponding to those in FIG. 2 are denoted by the same reference numerals.

[0040]

According to the first aspect of the invention, the oxygen ion conductive solid electrolyte ceramics, which are attached to the exhaust system of the engine and function as oxygen pumps, are formed on the inner and outer circumferences of the solid electrolyte ceramics. The first and second electrodes, and the porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover at least the first electrode, and thus the first and second electrodes are provided. When an electric current flows through the solid electrolyte ceramics through oxygen, oxygen is pumped by ionic conduction in the opposite direction to this electric current, and the engine is operated arbitrarily at an air-fuel ratio that gives the best fuel economy or power performance. However, the oxygen concentration in the exhaust gas can be set within the window range where the three-way catalyst can be purified, and the three-way catalyst can be used to efficiently purify the exhaust gas. There is an effect.

According to the second aspect of the invention, the oxygen ion conductive solid electrolyte ceramics mounted on the exhaust system of the engine and functioning as an oxygen pump, and the solid electrolyte ceramics are formed on the inner circumference and the outer circumference respectively. First and second electrodes, an exhaust gas purifying ceramic exhaust pipe having a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover at least the first electrode, and an upstream of the ceramic exhaust pipe And the first and second gas concentration detecting means for detecting the oxygen concentration in the downstream exhaust gas, and the solid electrolyte ceramics of the ceramic exhaust pipe according to the outputs of the first and second gas concentration detecting means. Since the control means for controlling the flowing current is provided, the oxygen concentration in the exhaust gas downstream of the ceramic exhaust pipe is adjusted to a predetermined value. When the current is controlled to flow through the solid electrolyte ceramics through the first and second electrodes, oxygen is pumped by ionic conduction in the opposite direction of this current, and the best fuel efficiency is the air-fuel ratio and power performance. Even if the engine is operated arbitrarily with such an air-fuel ratio, the oxygen concentration in the exhaust gas can be set within the window range where the three-way catalyst can be purified, and the three-way catalyst can be used to efficiently purify the exhaust gas. Has the effect of.

According to the third aspect of the invention, the oxygen ion conductive solid electrolyte ceramics mounted on the exhaust system of the engine and functioning as an oxygen pump, and the solid electrolyte ceramics are formed on the inner circumference and the outer circumference respectively. First and second electrodes, an exhaust gas purifying ceramic exhaust pipe having a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover at least the first electrode, and an upstream of the ceramic exhaust pipe And first and second gas concentration detecting means for detecting the oxygen concentration in the exhaust gas, and first temperature detecting means for detecting the temperature of the exhaust gas at least upstream of the ceramic exhaust pipe, and the ceramic exhaust Second temperature detecting means for detecting the temperature of the tube,
The current flowing through the solid electrolyte ceramics of the ceramic exhaust pipe is controlled according to the outputs of the first and second gas concentration detecting means, and the ceramic is based on the outputs of the first and second temperature detecting means. Since the control means for controlling the temperature of the exhaust pipe is provided, the oxygen concentration in the exhaust gas downstream of the ceramic exhaust pipe is feedback-controlled to a predetermined value, and a current flows through the solid electrolyte ceramics through the first and second electrodes. At this time, oxygen is pumped by ionic conduction in the opposite direction to this current, and even if the engine is arbitrarily operated at an air-fuel ratio that gives the best fuel economy or an air-fuel ratio that gives the best power performance, the oxygen in the exhaust gas The concentration can be adjusted to a window range that can be purified with a three-way catalyst, the exhaust gas can be efficiently purified using the three-way catalyst, and the ceramic exhaust pipe At least the temperature of the exhaust gas upstream and the temperature of the ceramic exhaust pipe are detected, and the temperature of the ceramic exhaust pipe is controlled based on the detected output, so that it is possible to control the temperature range with high efficiency as an oxygen pump. There is.

According to the fourth aspect of the present invention, the three-way catalyst is provided downstream of or integrally with the ceramic exhaust pipe for purifying exhaust gas.
In addition to the effects similar to those of the inventions of the items 3 and 4, it is possible to further control the exhaust gas of the engine within a gas concentration range in which it can be purified by the three-way catalyst.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an exhaust gas purifying apparatus for an engine according to the present invention.

FIG. 2 is a partially cut configuration diagram showing an embodiment of a ceramic exhaust pipe for purifying exhaust gas according to the present invention.

FIG. 3 is a diagram for explaining an oxygen pump action of an oxygen ion conductive solid electrolyte ceramics.

FIG. 4 is a configuration diagram showing another embodiment of the ceramic exhaust pipe for purifying exhaust gas according to the present invention.

FIG. 5 is a configuration diagram showing another embodiment of the ceramic exhaust pipe for purifying exhaust gas according to the present invention.

FIG. 6 is a configuration diagram showing another embodiment of the ceramic exhaust pipe for purifying exhaust gas according to the present invention.

FIG. 7 is a partially cut configuration view showing another embodiment of the exhaust gas purifying ceramic exhaust pipe according to the present invention.

FIG. 8 is a configuration diagram showing a conventional exhaust gas purifying apparatus for an internal combustion engine.

FIG. 9 is a diagram showing a relationship between a purification rate of a three-way catalyst and an air-fuel ratio.

[Explanation of symbols]

 1 Exhaust Gas Purifying Ceramic Exhaust Pipe 2 Atmosphere Side Electrode 3 Exhaust Gas Side Electrode 4 Porous Electrode Protective Layer 5 Oxygen Ion Conducting Solid Electrolyte Ceramics 6 Internal Combustion Engine 8 Three-Way Catalyst 9,10 Air Fuel Ratio Sensor 11,12 Air Fuel Ratio Sensor Control device 13,14 Temperature sensor 16 Ceramic exhaust pipe control device 19 Heater

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location F01N 9/00 Z

Claims (4)

[Claims] 1. An oxygen ion conductive solid electrolyte ceramic, which is attached to an exhaust system of an engine and functions as an oxygen pump, and first and second electrodes respectively formed on an inner circumference and an outer circumference of the solid electrolyte ceramic. A ceramic exhaust pipe for purifying exhaust gas, comprising: a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover at least the first electrode. 2. An oxygen ion conductive solid electrolyte ceramic which is attached to an exhaust system of an engine and functions as an oxygen pump, first and second electrodes respectively formed on an inner circumference and an outer circumference of the solid electrolyte ceramic, at least the above. Exhaust gas purifying ceramic exhaust pipe having a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover the first electrode, and oxygen concentration in exhaust gas upstream and downstream of the ceramic exhaust pipe And first and second gas concentration detecting means for detecting the above, and control means for controlling a current flowing through the solid electrolyte ceramics of the ceramic exhaust pipe in accordance with the outputs of the first and second gas concentration detecting means. An exhaust gas purifying device for an engine, which is characterized by being provided. 3. An oxygen ion conductive solid electrolyte ceramic, which is attached to an exhaust system of an engine and functions as an oxygen pump, first and second electrodes respectively formed on an inner circumference and an outer circumference of the solid electrolyte ceramic, at least the above. Exhaust gas purifying ceramic exhaust pipe having a porous electrode protective layer formed on the inner periphery of the solid electrolyte ceramic so as to cover the first electrode, and oxygen concentration in exhaust gas upstream and downstream of the ceramic exhaust pipe For detecting the temperature of the exhaust gas at least upstream of the ceramic exhaust pipe, and second temperature detecting means for detecting the temperature of the ceramic exhaust pipe. Depending on the outputs of the temperature detecting means and the first and second gas concentration detecting means, the solid electrolyte ceramic of the ceramic exhaust pipe is And a control means for controlling the temperature of the ceramic exhaust pipe based on the outputs of the first and second temperature detection means. . 4. The exhaust gas purifying apparatus for an engine according to claim 2, further comprising a three-way catalyst provided downstream of or integrally with the ceramic exhaust pipe for purifying exhaust gas. .