JP2003083152A - Air-fuel ratio sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the an air-fuel ratio sensor from being damaged by wetting and the combustion of soot and the like when being heated by a heater. SOLUTION: This air-fuel ratio sensor comprises the air-fuel ratio sensor 37, the heater 76 for heating the air-fuel ratio sensor 37, an energizing means 60 for energizing the heater 76 to keep an arbitrary temperature, and a determining means 35 for determining whether the water is attached to an inner wall of an exhaust passage 19 or not, and the heater 76 is energized to keep the air-fuel sensor 37 at a temperature lower than an ordinary temperature when the attachment of the water to a wall face of the exhaust passage 19 is determined.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio sensor, and more particularly to heater control of the air-fuel ratio sensor.

[0002]

2. Description of the Related Art In recent years, in the air-fuel ratio control of an internal combustion engine, an air-fuel ratio sensor is provided in the exhaust system of the internal combustion engine, and the fuel injection amount and intake are adjusted so that the output signal of the air-fuel ratio sensor matches the target air-fuel ratio. Feedback control such as air amount is being performed. As the air-fuel ratio sensor, an oxygen sensor (O ₂ sensor) for determining whether the air-fuel ratio of the exhaust gas of the internal combustion engine is lean or rich from the oxygen concentration contained in the exhaust gas discharged from the internal combustion engine, and A limiting current type oxygen concentration detecting element is used which outputs a limiting current in proportion to the oxygen concentration contained in the exhaust gas discharged from an internal combustion engine. The limiting current type oxygen concentration detection element is for linearly detecting the exhaust air-fuel ratio of the internal combustion engine in a wide range from the oxygen concentration.

It is essential that these air-fuel ratio sensors be kept in the active state in order to maintain the detection accuracy of the air-fuel ratio. Normally, the heaters attached to the air-fuel ratio sensor are energized from the time of engine start to heat them for early activation, and further, energization control of the heaters is performed in order to maintain the activated state.

However, when the temperature is low immediately after the engine is started, the moisture in the exhaust gas condenses and comes into contact with the heating air-fuel ratio sensor, so that the temperature of a part of the air-fuel ratio sensor drops sharply and the sensor element May be damaged. Therefore, in Japanese Unexamined Patent Application Publication No. 9-184443, the cumulative heat quantity given to the catalyst is obtained according to the operation history from the engine start time, and this is compared with a predetermined value, so that the accumulated heat quantity in the catalyst container immediately after the engine start. It is determined whether or not the evaporation of water in the condensing exhaust gas has been completed, and while the accumulated heat quantity is below a predetermined value,
It is prohibited to energize the heater because the evaporation of water is not completed.

[0005]

However, when the heater is completely de-energized, soluble organic fractions such as soot and unburned hydrocarbons (HC) in the exhaust gas (Soluble Organic Func).
tion: SOF) will adhere to the sensor element portion. If the heater is energized after the water content has evaporated, the soot and SOF may rapidly burn and the temperature of the sensor element part may rise excessively, resulting in damage to the element. Also, soot and S
The combustion of OF changes the oxygen concentration in the exhaust gas, making it difficult to measure the air-fuel ratio with high accuracy.

The present invention has been made in view of the above problems, and an object of the present invention is to prevent damage to the air-fuel ratio sensor due to combustion of water, soot, etc. when the sensor is heated by a heater. Especially.

[0007]

In order to achieve the above object, the air-fuel ratio sensor of the present invention employs the following means. That is, an air-fuel ratio sensor that detects the air-fuel ratio of the exhaust gas flowing through the exhaust passage of the internal combustion engine, a heater that heats the air-fuel ratio sensor, and an energizing unit that energizes the heater to maintain the heater at an arbitrary temperature. A determining means for determining whether or not moisture adheres to the inner wall of the exhaust passage, and the determining means determines that the energizing means has moisture adhering to at least the wall surface of the exhaust passage. At times, it is characterized in that the heater is energized to keep the temperature lower than usual.

The greatest feature of the present invention is that the heater of the air-fuel ratio sensor is always energized while the engine is operating, and the set temperature is gradually increased to damage the elements of the air-fuel ratio sensor due to combustion of water, soot, etc. To prevent.

In the air-fuel ratio sensor thus constructed,
When it is determined by the determination means that water is attached, the heater is energized to maintain a temperature lower than usual. The temperature at this time is a temperature at which the element is not damaged even if the element is exposed to water.

Further, at this time, for example, a part of SOF or the like attached to the air-fuel ratio sensor is burned. After that, when it is determined by the determination means that the adhesion of water has been eliminated, it is possible to maintain the active state of the element portion of the air-fuel ratio sensor by raising the temperature of the heater and maintaining the predetermined temperature.

In this way, it is possible to prevent damage to the elements of the air-fuel ratio sensor due to water exposure, while preventing damage to the elements of the air-fuel ratio sensor due to overheating due to rapid combustion of SOF and soot.

In the present invention, the energizing means energizes the heater so as to maintain the air-fuel ratio sensor at the first predetermined temperature when the determining means determines that water is attached to the wall surface of the exhaust passage. Then, when it is determined that the adhesion of water has been eliminated, the air-fuel ratio sensor is maintained at a second predetermined temperature higher than the first predetermined temperature to remove the particulate matter adhered to the air-fuel ratio sensor. The heater can be energized for a predetermined time to burn, and then the heater can be energized to maintain a third predetermined temperature higher than the second predetermined temperature.

In the air-fuel ratio sensor thus constructed,
When the determination means determines that the water is attached, the heater is energized to maintain the first predetermined temperature. The first predetermined temperature is a temperature at which the element is not damaged even when the element is exposed to water, for example, a temperature at which a part of the SOF in the exhaust gas is burned. After that, when the determination unit determines that the adhesion of water has been eliminated, the heater is energized to maintain the second predetermined temperature. The second predetermined temperature is a temperature at which the element is not damaged even if the particulate matter such as SOF or soot adhering to the air-fuel ratio sensor burns and the temperature rises. By maintaining the second predetermined temperature, the particulate matter attached to the air-fuel ratio sensor can be burned and removed. Further, when the second predetermined temperature is maintained for a predetermined time, the heater is energized to maintain the third predetermined temperature. Since the particulate matter adhering to the air-fuel ratio sensor is removed by maintaining the second predetermined temperature for a predetermined time, even if the temperature is raised to the third predetermined temperature thereafter, the air-fuel ratio sensor due to the combustion of soot and the like. No damage will occur. Then, by maintaining the third predetermined temperature, the active state of the element portion of the air-fuel ratio sensor can be maintained.

In this way, the temperature of the air-fuel ratio sensor is raised stepwise to prevent the element of the air-fuel ratio sensor from being damaged by water, while the element is damaged by overheating caused by the combustion of the particulate matter. It is also possible to prevent

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the air-fuel ratio sensor according to the present invention will be described below with reference to the drawings.
Here, a case where the air-fuel ratio sensor according to the present invention is applied to a diesel engine for driving a vehicle will be described as an example.

FIG. 1 is a diagram showing a schematic structure of an engine 1 to which an air-fuel ratio sensor according to this embodiment is applied and its intake and exhaust systems.

The engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders 2.

The engine 1 is equipped with a fuel injection valve 3 for directly injecting fuel into the combustion chamber of each cylinder 2. Each fuel injection valve 3 is connected to a pressure accumulator (common rail) 4 that accumulates fuel to a predetermined pressure. A common rail pressure sensor 4a that outputs an electric signal corresponding to the pressure of the fuel in the common rail 4 is attached to the common rail 4.

The common rail 4 communicates with a fuel pump 6 via a fuel supply pipe 5. This fuel pump 6
Is a pump that operates using the rotational torque of the output shaft (crankshaft) of the engine 1 as a drive source. A pump pulley 6a attached to the input shaft of the fuel pump 6 is attached to the output shaft (crankshaft) of the engine 1. The crank pulley 1a is connected to the crank pulley 1a via a belt 7.

In the fuel injection system thus constructed, when the rotational torque of the crankshaft is transmitted to the input shaft of the fuel pump 6, the fuel pump 6 is transmitted from the crankshaft to the input shaft of the fuel pump 6. The fuel is discharged at a pressure according to the rotating torque.

The fuel discharged from the fuel pump 6 is
It is supplied to the common rail 4 through the fuel supply pipe 5, accumulated in the common rail 4 up to a predetermined pressure, and distributed to the fuel injection valve 3 of each cylinder 2. When a drive current is applied to the fuel injection valve 3, the fuel injection valve 3 opens, and as a result, fuel is injected from the fuel injection valve 3 into the cylinder 2.

Next, an intake branch pipe 8 is connected to the engine 1, and each branch pipe of the intake branch pipe 8 communicates with a combustion chamber of each cylinder 2 through an intake port (not shown). There is.

The intake branch pipe 8 is connected to the intake pipe 9,
The intake pipe 9 is connected to the air cleaner box 10. An air flow meter 11 for outputting an electric signal corresponding to the mass of the intake air flowing through the intake pipe 9 and an intake air temperature flowing through the intake pipe 9 are provided in the intake pipe 9 downstream of the air cleaner box 10. The intake air temperature sensor 12 that outputs the electric signal is attached.

An intake throttle valve 13 for adjusting the flow rate of intake air flowing through the intake pipe 9 is provided at a portion of the intake pipe 9 located immediately upstream of the intake branch pipe 8. The intake throttle valve 13 is provided with an intake throttle actuator 14 configured by a step motor or the like for driving the intake throttle valve 13 to open and close.

The intake pipe 9 located between the air flow meter 11 and the intake throttle valve 13 is provided with a compressor housing 15a of a centrifugal supercharger (turbocharger) 15 which operates by using heat energy of exhaust gas as a drive source. The
An intercooler 16 for cooling the intake air that has become hot due to being compressed in the compressor housing 15a is provided in the intake pipe 9 downstream of the compressor housing 15a.
Is provided.

In the intake system constructed as described above, the intake air flowing into the air cleaner box 10 is cleaned by an air cleaner (not shown) in the air cleaner box 10 to remove dust and dirt from the intake air, and then the intake pipe 9 Through the compressor housing 15a.

The intake air that has flowed into the compressor housing 15a is compressed by the rotation of the compressor wheel installed in the compressor housing 15a. The intake air, which has been compressed in the compressor housing 15a and has reached a high temperature, is cooled by the intercooler 16 and then flows into the intake branch pipe 8 with its flow rate adjusted by the intake throttle valve 13 if necessary. The intake air that has flowed into the intake branch pipe 8 is distributed to the combustion chamber of each cylinder 2 through each branch pipe, and is burned using the fuel injected from the fuel injection valve 3 of each cylinder 2 as an ignition source.

On the other hand, an exhaust branch pipe 18 is connected to the engine 1, and each branch pipe of the exhaust branch pipe 18 communicates with a combustion chamber of each cylinder 2 through an exhaust port (not shown).

The exhaust branch pipe 18 serves as the centrifugal supercharger 15.
Is connected to the turbine housing 15b. The turbine housing 15b is connected to an exhaust pipe 19, and the exhaust pipe 19 is connected downstream to a muffler (not shown).

In the middle of the exhaust pipe 19, a storage reduction type N
A particulate filter carrying an Ox catalyst (hereinafter,
Simply called a filter. ) 20 are provided. An oxidation catalyst 41 is provided downstream of the filter 20. Between the filter 20 and the oxidation catalyst 41, an exhaust temperature sensor 24 that outputs an electric signal corresponding to the temperature of the exhaust gas flowing in the exhaust pipe 19, and an air-fuel ratio sensor that outputs an electric signal corresponding to the air-fuel ratio of the exhaust gas. 37 is attached.

Exhaust pipe 1 downstream of the above-mentioned oxidation catalyst 41
An exhaust throttle valve 21 for adjusting the flow rate of the exhaust gas flowing through the exhaust pipe 19 is provided in the valve 9. This exhaust throttle valve 2
1, an exhaust throttle valve 2 which is composed of a step motor or the like
An exhaust throttle actuator 22 for driving to open and close 1 is attached.

In the exhaust system thus constructed, the air-fuel mixture (burnt gas) burned in each cylinder 2 of the engine 1 is discharged to the exhaust branch pipe 18 through the exhaust port, and then the exhaust branch pipe 18 is exhausted. Turbine housing 15b of centrifugal supercharger 15
Flow into. The exhaust gas that has flowed into the turbine housing 15b uses the thermal energy of the exhaust gas to rotate the turbine wheel that is rotatably supported in the turbine housing 15b. At that time, the rotational torque of the turbine wheel is transmitted to the compressor wheel of the compressor housing 15a described above.

The exhaust gas discharged from the turbine housing 15b flows into the filter 20 through the exhaust pipe 19, PM in the exhaust gas is collected, and harmful gas components are removed or purified. The exhaust gas, in which the PM is collected by the filter 20 and the harmful gas component is removed or purified, is discharged into the atmosphere through the muffler after the flow rate is adjusted by the exhaust throttle valve 21 as necessary. Hydrocarbons (HC) and carbon monoxide (CO) that could not be purified by the filter 20 are
It is oxidized and purified by the oxidation catalyst 41 provided downstream.

The exhaust branch pipe 18 and the intake branch pipe 8 are exhaust gas recirculation passages (hereinafter referred to as EGR passages) for recirculating a part of the exhaust gas flowing in the exhaust branch pipe 18 to the intake branch pipe 8. )
25 are communicated with each other. In the middle of the EGR passage 25, an exhaust valve (hereinafter, referred to as EG, which is composed of a solenoid valve or the like, flows through the EGR passage 25 according to the magnitude of the applied power.
R gas. ) Flow rate adjustment valve (hereinafter,
Use EGR valve. ) 26 are provided.

An EGR valve 26 is provided in the middle of the EGR passage 25.
An EGR cooler 27 that cools the EGR gas flowing through the EGR passage 25 is provided further upstream. A cooling water passage (not shown) is provided in the EGR cooler 27, and a part of the cooling water for cooling the engine 1 circulates.

In the exhaust gas recirculation mechanism constructed as described above, when the EGR valve 26 is opened, the EGR passage 25 is brought into a conductive state, and a part of the exhaust gas flowing through the exhaust branch pipe 18 is part of the EGR passage 25. To the intake branch pipe 8 via the EGR cooler 27.

At this time, in the EGR cooler 27, heat exchange is performed between the EGR gas flowing in the EGR passage 25 and the cooling water of the engine 1 to cool the EGR gas.

The EGR gas recirculated from the exhaust branch pipe 18 to the intake branch pipe 8 through the EGR passage 25 is introduced into the combustion chamber of each cylinder 2 while being mixed with the fresh air flowing from the upstream side of the intake branch pipe 8. Get burned.

The filter 20 is made of, for example, a porous material such as cordierite, and alumina is used as a carrier on the inner wall surface of the pores, and potassium (K), sodium (Na) and lithium are supported on the carrier. At least one selected from alkali metals such as (Li) or cesium (Cs), alkaline earths such as barium (Ba) or calcium (Ca), and rare earths such as lanthanum (La) or yttrium (Y). Tsuto, platinum (P
and a precious metal such as t) are supported. In the present embodiment, barium (B
An occlusion reduction type NOx catalyst constituted by supporting a) and platinum (Pt) and further adding ceria (Ce ₂ O ₃ ) having an O ₂ storage capacity was adopted.

The NOx catalyst thus constructed is
When the exhaust gas flowing into the Ox catalyst has a high oxygen concentration, it absorbs nitrogen oxides (NOx) in the exhaust gas.

On the other hand, the NOx catalyst releases the nitrogen oxides (NOx) absorbed when the oxygen concentration of the exhaust gas flowing into the NOx catalyst decreases. At that time, if a reducing component such as hydrocarbon (HC) or carbon monoxide (CO) is present in the exhaust gas, the NOx catalyst converts the nitrogen oxide (NOx) released from the NOx catalyst into nitrogen (N). It can be reduced to ₂ ).

By the way, when the engine 1 is in the lean burn operation, the air-fuel ratio of the exhaust gas discharged from the engine 1 becomes a lean atmosphere and the oxygen concentration of the exhaust gas becomes high.
Nitrogen oxides (NOx) contained in the exhaust gas will be absorbed by the NOx catalyst, but if the lean burn operation of the engine 1 is continued for a long period of time, the NOx absorption capacity of the NOx catalyst will be saturated, and Nitrogen oxides (NOx) are released into the atmosphere without being removed by the NOx catalyst.

In particular, in a diesel engine such as the engine 1, a lean air-fuel ratio mixture is burned in most operating regions, and accordingly, the exhaust air-fuel ratio becomes lean air-fuel ratio in most operating regions. , N of NOx catalyst
Ox absorption capacity is easily saturated.

Therefore, when the engine 1 is in the lean burn operation, the oxygen concentration in the exhaust flowing into the NOx catalyst is reduced and the concentration of the reducing agent is increased before the NOx absorption capacity of the NOx catalyst is saturated, and the NOx concentration is increased. It is necessary to release and reduce nitrogen oxides (NOx) absorbed by the catalyst.

As a method of lowering the oxygen concentration in this way, fuel is added to the exhaust gas or the amount of recirculated EGR gas is increased to increase the soot generation amount to the maximum and then the EGR gas amount is further increased. It is conceivable that there are methods such as low temperature combustion for increasing the fuel consumption and changing the fuel injection timing and the number of times into the cylinder 2,
In the present embodiment, the exhaust pipe 19 upstream of the filter 20.
The reducing agent supply mechanism for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust gas is added, and the oxygen concentration of the exhaust gas flowing into the filter 20 is reduced by adding the fuel into the exhaust gas from the reducing agent supply mechanism. At the same time, the concentration of the reducing agent was increased.

As shown in FIG. 1, the reducing agent supply mechanism is mounted so that its injection hole faces the inside of the exhaust branch pipe 18, and is provided with an electronic control unit (ECU).
nit) 35 to open a valve by a signal from the reducing agent injection valve 28 to inject fuel, and a reducing agent supply path 29 that guides the fuel discharged from the fuel pump 6 to the reducing agent injection valve 28.
And the reducing agent supply path 29 is provided in the reducing agent supply path 29.
A shutoff valve 31 for shutting off the flow of fuel inside. Here, the ECU 35 is a unit that controls the operating state of the engine 1 in accordance with the operating conditions of the engine 1 and the driver's request.

In such a reducing agent supply mechanism, the high-pressure fuel discharged from the fuel pump 6 is applied to the reducing agent injection valve 28 via the reducing agent supply passage 29. And ECU3
The reducing agent injection valve 28 is opened by a signal from 5 and fuel as a reducing agent is injected into the exhaust branch pipe 18.

The reducing agent injected from the reducing agent injection valve 28 into the exhaust branch pipe 18 reduces the oxygen concentration of the exhaust gas flowing from the upstream side of the exhaust branch pipe 18.

The exhaust gas having a low oxygen concentration formed in this manner flows into the filter 20 and releases nitrogen oxides (NOx) absorbed in the filter 20 while releasing nitrogen (N ₂ ).
Will be reduced to.

After that, the reducing agent injection valve 28 is closed by a signal from the ECU 35, and the addition of the reducing agent into the exhaust branch pipe 18 is stopped.

In this embodiment, fuel is injected into the exhaust gas to add the fuel, but instead of this, the amount of recirculated EGR gas is increased to increase the amount of soot generated. After reaching the maximum, low temperature combustion may be performed to further increase the EGR gas amount, and fuel may be injected from the fuel injection valve 3 in the expansion stroke, exhaust stroke, etc. of the engine 1.

Next, the air-fuel ratio sensor according to this embodiment will be described.

FIG. 2 is a schematic configuration diagram of the air-fuel ratio sensor according to this embodiment.

The air-fuel ratio sensor 37 comprises an air-fuel ratio sensor element (hereinafter referred to as a sensor element) 52 and a heater 54, and a voltage is applied to the sensor element 52 from an air-fuel ratio sensor circuit (hereinafter referred to as a sensor circuit) 53. Then, electric power is supplied to the heater 54 from the battery 55 through the heater control circuit 56. The sensor circuit 53 applies an analog applied voltage from an air-fuel ratio control unit (A / FCU) 60 including a microcomputer to the sensor element 52 via a low pass filter (LPF) 57.

The A / FCU 60 forms a part of the ECU 35 together with the sensor circuit 53, the heater control circuit 56 and the LPF 57, and converts the digital data into a rectangular analog voltage by the D / A converter provided therein, and then the LPF 57.
To the sensor circuit 53 via the. The LPF 57 outputs a smoothing signal from which a high frequency component of a rectangular analog voltage signal has been removed, and prevents a detection error in the output current of the sensor element 52 due to high frequency noise. As the voltage of this smoothing signal is applied to the sensor element 52, the A / FCU 60
Detects the current flowing through the sensor element 52 that changes in proportion to the oxygen concentration in the gas to be detected, that is, the exhaust gas, and the voltage applied to the sensor element 52 at that time. A / FCU60
Has an internal A / D to detect these currents and voltages.
Converters are provided, and these A / D converters receive an analog voltage corresponding to a current flowing through the sensor element 52 from the sensor circuit 53 and a voltage applied to the sensor element 52, and convert the digital data.

By the way, the output signal of the air-fuel ratio sensor 37 cannot be used for air-fuel ratio control unless the sensor element 52 is activated. Therefore, the A / FCU 60 supplies electric power from the battery 55 to the heater 54 built in the sensor element 52 to energize the heater 54 at the time of engine start, early activates the sensor element 52, and the sensor element 52 is activated. After that, electric power is supplied to the heater 54 so as to maintain the active state. The voltage of the battery 55 is converted into digital data by an A / D converter provided inside the A / FCU 60.

However, the resistance of the sensor element 52 is
It depends on the temperature of the element 52, that is, the temperature of the sensor element 52.
Paying attention to the fact that the sensor element 5
The temperature at which the resistance of 2 maintains the activated state of the sensor element 52.
Heater 54 to have a resistance value corresponding to
Target the temperature of the sensor element 52 by supplying power to
The temperature is controlled to be maintained at 730 ° C., for example.
The A / FCU 60 also heats the sensor element 52.
From the data control circuit 56 to the voltage and current of the heater 54.
A / D that receives analog voltage and converts it to digital data
A converter is provided inside. These digital data
By using, for example, the resistance value of the heater 54 is calculated and calculated.
Based on the resistance value, the power supply according to the operating condition of the engine is heated.
To prevent excessive heating of the heater 54
The temperature of the heater 54 is controlled. The embodiment of the present invention
Then, as the air-fuel ratio sensor 37, the limiting current type oxygen concentration detection is performed.
However, the present invention is not limited to this.
Whether the air-fuel ratio is rich or lean as the air-fuel ratio sensor 37
Type oxygen sensor (hereinafter, referred to as O ₂
It is applicable also when using (as a sensor).

Air-fuel ratio control unit (A / FCU) 60
Is, for example, a CPU interconnected by a bidirectional bus,
It comprises a ROM, a RAM, a pack-up RAM, an input port, an output port, an A / D converter and a D / A converter, and controls the heater of the air-fuel ratio sensor 37. In addition, the ECU 35
Includes a common rail pressure sensor 4a and an air flow meter 1
1, various sensors such as an intake air temperature sensor 12, an exhaust gas temperature sensor 24, a crank position sensor 33, a water temperature sensor 34, an accelerator opening sensor 36, an air-fuel ratio sensor 37, etc. are connected via electrical wiring, and the outputs of the various sensors described above are connected. A signal is input to the ECU 35.

On the other hand, the ECU 35 has a fuel injection valve 3, an intake throttle actuator 14, an exhaust throttle actuator 22, a reducing agent injection valve 28, an EGR valve 26, and a shutoff valve 31.
Etc. are connected via electrical wiring, and the above-mentioned parts are connected to the ECU.
35 can be controlled.

The ROM stores various application programs and control maps.

The RAM stores output signals from the respective sensors, calculation results of the CPU, and the like. The calculation result is, for example, the engine speed calculated based on the time interval at which the crank position sensor 33 outputs a pulse signal. These data are used by the crank position sensor 33.
Each time, outputs a pulse signal, it is rewritten with the latest data.

The backup RAM is a non-volatile memory that can store data even after the operation of the engine 1 is stopped.

The CPU operates according to the application program stored in the ROM, and controls the fuel injection valve, intake throttle, exhaust throttle, EGR control, N.
Ox purification control, poisoning elimination control, PM combustion control, etc. are executed.

For example, in the NOx purification control, the CPU
The so-called rich spike control is executed to reduce the oxygen concentration in the exhaust gas flowing into the filter 20 in a spike-like (short time) manner in a relatively short cycle.

In the rich spike control, the CPU determines whether or not the rich spike control execution condition is satisfied every predetermined period. Examples of the rich spike control execution condition include conditions in which the filter 20 is in the active state, the output signal value of the exhaust temperature sensor 24 (exhaust temperature) is equal to or less than a predetermined upper limit value, and the like.

When it is determined that the rich spike control execution condition as described above is satisfied, the CPU causes the reducing agent injection valve 28 to inject spiked reducing agent fuel. By controlling
The air-fuel ratio of the exhaust gas flowing into the filter 20 is temporarily set to a predetermined target rich air-fuel ratio.

Specifically, the CPU has the engine speed stored in the RAM, the output signal of the accelerator opening sensor 36 (accelerator opening), the output signal value of the air flow meter 11 (intake air amount), the air-fuel ratio. The output signal of the sensor 37, the fuel injection amount, etc. are read.

The CPU uses the engine speed, the accelerator opening, the intake air amount, and the fuel injection amount as parameters for the R
The reducing agent addition amount control map of the OM is accessed, and the reducing agent addition amount (target addition amount) necessary for setting the air-fuel ratio of the exhaust gas to the preset target air-fuel ratio is calculated.

Subsequently, the CPU accesses the reducing agent injection valve control map of the ROM using the target addition amount as a parameter, and the reducing agent necessary for injecting the target addition amount of the reducing agent from the reducing agent injection valve 28. The valve opening time (target valve opening time) of the injection valve 28 is calculated.

When the target opening time of the reducing agent injection valve 28 is calculated, the CPU opens the reducing agent injection valve 28.

The CPU closes the reducing agent injection valve 28 when the target valve opening time elapses from the time when the reducing agent injection valve 28 is opened.

As described above, when the reducing agent injection valve 28 is opened for the target valve opening time, the target addition amount of fuel is injected from the reducing agent injection valve 28 into the exhaust branch pipe 18. Then, the reducing agent injected from the reducing agent injection valve 28 mixes with the exhaust gas that has flowed from the upstream side of the exhaust branch pipe 18 to form an air-fuel mixture having a target air-fuel ratio, and then flows into the filter 20.

As a result, the air-fuel ratio of the exhaust gas flowing into the filter 20 is such that the oxygen concentration changes in a relatively short cycle, so that the filter 20 absorbs and releases / reduces nitrogen oxides (NOx). And will be repeated alternately in a short cycle.

Next, in the poisoning elimination control, the CPU carries out the poisoning elimination processing in order to eliminate the poisoning caused by the oxide of the filter 20.

Here, sulfur (S) is used as the fuel for the engine 1.
When such a fuel is burned in the engine 1, sulfur oxides (SOx) such as sulfur dioxide (SO ₂ ) and sulfur trioxide (SO ₃ ) are generated.

Sulfur oxide (SOx) flows into the filter 20 together with the exhaust gas, and is absorbed by the filter 20 by the same mechanism as nitrogen oxide (NOx).

Specifically, when the oxygen concentration of the exhaust gas flowing into the filter 20 is high, the sulfur oxides (SO ₂ ) and SO ₃ such as sulfur dioxide (SO ₂ ) and sulfur trioxide (SO ₃ ) in the inflowing exhaust gas.
x) is oxidized on the surface of platinum (Pt) and absorbed by the filter 20 in the form of sulfate ions (SO ₄ ²⁻ ). Furthermore,
The sulfate ions (SO ₄ ^2- ) absorbed by the filter 20 are
Sulfate (BaS) combined with barium oxide (BaO)
O ₄ ) is formed.

By the way, the sulfate (BaSO ₄ ) is more stable than barium nitrate (Ba (NO ₃ ) ₂ ) and hard to decompose, and is decomposed even when the oxygen concentration of the exhaust gas flowing into the filter 20 becomes low. Instead, they remain in the filter 20.

Sulfate (BaS) in the filter 20
As the amount of O ₄ ) increases, the nitrogen oxides (N
Barium oxide (B) which can participate in the absorption of Ox)
Since the amount of aO) decreases, so-called SOx poisoning occurs in which the NOx absorption capacity of the filter 20 decreases.

As a method of eliminating SOx poisoning of the filter 20, the ambient temperature of the filter 20 is raised to a high temperature range of about 600 to 650 ° C. and the oxygen concentration of the exhaust gas flowing into the filter 20 is lowered. ,
Barium sulfate (BaSO) absorbed by the filter 20
₄₎ the SO ₃ ^- and SO ₄ ^- and pyrolyzed, followed by SO ₃ ^- and SO ₄
^- it can be exemplified a method of reducing the ^- is reacted with a hydrocarbon in the exhaust gas (HC) and carbon monoxide (CO) gaseous SO ₂ and.

Therefore, in the poisoning elimination processing according to the present embodiment, the CPU first executes the catalyst temperature raising control for raising the bed temperature of the filter 20, and then lowers the oxygen concentration of the exhaust gas flowing into the filter 20. I did it.

In the catalyst temperature raising control, the CPU causes, for example, secondary fuel injection from the fuel injection valve 3 and addition of fuel from the reducing agent injection valve 28 into the exhaust during the expansion stroke of each cylinder 2. Alternatively, those unburned fuel components may be oxidized in the filter 20, and the heat generated during the oxidation may raise the bed temperature of the filter 20.

However, if the temperature of the filter 20 rises excessively,
Since the heat deterioration of the filter 20 may be induced, it is preferable that the secondary injection fuel amount and the additional fuel amount be feedback-controlled based on the output signal value of the exhaust temperature sensor 24.

When the bed temperature of the filter 20 rises to a high temperature range of about 600 ° C. to 650 ° C. by the catalyst temperature raising process as described above, the CPU injects the reducing agent to reduce the oxygen concentration of the exhaust gas flowing into the filter 20. Fuel is injected from the valve 28.

When excessive fuel is injected from the reducing agent injection valve 28, those fuels burn rapidly in the filter 20 and the filter 20 overheats, or excessive fuel injected from the reducing agent injection valve 28 is injected. Since the filter 20 may be unnecessarily cooled by the fuel, the CPU uses the air-fuel ratio sensor 37.
It is preferable that the fuel injection amount from the reducing agent injection valve 28 is feedback-controlled based on the output signal of.

When the poisoning elimination processing is executed in this way,
When the bed temperature of the filter 20 is high, the oxygen concentration of the exhaust gas flowing into the filter 20 is low, so that barium sulfate (BaSO ₄ ) absorbed by the filter 20 is thermally decomposed into SO ₃ ⁻ and SO ₄ ^−. The SO ₃ ⁻ and SO ₄ ⁻ react with hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas and are reduced, whereby SOx poisoning of the filter 20 is eliminated.

On the other hand, depending on the operating condition of the engine, the PM captured by the filter 20 remains unburned and accumulated, which causes the filter 20 to be clogged. As one of the methods for effectively removing the unburned PM, the temperature increase control by the fuel addition is effective.

As described above, the air-fuel ratio sensor 37 is used to measure the air-fuel ratio of the exhaust gas, and the feedback control of the fuel injection amount and the reducing agent addition amount is performed, so that the catalyst and the filter can be effectively operated. Becomes

Next, the cup type air-fuel ratio sensor used in the present embodiment will be described below. In this cup-type air-fuel ratio sensor, element cracking may occur due to water contact during cold start.

FIG. 3 is a sectional view of a cup type air-fuel ratio sensor. The sensor body 70 of the cup-type air-fuel ratio sensor has a diffusion resistance layer 71 having a cup-shaped cross section, and the diffusion resistance layer 71 is internally fitted into a mounting hole portion of an exhaust pipe of an engine at an opening end 71a thereof. There is. The diffusion resistance layer 71 is formed by a plasma spraying method of ZrO ₂ or the like. Further, the sensor main body 70 has a solid electrolyte layer 72, and this solid electrolyte layer 72 is made of an oxygen ion conductive oxide sintered body, and the inside of the resistance diffusion layer is provided via an exhaust side electrode layer 73 having a cup-shaped cross section. It is evenly fitted inside the peripheral wall. An atmosphere-side electrode layer 74 is uniformly adhered to the inner surface of the solid electrolyte layer 72 in a cup-shaped cross section. In this case, both the exhaust-side electrode layer 73 and the atmosphere-side electrode layer 74 are formed so that a noble metal having a high catalytic activity such as platinum (Pt) has sufficient permeability by chemical plating or the like. The area and thickness of the exhaust side electrode layer 73 are about 10 to 100 mm ² and about 0.5 to 2.0 μm. On the other hand, the area and thickness of the atmosphere-side electrode layer 74 are 10 mm ² and 0.5 to 2.0 μm. The sensor body 70 is surrounded by a protective cover 78. The protection cover 78 is provided to prevent the sensor body 70 from directly contacting the exhaust gas and to keep the sensor body 70 warm. The protective cover 78 has a large number of small holes for communicating the inside and outside of the cover.

When the engine is cold started, it is necessary to supply a large amount of electric power to the heater 76 in order to heat the sensor main body 70 early.
6, the power is supplied at a duty ratio of 100%. However, when the water condensed by the filter 20 provided upstream in the exhaust pipe 19 stays at the bottom of the exhaust pipe 19 and the exhaust system at the time of cold start of the engine is not yet warm, this condensed water is discharged together with the exhaust. Are scattered and pass through the small holes of the protective cover 78 and come into contact with the sensor main body 70, causing element breakage of the sensor main body 70.

Therefore, in the present embodiment, first, the wall temperature of the exhaust pipe 19 around the air-fuel ratio sensor 37 is estimated, and the presence or absence of water in the exhaust pipe 19 is estimated from the wall temperature. When it is estimated that water is present, the output of the heater 76 is adjusted to keep the temperature low to prevent element cracking. In particular, in an engine with a low exhaust temperature such as a diesel engine, it takes time for the catalyst temperature to rise, so feedback control of the air-fuel ratio sensor is not performed until the temperature of the catalyst rises. Suitable for raising temperature.

Here, a flow for calculating the exhaust pipe temperature around the air-fuel ratio sensor 37 will be described.

FIG. 4 is a flow chart showing the flow of the exhaust pipe temperature calculation routine.

In step S501, the intake air temperature sensor 1
The outside air temperature T _0A detected by 2 is read.

In step S502, the exhaust temperature T _EA2 downstream of the filter 20 detected by the exhaust temperature sensor is read.

In step S503, the exhaust temperature T _EA1 near the air-fuel ratio sensor 37 is estimated from the outside air temperature T _0A and the exhaust temperature T _EA2 as follows.

From the outside air temperature T _0A and the thermal conductivity k1 of the exhaust pipe, a decrease T _D of the exhaust temperature from the vicinity of the air-fuel ratio sensor 37 to the vicinity of the exhaust temperature sensor is estimated. The exhaust temperature T _EA1 near the air-fuel ratio sensor 37 is the exhaust temperature T _EA2 plus the decrease T _D in the exhaust temperature, and therefore the following equation is established.

T _EA1 = T _EA2 + T _D (1) In step S 504, the air-fuel ratio is calculated from the exhaust temperature T _EA1 near the air-fuel ratio sensor 37 calculated in step S 503 and the heat transfer coefficient k 2 to the exhaust pipe to air. The exhaust pipe temperature Tw around the sensor 37 is calculated.

The exhaust pipe temperature Tw may be calculated as follows. First, the exhaust gas temperature T near the air-fuel ratio sensor 37
_EA1 is calculated from a two-dimensional map of the engine speed NE detected by the crank position sensor 33 and the intake air amount GA detected by the air flow meter 11. This 2
The dimension map is created from experimental values, and the larger the NE and the larger the GA, the higher the exhaust temperature T _EA1 . The exhaust pipe temperature Tw is calculated from the following equation from the exhaust gas temperature T _EA1 thus calculated and the outside air temperature T _0A detected by the intake air temperature sensor 12.

Tw = α (T _EA1 −T _0A ) where α is a constant.

Next, a method of calculating the element temperature of the air-fuel ratio sensor 37 will be described. The element temperature is calculated from the element DC impedance Zdc. This impedance Zd
c is obtained by applying the negative voltage Vneg to the sensor element, detecting the current Ineg at that time, and using the following equation.

Zdc = Vneg / Ineg In general, there is a correlation that the element DC impedance is attenuated as the element temperature rises. For example, the element DC impedance when the sensor element temperature is 730 ° C. is 30Ω. By mapping the relationship between the element temperature and the sensor element impedance in advance, the temperature of the sensor element can be obtained from the sensor element impedance. Therefore, if the temperature of the sensor element is fed back to control the energization of the heater 76, the temperature of the sensor element can be maintained at an arbitrary temperature.

In the conventional energization control of the air-fuel ratio sensor, the energization of the heater 76 is started after the temperature of the exhaust gas is raised and the water is removed in order to prevent the element from being damaged by the water. However, soot and the like in the exhaust adhered to the element when the heater 76 was not energized, and the soot and the like rapidly burned after the heater 76 was energized. Due to the rapid combustion of the soot, the element was damaged, and the measurement accuracy was deteriorated due to the fluctuation of the air-fuel ratio.

Therefore, in the present embodiment, when the exhaust pipe temperature is low, the heater is energized so as to maintain the temperature at which the soot and the like are burned, and excessive temperature rise and fluctuation of the air-fuel ratio due to rapid combustion of the soot and the like. To prevent.

FIG. 5 is a flow chart showing the flow of air-fuel ratio sensor energization control according to the present embodiment.

FIG. 6 is a time chart showing changes in the heater set temperature and the exhaust pipe temperature when the energization control of the air-fuel ratio sensor according to the present embodiment is performed.

The air-fuel ratio sensor energization control is started at the same time when the ignition switch is turned on.

In step S601, the exhaust temperature, the intake air amount, and the outside air temperature (intake air temperature) are read. CP
U accesses the RAM and reads the output signals of the exhaust temperature sensor 24 and the intake air temperature sensor 12.

In step S602, the exhaust pipe temperature Tw is calculated. The CPU calculates the exhaust pipe temperature Tw from the exhaust temperature and the outside air temperature as described in step S504.

In step S603, it is determined whether the exhaust pipe temperature Tw is lower than a predetermined temperature (for example, 30 ° C.).
Here, when the exhaust pipe temperature Tw is lower than a predetermined temperature (for example, 30 ° C.), moisture such as condensed water exists in the exhaust pipe, and the element may be damaged due to energization of the heater. Therefore, the method of energizing the heater is classified according to whether the exhaust pipe temperature is lower than a predetermined temperature (for example, 30 ° C.). If an affirmative decision is made in step S603, step S6
04, on the other hand, if the negative determination is made, the process proceeds to step S605.

In step S604, the air-fuel ratio sensor 37
Is maintained at, for example, 200 ° C. This temperature is a temperature at which the element is not damaged even if a small amount of water adheres to the element. At such a temperature, part of the SOF attached to the air-fuel ratio sensor 37 burns. Even if the temperature rises due to the combustion of SOF, the temperature of the air-fuel ratio sensor 37 is low, so the temperature of the element does not rise excessively.
Even if moisture adheres, the element is less likely to be damaged at this temperature. Exhaust pipe temperature and heater 7 at this time
The set temperature of 6 is shown by the area in FIG.

In step S605, the air-fuel ratio sensor 37
Is maintained at, for example, 500 ° C. At such a temperature, the soot attached to the air-fuel ratio sensor 37 burns. Here, the reason why the temperature is maintained at, for example, 500 ° C. is to prevent the element from being damaged due to overheating due to combustion of soot. Further, at this time, since the water is evaporated, it does not adhere to the air-fuel ratio sensor 37 and damage the element. The temperature of the exhaust pipe and the set temperature of the heater 76 at this time are shown in the region in FIG.

In step S606, step S605
The energization time of the heater started at is counted.

In step S607, step S606 is performed.
It is determined whether the count time of is longer than the time required for burning the soot. The time required for burning soot is obtained in advance by experiments or the like. When the soot is burned and removed, when the temperature is further raised, the soot will not burn and the temperature of the element will not excessively rise. If a positive determination is made, the process proceeds to step S608, while if a negative determination is made, the process proceeds to step S606.

In step S608, the air-fuel ratio sensor 37
Is maintained at, for example, 730 ° C. At such a temperature, the element is activated and the air-fuel ratio of exhaust gas can be measured. The temperature of the exhaust pipe and the set temperature of the heater 76 at this time are shown in the region in FIG.

In this way, the energization of the heater 76 can be controlled stepwise.

Here, in the conventional air-fuel ratio sensor, when the sensor element is likely to be exposed to water, energization of the heater is stopped. However, when energization to the heater is stopped, soot or the like in the exhaust adheres to the air-fuel ratio sensor, and when the heater is energized, it may burn rapidly and damage the sensor element due to overheating.

In this respect, according to the air-fuel ratio sensor of the present embodiment, by energizing the heater stepwise, damage to the element due to rapid combustion of soot and the like while preventing damage to the element due to moisture and It is possible to suppress fluctuations in measured values due to fluctuations in the air-fuel ratio around the sensor.

Although the air-fuel ratio sensor has been described in the present embodiment, the present application is not limited to the air-fuel ratio sensor, and all heater heating exhaust sensors (for example, O ₂ sensor, NOx sensor,
HC sensor, etc.).

[0121]

With the air-fuel ratio sensor according to the present invention, it is possible to prevent damage to the element due to water, while suppressing overheating of the sensor due to rapid combustion of soot and the like. . Further, the fluctuation of the measured air-fuel ratio due to the combustion of the soot adhering to the element and the fluctuation of the air-fuel ratio near the element can be suppressed.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an engine to which an air-fuel ratio sensor according to an embodiment of the present invention is applied and an intake / exhaust system thereof.

FIG. 2 is a schematic configuration diagram of an air-fuel ratio sensor.

FIG. 3 is a sectional view of a cup-type air-fuel ratio sensor.

FIG. 4 is a flowchart showing a flow of an exhaust pipe temperature calculation routine.

FIG. 5 is a flowchart showing a flow of air-fuel ratio sensor energization control according to the embodiment of the present invention.

FIG. 6 is a time chart diagram showing changes in heater set temperature and exhaust pipe temperature when energization control of the air-fuel ratio sensor according to the embodiment of the present invention is performed.

[Explanation of symbols]

1 ... Engine 1a: Crank pulley 2 ... Cylinder 3 ... Fuel injection valve 4 ... Common rail 4a ... Common rail pressure sensor 5 ... Fuel supply pipe 6 ... Fuel pump 6a ... Pump pulley 8 ... Intake branch pipe 9 ... Intake pipe 18 ... Exhaust branch pipe 19 ... Exhaust pipe 20 ... Particulate filter 21 ... Exhaust throttle valve 24 ... Exhaust gas temperature sensor 25 ... EGR passage 26 ... EGR valve 27 ... EGR cooler 28 ... Reducing agent injection valve 29 ... Reductant supply path 31 ... Shut-off valve 33 ... Crank position sensor 34 ... Water temperature sensor 35 ... ECU 36 ... Accelerator opening sensor 37 ... Air-fuel ratio sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G01N 27/46 371G 331 311G F term (reference) 2G004 BB01 BD04 BG09 BG13 BG15 BJ02 BL08 BL09 BL19 BM04 3G084 AA01 BA00 BA08 BA20 BA24 CA01 CA02 DA19 DA27 DA28 EA02 EA04 EA11 EB08 EB12 EB22 EC04 FA02 FA07 FA10 FA27 FA29 FA33

Claims (2)

[Claims] 1. An air-fuel ratio sensor for detecting an air-fuel ratio of exhaust gas flowing through an exhaust passage of an internal combustion engine, a heater for heating the air-fuel ratio sensor, and a heater which is energized to maintain the heater at an arbitrary temperature. An energizing unit; and a determining unit that determines whether or not moisture adheres to the inner wall of the exhaust passage, wherein the energizing unit determines that the moisture adheres to at least the wall surface of the exhaust passage. When the determination is made, the air-fuel ratio sensor is energized to keep the temperature of the heater lower than normal. 2. The energizing means energizes the heater so as to maintain the air-fuel ratio sensor at the first predetermined temperature when the determining means determines that the water adheres to the wall surface of the exhaust passage, and thereafter, In order to burn the particulate matter adhering to the air-fuel ratio sensor by maintaining the air-fuel ratio sensor at a second predetermined temperature higher than the first predetermined temperature when it is determined that the water adhesion has been eliminated. The air-fuel ratio sensor according to claim 1, wherein the heater is energized for a predetermined time, and then the heater is energized so as to be maintained at a third predetermined temperature higher than the second predetermined temperature.