JP2013245617A - Exhaust emission control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control system having a DOC, a DPF and an SCR, capable of controlling degradation in the fuel consumption, and improving the NOx purification performance.SOLUTION: A bypass pipe 6 with an NOx adsorbent 9 having a heater 8 being interposed therebetween is fitted to a conduit 1 on the upstream side of a DOC 2 via a flow path changing means 7. When the temperature of the DOC 2 is equal to or lower than the lower limit value α of the first temperature range, exhaust gas G is allowed to flow in the bypass pipe 6 to absorb NOx in the NOx adsorbent 9. When the differential pressure of the DPF 3 is equal to or higher than the pressure P, and when the temperature of the DOC 2 is below the lower limit of the first temperature range, the NOx adsorbent 9 is heated by the heater 8 to emit NOx. When the temperature of the DOC 2 is within the first temperature range, the NOx adsorbent 9 is heated only by the heat of exhaust gas G to emit NOx. When the temperature of the DOC 2 exceeds the upper limit of the first temperature range, the exhaust gas G is allowed to flow in the conduit 1, and the fuel is supplied to the DOC 2 to perform the forced regeneration.

Description

  The present invention relates to an exhaust gas purification system, and more specifically, in an exhaust gas purification system including a DOC, a DPF, and an SCR connected in series, exhaust gas purification that can suppress deterioration in fuel consumption and improve NOx purification performance. About the system.

As an example of a general diesel engine exhaust gas purification system, as shown in FIG. 6, an oxidation catalyst (DOC) 2 and a DPF (Diesel Particulate Filter) connected in series to the exhaust gas G flow path. ) 3 and selective catalytic reduction (SCR) 4 (for example, see Patent Document 1). In this exhaust gas purification system, the exhaust gas G of the diesel engine was decomposed and removed of hydrocarbons (HC) and carbon monoxide (CO) in DOC2, and then particulate matter (PM) was collected and removed in DPF3. Later, in the SCR 4, the nitrided oxide (NOx) is purified using ammonia (NH ₃ ) generated from the urea water supplied from the urea injection nozzle 5. The DOC ₂ also has a function of oxidizing nitrogen monoxide (NO) that occupies most of NOx contained in the exhaust gas G to generate nitrogen dioxide (NO ₂ ). By utilizing this function, it becomes possible to promote the combustion (PM regeneration) of PM collected by the DPF 3 and to improve the NOx purification efficiency of the SCR 4.

However, in the exhaust gas purification system described above, when the temperature of DOC2 becomes a low temperature of about 150 ° C. or less, such as during a cold start, DOC2 activity cannot be obtained sufficiently, and NO → NO ₂ production reaction occurs. It may be difficult to progress. Moreover, when the temperature of DOC2 becomes a high temperature of about 400 ° C. or more, the decomposition reaction of NO ₂ → NO becomes more dominant than the generation reaction. For this reason, it becomes difficult to supply NO ₂ having a sufficient concentration to the subsequent DPF 3 or SCR 4 under the condition that the temperature of the DOC 2 is low or high.

In the case where it is difficult to supply a sufficient concentration of NO ₂ to the subsequent DPF 3 or SCR 4 as described above, a part of the exhaust gas G is recirculated to the intake air, so that the combustion temperature is suppressed to a low level. It is facilitated by installing an exhaust gas recirculation (EGR) system that suppresses the generation of gas.

In such a situation, in the DPF 3, it becomes impossible to perform PM regeneration using the oxidizing power of NO ₂ , so forced regeneration that burns and removes PM by fuel injection is performed, which deteriorates fuel consumption. Become. Further, in the SCR4, decreases the reduction reaction of NOx by NH _3, there is a possibility that the NOx purification performance is decreased.

JP 2011-190720 A

  An object of the present invention is to provide an exhaust gas purification system that can suppress deterioration of fuel consumption and improve NOx purification performance in an exhaust gas purification system including a DOC, DPF, and SCR connected in series. .

  An exhaust gas purification system of the present invention that achieves the above object is provided with an exhaust gas that includes a DOC, a DPF, and an SCR connected in series toward the downstream side of an exhaust gas pipe of a diesel engine mounted on a vehicle. In the purification system, a bypass pipe provided with a NOx adsorbent having a heating means is attached to a pipe line upstream of the DOC via a flow path switching means, and the temperature of the DOC is set in advance. When the temperature is lower than the lower limit of the temperature range, the flow path switching means switches the exhaust gas flow path to the bypass pipe so that NOx in the exhaust gas is adsorbed to the NOx adsorbent, and the difference in DPF When the pressure is equal to or higher than a preset pressure and the temperature of the DOC is less than the lower limit of the first temperature range, the heating means is used after the adsorption amount of the NOx adsorbent is saturated. The NOx adsorbent is heated to desorb and release the adsorbed NOx, and when the DOC temperature is within the first temperature range, the NOx adsorbent is heated only by the heat of the exhaust gas. When the adsorbed NOx is desorbed and released, and the temperature of the DOC exceeds the upper limit of the first temperature range, the flow path switching means causes the exhaust gas flow path to pass through the pipe. The fuel is supplied to the DOC after switching to a road.

  According to the exhaust gas purification system of the present invention, even when the temperature of the DOC is low, NOx containing nitrogen dioxide is supplied to the subsequent DPF and SCR, so PM combustion by the oxidizing power of nitrogen dioxide in the DPF Is promoted, and the frequency of forced regeneration by fuel injection is reduced, so that deterioration of fuel consumption can be suppressed. Further, NOx in the exhaust gas is adsorbed and the NOx reduction reaction is promoted in the SCR by nitrogen dioxide contained in the supplied NOx, so that the NOx purification performance can be improved.

1 is a configuration diagram of an exhaust gas purification system according to an embodiment of the present invention. It is a flowchart explaining the function of ECU when the DOC temperature is less than a preset temperature in the exhaust gas purification system according to the embodiment of the present invention. It is a flowchart explaining the function of ECU when DPF in the exhaust gas purification system which consists of embodiment of this invention is at the time of PM reproduction | regeneration. It is a graph showing the temperature dependence of NO → NO ₂ formation reaction in the DOC. It is a graph showing the driving mode in an Example. It is a block diagram of the conventional exhaust gas purification system.

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

  FIG. 1 shows an exhaust gas purification system according to an embodiment of the present invention.

  This exhaust gas purification system includes a DOC 2, a DPF 3, and an SCR 4 that are sequentially arranged in series from the upstream side in a pipeline 1 through which exhaust gas G of a diesel engine flows.

  The DOC 2 is formed by supporting rhodium, cerium oxide, platinum, aluminum oxide or the like on a metal carrier formed into a structure having a function of mixing the exhaust gas G. The DPF 3 is formed of a monolith honeycomb type wall flow filter in which the inlets and outlets of the porous ceramic honeycomb channels (cells) are alternately plugged.

In addition, SCR4 contains titania-vanadia, β-type zeolite, chromium oxide, manganese oxide, molybdenum oxide, titanium oxide, tungsten oxide, etc. on a carrier such as a honeycomb structure formed of cordierite, aluminum oxide, titanium oxide or the like. NOx in the exhaust gas G is reduced and purified using NH ₃ formed by hydrolysis of urea water supplied from a urea injection nozzle 5 provided at the inlet and supported.

  A bypass pipe 6 is attached in parallel to the pipe line 1 on the upstream side of the DOC 2 via a flow path switching means 7. The bypass pipe 6 is provided with a NOx adsorbent 9 that is heated by a heater 8. Preferred examples of the NOx adsorbent 9 include a metal-supported zeolite system, a material containing a metal oxide or an alkaline earth metal compound, and the like.

  The flow path switching means 7 is composed of two switching valves 7a and 7b respectively installed at the branch part and the junction part of the pipe line 1 and the bypass pipe 6. The flow path of the exhaust gas G can be arbitrarily set by appropriately remotely controlling the opening degrees of the two switching valves 7a and 7b. The flow path switching means 7 is not limited to this configuration as long as the flow path of the exhaust gas G can be arbitrarily set.

  A first temperature sensor 10 and a second temperature sensor 11 that measure the temperature of the exhaust gas G are installed at the inlets of the DOC 2 and the DPF 3, respectively. The measured values of the temperature sensors 10 and 11 are used to estimate the temperatures of the DOC 2 and the DPF 3 that are generally difficult to directly measure. Further, a first NOx sensor 12 and a second NOx sensor 13 for measuring the NOx concentration in the exhaust gas G are respectively installed at the inlet of the DOC 2 and the pipe 1 upstream of the pipe 1 and the bypass branch. Further, a pair of pressure sensors 14a and 14b are disposed at the inlet and the outlet of the DPF 3, and the differential pressure of the DPF 3 can be obtained from the measured values of both.

  The flow path switching means 7, the heater 8, the two temperature sensors 10, 11, the two NOx sensors 12 and 13, and the pair of pressure sensors 14 a and 14 b are respectively connected to the ECU 15 by signal lines (indicated by alternate long and short dash lines). Connected through.

  The function of the ECU 15 in such an exhaust gas purification system will be described below with reference to FIGS. The flow path of the exhaust gas G is switched by operating the flow path switching means 7.

As shown in FIG. 2, the ECU 15 compares the temperature of the DOC 2 estimated from the measured value of the first temperature sensor 10 with a preset lower limit value α of the first temperature range (S10), and is less than the temperature α. If it is determined, the flow path is switched so that the exhaust gas G flows to the bypass pipe 6, and NOx in the exhaust gas G is adsorbed to the NOx adsorber 9 (S12). This first temperature range is a temperature range of the activation temperature K at which the generation reaction (conversion reaction) of NO → NO ₂ proceeds. As the lower limit α, for example, as shown in FIG. 4, a value of about 250 ° C. is exemplified under the condition that the abundance ratio of NO ₂ is 50% or more. Further, as the upper limit value β, a value of about 420 ° C. is exemplified.

  When it is determined that the temperature of the DOC 2 is equal to or higher than the temperature α, the purification process of the exhaust gas G similar to the conventional one is performed by maintaining or switching the flow path so that the exhaust gas G flows through the pipeline 1. Perform (S20).

  Next, it is determined from the measured value of the first NOx sensor 12 whether or not the adsorption amount of the NOx adsorbent 9 is saturated (S14), and when it is determined that it is saturated, the flow path is set so that the exhaust gas G flows through the pipe line 1. Switching (S16).

Then, as shown in FIG. 3, the ECU 15 compares the differential pressure of the DPF 3 obtained from the measured values of the pair of pressure sensors 14a and 14b with a preset pressure P that requires PM regeneration (S30). If it is determined that the temperature is above, it is further determined whether or not the temperatures of the DOC2 and DPF3 are within the appropriate temperature range (S32). The appropriate temperature range of DOC2 means the activation temperature K described above. Further, the temperature suitable range of DPF 3, means the temperature range (second temperature range), in particular a temperature range of about two hundred and fifty to three hundred and eighty ° C. is exemplified above is PM combustion by NO ₂ than the PM deposit .

  When the temperatures of DOC2 and DPF3 are within the appropriate temperature range, the flow path is switched so that the exhaust gas G flows through the bypass pipe 6 (S42), and the NOx adsorbent 9 is heated and adsorbed only by the heat of the exhaust gas G. The NOx being desorbed is desorbed and supplied to the subsequent DPF 3 and SCR 4.

  On the other hand, when the temperatures of DOC2 and DPF3 are not within the appropriate temperature range, the temperatures of DOC2 are compared (S34, S38). When the temperature of DOC2 exceeds the upper limit β, forced regeneration is performed by burning and removing PM by fuel injection. (S36). When the temperature of the DOC 2 is lower than the lower limit value α, the NOx adsorbent 9 is heated by the heater 8 to desorb the adsorbed NOx (S40), and then the exhaust gas G flows through the bypass pipe 6. Thus, the flow path is switched to supply NOx to the subsequent DPF 3 and SCR 4 (S42).

  Then, the ECU 15 compares the differential pressure of the DPF 3 obtained from the measured values of the pair of pressure sensors again with the pressure P that requires PM regeneration (S44). The flow path is switched so that the gas G flows through the pipe line 1 (S46), and the PM regeneration is finished. On the other hand, when it is determined that the differential pressure of the DPF 3 exceeds the pressure P, the measured value (first NOx concentration) of the first NOx sensor 12 and the measured value (second NOx concentration) of the second NOx sensor 13 are compared. If the second NOx concentration is higher (S48), it is assumed that NOx emission from the diesel engine continues, and the process returns to the step of comparing the differential pressure of the DPF 3 with the pressure P (S44). Further, when the second NOx concentration is equal to or lower than the first NOx concentration, the flow is switched so that the exhaust gas G flows through the pipe line 1 (S50), and then forced to remove PM by fuel injection. Reproduction is performed (S52).

Thus, even when the temperature is generated reaction of NO → NO ₂ of DOC2 it is low to hardly proceed, since then supplied the NOx containing NO ₂ from the bypass pipe 6 downstream of the DPF 3 and SCR4, DPF 3 In this case, PM combustion due to the oxidizing power of NO ₂ is promoted and the frequency of forced regeneration by fuel injection is reduced, so that deterioration of fuel consumption can be suppressed. Further, the adsorbed to the NOx adsorber 9 in the bypass pipe 6 NOx in the exhaust gas, so to supply the NO ₂ contained in the NOx in SCR4, since the reduction reaction of NOx with ammonia is promoted in SCR4, The NOx purification performance can be improved.

A vehicle of the same specification equipped with the exhaust gas purification system (example) of the present invention and the conventional exhaust gas purification system (comparative example) shown in FIG. 6 is run 10 times in the low-load running mode shown in FIG. Table 1 shows the experimental results comparing the amount of remaining PM and the amount of fuel consumed in the DPF 3 at the time.

  From the experimental results shown in Table 1, it can be seen that the exhaust gas purification system of the present invention suppresses the deterioration of fuel consumption compared to the conventional exhaust gas purification system.

1 Pipeline 2 DOC
3 DPF
4 SCR
5 Urea injection nozzle 6 Bypass pipe 7 Flow path switching means 8 Heater 9 NOx adsorber 10 First temperature sensor 11 Second temperature sensor 12 First NOx sensor 13 Second NOx sensors 14a, 14b A pair of pressure sensors 15 ECU

Claims (5)

In an exhaust gas purification system provided with DOC, DPF, and SCR connected in series toward the downstream side in an exhaust gas pipeline of a diesel engine mounted on a vehicle,
A bypass pipe provided with a NOx adsorbent having a heating means is attached to a pipe line upstream of the DOC via a flow path switching means,
When the temperature of the DOC is equal to or lower than a lower limit of a first temperature range set in advance, the flow path switching unit switches the flow path of the exhaust gas to the bypass pipe, and the NOx in the exhaust gas is Adsorbed on NOx adsorbent,
When the differential pressure of the DPF exceeds a preset pressure,
When the temperature of the DOC is lower than the lower limit of the first temperature range, after the adsorption amount of the NOx adsorbent is saturated, the NOx adsorbed by heating the NOx adsorbent by the heating means is desorbed. Let it release,
When the temperature of the DOC is within the first temperature range, the NOx adsorbent is heated only by the heat of the exhaust gas to desorb and release the adsorbed NOx,
When the temperature of the DOC exceeds the upper limit of the first temperature range, the fuel is supplied to the DOC after the flow path switching means switches the flow path of the exhaust gas to the pipeline. A featured exhaust gas purification system.   When the temperature of the DOC is in the first temperature range and the temperature of the DPF is in a preset second temperature range, the NOx adsorbent is heated only by the heat of the exhaust gas. The exhaust gas purification system according to claim 1, wherein the adsorbed NOx is desorbed and released. 3. The exhaust gas purification system according to claim 1, wherein the first temperature range is an activation temperature at which a generation reaction of NO → NO ₂ proceeds. If the second temperature range is less than the lower limit, the NO → NO ₂ formation reaction does not proceed, and if the second temperature range exceeds the upper limit, the NO ₂ → NO decomposition reaction prevails over the NO → NO ₂ generation reaction. The exhaust gas purification system according to claim 2 or 3. After the NOx adsorbed by heating the NOx adsorbent is desorbed and released, when the differential pressure of the DPF becomes equal to or lower than the preset pressure, the exhaust gas is switched by the flow path switching means. While switching the flow path to the conduit,
When the differential pressure of the DPF exceeds the preset pressure, the first NOx concentration of the exhaust gas at the DOC inlet and the second NOx of the exhaust gas at the exhaust outlet of the diesel engine Compare the concentration with
When the second NOx concentration is greater, the differential pressure of the DPF is again compared with the preset pressure. When the second NOx concentration is equal to or lower than the first NOx concentration, the flow path switching is performed. The exhaust gas purification system according to any one of claims 1 to 4, wherein fuel is supplied to the DOC after the exhaust gas flow path is switched to the pipe line by means.

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