Exhaust purification device for internal combustion engine

Abstract

An exhaust gas purification device includes a diesel particulate filter (DPF) that collects particulate matter (PM) from an exhaust gas, a urea water spray unit that sprays urea water into the exhaust gas, a selective catalytic reduction (SCR) device that reduces and purifies NO.sub.x of the exhaust gas, a capacitance detecting unit that detects capacitance of the DPF, a PM accumulation calculating unit that calculates an amount of accumulated PM on the basis of the capacitance, an NO.sub.2 consumption estimating unit that calculates an amount of consumed NO.sub.2 on the basis of the amount of accumulated PM, and a control unit that controls an engine such that a ratio of NO to NO.sub.2 flowing into the SCR approaches 1:1 on the basis of the estimated consumption.

Claims

1. An exhaust gas purification device of an internal combustion engine, comprising: a first filter in an exhaust passage of the internal combustion engine and configured to collect particulate matter in an exhaust gas; a urea water injector at the exhaust passage downstream of the first filter and configured to spray urea water into the exhaust gas; a selective catalytic reduction catalyst in the exhaust passage downstream of the urea water injector and configured to reduce and purify a nitrogen compound in the exhaust gas by using ammonia produced from the urea water; a bypass passage that branches off from the exhaust passage at a position upstream of the first filter so as to bypass the first filter and return to the exhaust passage upstream of the selective catalytic reduction catalyst; a second filter in the bypass passage and configured to collect particulate matter in the exhaust gas flowing through the bypass passage; an electrostatic capacity detector configured to detect an electrostatic capacity of the second filter; and an electronic control unit configured to calculate an amount of accumulated particulate matter collected by the second filter, based on the detected electrostatic capacity, estimate an amount of nitrogen dioxide consumed by the particulate matter that has accumulated in the second filter, based on the calculated amount of accumulated particulate matter, and control a combustion state of the internal combustion engine such that a ratio of nitrogen monoxide to nitrogen dioxide in the exhaust gas flowing into the selective catalytic reduction catalyst approaches 1 to 1, based on the estimated amount of consumed nitrogen dioxide, wherein the electrostatic capacity detector includes a pair of electrodes disposed in a corresponding pair of cells that oppose each other with at least one cell in the second filter interposed therebetween.

2. The exhaust gas purification device of an internal combustion engine according to claim 1, wherein when a forced regeneration of the second filter is carried out, the pair of electrodes functions as a heater.

3. The exhaust gas purification device of an internal combustion engine according to claim 1, wherein a capacity of the second filter is smaller than a capacity of the first filter.

4. The exhaust gas purification device of an internal combustion engine according to claim 1 further comprising an orifice disposed in the bypass passage for adjusting a flow rate of the exhaust gas flowing through the bypass passage.

5. The exhaust gas purification device of an internal combustion engine according to claim 1, wherein the internal combustion engine is a diesel engine.

6. The exhaust gas purification device of an internal combustion engine according to claim 5, wherein the first filter and the second filter are diesel particulate filters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an overall configuration diagram schematically illustrating an exhaust gas purification device of an internal combustion engine according to an embodiment of the present invention.

(2) FIG. 2 illustrates an exemplary map for calculating an amount of consumed NO2 from the amount of accumulated PM in the exhaust purification gas device of the internal combustion engine according to an embodiment of the present invention.

(3) FIG. 3 is an overall configuration diagram schematically illustrating an exhaust gas purification device of an internal combustion engine according to another embodiment of the present invention.

DETAILED DESCRIPTION

(4) Hereinafter, with reference to FIGS. 1 and 2, an exhaust gas purification device of an internal combustion engine according to embodiments of the present invention will be described. Identical parts are given identical reference numerals and symbols, and their names and functions are identical as well. Therefore, detailed description of such parts will not be repeated.

(5) As illustrated in FIG. 1, a diesel engine (hereinafter, simply referred to as engine) 10 has an intake manifold 10a and an exhaust manifold 10b. An intake passage 11 for introducing fresh air is connected to the intake manifold 10a, and an exhaust passage 12 for discharging an exhaust gas to the atmosphere is connected to the exhaust manifold 10b. A pre-stage (upstream) post-treatment device 14 and a post-stage (downstream) post-treatment device 20 are provided in the exhaust passage 12. The pre-stage post-treatment device 14 is arranged upstream of the post-stage post-treatment device 20.

(6) The pre-stage post-treatment device 14 is constituted by a diesel oxidation catalyst (hereinafter referred to as DOC) 15 and a DPF 16 disposed in a casing 14a. The DOC 15 is arranged upstream of the DPF 16. An in-pipe injection device 13 is provided upstream of the DOC 15. A DPF inlet temperature sensor 18 is provided upstream of the DPF 16. A DPF outlet temperature sensor 19 is provided downstream of the DPF 16.

(7) The in-pipe injection device 13 injects unburned fuel (primarily HC) into the exhaust passage 12, in response to an instruction signal from an electronic control unit (hereinafter referred to as ECU) 40. The in-pipe injection device 13 may be omitted if post-injection through multiple-injection of the engine 10 is carried out.

(8) The DOC 15 includes, for example, a ceramic carrier having a cordierite honeycomb structure, with a catalyst component supported on a surface of the ceramic carrier. Upon unburned fuel (primarily HC) being supplied by the in-pipe injection device 13 or through post-injection, the DOC 15 oxidizes the unburned fuel, thereby causing the exhaust gas temperature to rise. The DOC 15 also oxidizes NO in the exhaust gas to produce NO.sub.2, thereby causing the ratio of NO.sub.2 to NO in the exhaust gas to increase.

(9) The DPF 16 includes, for example, a number of cells defined by porous partition walls disposed along a flowing direction of the exhaust gas. The cells are plugged alternatingly at the upstream side and the downstream side. The DPF 16 collects PM in the exhaust gas into the small cavities and on the surfaces of the partition walls. Upon the amount of accumulated PM reaching a predetermined amount, a so-called forced regeneration for burning and removing the PM is performed. The forced regeneration is performed by supplying unburned fuel (primarily HC) to the DOC 15 by the in-pipe injection device 13 or through post-injection and by raising the temperature of the DPF 16 to the PM-burning temperature (e.g., approximately 600 degrees C.).

(10) The DPF 16 of this embodiment is provided with a pair of electrodes 17a and 17b disposed inside a corresponding pair of cells that oppose each other with at least one cell interposed therebetween. The paired electrodes 17a and 17b form a capacitor. The paired electrodes 17a and 17b are electrically connected to the ECU 40.

(11) The DPF inlet temperature sensor 18 detects the temperature of the exhaust gas flowing into the DPF 16 (hereinafter referred to as inlet temperature T.sub.IN). The DPF outlet temperature sensor 19 detects the temperature of the exhaust gas flowing out of the DPF 16 (hereinafter referred to as outlet temperature T.sub.OUT). The inlet temperature T.sub.IN and the outlet temperature T.sub.OUT are introduced to the ECU 40 that is electrically connected to the DPF inlet temperature sensor 18 and the DPF outlet temperature sensor 19.

(12) The post-stage post-treatment device 20 includes a urea water spraying device 21 and an SCR catalyst 22 disposed in a casing 20a. The urea water spraying device 21 is arranged upstream of the SCR catalyst 22.

(13) The urea water spraying device 21 sprays or injects urea water (urea solution) from a urea water tank (not illustrated) into the exhaust passage 12 between the pre-stage post-treatment device 14 and the post-stage post-treatment device 20, in response to an instruction signal from the ECU 40. The sprayed urea water undergoes hydrolysis with the heat of the exhaust gas, and ammonia (NH.sub.3) is produced. Ammonia (NH.sub.3) is then supplied to the SCR catalyst 22 on the downstream side as a reducing agent.

(14) The SCR catalyst 22 includes, for example, a ceramic carrier having a honeycomb structure, with a copper zeolite or an iron zeolite supported on a surface of the ceramic carrier. The SCR catalyst 22 adsorbs ammonia (NH.sub.3) supplied as the reducing agent and reduces and purifies NOx in the exhaust gas passing therethrough with the adsorbed ammonia (NH.sub.3).

(15) The ECU 40 controls the engine 10, the in-pipe injection device 13, the urea water spraying device 21, and other components, and includes known CPU, ROM, RAM, input port, output port, and so on. The ECU 40 further includes, as part of its functional elements, an electrostatic capacity calculating unit 41, a PM accumulation amount calculating unit 42, a NO.sub.2 consumption amount estimating unit 43, a NOx inflow ratio calculating unit 44, and a NOx inflow ratio adjusting unit 45. The description continues with a premise that these functional elements are included in the ECU 40, which is an integrated piece of hardware, but some of these functional elements can be provided in a separate piece of hardware. In this embodiment, the electrostatic capacity calculating unit 41 and the electrodes 17a and 17b constitute an electrostatic capacity detecting unit according to the present invention.

(16) The electrostatic capacity calculating unit 41 calculates an electrostatic capacity (capacitance) C between the electrodes 17a and 17b on the basis of signals entered from the paired electrodes 17a and 17b. The electrostatic capacity C is calculated by Expression 1, where represents a dielectric constant of a medium between the electrodes 17a and 17b, S represents the area of the electrodes 17a and 17b, and d represents the distance between the electrodes 17a and 17b.

(17) $\begin{array}{cc}C=\mathrm{.Math.}\frac{S}{d}& \mathrm{Expression}1\end{array}$

(18) The PM accumulation amount calculating unit 42 calculates the amount of accumulated PM collected by the DPF 16, on the basis of the electrostatic capacity C calculated by the electrostatic capacity calculating unit 41 and an average T.sub.AVE of the inlet temperature T.sub.IN detected by the DPF inlet temperature sensor 18 and the outlet temperature T.sub.OUT detected by the DPF outlet temperature sensor 19. The amount of accumulated PM can be calculated by using an approximation formula, a map, or the like, which may be prepared or obtained in advance through an experiment or the like.

(19) The NO.sub.2 consumption amount estimating unit 43 estimates the amount of NO.sub.2 consumed by the PM that has accumulated in the DPF 16, on the basis of the amount of accumulated PM calculated by the PM accumulation amount calculating unit 42. More specifically, the ECU 40 stores a consumption amount map (see FIG. 2) that defines relation between the amount of accumulated PM and the amount of consumed NO.sub.2, which is obtained in advance through an experiment or the like. The NO.sub.2 consumption amount estimating unit 43 reads a value corresponding to the amount of accumulated PM from the consumption amount map and thus estimates the amount of consumed NO.sub.2.

(20) The NOx inflow ratio calculating unit 44 calculates the ratio of NO to NO.sub.2 in the exhaust gas that has passed through the DPF 16 and flows into the SCR catalyst 22. The calculation method will be described below in further detail. The NOx inflow ratio calculating unit 44 first calculates the amount of NO and the amount of NO.sub.2 in the exhaust gas emitted from the engine 10, on the basis of the running condition of the engine 10. The NOx inflow ratio calculating unit 44 also calculates the amount of NO.sub.2 produced from NO through oxidation by the DOC 15. The NOx inflow ratio calculating unit 44 then calculates the amount of NO.sub.2 flowing into the DPF 16 from the calculated amounts of NO, NO.sub.2, and produced NO.sub.2. The NOx inflow ratio calculating unit 44 further calculates the ratio of NO to NO.sub.2 in the exhaust gas flowing into the SCR catalyst 22 by subtracting the amount of NO.sub.2 consumed by the PM from the calculated amount of NO.sub.2 flowing into the DPF 16.

(21) The NOx inflow ratio adjusting unit 45 controls the combustion state of the engine 10 such that the ratio of NO to NO.sub.2 calculated by the NOx inflow ratio calculating unit 44 approaches 1 to 1. Accordingly, the ratio of NO to NO.sub.2 in the exhaust gas flowing into the SCR catalyst 22 is kept at an ideal ratio of 1 to 1, and the purification rate of NOx by the SCR catalyst 22 improves. The control for bringing the ratio of NO to NO.sub.2 at 1 to 1 can be implemented, for example, by adjusting parameters, such as the fuel injection timing of the engine 10 (ignition timing), the amount of recirculated exhaust gas, and the air-fuel ratio.

(22) Operations and advantages of the exhaust gas purification device of an internal combustion engine according to this embodiment will now be described.

(23) Conventionally, in an exhaust gas purification system that has a DPF disposed on an upstream side and an SCR catalyst disposed on a downstream side, NO.sub.2 is consumed by PM that has accumulated in the DPF. Thus, it is difficult to keep the ratio of NO to NO.sub.2 of the exhaust gas that flows into the SCR catalyst at 1 to 1. Therefore, the reaction of ammonia (NH.sub.3) with NO and NO.sub.2 decreases in the SCR catalyst. This can lead to a problem, i.e., the purification rate of NOx deteriorates. The amount of PM that has accumulated in a DPF is typically estimated on the basis of a pressure difference across the DPF detected by a differential pressure sensor. However, the flow rate of the exhaust gas varies with the running condition. This can lead to another problem, i.e., an accurate amount of accumulated PM cannot be grasped on the basis of the pressure difference across the DPF.

(24) On the contrary, the exhaust gas purification device of the internal combustion engine according to this embodiment calculates the amount of accumulated PM from the electrostatic capacity C between the electrodes 17a and 17b that is not affected by the flow rate of the exhaust gas, and keeps the ratio of NO to NO.sub.2 that flow into the SCR catalyst 22 at 1 to 1 with the amount of NO.sub.2 consumed by the PM taken into consideration.

(25) Therefore, the exhaust gas purification device of the internal combustion engine according to this embodiment can precisely detect the amount of PM that has accumulated in the DPF 16, and keep the ratio of NO to NO.sub.2 that flow into the SCR catalyst 22 at an ideal ratio of 1 to 1. Thus, the NOx purification rate can effectively be improved. Furthermore, because the reaction of ammonia (NH.sub.3) with NO and NO.sub.2 is facilitated in the SCR catalyst 22, excess ammonia can effectively be reduced.

(26) It is to be noted that the present invention is not limited to the above-described embodiments and can be implemented with changes and modifications, as appropriate, within the scope that does not depart from the spirit of the present invention.

(27) For example, as illustrated in FIG. 3, a bypass passage 12a may be connected to the exhaust passage 12 so as to bypass the DPF 16, and a DPF 16a for measurement (second filter) with a small capacity may be provided in the bypass passage 12a. In this case, it is preferred that a pair of electrodes 17a and 17b be disposed inside a corresponding pair of cells that oppose each other with at least one cell in the DPF 16a interposed therebetween. It is also preferred that the bypass passage 12a be provided with an orifice 12b (throttle) that regulates the flow rate of the exhaust gas. When the forced regeneration of the DPF 16a is executed, the paired electrodes 17a and 17b may be used as a heater by applying a voltage across the electrodes 17a and 17b.

(28) The engine 10 is not limited to a diesel engine, and an embodiment can be widely applied to other internal combustion engines including a gasoline engine.