JP4363074B2 - Exhaust gas purification system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification system that includes a continuously regenerating diesel particulate filter and purifies engine exhaust gas.
[0002]
[Prior art]
Emissions of particulate matter (PM: particulate matter: hereinafter referred to as PM) emitted from diesel engines have been strengthened year by year along with NOx, CO and HC. Improvement alone has made it impossible to respond. Therefore, a technology has been developed that collects PM discharged from the engine with a filter called a diesel particulate filter (DPF) and reduces the amount of PM discharged to the outside. Yes.
[0003]
The DPF that directly collects PM includes ceramic monolith honeycomb wall flow type filters and fiber type filters made of ceramics and metal fibers. Exhaust gas purification using these DPFs Similar to other exhaust gas purification systems, the system is installed in the middle of the exhaust passage of the engine and purifies exhaust gas generated by the engine.
[0004]
When the DPF collects PM, the exhaust pressure rises in proportion to the amount collected, so it is necessary to regenerate the DPF by removing the collected PM by burning or the like. Various methods have been proposed for this reproduction method, and there are an intermittent reproduction system and a continuous reproduction system.
[0005]
And most of the systems of the intermittent regeneration method have an exhaust purification system of two exhaust systems for PM collection and regeneration of the collected PM, and at the time of PM collection by a filter of each system The regeneration is switched alternately, and regeneration is performed by a system dedicated to removing PM, and there are an electric heater heating type, a burner heating type, a backwash type and the like.
[0006]
However, this intermittent regeneration system has advantages that regeneration is not easily influenced by operating conditions and regeneration efficiency is high, but PM is burned by receiving energy supply from the outside, resulting in deterioration of fuel consumption. In addition, there is a problem that the amount of collection of one filter increases, and combustion control becomes difficult at the time of re-combustion. In addition, since a two-system DPF system is required, the system becomes large and complicated, and the cost becomes high, so there is a problem that it is not suitable for a passenger car.
[0007]
On the other hand, the continuous regeneration system is a system proposed to solve the problems of the intermittent regeneration system, and the PM oxidation start exhaust temperature is higher than that of a normal filter due to the PM oxidation reaction by the catalyst and NO _2. This is a system that realizes continuous regeneration of PM by lowering the regeneration temperature of PM using an oxidation catalyst, and oxidizes PM with exhaust heat from the engine to regenerate DPF without receiving energy from the outside. In this case, since PM regeneration is basically continuous, there is an advantage that a simpler one-system DPF system is obtained and regeneration control is simplified.
[0008]
The NO ₂ regeneration type DPF system 1X shown as an example in FIG. 8 is a system for regenerating DPF by oxidizing PM with NO ₂ (nitrogen dioxide), and an oxidation catalyst 3Aa is disposed upstream of a normal wall flow filter 3Ab. Then, NO (nitrogen monoxide) in the exhaust gas is oxidized, and the generated NO ₂ oxidizes the PM collected by the downstream filter 3Ab to CO ₂ (carbon dioxide), and the PM is removed. Yes. Since this NO ₂ has a lower energy barrier than O _2, it lowers the PM oxidation temperature (DPF regeneration temperature), and it is assumed that PM combustion occurs continuously with thermal energy in the exhaust gas without supplying energy from the outside. ing. However, this regeneration temperature is affected by the NO ₂ concentration.
[0009]
In FIG. 8, E is a diesel engine, 2 is an exhaust passage, 4 is a fuel pump system, 5 is an electronic control box, 7 is a battery, 8 is a silencer, and 9 is a fuel tank.
[0010]
FIG. 9 shows an improved system 1Y of the NO ₂ regeneration type DPF system shown in FIG. In this improved system 1Y, the porous catalyst coat layer 31 of the oxidation catalyst 32A is applied to the porous wall surface 30 of the wall flow filter 3B, and oxidation of NO and oxidation of PM by NO ₂ generated thereby are performed, and the wall flow filter 3B. The system is simplified to be configured to do on the wall surface. However, since the catalyst is applied to the wall surface of the wall flow filter 3B, the initial filter pressure loss tends to increase.
[0011]
In FIG. 10, a porous catalyst coating layer 31 of an oxidation catalyst 32A such as platinum and a PM oxidation catalyst 32B such as an oxide is applied to the porous wall surface 30 of the wall flow filter 3C, and the PM accumulated in the filter 3C is applied. A system 1Z that directly burns with a catalyst at low temperature and continuously regenerates is shown. This system 1Z has the advantage that it is not affected by the NO ₂ concentration like the system 1X, but it has a problem that it is difficult to burn PM mainly composed of solid soot with a catalyst because of the contact surface area. .
[0012]
However, in these continuous regeneration systems, the oxidation start temperature of PM is lowered, but an exhaust temperature of about 350 ° C. is still necessary. Therefore, under engine operating conditions such as low load operation and idle operation, the exhaust temperature And there is a problem that PM regeneration does not occur, and there is a problem that the oxidation start temperature of PM rises under operating conditions with a small amount of NOx emission.
[0013]
Therefore, if the engine operating state where the exhaust temperature is low, such as idling or low load, or the low NOx engine operating state continues, PM does not become oxidized even if PM accumulates. There is a risk of inviting and troubles such as engine stoppage.
[0014]
Therefore, in these continuous regeneration type DPF systems, it is necessary to regenerate the DPF by calculating the PM accumulation amount in the filter from the engine operating conditions or estimating the PM accumulation amount from the filter pressure loss corresponding to the PM accumulation amount. When the conditions are set and this DPF regeneration requirement is satisfied, the exhaust gas temperature is forcibly raised, and the accumulated PM is forcibly burned and removed. If there is a large amount of PM regeneration, a large amount of accumulated PM burns rapidly, causing abnormal combustion in which the temperature in the filter becomes high, and the filter is damaged or damaged by cracks. is there.
[0015]
In order to solve these problems, a system has been proposed in which the filter is formed so as to be able to be energized, and the PM collected by the filter is burned and removed by the heat generated by the electrical resistance of the filter.
[0016]
As one of them, in order to burn and remove PM at a relatively low temperature, a filter is formed with a porous molded body having excellent heat resistance and shaking electric conductivity, and this filter itself according to the increase in the internal pressure of the filter. A PM trap filter has been proposed in which the PM is combusted and removed by raising the temperature above the ignition temperature of PM by resistance heating due to this energization (see, for example, Patent Document 1).
[0017]
In addition, it is composed of a self-heatable filter formed of a composite made of porous conductive ceramics and ceramics having PTC characteristics. The filter is energized and heated, and temperature controlled by PTC characteristics, An exhaust gas treatment filter that oxidatively decomposes adsorbed PM has been proposed (see, for example, Patent Document 2).
[0018]
[Patent Document 1]
Japanese Patent Laid-Open No. 07-253013 [Patent Document 2]
Japanese Patent Laid-Open No. 06-323129
[Problems to be solved by the invention]
However, when these filters are formed so that they can be energized and the filter itself is energized, the filter material is special and the heating energy required to heat the entire filter increases. This increases the efficiency of the engine system as a whole and causes a problem that fuel efficiency deteriorates.
[0020]
The present invention has been made in order to solve the above-mentioned problems, and its purpose is low power consumption and sufficient regeneration capacity without being affected by engine operating conditions such as exhaust gas flow rate and exhaust gas temperature. And an exhaust gas purification system capable of performing stable PM regeneration.
[0021]
[Means for Solving the Problems]
An exhaust gas purification system for achieving the above object is disposed in an exhaust passage of an engine and purifies particulate matter in exhaust gas in an exhaust gas purification system that purifies particulate matter in exhaust gas. The filter body is formed of a wall flow type honeycomb structure of porous ceramics having heat resistance and non-conductivity, and a blocking member for staggered plugging at both ends of the filter body is formed of electrodes. The electrode is provided with an overhanging portion that protrudes from the honeycomb wall to the inside of the channel so that the aperture diameter of the open portion on the opposite side of the plug is smaller than the aperture of the honeycomb channel. In addition to constituting the inlet-side electrode and the outlet-side electrode of the filter body, it is always necessary to purify particulate matter between the inlet-side electrode and the outlet-side electrode at both ends of the filter body. When the particulate matter is accumulated between the inlet side electrode and the outlet side electrode, the particulate matter is accumulated between the protruding portion and the plugged portion of the electrode on the opposite side. It is formed and configured to be conductive.
[0022]
In the exhaust gas purification system, the filter body is formed of cordierite. The exhaust gas purification system can also be applied to a continuous regeneration exhaust gas purification system using at least one of an oxidation catalyst and a PM oxidation catalyst.
[0023]
The present invention relates to an exhaust gas purification system using a wall flow type honeycomb structure filter, by arranging electrodes made of metal or the like in the closed portions at both ends of the filter body, and applying a high voltage to the electrodes. Since the PM in the PM is an electrical conductor and has conductivity, as the PM accumulates, a current flows directly through the PM, and the PM directly generates heat and burns and is removed.
[0024]
Therefore, the regeneration capability is not affected by engine operating conditions such as the exhaust gas flow rate and temperature, and stable and reliable PM regeneration can be performed under all operating conditions.
[0025]
Further, when the amount of accumulated PM that can be energized is reached, energization, heat generation, and combustion are immediately regenerated, so that an increase in exhaust pressure due to clogging of the filter is small, and deterioration in fuel consumption of the engine is reduced.
[0026]
In addition, when the PM accumulation amount is small and energization is not possible, the energization is not performed, and the entire filter main body is not heated to regenerate PM, but only the portion where the PM trapping amount exceeds a certain level. Since the heat is regenerated by energization, the heat loss is small, the regeneration can be performed with low energy, and the filter of the exhaust gas purification system can be efficiently regenerated with a small amount of power consumption.
[0027]
In addition, since regeneration can be performed while PM is being collected by a filter, a continuous regeneration system can be provided, and two filter systems for regeneration and collection are not required. Can be miniaturized.
[0028]
Furthermore, when applied to a continuously regenerative exhaust gas purification system using at least one of an oxidation catalyst and a PM oxidation catalyst, the combustion start temperature (oxidation start temperature) of PM can be lowered, so that a lower applied voltage, and More efficient playback with less power consumption.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an exhaust gas purification system according to an embodiment of the present invention will be described with reference to the drawings.
[0030]
FIG. 1 shows the configuration of the exhaust gas purification system 1. In the exhaust gas purification system 1, an electrode-attached DPF 3 is provided between the engine E in the exhaust passage (exhaust pipe) 2 connected to the exhaust manifold of the diesel engine E and the silencer 8. The arrangement of the electrode-attached DPF 3 is preferably close to the engine E in consideration of the regenerative energy.
[0031]
The diesel engine E also injects fuel supplied from the fuel tank 9 under the control of an electronic control box (ECU: engine control unit) 5 that is supplied by the fuel pump system 4 and uses the battery 7 as a power source. The generated exhaust gas G passes through the exhaust passage 2 to become exhaust gas Gc purified by the electrode-attached DPF 3 and is discharged into the atmosphere via the silencer 8.
[0032]
As shown in FIGS. 2 to 5, the electrode-attached DPF 3 includes a filter body 11, an inlet side electrode 12, and an outlet side electrode 13, and the inlet side electrode 12 and the outlet side electrode 13 are connected to a power source 14, and During the operation, a high voltage Va is constantly applied to the filter body 11 between the inlet side electrode 12 and the outlet side electrode 13.
[0033]
The voltage Va applied to the electrodes 12 and 13 is increased by a booster (not shown). Preferably, this applied voltage Va is efficiently re-burned by the amount of collected PM and engine outlet exhaust pressure. Electronic control to occur.
[0034]
This filter body 11 is made of cordierite, and is formed of a monolith honeycomb wall flow type filter in which the inlet and outlet of a channel (passage: cell) 11c partitioned by a porous ceramic wall 11w are alternately sealed. Is done.
[0035]
The staggered plugging on the inlet side is performed by a closing member formed of a metal material that becomes the inlet side electrode 12, and the staggered plugging on the outlet side is formed of a metal material that becomes the outlet side electrode 13. With a closed member. The electrodes 12 and 13 seal the both ends of the honeycomb filter. As shown in FIGS. 3 and 5, the apertures 12h and 13h in the open portion have a diameter larger than that of the honeycomb channel 11c. The electrodes 12 and 13 are formed so as to protrude from the honeycomb wall 11w to the inside of the channel 11c.
[0036]
In this configuration, PM (particulate matter) in the exhaust gas G is collected on the surface of the wall 11w when the exhaust gas G passes through the porous ceramic wall 11w.
[0037]
Next, filter regeneration will be described.
[0038]
As shown in FIG. 5, when PM is not collected and accumulated on the wall 11w of the filter, the cordierite forming the filter body 11 is an insulating material having a very large electric resistance. Even if the high voltage Va is applied to the electrode 12 and the outlet electrode 13 by the power supply 14, no current flows and no power consumption occurs in the ammeter 15 as shown in the figure.
[0039]
On the other hand, as shown in FIG. 6, when PM is collected and accumulated on the wall 11 w of the filter, PM (Soot), which is the main component of PM, has conductivity and a current flows. If the voltage Va is applied between the outlet side electrode 13 and the outlet side electrode 13, the gap between the protruding portion 12a of the hole 12h in the open portion of the inlet side electrode 12 and the plugged portion 13a of the outlet side electrode 13 is accumulated. Conducted by PM.
[0040]
This conduction, current flows, heated and directly PM is heated by the electric resistance of the PM, and ignited to become more ignition combustion temperature combustion becomes CO ₂ is removed. Since this combustion propagates and combusts in the downstream direction along the flow of the exhaust gas G, the PM accumulated on the wall 11w of the filter is removed by combustion.
[0041]
And when PM is burned and removed, there is no PM to conduct electricity, so no current flows. While this is repeated and collected, the PM accumulated above a certain level is heated by an electric current to be removed by combustion.
[0042]
That is, when the voltage Va is constantly applied to the electrodes 12 and 13 and PM is collected and the electric resistance between the electrodes 12 and 13 is reduced by the PM and energization occurs, the PM in that portion generates heat, combustion occurs and regeneration occurs. Done.
[0043]
When the PM is burned and removed, it does not conduct unless the accumulated amount of PM exceeds a certain level. Therefore, power consumption does not occur, and conduction only occurs when PM exceeding a certain level is collected, and a part of the PM is burned and removed. Since the conduction is lost and the combustion is propagated by the heat generated by the combustion of the PM, the PM can be efficiently burned and removed with a very small amount of power consumption.
[0044]
In addition, the entire filter body 11 is not energized, but the accumulated PM is locally energized. If the accumulated amount of PM is locally large, the heat generated by the current in that portion increases, The efficiency is very good.
[0045]
Next, the voltage Va applied to the electrodes 12 and 13 will be described.
[0046]
The applied voltage Va of the electrodes 12 and 13 may be a constant voltage, but in order to use power energy more efficiently, the PM emission amount is calculated from the engine operating state such as engine rotation and load, and the PM When engine operating conditions with a large amount of discharge continue, control to increase the applied voltage Va is performed to speed up the regeneration of PM. This applied voltage control flow is shown in FIG.
[0047]
The applied voltage control flow of FIG. 7 starts with the start of the engine. While the engine is operating, the steps from Step S11 to Step S16 are repeatedly executed, and the control is stopped and stopped when the engine is stopped. is there. When the applied voltage control flow of FIG. 7 starts, the applied voltage Va is set to the preset initial applied voltage V1 in step S11. Also, the voltage rise time tu is set to zero.
[0048]
In the next step S12, the PM emission amount Mc is calculated from the engine rotational speed Ne and the engine load Q with reference to map data inputted in advance.
[0049]
In the next step S13, the measured exhaust pressure Pm and the PM discharge amount Mc are checked, the measured exhaust pressure Pm is smaller than a predetermined determination value P1, and the PM exhaust amount Mc is smaller than a predetermined determination value M1. In this case, the process returns to step S12 without changing the applied voltage Va.
[0050]
If it is determined in step S13 that the measured exhaust pressure Pm is greater than the predetermined determination value P1 or the PM discharge amount Mc is greater than the predetermined determination value M1, the process goes to step S14 to apply the voltage Va applied to the electrode. Is increased (Va = Va + ΔVa), and the voltage increase time tu is counted (tu = tu + Δtu), and then, in step S15, which goes to step S15, it is checked whether the set energization time ta is equal to or less than the predetermined determination value t1. To do. If the voltage increase time tu is equal to or shorter than the predetermined set energization time t1, the process returns to step S14, and the applied voltage Va is increased (Va = Va + ΔVa) in step S14 until the voltage increase time tu exceeds the predetermined set energization time t1. If the voltage rise time tu exceeds the predetermined set energization time t1, the process returns to step S11, the applied voltage Va is returned to the initial applied voltage V1, and step S12 and subsequent steps are repeated.
[0051]
Steps S11 to S15 are repeated, an interrupt is generated when the engine key is turned off, and the control flow is stopped and terminated.
[0052]
By the control according to the applied voltage control flow, when the PM discharge amount Mc is low and when the measured exhaust pressure Pm is low, the applied voltage Va is continued as the predetermined initial applied voltage V1, and the engine operating state is PM discharged. When the amount Mc is large, or when clogging progresses and the measured exhaust pressure Pm increases, the applied voltage Va continues to rise until the voltage rise time tu exceeds the predetermined set energization time t1, and the voltage When the rising time tu exceeds the predetermined set energization time t1, the applied voltage Va can be returned to the predetermined initial applied voltage V1.
[0053]
Note that the present invention is not limited to the exhaust gas purification system 1 described above, but can be applied to other DPF systems, in particular, continuous regeneration type DPF systems.
[0054]
For example, in the NO ₂ regeneration type DPF system 1X in which the oxidation catalyst 3Aa as shown in FIG. 8 is arranged in front of the filter 3Ab, the filter 3Ab may be a filter with an electrode, or the NO ₂ regeneration type DPF system as shown in FIG. The wall flow filter 3B carrying the oxidation catalyst 32A of the system 1Y is used as a filter with an electrode carrying the oxidation catalyst 32A, or an oxidation catalyst 32A such as platinum and a PM oxidation catalyst such as oxide as shown in FIG. The wall flow filter 3C carrying 32B can be a filter with an electrode carrying an oxidation catalyst 32A such as platinum and a PM oxidation catalyst 32B such as oxide.
[0055]
According to the continuous regeneration type DPF system to which the filter with an electrode of the present invention is applied, the combustion start temperature of PM is lowered, so that the DPF can be regenerated more efficiently with less power consumption. .
[0056]
【The invention's effect】
As described above, according to the exhaust gas purification system of the present invention, since the electrodes to which voltage is constantly applied are arranged at both ends of the filter body, when PM accumulates and can be energized, this PM A current flows locally in the accumulated portion, and this current causes the PM to generate heat and ignite, and combustion is started.
[0057]
Therefore, regeneration performance is not affected by engine operating conditions such as exhaust gas flow rate and temperature, and stable and reliable PM regeneration can be performed under all operating conditions. In addition, PM does not accumulate to the accumulation amount enough to shift to runaway combustion, and the filter can be prevented from being melted or broken.
[0058]
Further, when the amount of accumulated PM that can be energized is reached, energization, heat generation, and combustion are immediately regenerated, so that an increase in exhaust pressure due to clogging of the filter is small, and deterioration in fuel consumption of the engine is reduced.
[0059]
In addition, since the PM is directly heated rather than energizing the filter body to heat the entire filter, the heat loss is small, and only when the PM is accumulated, the current flows only in the accumulated portion. When PM is burned out, current stops flowing again, so power consumption becomes very small.
[0060]
In addition, since this exhaust gas purification system can regenerate while collecting PM with a filter, it becomes a continuous regeneration type DPF system, which can be downsized as a single DPF system, and also has regeneration control. However, the application of a constant voltage or the control of the applied voltage is very simple control.
[0061]
Further, in a continuous regeneration type exhaust gas purification system using at least one of an oxidation catalyst and a PM oxidation catalyst, the combustion start temperature (oxidation start temperature) of PM is lowered, and therefore, with a lower applied voltage and less power consumption. Can be played more efficiently.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an exhaust gas purification system according to an embodiment of the present invention.
FIG. 2 is a diagram schematically showing a filter main body and electrodes according to the present invention.
FIG. 3 is a partial cross-sectional view schematically showing the structure of a filter body according to the present invention.
FIG. 4 is a partially enlarged cross-sectional view showing a configuration of a filter wall according to the present invention.
FIG. 5 is a diagram showing a state where PM is not deposited on the wall of the filter.
FIG. 6 is a diagram showing a state where PM is deposited on the wall of the filter and combustion is started by energization.
FIG. 7 is a diagram illustrating an example of an applied voltage control flow.
FIG. 8 is a system configuration diagram showing an example of a conventional exhaust gas purification system.
FIG. 9 is a system configuration diagram showing another example of a conventional exhaust gas purification system.
FIG. 10 is a system configuration diagram showing another example of a conventional exhaust gas purification system.
[Explanation of symbols]
1 Exhaust gas purification system 2 Exhaust passage (exhaust pipe)
3 DPF with electrode
DESCRIPTION OF SYMBOLS 11 Filter main body 12 Inlet side electrode 13 Outlet side electrode 14 Power supply E Diesel engine G Exhaust gas Gc Purified exhaust gas Va Applied voltage (high voltage)

Claims (1)

In an exhaust gas purification system that is disposed in the exhaust passage of an engine and purifies particulate matter in the exhaust gas, the filter body that purifies particulate matter in the exhaust gas is porous with heat resistance and non-conductivity It is formed of a ceramic wall flow type honeycomb structure, and a blocking member for staggered plugging at both ends of the filter body is formed by an electrode. The electrode has a hole in an open portion opposite to the plugging. Provided an overhanging portion that protrudes from the honeycomb wall to the inside of the channel such that the diameter of the filter is smaller than the diameter of the honeycomb channel, and these electrodes constitute an inlet side electrode and an outlet side electrode of the filter body, and When purifying particulate matter between the inlet-side electrode and the outlet-side electrode at both ends of the filter body, a voltage is always applied to form a particulate between the inlet-side electrode and the outlet-side electrode. When the quality has accumulated, the exhaust gas purification system, characterized in that between the protruding portion and plugged portion opposite the electrode was formed to conduct the accumulated particulate matter.

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