JP4934082B2 - Exhaust purification device - Google Patents

Description

  The present invention relates to an exhaust purification device for reducing NOx in exhaust gas.

  Conventionally, a diesel engine is equipped with a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen in the middle of an exhaust pipe through which exhaust gas flows, and the selective reduction catalyst A required amount of a reducing agent is added to the upstream side of the catalyst so that the reducing agent undergoes a reduction reaction with NOx (nitrogen oxide) in the exhaust gas on the selective catalytic reduction catalyst, thereby reducing the NOx emission concentration. There is what I did.

On the other hand, in the field of industrial flue gas denitration treatment in plants and the like, the effectiveness of a method for reducing and purifying NOx using NH ₃ (ammonia) as a reducing agent is already widely known. Since it is difficult to ensure safety with respect to traveling with a toxic substance such as NH ₃ , in recent years, the use of non-toxic urea water as a reducing agent has been studied (for example, , See Patent Document 1).

That is, when urea water is added to the exhaust gas upstream of the selective catalytic reduction catalyst, the urea water is decomposed into NH ₃ and CO ₂ (carbon dioxide) in the exhaust gas, and the exhaust gas is exhausted on the selective catalytic reduction catalyst. The inside NOx is reduced and purified well by NH ₃ .
JP 2002-161732 A

  However, in the exhaust gas purification apparatus using the selective reduction catalyst using urea water as a reducing agent in this way, a urea water tank for storing urea water must be newly added. As a result, there is a problem that it is difficult to secure the installation space for the vehicle, and that it is unavoidable that the rise in cost is unavoidable, and that the infrastructure must be improved so that the driver can easily obtain urea water. The problem was also invited.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an exhaust purification device capable of effectively reducing and purifying NOx in exhaust gas without using urea water as a reducing agent. Yes.

In the present invention, a three-way catalyst having NOx and NH ₃ adsorption capacity is disposed in the middle of an exhaust pipe of a diesel engine, and the NOx and H ₂ in the exhaust gas are exhausted by the three-way catalyst when the accelerator is off while the engine is operating. Exhaust gas having a control device that performs premixed compression ignition on the diesel engine so that the excess air ratio can be reduced deeply to the extent that NH ₃ is generated from the exhaust gas and smokeless combustion can be maintained even at the low excess air ratio This relates to a purification device.

Thus, when the excess air ratio is deeply lowered by the control device when the accelerator is off while the engine is running, the reduction action of the three-way catalyst is activated and NOx becomes N ₂ , and this N ₂ becomes H ₂ in the exhaust gas. As a result, NH ₃ is produced and adsorbed and stored in the three-way catalyst.

That is, when premixed compression ignition is performed with the air excess ratio lowered deeply, the ratio of the fuel input amount to the unit air amount increases to a combustion temperature of 2000 K or more, and the moisture (H ₂ O) in the intake air is H ₂ , O ₂ , Pyrolysis into OH and H, and the originally stable N ₂ reacts with these to form an environment where NOx is likely to be generated. However, the reduction reaction of NOx also proceeds due to the active reduction action of the three-way catalyst. During the reaction, it reacts with excess H ₂ to produce NH ₃ .

  At this time, it is preferable to reduce the excess air ratio deeply to about 0.7. However, even if the excess air ratio is lowered so far, the pre-mixed compression ignition improves the combustibility significantly. It is possible to keep smokeless combustion without generating a large amount of soot).

  That is, after the main injection of fuel, which should normally be performed near the compression top dead center, is performed at a timing earlier than the compression top dead center, and fuel premixing is promoted by the advance injection of fuel into the cylinder. Since ignition and combustion are performed, combustion is performed by simultaneous whole area ignition in a state where the fuel is well dispersed and mixed and thinned uniformly, and the combustibility is remarkably improved.

  However, when the accelerator is off while the engine is running, the engine output is not particularly required, such as when the vehicle is decelerating or when the vehicle is idling, so simply increase the fuel injection amount to increase the excess air ratio. Not only lowering, but also using an air throttle as appropriate, the excess air ratio is lowered to prevent unnecessary engine output.

Then, the accelerator-off state is released and the accelerator travels to a normal driving state, the excess air ratio is returned to a normal state of about 1.6 or more, and the three-way catalyst is in an aerobic state. As a result, NOx already adsorbed on the three-way catalyst and newly adsorbed NOx are reduced and purified to N ₂ by NH ₃ stored when the accelerator is off.

  In the present invention, it is preferable that a particulate filter is provided immediately after the three-way catalyst so that fuel can be added to the exhaust gas upstream of the three-way catalyst. For example, the oxidation of a three-way catalyst exposed to exhaust gas with a large amount of residual oxygen in a diesel engine is activated, and high-concentration HC generated from the added fuel is well oxidized by the three-way catalyst. The inflow of the heated exhaust gas further raises the bed temperature of the particulate filter immediately after that, so that the particulates are efficiently incinerated in a short time.

  Moreover, in the present invention, since premixed compression ignition is performed with a deep excess of the air excess ratio when the accelerator is off while the engine is running, the fuel injection of the diesel engine is stopped especially when the accelerator is off at the time of deceleration while traveling. As a result, low-temperature intake air does not flow directly into the three-way catalyst and the particulate filter without going through the combustion stroke, and the problem that these three-way catalyst and the particulate filter are rapidly deprived of heat is avoided. In addition, since the high temperature exhaust gas flows into the three-way catalyst and the particulate filter, the temperature rises rather positively, so the time required for completing the regeneration of the particulate filter is greatly reduced. .

  In addition, when the accelerator is off when idling while the vehicle is stopped, the exhaust temperature can be significantly increased compared to the conventional extremely low exhaust temperature. It is possible to complete the warm-up of the filter within a short time, and the time required for completing the regeneration of the particulate filter is greatly reduced.

  According to the exhaust emission control device of the present invention described above, various excellent effects as described below can be obtained.

(I) According to the invention described in claim 1 of the present invention, the three-way catalyst, which has been applied only to gasoline vehicles so far, is applied to a diesel engine with the ability to adsorb NOx and NH _3. , entrapment adsorbed to generate and three-way _catalyst, NH ₃ from NOx when accelerator pedal during engine operation, since the NOx in the exhaust gas to be reduced and purified using the NH _3, reservoir urea water Maintenance of infrastructure that can eliminate urea water tanks for storage and eliminate problems related to securing the mounting space for the urea water tank and rising costs, and allowing the driver to easily obtain urea water Can be made unnecessary and can be widely used as an exhaust emission control device that can be easily operated simply by supplying fuel indispensable for running the vehicle.

  (II) According to the invention described in claim 2 of the present invention, when the accelerator is turned off during engine operation, the bed temperature of the three-way catalyst and the particulate filter tends to be low, the excess air ratio is reduced deeply and the premix compression is performed. By performing ignition, high-temperature exhaust gas can be sent from the diesel engine to raise the bed temperature of the three-way catalyst and particulate filter, so the excess air ratio was returned to the normal vicinity and the process shifted to forced regeneration by adding fuel. At this time, the particulate filter can be regenerated more efficiently and quickly than before, and the time taken to complete the regeneration of the particulate filter can be greatly reduced.

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

  FIG. 1 shows an example of an embodiment of the present invention. In FIG. 1, reference numeral 1 shows a diesel engine equipped with a turbocharger 2, and intake air 4 guided from an air cleaner 3 passes through an intake pipe 5 and the turbocharger. 2, the intake air 4 pressurized by the compressor 2 a is sent to the intercooler 6 to be cooled, and the intake air 4 is further guided from the intercooler 6 to the intake manifold 7. 1 is distributed to each cylinder 8 (FIG. 1 illustrates the case of inline 6 cylinders).

  Further, the exhaust gas 9 discharged from each cylinder 8 of the diesel engine 1 is sent to the turbine 2b of the turbocharger 2 through the exhaust manifold 10, and the exhaust gas 9 driving the turbine 2b is discharged to the exhaust pipe 11 (exhaust gas). It is designed to be discharged out of the vehicle through the flow path).

  In the middle of the exhaust pipe 11, a filter case 12 is interposed, and a catalyst regeneration type particulate filter 13 that integrally supports an oxidation catalyst is accommodated on the rear stage side in the filter case 12. Has been.

  In other words, this particulate filter 13 has a porous honeycomb structure made of ceramic, and the inlets of the respective flow paths partitioned in a lattice pattern are alternately sealed, and the flow paths in which the inlets are not sealed are used. The outlet is sealed, and only the exhaust gas 9 that has permeated through the porous thin wall defining each flow path is discharged to the downstream side.

In addition, a flow-through type three-way catalyst 14 having NOx and NH ₃ adsorption ability is provided in the preceding stage of the particulate filter 13 in the filter case 12, and more specifically, conventionally known. Zeolite catalyst (for example, Cu / zeolite catalyst or Fe / zeolite) having excellent ability to physically adsorb NOx and NH ₃ in the exhaust gas 9 to a three-way catalyst composed of Pt, Pd, Rh, ZeO, CeO, etc. A three-way catalyst 14 in which a catalyst or the like is coated, or this type of zeolitic catalyst is mixed and supported together from the raw material stage is applied.

Here, the conventionally known three-way catalyst simultaneously performs oxidation of CO and HC in the exhaust gas 9 and reduction of NOx, and removes the three harmful gas components in the exhaust gas 9 as harmless CO ₂ , H ₂ O, N it is intended to purify the _2, but in which the excess air ratio of 1.0 works only effective in the vicinity of the stoichiometric air-fuel ratio before and after, in a diesel engine air excess ratio is also 1.6 or more under normal running state The remaining oxygen amount in the exhaust gas 9 is too large to effectively use the three-way catalyst, and there has been a history of being applied only to gasoline vehicles so far.

  Note that the purification characteristics of the conventionally known three-way catalyst greatly change depending on the excess air ratio (air-fuel ratio), and when the excess air ratio is high (when the air-fuel ratio is low), the amount of residual oxygen in the exhaust gas 9 is large. Therefore, it is known that when the excess air ratio is low (when the air-fuel ratio is high), the reduction action becomes active and the oxidation action becomes inactive when the oxidation action becomes active and the reduction action becomes inactive. Yes.

  Zeolite, on the other hand, is an alumina silicate porous crystal material, characterized by having uniform molecular level pores regularly oriented in the crystal, through which various molecules are hollow or In addition to these properties, it has the property of adsorbing inside pores, and has the function of molecular sieving based on the uniform pores (adsorbs only molecules smaller than the pore diameter), crystal structure It also has the property of strongly adsorbing polar substances by the action of the cations and anions therein and the property of having a catalytic action.

Also, this type of zeolite is classified into various types based on the type of its backbone structure, the ability to adsorb NOx and NH ₃ into the pores, and the ability to promote the reaction between NOx and NH _3, What is necessary is just to select suitably the thing excellent in high heat resistance and high durability, and it is also possible to select from the zeolite related compounds provided with the same property.

Further, the diesel engine 1 is equipped with a fuel injection device 15 composed of an injector (not shown) provided for each cylinder 8, and an electromagnetic valve of each injector in the fuel injection device 15 is connected to an engine control computer (ECU). : The control device 16 forming the Electronic Control Unit) appropriately controls the injection timing and the injection amount (valve opening time) based on the load and the rotational speed, but when the accelerator is off while the engine is running, Premixed compression ignition that lowers the excess air ratio (λ) deeply to the extent that NH ₃ is generated from NOx and H ₂ in the exhaust gas 9 by the three-way catalyst 14 and can maintain smokeless combustion even at the low excess air ratio. It is supposed to be executed.

  However, when the accelerator is off while the engine is running, the engine output is not particularly required, such as when the vehicle is decelerating or when the vehicle is idling, so simply increase the fuel injection amount to increase the excess air ratio. In addition to reducing the air intake rate, the air throttle (the turbocharger 2 can be used as a variable geometry turbocharger to reduce the turbine efficiency to reduce the intake air amount) can be used to reduce the excess air ratio. To avoid unnecessary engine power.

In addition, the premixed compression ignition used in this case is that the main injection that should normally be performed in the vicinity of the compression top dead center is performed at a timing earlier than the compression top dead center, and the fuel is put into the cylinder 8 in advance. Refers to a combustion method that promotes premixing of fuel and then performs ignition and combustion, and is conventionally known as a combustion method for suppressing the generation of NOx However, NOx is more easily generated by lowering the excess air ratio deeply, and by using this NOx, NH ₃ can be generated and adsorbed on the three-way catalyst 14 as described later.

  Moreover, in this embodiment, when the forced regeneration of the particulate filter 13 becomes necessary, the main injection of fuel performed near the compression top dead center (crank angle 0 °) is followed by the compression top dead center. Post-injection is added at a late non-ignition timing (starting time is in the range of crank angle 90 ° to 120 °).

  That is, unburned fuel is added to the exhaust gas 9 by this post injection, and high-concentration HC generated from the unburned fuel undergoes an oxidation reaction while passing through the three-way catalyst 14, The inflow of the exhaust gas 9 raised in temperature by the reaction heat raises the bed temperature of the particulate filter 13 immediately after that to burn out the particulate filter 13 so that the particulate filter 13 can be regenerated.

  1 is an exhaust brake provided near the outlet of the turbine 2b in the exhaust pipe 11, 19 is an EGR line for recirculating the exhaust gas 9 from the exhaust side to the intake side, and 20 is the recirculation thereof. A water-cooled EGR cooler 21 for cooling a part of the circulated exhaust gas 9 and an EGR valve are respectively shown.

Thus, when the fuel injection device 15 and the intake valve 17 are controlled by the control device 16 when the accelerator is off while the engine is running and the excess air ratio is deeply reduced, the reduction action of the three-way catalyst 14 is activated and the NOx is increased. Becomes N ₂ , and N ₂ is combined with H ₂ in the exhaust gas 9 to generate NH _{3, which} is adsorbed and stored in the three-way catalyst 14 as it is.

That is, when premixed compression ignition is performed with the air excess ratio lowered deeply, the ratio of the fuel input amount to the unit air amount increases to a combustion temperature of 2000 K or more, and the moisture (H ₂ O) in the intake air is H ₂ , O ₂ , which decomposes thermally into OH and H and reacts with the originally stable N ₂ to generate NOx, but the NOx reduction reaction is promoted by the active reduction action of the three-way catalyst 14, and this During the reduction reaction, it reacts with excess H ₂ to produce NH ₃ .

  At this time, it is preferable to reduce the excess air ratio deeply to about 0.7. However, even if the excess air ratio is lowered so far, the pre-mixed compression ignition improves the combustibility significantly. It is possible to keep smokeless combustion without generating a large amount of soot).

  That is, after the main injection of fuel, which should normally be performed near the compression top dead center, is performed at a timing earlier than the compression top dead center, and fuel premixing is promoted by the advance injection of fuel into the cylinder. Since ignition and combustion are performed, combustion is performed by simultaneous whole area ignition in a state where the fuel is well dispersed and mixed and thinned uniformly, and the combustibility is remarkably improved.

  However, when the accelerator is off while the engine is running, the engine output is not particularly required, such as when the vehicle is decelerating or when the vehicle is idling, so simply increase the fuel injection amount to increase the excess air ratio. Not only lowering, but also using an air throttle as appropriate, the excess air ratio is lowered to prevent unnecessary engine output.

  In addition, you may make it use together control of the recirculation amount of the exhaust gas 9 by the EGR line 19 for adjustment of the excess air ratio at the time of such an accelerator off, and adjustment of the ignition timing of premix compression ignition.

Then, the accelerator-off state is released and the vehicle is shifted to a normal driving state where the accelerator is depressed, the excess air ratio is returned to a normal state of about 1.6 or more, and the three-way catalyst 14 is in an aerobic state. Then, NOx already adsorbed on the three-way catalyst 14 and newly adsorbed NOx are reduced and purified to N ₂ by NH ₃ stored when the accelerator is off.

  Further, when it is necessary to perform forced regeneration on the particulate filter 13, if the control device 16 performs fuel addition by post-injection in the fuel injection device 15 and starts forced regeneration, a large amount of oxygen remains in the diesel engine 1. The oxidation action of the three-way catalyst 14 exposed to the exhaust gas 9 is activated, and the high-concentration HC generated from the added fuel is oxidized well by the three-way catalyst 14, and the exhaust gas 9 heated by the reaction heat is heated. The bed temperature immediately after the particulate filter 13 is further raised by the inflow, and the particulates are efficiently incinerated in a short time.

  At this time, a part of the fuel of the main injection is distributed as after-injection at a combustible timing immediately after the main injection, and burned at a slightly later timing to lower the thermal efficiency of the diesel engine 1, The amount of heat not used for power may be increased to further increase the exhaust gas temperature.

  In addition, when the particulate filter 13 is forcibly regenerated, an air throttle that throttles the opening of the intake valve 17 can be used together. In this way, the intake 4 is throttled by the intake valve 17, and the diesel engine 1 The amount of generated exhaust gas 9 due to combustion of the gas decreases with respect to the amount of input heat, so that the exhaust temperature can be further increased.

  In addition, when the particulate filter 13 is forcibly regenerated, an exhaust throttle that narrows the opening of the exhaust brake 18 can be used together. In this way, the exhaust is throttled when the accelerator is off, and the exhaust on the upstream side of this is throttled. By increasing the pressure of the gas 9, the exhaust temperature is raised, and the exhaust resistance of the engine is increased, so that the intake air 4 having a relatively low temperature does not easily flow into the cylinder 8 and the exhaust gas having a relatively high temperature is exhausted. The exhaust gas temperature can be further increased by increasing the amount of residual gas 9 and the air in the cylinder 8 containing a large amount of the relatively high-temperature exhaust gas 9 being compressed in the next compression stroke to reach the explosion stroke.

  Further, in the present embodiment, since premixed compression ignition is performed in which the excess air ratio is deeply reduced when the accelerator is off while the engine is operating, the fuel injection of the diesel engine 1 is performed particularly when the accelerator is off during deceleration during traveling. The low-temperature intake air does not flow into the three-way catalyst 14 and the particulate filter 13 without going through the combustion stroke, and the three-way catalyst 14 and the particulate filter 13 are suddenly deprived of heat. In addition, since the high temperature exhaust gas 9 flows into the three-way catalyst 14 and the particulate filter 13 to increase the temperature actively, the regeneration of the particulate filter 13 is completed. The time it takes to be significantly reduced.

  In addition, when the accelerator is off when idling while the vehicle is stopped, the exhaust temperature can be significantly increased compared to the conventional extremely low exhaust temperature. It is possible to complete the warm-up in a short time, and the time required for the regeneration of the particulate filter 13 to be completed is greatly reduced.

Therefore, according to the above-described embodiment, the three-way catalyst 14 that has been applied only to gasoline vehicles so far is applied to the diesel engine 1 with the ability to adsorb NOx and NH ₃ , and the accelerator is operating. entrapment and to generate NH ₃ from NOx is adsorbed in the three-way catalyst 14 at the time of off, since the NOx in the exhaust gas 9 to be reduced and purified using the NH _3, since to keep reservoir urea water This eliminates the need for a urea water tank and eliminates the problems associated with securing a space for mounting the urea water tank and increasing costs, and also eliminates the need for infrastructure that allows the driver to easily obtain urea water. Therefore, it can be widely used as an exhaust emission control device that can be operated simply by supplying fuel indispensable for running the vehicle.

  Further, when the accelerator is off while the engine is operating, the bed temperature of the three-way catalyst 14 and the particulate filter 13 tends to be low, the exhaust air at a high temperature is exhausted from the diesel engine 1 by performing premixed compression ignition with the excess air ratio lowered deeply. Since the gas 9 can be fed and the bed temperature of the three-way catalyst 14 and the particulate filter 13 can be raised, the particulate filter 13 is used when the excess air ratio is returned to the normal vicinity and the forced regeneration is performed by adding fuel. It is possible to regenerate more efficiently and quickly, and the time taken to complete the regeneration of the particulate filter 13 can be greatly reduced.

  Note that the exhaust emission control device of the present invention is not limited to the above-described embodiment. When adding fuel at the time of forced regeneration, instead of post-injection at the diesel engine side, Direct injection can also be adopted, and both post injection and direct injection in the middle of the exhaust pipe may be used together as appropriate, and various other modifications may be made without departing from the scope of the present invention. Of course.

It is the schematic which shows an example of the form which implements this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Diesel engine 9 Exhaust gas 11 Exhaust pipe 13 Particulate filter 14 Three-way catalyst 15 Fuel injection apparatus 16 Control apparatus 17 Intake valve

Claims (2)

A three-way catalyst with NOx and NH ₃ adsorption capacity is installed in the middle of the exhaust pipe of the diesel engine, and when the accelerator is off while the engine is running, the three-way catalyst removes NH ₃ from NOx and H ₂ in the exhaust gas. An exhaust emission control device comprising a control device for causing the diesel engine to perform premixed compression ignition so that the excess air ratio is deeply reduced to the extent that it is generated and smokeless combustion can be maintained even at the low excess air ratio.   2. The exhaust emission control device according to claim 1, wherein a particulate filter is disposed immediately after the three-way catalyst so that fuel can be added to the exhaust gas upstream of the three-way catalyst.

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