JP2022054622A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

To provide an exhaust emission control system capable of improving exhaust emission control efficiency by raising an exhaust gas temperature to a catalyst activation temperature while suppressing generation of CO2.SOLUTION: An exhaust emission control system includes: an SCR disposed in an exhaust pipe of an internal combustion engine; a DOC disposed on the upstream side of the SCR in the exhaust pipe; an ammonia supply device supplying ammonia into the exhaust pipe on the upstream side of the DOC; an ignition device burning ammonia supplied into the exhaust pipe; an oxygen sensor detecting an oxygen concentration in the exhaust pipe supplied with ammonia; and a control section (ECU) controlling supply of ammonia into the exhaust pipe by using the ammonia supply device on the basis of at least the oxygen concentration detected by the oxygen sensor.SELECTED DRAWING: Figure 1

Description

The present invention relates to an exhaust purification system for an internal combustion engine.

A selective reduction catalyst that reduces NOx to nitrogen and water using urea water as a reducing agent as an exhaust gas purification system for purifying NOx in the exhaust gas of diesel engines mounted on vehicles such as trucks and buses. A system using (SCR: Selective Catalytic Reduction) has been developed and realized. An exhaust gas purification system using SCR is disclosed in, for example, Patent Document 1.

The exhaust purification system using SCR supplies urea water stored in the urea water tank to the exhaust pipe upstream of SCR, hydrolyzes urea with the heat of the exhaust gas to generate ammonia, and NOx is produced by this ammonia in SCR. Is designed to be reduced. An appropriate amount of urea water is injected, for example, by a urea water injector provided in an exhaust pipe constituting an exhaust passage.

Further, a technique of supplying ammonia into an exhaust pipe instead of urea water is also known (see, for example, Patent Document 2).

The SCR has an optimum temperature at which the catalytic reaction is activated (hereinafter, this is referred to as "activation temperature"). The activation temperature depends on the type of catalyst, but is, for example, about 200 ° C.

Here, when the exhaust temperature is lowered, the temperature becomes lower than the activation temperature of SCR, and the reaction efficiency of the catalyst is lowered. The same applies when a NOx storage catalyst such as LNT (Lean NOx Trap) is used as the catalyst, and the reaction efficiency of the catalyst decreases when the temperature becomes lower than the activation temperature of LNT.

In consideration of this, for example, as disclosed in Patent Document 3, there is a technique for preventing a decrease in the reaction efficiency of the catalyst by raising the temperature in the exhaust pipe. In Patent Document 3, unburned fuel is supplied to an oxidation catalyst (DOC: Diesel Oxidation Catalyst) provided on the upstream side of the LNT, and the exhaust temperature is raised by the heat generated by the oxidation of the unburned fuel by the DOC. , A configuration that enhances the catalytic reaction of LNT on the downstream side is disclosed.

Japanese Unexamined Patent Publication No. 2012-7557 Japanese Unexamined Patent Publication No. 2013-124569 Japanese Unexamined Patent Publication No. 2013-194702

However, if the temperature of the exhaust gas is raised by direct injection of unburned fuel into the exhaust pipe or post-injection into the engine, there is a drawback that CO2 is generated.

The present invention has been made in consideration of the above points, and is an exhaust gas purification system capable of improving the exhaust purification efficiency by raising the exhaust temperature to the activation temperature of the catalyst while suppressing the generation of CO2. offer.

One aspect of the exhaust gas purification system for an internal combustion engine of the present invention is:
An exhaust purification system that purifies the exhaust gas of an internal combustion engine.
With the NOx reduction catalyst or NOx storage catalyst arranged in the exhaust pipe of the internal combustion engine,
An oxidation catalyst arranged in the exhaust pipe on the upstream side of the NOx reduction catalyst or the NOx storage catalyst,
An ammonia supply device that supplies ammonia into the exhaust pipe on the upstream side of the oxidation catalyst,
An ignition device that burns the ammonia supplied into the exhaust pipe,
An oxygen sensor that detects the oxygen concentration in the exhaust pipe to which the ammonia is supplied, and
A control unit that controls the supply of ammonia into the exhaust pipe by the ammonia supply device based on the oxygen concentration detected by the oxygen sensor.
To prepare for.

According to the present invention, it is possible to improve the exhaust gas purification efficiency by raising the exhaust temperature to the activation temperature of the catalyst while suppressing the generation of CO2.

The figure which showed the main part structure of the exhaust gas purification system of Embodiment 1. A flowchart used to explain the temperature rise control according to the first embodiment. FIG. 3A is a diagram showing a configuration of an exhaust pipe according to the second embodiment, FIG. 3A is a schematic cross-sectional view of the exhaust pipe cut along a plane parallel to the longitudinal direction of the exhaust pipe, and FIG. 3B shows the exhaust pipe in the longitudinal direction of the exhaust pipe. Schematic cross-section cut on a plane perpendicular to FIG. 4A is a diagram showing another configuration of the exhaust pipe according to the second embodiment, FIG. 4A is a schematic cross-sectional view of the exhaust pipe cut along a plane parallel to the longitudinal direction of the exhaust pipe, and FIG. 4B shows the exhaust pipe of the exhaust pipe. Schematic cross-section cut along a plane perpendicular to the longitudinal direction The figure which shows the state of the ammonia supply by Embodiment 3.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<1> Embodiment 1
<1-1> Configuration of Exhaust Gas Purification System FIG. 1 is a diagram showing a main configuration of an exhaust gas purification system according to the present embodiment. In this embodiment, an embodiment in which the present invention is applied to the exhaust gas purification system 100 of the diesel engine 10 will be described. However, the exhaust purification system according to the present embodiment can be applied not only to the exhaust purification system 100 of the diesel engine 10 but also to the exhaust purification system of a gasoline engine.

The exhaust purification system 100 is mounted on a vehicle such as a truck, and purifies NOx in the exhaust gas of the engine 10.

The engine 10 includes, for example, a combustion chamber, a fuel injection device that injects fuel in the combustion chamber, an engine ECU that controls the fuel injection device, and the like (not shown). The engine 10 generates power by burning and expanding a mixture of fuel and air in a combustion chamber. The engine 10 is connected to an intake pipe 20 that introduces air into the combustion chamber and an exhaust pipe 30 that discharges the exhaust gas after combustion discharged from the combustion chamber to the outside of the vehicle.

The exhaust gas purification system 100 has a DOC (Diesel Oxidation Catalyst) 101, a DPF (Diesel Particulate Filter) 102 and an SCR (Selective Catalytic Reduction) 103. The exhaust gas purification system 100 may have LNT (Lean NOx Trap) or the like as a catalyst in addition to SCR103 or in place of SCR.

DOC101 is formed by supporting rhodium, platinum or the like on a carrier made of aluminum oxide, metal or the like. DOC101 oxidizes and removes unburned components (hydrocarbon HC and carbon monoxide CO) in the exhaust, heats and raises the exhaust gas with the reaction heat at this time, and oxidizes NO in the exhaust to NO2.

The DPF 102 is composed of a filter with a catalyst, and collects particulate matter (PM: Particulate Matter) contained in the exhaust gas and burns and removes the collected PM.

The SCR 103 has, for example, a cylindrical shape and has a honeycomb carrier made of ceramic. A catalyst such as zeolite or vanadium is supported or coated on the wall surface of the honeycomb. The SCR 103 occludes ammonia and selectively reduces and purifies NOx from the exhaust gas by the occluded ammonia.

The exhaust purification system 100 includes an ammonia supply device 110 that supplies ammonia into the exhaust pipe 30. The ammonia supply device 110 includes an ammonia tank 111 for storing liquefied ammonia (liquefied NH3), a shutoff valve 112, a pressure reducing valve 113, a flow rate adjusting valves 114 and 115, and an ammonia injection port 116 and 117.

Further, the exhaust gas purification system 100 has an ECU (Electronic Control Unit) 130. The ECU 130 controls the operation of the exhaust gas purification system 100.

The ECU 130 includes, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an input port, an output port, and the like. The functions described later in the ECU 130 are realized, for example, by the CPU referring to a control program and various data stored in a ROM, RAM, or the like. However, the function is not limited to processing by software, and of course, it can be realized by a dedicated hardware circuit.

The ECU 130 communicates with the engine ECU (not shown) of the engine 10 to control them and acquire their states.

The ECU 130 functions as a control unit that controls the supply of ammonia to the DOC 101 by the ammonia supply device 110.

Specifically, the ECU 130 inputs the temperature information detected by the temperature sensor 141 provided at the entrance of the SCR 103 and the temperature sensor 142 provided at the entrance of the DOC 101, and based on the temperature information, the isolation valve 112, By controlling the opening degrees of the flow control valves 114 and 115, the amount of ammonia injected from the ammonia injection ports 116 and 117 is controlled.

In addition to this, a lambda sensor 160 as an oxygen sensor is provided in the exhaust pipe 30. The lambda sensor 160 is arranged on the upstream side of the ammonia injection port 117. The oxygen concentration detected by the lambda sensor 160 is sent to the ECU 130.

The ECU 130 controls the supply of ammonia into the exhaust pipe 30 by the ammonia supply device 110 based on the oxygen concentration detected by the lambda sensor 160. Specifically, the ECU 130 increases the opening degree of the flow rate adjusting valve 115 and increases the amount of ammonia supplied as the detected oxygen concentration increases.

<1-2> Temperature rise control Next, the temperature rise control according to the present embodiment will be described.

FIG. 2 is a flowchart provided for explaining the temperature rise control according to the present embodiment. The flowchart of FIG. 2 is executed by the ECU 130.

When the temperature rise control is started, the ECU 130 acquires the exhaust temperature information by inputting the temperature information detected by the temperature sensor 141 in step S11.

In the subsequent step S12, the ECU 130 determines whether or not there is an exhaust gas temperature rise request. Specifically, the ECU 130 determines that if the exhaust temperature is less than the SCR active temperature (for example, 200 ° C.), there is a request for raising the exhaust gas, and if the exhaust temperature is equal to or higher than the SCR active temperature, there is no request for raising the exhaust gas. Judge. The threshold value (SCR activity temperature) used in step S12 is, of course, preferably set appropriately depending on the type and structure of the SCR catalyst used.

When a negative result is obtained in step S12 (step S12; NO), the ECU 130 ends the temperature rise control.

On the other hand, when the ECU 130 obtains an affirmative result in step S12 (step S12; YES), the ECU 130 proceeds to step S13. In step S13, the ECU 130 determines whether or not the exhaust temperature detected by the temperature sensor 142 is lower than the ammonia combustion temperature of the DOC 101.

When the ECU 130 obtains a positive result in step S13 (step S13; YES),
The process proceeds to step S14, the oxygen concentration information from the lambda sensor 160 is acquired, and in the following step S15, the amount of ammonia to be supplied is calculated based on the oxygen concentration.

In the subsequent step S16, the ECU 130 supplies the calculated amount of ammonia into the exhaust pipe 30 and ignites the ignition device 150. As a result, the ignition device 150 burns ammonia to raise the exhaust temperature. The amount of ammonia supplied into the exhaust pipe 30 can be adjusted by adjusting the opening degree of the flow rate adjusting valve 115.

On the other hand, when the negative result is obtained in step S13 (step S13; NO), the ECU 130 moves to step S17 and controls the isolation valve 112 and the flow rate adjusting valve 115 to be in the open state to control the ammonia injection port 117. Ammonia is supplied to DOC101 by injecting ammonia from the DOC101. In step S17, the ECU 130 does not ignite the ignition device 150.

The ECU 130 returns to step S11 after performing the processing of step S16 or step S17, or while performing the processing of step S16 or step S17.

The threshold value (ammonia combustion temperature of DOC101) used in step S13 is, of course, preferably set appropriately depending on the type and structure of the DOC used.

In short, in the control of the present embodiment, when the exhaust temperature is less than a predetermined threshold value, if ammonia is supplied to the DOC 101, the ammonia is not sufficiently burned (oxidized) by the DOC 101, and the temperature rise effect is obtained. Since it becomes low, first, ammonia is burned by the ignition device 150 to raise the exhaust temperature to a predetermined threshold value. Then, when the exhaust temperature reaches a predetermined threshold value, ammonia is supplied to the DOC 101 to raise the temperature by the ammonia.

When the exhaust temperature is raised to the active temperature of the SCR by the combustion of ammonia by the DOC 101, the ECU 130 controls the flow rate adjusting valve 114 to be in the open state to supply ammonia to the SCR 103. At this time, the ECU 130 may control the flow rate adjusting valve 1 in the closed state or may control it in the open state.

Here, the amount of ammonia supplied based on the oxygen concentration calculated in step S15 will be described. In the present embodiment, the ammonia supply amount is determined in consideration of the following points.

(1) The concentration of ammonia injected into the exhaust pipe 30 is a concentration that can be ignited (combusted) by the ignition device 150.

(2) If the amount of ammonia is too small with respect to O2, NOx, which is a harmful substance, is generated during combustion by the ignition device 150, so supply an amount of ammonia in consideration of this. Specifically, since the chemical formula of complete combustion is represented by 4NH3 + 3O2 → 2N2 + 6H2O, the ammonia supply device 110 is based on the oxygen concentration detected by the lambda sensor 160, in the ammonia combustion space, of ammonia. It is preferable to supply ammonia so that the number of molecules is 4/3 or more of the number of molecules of oxygen. That is, if the number of molecules of ammonia is less than 4/3 of the number of molecules of oxygen, NOx is generated, which is not preferable.

(3) Combustion of ammonia does not consume all the oxygen in the exhaust. This is because HC and CO are contained in the exhaust gas, and in the case of a diesel engine, HC and CO are burned with excess oxygen in the DOC101 to convert them into CO2 and H2O. Therefore, if O2 is used up for ammonia combustion, there arises a problem that HC and CO cannot be purified by DOC101.

In the present embodiment, in consideration of the above (1), (2), and (3), the amount of ammonia to be injected from the ammonia injection port 117 and the injection timing are optimized.

According to the above configuration, ammonia is supplied into the SCR 103 arranged in the exhaust pipe 30 of the internal combustion engine, the DOC 101 arranged in the exhaust pipe 30 on the upstream side of the SCR 103, and the exhaust pipe 30 on the upstream side of the DOC 101. An ammonia supply device 110, an ignition device 150 that burns the ammonia supplied in the exhaust pipe 30, an oxygen sensor (lambda sensor 160) that detects the oxygen concentration in the exhaust pipe 30 to which the ammonia is supplied, and at least oxygen. CO2 is generated by providing a control unit (ECU 130) that controls the supply of ammonia into the exhaust pipe 30 by the ammonia supply device 110 based on the oxygen concentration detected by the sensor (lambda sensor 160). While suppressing the exhaust gas temperature, the exhaust gas temperature can be raised to the activation temperature of the catalyst to improve the exhaust gas purification efficiency.

Here, what is generated by the combustion of ammonia in DOC101 is N2 and H2O, which are harmless and have no greenhouse effect. Some ammonia becomes NOx, but since it is purified to N2 and H2O by SCR103 downstream, there is no impact on the environment.

Further, according to the present embodiment, since the ammonia supply device 110 is shared by the DOC 101 and the SCR 103, it is possible to suppress an increase in the size of the exhaust gas purification system. However, the SCR 103 may be configured to supply urea water instead of ammonia.

As described above, the preliminary temperature rise in step S16 may be performed only by combustion of ammonia by the ignition device 150, or may be performed in combination with the temperature rise of the exhaust temperature by the engine.

Further, by combining the combustion of ammonia by the ignition device 150 and the raising of the exhaust temperature by the electric heater (not shown) provided in the exhaust pipe 30, the preliminary raising of the temperature in step S16 may be performed. good.

In the present embodiment, the case where the present invention is applied to the exhaust gas purification system 100 using SCR103 as a catalyst for purifying NOx has been described, but the present invention is not limited to this, and the point is that it is a catalytic reaction. Can be widely applied when a NOx reduction catalyst or a NOx storage catalyst depending on the temperature is used. That is, the control unit (ECU 130) determines whether or not there is an exhaust temperature increase request based on the relationship between the activation temperature of the NOx reduction catalyst or the NOx storage catalyst and the exhaust temperature, and determines that there is an exhaust temperature increase request. In this case, ammonia may be supplied to the exhaust pipe 30 on the upstream side of the DOC 101.

<2> Embodiment 2
3 and 4 are schematic cross-sectional views showing the configuration of the exhaust pipe 30 according to the second embodiment.

FIG. 3 is a diagram showing a first configuration example of the exhaust pipe according to the second embodiment, and FIG. 3A is a schematic cross-sectional view of the exhaust pipe 30 cut along a plane parallel to the longitudinal direction of the exhaust pipe 30. 3B is a schematic cross-sectional view of the exhaust pipe 30 cut along a plane perpendicular to the longitudinal direction of the exhaust pipe 30.

FIG. 4 is a diagram showing a second configuration example of the exhaust pipe according to the second embodiment, and FIG. 4A is a schematic cross-sectional view of the exhaust pipe 30 cut along a plane parallel to the longitudinal direction of the exhaust pipe 30. 4B is a schematic cross-sectional view of the exhaust pipe 30 cut along a plane perpendicular to the longitudinal direction of the exhaust pipe 30.

First, the configuration of FIG. 3 will be described. The exhaust pipe 30 on the upstream side of the DOC 101 is provided with a partition wall 170 for dividing the exhaust flow, and the ammonia injection port 117 and the ignition device 150 of the ammonia supply device 110 are provided in one flow path 30a divided by the partition wall 170. It is provided.

As a result, the above requirements (1), (2) and (3) can be satisfied. That is, in the flow path 30a divided by the partition wall 170, a sufficient concentration that allows ammonia combustion by the ignition device 150 and a sufficient amount that the number of molecules of ammonia is 4/3 or more with respect to the number of molecules of oxygen. Ammonia can be supplied. Therefore, it is possible to reliably perform preliminary ammonia combustion until the exhaust temperature becomes equal to or higher than the ammonia combustion temperature of DOC101, and it is possible to prevent the generation of NOx at this time. Thereby, the above requirements (1) and (2) can be satisfied. On the other hand, the O2 in the exhaust reaches the DOC 101 through the other flow path 30b where the ammonia combustion by the ignition device 150 is not performed, so that the above requirement (3) can be satisfied.

Next, the configuration of FIG. 4 will be described. As can be seen from FIG. 4B, the configuration of FIG. 4 is characterized in that the cross sections of the flow paths 30a and 30b divided by the partition wall 170 have a substantially circular shape. As a result, the throttle valve can be easily provided in the flow path 30a, and the exhaust flow rate of the flow path 30a can be adjusted by controlling the opening degree of the throttle valve by the ECU 130.

Here, depending on the operating state, the exhaust flow rate in the exhaust pipe 30 becomes very fast, and even if the supply amount of ammonia is increased, the ammonia concentration cannot be increased, and as a result, the ammonia may not be ignited. In such a case, the throttle valve (not shown) provided in the flow path 30a is throttled (that is, the aperture ratio is reduced) to increase the ammonia concentration, and the ignition device 150 can ignite the ammonia. It will be like.

<3> Embodiment 3
FIG. 5 is a diagram showing a state of ammonia supply according to the third embodiment.

In the present embodiment, the ammonia supply device 110 intermittently supplies ammonia into the exhaust pipe 30. As a result, as shown in FIG. 5, a region having a high ammonia concentration (NH3 rich region) and a region having a low ammonia concentration (NH3 non-injection region) alternately exist in the exhaust flow. The region having a high ammonia concentration (NH3 rich) may be formed by supplying an amount of ammonia that satisfies the requirements of (1) and (2) above. The above requirement (3) is achieved when the region where the ammonia concentration is low (NH3 non-injection region) reaches DOC101.

Further, if ammonia is intermittently supplied into the exhaust pipe 30 as in the present embodiment, it is possible to increase the ignition probability while saving ammonia as compared with the case where the overall ammonia concentration is increased. ..

The above-described embodiment is merely an example of the embodiment of the present invention, and the technical scope of the present invention should not be construed in a limited manner by these. That is, the present invention can be implemented in various forms without departing from its gist or its main features.

The present invention has an effect that the exhaust gas temperature can be raised to the activation temperature of the catalyst to improve the exhaust gas purification efficiency while suppressing the generation of CO2, and the catalytic reaction depends on the temperature (has an activation temperature). ) Widely applicable to exhaust gas purification systems using catalysts.

10 Diesel engine (engine)
20 Intake pipe 30 Exhaust pipe 30a, 30b Channel 100, 200 Exhaust purification system 101 DOC (oxidation catalyst)
102 DPF (Diesel Particulate Filter)
103 SCR (selective reduction catalyst)
110 Ammonia supply device 111 Ammonia tank 112 Isolation valve 113 Pressure reducing valve 114, 115 Flow control valve 116, 117 Ammonia injection port 130 ECU (control unit)
141, 142 Temperature sensor 150 Ignition system 160 Lambda sensor 170 Bulkhead

Claims (5)

An exhaust purification system that purifies the exhaust gas of an internal combustion engine.
With the NOx reduction catalyst or NOx storage catalyst arranged in the exhaust pipe of the internal combustion engine,
An oxidation catalyst arranged in the exhaust pipe on the upstream side of the NOx reduction catalyst or the NOx storage catalyst,
An ammonia supply device that supplies ammonia into the exhaust pipe on the upstream side of the oxidation catalyst,
An ignition device that burns the ammonia supplied into the exhaust pipe,
An oxygen sensor that detects the oxygen concentration in the exhaust pipe to which the ammonia is supplied, and
A control unit that controls the supply of ammonia into the exhaust pipe by the ammonia supply device based on the oxygen concentration detected by the oxygen sensor.
Exhaust purification system equipped with. The exhaust pipe on the upstream side of the oxidation catalyst is provided with a partition wall for dividing the exhaust flow.
The ammonia injection port of the ammonia supply device and the ignition device are provided in one flow path divided by the partition wall.
The exhaust purification system according to claim 1. The ammonia supply device intermittently supplies ammonia into the exhaust pipe.
The exhaust purification system according to claim 1. The ammonia supply device supplies ammonia so that the number of molecules of ammonia is 4/3 or more of the number of molecules of oxygen in the combustion space of ammonia based on the oxygen concentration detected by the oxygen sensor. ,
The exhaust gas purification system according to any one of claims 1 to 3. In the NOx reduction type catalyst or NOx storage catalyst, the catalytic reaction depends on the temperature, and the catalytic reaction depends on the temperature.
The control unit determines whether or not there is a request for raising the temperature of the exhaust gas based on the relationship between the activation temperature of the NOx reduction catalyst or the NOx storage catalyst and the exhaust temperature.
The exhaust gas purification system according to any one of claims 1 to 4.

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