JP2006161697A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of providing high NOx reduction rate from a relatively low temperature zone. <P>SOLUTION: This device is provided with NOx storage reducing catalyst 5 provided in a middle of an exhaust pipe 4 leading exhaust gas from a diesel engine 1, a control device 14 controlling fuel injection to each cylinder of the diesel engine 1 to remain suitable amount of unburnt fuel as reducing agent to the NOx storage reducing catalyst 5, and a plasma generation device 7 performing electric discharge in exhaust gas 3 in an upstream of the NOx storage reducing catalyst and decomposing unburnt fuel into H<SB>2</SB>and CO. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device.

Conventionally, exhaust purification is carried out with an exhaust purification catalyst installed in the middle of the exhaust pipe. As this type of exhaust purification catalyst, NOx in exhaust gas is oxidized when the exhaust air-fuel ratio is lean. Thus, a NOx occlusion reduction catalyst having the property of temporarily storing in the form of nitrate and decomposing and releasing NOx through the intervention of unburned HC, CO, etc. when the O ₂ concentration in the exhaust gas decreases is reduced and purified. Are known.

In such a NOx occlusion reduction catalyst, when the occlusion amount of NOx increases and reaches the saturation amount, no more NOx can be occluded, so that the exhaust gas flowing into the NOx occlusion reduction catalyst periodically It is necessary to decompose and release NOx by reducing the O ₂ concentration.

For example, when used in a gasoline engine, the operating air-fuel ratio of the engine is reduced (the engine is operated at a rich air-fuel ratio), thereby reducing the O ₂ concentration in the exhaust gas and unburned HC in the exhaust gas. It is possible to promote the decomposition and release of NOx by increasing reducing components such as CO and CO. However, when the NOx storage reduction catalyst is used as an exhaust purification device of a diesel engine, it is difficult to operate the engine at a rich air-fuel ratio. is there.

For this reason, a means for adding fuel (HC) to the exhaust gas upstream of the NOx storage reduction catalyst is newly provided, and the added fuel reacts with O ₂ on the NOx storage reduction catalyst as a reducing agent, It is necessary to actively reduce the O ₂ concentration in the exhaust gas to regenerate the NOx storage reduction catalyst (see, for example, Patent Document 1).
JP 2000-356127 A

However, only by adding fuel upstream of the NOx storage reduction catalyst in this way, a part of the HC generated by evaporation of the added fuel reacts with O ₂ in the exhaust gas on the surface of the NOx storage reduction catalyst. (Combustion), and NOx decomposition and release is started after the O ₂ concentration in the atmosphere around the NOx storage reduction catalyst becomes almost zero, so that HC is O ₂ on the surface of the NOx storage reduction catalyst. NOx is efficiently removed from the NOx occlusion reduction catalyst under operating conditions where the combustion temperature (about 220-250 ° C) required for reaction (combustion) with NOx cannot be obtained (for example, slow driving in cities with heavy traffic). There was a problem that the NOx occlusion reduction catalyst could not be decomposed and released, and the regeneration of the NOx occlusion reduction catalyst did not proceed efficiently, so that the recovery rate of the NOx occlusion site in the catalyst volume was reduced and the occlusion capacity was lowered.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an exhaust emission control device capable of obtaining a high NOx reduction rate from a relatively low temperature range.

The present invention relates to a NOx occlusion reduction catalyst provided in an exhaust pipe for leading exhaust gas from an engine, and fuel injection of the engine so that a large amount of unburned fuel can be left in the exhaust gas as a reducing agent when the NOx occlusion reduction catalyst is regenerated. And a plasma generator for generating plasma by discharging into the exhaust gas upstream of the NOx storage reduction catalyst and decomposing unburned fuel into H ₂ and CO by the plasma. The present invention relates to an exhaust emission control device.

  Thus, when the fuel injection control device controls the fuel injection into each cylinder of the engine to leave a large amount of unburned fuel in the exhaust gas, the unburned fuel becomes an exhaust gas as a reducing agent for the NOx storage reduction catalyst. Will be led with.

When the exhaust gas containing a large amount of unburned fuel is discharged by the former plasma generator, the unburned fuel (HC) in the exhaust gas is decomposed into H ₂ and CO by the partial oxidation reaction by the plasma. On the surface of the NOx storage reduction catalyst, H ₂ and CO having high reactivity react (combust) with O ₂ in the exhaust gas from a combustion temperature lower than the combustion temperature of conventional HC.

As a result, the O ₂ concentration in the atmosphere around the NOx storage reduction catalyst becomes almost zero, and the decomposition and release of NOx is started, and the NOx efficiency is improved by the highly reactive H ₂ and CO on the surface of the NOx storage reduction catalyst. As a result of the reduction treatment to N ₂ well, a higher NOx reduction rate can be obtained from a relatively low temperature range than when HC produced from unburned fuel is reacted as it is on the NOx storage reduction catalyst.

  Further, in the present invention, a fuel addition device may be additionally provided so that the fuel can be directly added to the exhaust pipe upstream from the plasma generation device. Can be added directly into the exhaust pipe, and a reducing atmosphere necessary for regeneration of the NOx storage reduction catalyst can be realized more reliably.

According to the above-described exhaust purification device of the present invention, the fuel is added by controlling the fuel injection into each cylinder on the engine side to leave a lot of unburned fuel in the exhaust gas, and the unburned fuel in the exhaust gas is added. Fuel (HC) is decomposed into H ₂ and CO by the plasma generated by the discharge of the previous plasma generator, and a high NOx reduction rate is obtained from a relatively low temperature region by these highly reactive H ₂ and CO. Therefore, it is included in the exhaust gas that is discharged outside the vehicle even under operating conditions where low-exhaust and low-exhaust temperature driving conditions are likely to continue, such as slow driving in cities with heavy traffic. It is possible to effectively reduce NOx as compared with the prior art, and to achieve an excellent effect that the practicality of the exhaust purification device using the NOx storage reduction catalyst can be greatly improved.

  Embodiments of the present invention will be described below with reference to the drawings.

  1 to 3 show an example of an embodiment of the present invention. As shown in FIG. 1, in the exhaust purification apparatus of this embodiment, exhaust gas discharged from a diesel engine 1 via an exhaust manifold 2 is shown. A NOx occlusion reduction catalyst 5 having a flow-through type honeycomb structure is provided in the middle of the exhaust pipe 4 through which the gas flow 3 circulates. A plasma generator 7 for generating plasma by discharging is provided.

  The plasma generator 7 is configured so that the electrodes 8 and 9 are arranged to face each other and discharge can be performed between them. If the distance between the electrodes 8 and 9 can be set substantially uniformly, Various shapes such as a plate shape, a rod shape, and a cylindrical shape can be appropriately employed.

  Each electrode 8 and 9 has a structure in which a power source 11 is connected via an inverter 10. In particular, in this embodiment, a vehicle-mounted battery is assumed as the power source 11. An AC high voltage having an appropriate voltage and frequency that can be discharged can be applied to the electrodes 8 and 9.

  In addition, an accelerator in a driver's seat (not shown) is provided with an accelerator sensor 12 (load sensor) that detects the accelerator opening as a load of the diesel engine 1, and the rotational speed is set at an appropriate position of the diesel engine 1. A rotation sensor 13 for detection is provided, and an accelerator opening signal 12a and a rotation speed signal 13a from the accelerator sensor 12 and the rotation sensor 13 constitute an engine control computer (ECU: Electronic Control Unit) 14 (fuel injection). Is input to the control device).

  On the other hand, in the control device 14, the fuel injection timing and injection toward the fuel injection device 15 that injects the fuel into each cylinder according to the current operation state determined from the accelerator opening signal 12a and the rotation speed signal 13a. A fuel injection signal 15a for commanding the amount is output.

  Here, the fuel injection device 15 is constituted by a plurality of injectors (not shown) provided for each cylinder, and the electromagnetic valves of these injectors are appropriately opened by a fuel injection signal 15a from the control device 14. Thus, the fuel injection timing and the injection amount (valve opening time) are appropriately controlled.

  However, in this embodiment, the control device 14 determines the fuel injection signal 15a in the normal mode based on the accelerator opening signal 12a and the rotation speed signal 13a, while periodically regenerating from the normal mode to the regeneration mode. When the mode is switched to the regeneration mode, the non-ignition timing (starting time is determined to be the crank angle) after the main injection of the fuel performed near the compression top dead center (crank angle 0 °). An appropriate number of post injections (injection in one or more times) is performed in the range of 90 ° to 120 °.

  That is, when post-injection is performed at the non-ignition timing later than the compression top dead center following main injection, unburned fuel (mainly HC: hydrocarbon) is added to the exhaust gas 3 by this post-injection. As a result, the HC generated by the unburned fuel is guided to the NOx storage reduction catalyst 5 as a reducing agent.

  In addition, when it is determined from the control device 14 that the exhaust gas temperature is low based on the accelerator opening signal 12a and the rotation speed signal 13a, the discharge command signal 10a to the inverter 10 of the plasma generator 7 is determined. Is output, and the plasma generator 7 is operated by the inverter 10 that has received the discharge command signal 10a, so that discharge is performed in the exhaust gas 3.

  Further, an injection nozzle 16 is provided through the exhaust pipe 4 upstream of the plasma generator 7, and a connection between the injection nozzle 16 and a light oil tank 17 provided at a required place is connected by a light oil supply pipe 18. The light oil (fuel as the reducing agent) in the light oil tank 17 is discharged into the exhaust pipe 4 through the injection nozzle 16 by driving the supply pump 19 provided in the middle of the light oil supply pipe 18 and opening the light oil injection valve 20. The fuel injection device 21 is constituted by the injection nozzle 16, the light oil tank 17, the light oil supply pipe 18, the supply pump 19, and the light oil injection valve 20.

  Then, auxiliary fuel addition of the fuel addition device 21 is performed by a drive command signal 19 a to the supply pump 19 and a valve opening command signal 20 a to the light oil injection valve 20 that are output from the control device 14 as necessary. It is to be executed appropriately.

  Thus, the fuel injection control in the control device 14 is periodically switched from the normal mode to the regeneration mode, and after the main injection of the fuel performed near the compression top dead center, the post-ignition timing is delayed after the compression top dead center. When the injection is performed, a large amount of unburned fuel (mainly HC: hydrocarbon) remains in the exhaust gas 3 by this post injection, and the unburned fuel is introduced together with the exhaust gas 3 as a reducing agent to the NOx storage reduction catalyst 5. Will be.

When the exhaust gas 3 containing a large amount of unburned fuel is discharged by the former plasma generator 7, the unburned fuel (HC) in the exhaust gas 3 is decomposed into H ₂ and CO by a partial oxidation reaction by plasma. Therefore, highly reactive H ₂ and CO react with O ₂ in the exhaust gas 3 from the combustion temperature lower than the combustion temperature of the conventional HC on the surface of the NOx occlusion reduction catalyst 5 in the subsequent stage.

As a result, the O ₂ concentration in the atmosphere around the NO x storage reduction catalyst 5 becomes almost zero and the decomposition and release of NO x is started, and the NO x storage reduction catalyst 5 directly reacts with the highly reactive H ₂ and CO on the surface of the NO x storage reduction catalyst 5. As a result of efficient reduction treatment to N ₂ , a higher NOx reduction rate can be obtained from a relatively low temperature range than when HC generated from unburned fuel is reacted as it is on the NOx storage reduction catalyst 5. .

  Further, particularly in the present embodiment, the fuel addition device 21 is additionally provided so that the light oil can be directly added to the exhaust pipe 4 upstream of the plasma generation device 7. The fuel may be added directly into the exhaust pipe 4, and in this way, the reducing atmosphere necessary for the regeneration of the NOx storage reduction catalyst 5 is more reliably realized.

Therefore, according to the embodiment as described above, the fuel injection into each cylinder is controlled on the diesel engine 1 side and the post injection following the main injection is executed, and unburned in the exhaust gas 3 by this post injection. Fuel is added by leaving a large amount of fuel, and unburned fuel (HC) in the exhaust gas 3 is decomposed into H ₂ and CO by the plasma generator 7 in the previous stage, and these highly reactive H ₂ and CO Thus, a high NOx reduction rate can be obtained from a relatively low temperature range, so that, for example, a slow driving operation in a city with a lot of traffic, etc. However, NOx contained in the exhaust gas 3 discharged outside the vehicle can be reduced more effectively than before, and the practicality of the exhaust gas purification apparatus using the NOx storage reduction catalyst 5 can be greatly improved. .

In fact, according to the results of experiments conducted by the present inventor, as shown in the graph of FIG. 2, the case A in which HC produced from the added fuel is reacted as it is on the NOx occlusion reduction catalyst 5, and H ₂ is occluded by NOx. When the case B reacted on the reduction catalyst 5 and the case C reacted with CO on the NOx storage reduction catalyst 5 were compared, the case C had a higher NOx reduction rate from the lower temperature range than the case A. Further, it has been confirmed that the case B can obtain a higher NOx reduction rate from the lower temperature region than the case C. The vertical axis in the graph of FIG. 2 indicates the NOx reduction rate, and the horizontal axis indicates the catalyst temperature.

  Further, as shown in FIG. 3, the case X in which the unburned fuel is added in the apparatus configuration of the present embodiment described above, and the NOx occlusion reduction catalyst 5 without the plasma generator 7 are provided and the unburned fuel is provided. Comparison with case Y with fuel added confirms that case X can obtain a higher NOx reduction rate than case Y from the low load region (operation region where the exhaust temperature is low). The vertical axis in the graph of FIG. 3 indicates the NOx reduction rate, and the horizontal axis indicates the load of the diesel engine 1.

  Note that the exhaust emission control device of the present invention is not limited to the above-described embodiment, and performs post-injection at a non-ignition timing that is later than the compression top dead center following main injection, thereby removing unburned fuel. In addition to leaving a large amount, it may be possible to leave a large amount of unburned fuel (HC) by delaying or accelerating the injection timing of the main injection itself. Of course, changes can be made.

It is the schematic which shows an example of the form which implements this invention. It is a graph which shows the relationship between NOx reduction rate and catalyst temperature with a comparative example. It is a graph which shows the relationship between a NOx reduction rate and load with a comparative example.

Explanation of symbols

1 Diesel engine (engine)
3 Exhaust gas 4 Exhaust pipe 5 NOx storage reduction catalyst 7 Plasma generator 14 Control device (fuel injection control device)
DESCRIPTION OF SYMBOLS 15 Fuel injection apparatus 15a Fuel injection signal 21 Fuel addition apparatus

Claims (2)

NOx occlusion reduction catalyst installed in an exhaust pipe for leading exhaust gas from the engine, and fuel for controlling fuel injection of the engine so that a large amount of unburned fuel can be left in the exhaust gas as a reducing agent during regeneration of the NOx occlusion reduction catalyst An injection control device and a plasma generator for generating plasma by discharging into the exhaust gas upstream of the NOx storage reduction catalyst and decomposing unburned fuel into H ₂ and CO by the plasma are provided. Exhaust gas purification device.   The exhaust emission control device according to claim 1, further comprising a fuel addition device so that fuel can be directly added to an exhaust pipe upstream of the plasma generation device.

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