Patent Publication Number: US-2011047991-A1

Title: Exhaust gas purification apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust gas purification apparatus and, more specifically, to an exhaust gas purification apparatus having a urea SCR (selective catalytic reduction) system for reducing nitrogen oxides (NOx) in exhaust gas emitted from a diesel engine. 
     The urea SCR system has been developed for reducing NOx in exhaust gas emitted from a diesel engine. The urea SCR system employs an SCR catalyst for converting NOx into nitrogen (N2) and water (H2O) by chemical reaction between NOx and ammonia (NH3) generated by hydrolysis of urea water. 
     The SCR catalyst is provided in the exhaust passage between the engine and the muffler. Furthermore, an oxidation catalyst and an injection valve for injecting urea water into exhaust gas are provided upstream of the SCR catalyst. The oxidation catalyst oxidizes hydrocarbons (HC) and carbon monoxide (CO) in exhaust gas into water (H2O) and carbon dioxide (CO2) and also promotes the oxidation of nitrogen oxide (NO) into nitrogen dioxide (NO2). 
     A DPF (diesel particulate filter) is also provided in the exhaust passage between the engine and the muffler for reducing particulate matter (PM) such as carbon in exhaust gas. The exhaust gas purification apparatus including the urea SCR system and the DPF has many components provided between the engine and the muffler and requires a large space for mounting of such components to a vehicle. Therefore, the urea SCR system is required to be downsized and also to accomplish the NOx reduction efficiently for the usage of urea water. 
     Published Japanese Translation 2006-519331 of PCT International Publication discloses an exhaust gas purification apparatus that includes a platinum-containing precatalyst having a function of filtering PM in exhaust gas, an SCR catalyst provided downstream of the platinum-containing precatalyst, a first supply device provided upstream of the platinum-containing precatalyst for supplying ammonia or urea and a second supply device provided between the platinum-containing precatalyst and the SCR catalyst for supplying ammonia or urea. The platinum-containing precatalyst functions as a reduction catalyst under a temperature that is below about 250° C. and also as an oxidation catalyst under a temperature that is about 250° C. or higher. The SCR catalyst is activated as a reduction catalyst under a temperature that is about 250° C. or higher. 
     In the exhaust gas purification apparatus according to the above Published Japanese Translation, when the temperature of exhaust gas is under T1 that is between 220° C. and 270° C., ammonia or urea is supplied from the first supply device and then, ammonia or ammonia generated by the hydrolysis of urea in the platinum-containing precatalyst reduces NOx in the exhaust gas. When the temperature of exhaust gas exceeds T1, ammonia or urea is supplied from the second supply device and then, ammonia or ammonia generated in the SCR catalyst by the hydrolysis of urea reduces NOx in exhaust gas. Oxidation of ammonia in the platinum-containing precatalyst is prevented by supplying ammonia or urea from the second supply device. Thus, the exhaust gas purification apparatus uses the supplied ammonia or urea efficiently for the NOx reduction. 
     However, when urea is supplied from either of the first supply device and the second supply device in the exhaust gas purification apparatus of the above Published Japanese Translation, the time for which the supplied urea stays upstream of the platinum-containing precatalyst or the SCR catalyst before reaching the precatalyst or the catalyst should be long for ensuring the time that is long enough for urea to be hydrolyzed into ammonia. The distances between the first supply device and the platinum-containing precatalyst and between the second supply device and the SCR catalyst, respectively, should be long enough for the hydrolysis of urea. Therefore, there has been problems in that the apparatus increases its length and it is difficult to make it small. 
     The present invention is directed to providing an exhaust gas purification apparatus that improves the efficiency of NOx reduction relative to the urea water usage and makes possible downsizing of the apparatus. 
     SUMMARY OF THE INVENTION 
     An exhaust gas purification apparatus includes a first oxidation catalyst provided in a passage through which exhaust gas flows, a particulate matter collecting device provided downstream of the first oxidation catalyst, a SCR catalyst integrally formed with the particulate matter collecting device and having ammonia adsorption property, a second oxidation catalyst integrally formed with the particulate matter collecting device and oxidizing ammonia at a predetermined temperature or higher and a urea water supply device provided upstream of the SCR catalyst for supplying urea water. The urea water supply device supplies urea water only when a temperature of the second oxidation catalyst is below the predetermined temperature. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic view of an exhaust gas purification apparatus according to an embodiment of the present invention and its associated components; and 
         FIG. 2  is a schematic cross sectional view of the exhaust gas purification apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following will describe the embodiment of the exhaust gas purification apparatus according to the present invention with reference to the accompanying drawings. Referring to  FIGS. 1 and 2  showing the embodiment, the exhaust gas purification apparatus which is designated generally by reference numeral  101  and its associated components will be described. The exhaust gas purification apparatus  101  is employed in a vehicle equipped with a diesel engine. 
     Referring to  FIG. 1 , an engine assembly including an engine  1  and the exhaust gas purification apparatus  101  is designated generally by reference numeral  10 . The engine  1  has a plurality of cylinders  1 A each having an intake port  1 B to which an intake manifold  4  is connected for distributing intake air to the respective cylinders  1 A. The intake manifold  4  has an inlet  4 A to which an engine intake pipe  3  is connected and the engine intake pipe  3  is further connected to a compressor housing  8 A of a turbocharger  8 . The compressor housing  8 A is connected to an intake pipe  2  through which outside air is introduced. 
     On the other hand, an exhaust manifold  5  is connected to a plurality of exhaust ports  1 C of the engine  1  for collecting exhaust gas emitted from the respective exhaust ports  1 C. An outlet  5 A of the exhaust manifold  5  is connected to a turbine housing  8 B of the turbocharger  8 , to which the exhaust gas purification apparatus  101  having a substantially cylindrical shape is connected and disposed adjacent to a lateral side of the engine  1 . The exhaust gas purification apparatus  101  is connected to an exhaust pipe  6 , the downstream end of which is further connected to a muffler  7 . The intake pipe  2 , the turbocharger  8 , the engine intake pipe  3  and the intake manifold  4  cooperate to form an intake system of the vehicle, while the exhaust manifold  5 , the turbocharger  8 , the exhaust gas purification apparatus  101 , the exhaust pipe  6  and the muffler  7  cooperates to form an exhaust system of the vehicle. The engine  1 , the engine intake pipe  3 , the intake manifold  4 , the exhaust manifold  5  and the turbocharger  8  cooperate to form the aforementioned engine assembly  10 . 
     Referring to  FIG. 2 , the exhaust gas purification apparatus  101  includes a casing  11  having a substantially cylindrical shape. The casing  11  has an upstream end face  11 A to which the outlet  8 B 2  of the turbine housing  8 B of the turbocharger  8  is connected and a downstream end face  11 B to which the upstream end  6 A of the exhaust pipe  6  is connected. The casing  11  communicates internally with the turbine housing  8 B and the exhaust pipe  6 . 
     The cylindrical casing  11  houses therein a first oxidation catalyst layer  12  supporting a first oxidation catalyst and a diesel particulate filter (DPF)  13 D forming a particulate matter collecting device disposed downstream of the first oxidation catalyst layer  12  with respect to the flow of exhaust gas in the casing  11 . The first oxidation catalyst layer  12  and the DPF  13 D are made in the form of a layer extending perpendicular to the axis of a cylindrical portion  11 C of the casing  11  over the entire radial dimension of the interior of the cylindrical portion  11 C. The first oxidation catalyst layer  12  and the DPF  13 D are disposed spaced apart from each other thereby to form therebetween a space  16 . 
     The first oxidation catalyst layer  12  supports thereon the first oxidation catalyst for oxidizing hydrocarbons (HC) and carbon monoxide (CO) into water (H2O) and carbon dioxide (CO2) and also promoting the oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2). The first oxidation catalyst of the first oxidation catalyst layer  12  uses material such as platinum (Pt), palladium (Pd), rhodium (Rh), silver (Ag), iron (Fe), cupper (Cu), nickel (Ni), gold (Au) or a mixture of two or more of these materials. 
     The DPF  13 D is made of a porous material such as ceramic for capturing particulate matter (PM) contained in exhaust gas. For preventing the deterioration of the DPF  13 D caused by the accumulation of PM, PM accumulated in the DPF  13 D needs to be burned. 
     Furthermore, the DPF  13 D has an (urea) SCR catalyst  14  as a selective catalytic reduction catalyst supported thereon, e.g., by coating. The DPF  13 D and the SCR catalyst  14  cooperate to integrally form a DPF  13  with catalyst. 
     The selective catalytic reduction catalyst serves to promote the chemical reaction selectively among specific chemical substances. Specifically, the urea SCR catalyst (hereinafter referred to as SCR catalyst) catalyzes the reaction between nitrogen oxide (NOx) and ammonia (NH3) thereby to reduce NOx into nitrogen (N2) and water (H2O). Though the details will be described later, the SCR catalyst  14  has the above-described function and preferably has high ammonia adsorption property and also catalytic property under a low temperature. The ammonia adsorption property of the SCR catalyst  14  should preferably be over 20 mg/l, i.e. capable of adsorbing more than 20 mg of ammonia per litter of base material supporting the SCR catalyst  14 , and the SCR catalyst  14  should preferably be activated catalytically at 150° C. or higher. The SCR catalyst  14  should preferably be made of a zeolite replaced by a metal such as iron and the like. The SCR catalyst  14  being activated means rapid increase of the reduction rate of NOx by ammonia. 
     The DPF  13 D supporting the SCR catalyst  14  also has a second oxidation catalyst  15  supported thereon, e.g., by coating, for decreasing the combustion temperature of PM captured by the DPF  13 D. The DPF  13 D and the second oxidation catalyst  15  cooperate to integrally form the DPF  13  with catalyst. Thus, the DPF  13  with catalyst is formed by the DPF  13 D, the SCR catalyst  14  and the second oxidation catalyst  15 . The second oxidation catalyst  15  should preferably decrease the PM combustion temperature to a temperature between 400 and 650° C. 
     The second oxidation catalyst  15  functions not only to decrease the PM combustion temperature but also to oxidize and decompose ammonia under a predetermined temperature Tp° C. or higher. However, the second oxidation catalyst  15  neither oxidizes nor decomposes ammonia at a temperature below the predetermined temperature Tp° C. The predetermined temperature Tp° C. corresponds to a temperature at which the second oxidation catalyst  15  is activated. The oxidation catalyst being activated means that 50% of object substance for oxidation is oxidized to a predetermined level. Since the temperature at which the catalyst is activated depends on the proportion of component materials of the catalyst and the concentration of the catalyst in the region of the DPF where the catalyst is supported thereon, the temperature Tp° C. at which the second oxidation catalyst  15  is activated may be between 150 and 250° C. The second oxidation catalyst  15  may be Ag/CeO2 catalyst, i.e., silver (Ag) supported on ceria (CeO2). 
     The SCR catalyst  14  and the second oxidation catalyst  15  may be applied in either order to the DPF  13 D to be supported thereon, firstly the SCR catalyst  14  and then the second oxidation catalyst  15  or in the reverse order. Alternatively, a mixture of the SCR catalyst  14  and the second oxidation catalyst  15  may be supported on the DPF  13 D. The DPF  13 D and the SCR catalyst  14  may be formed integrally in a manner that the SCR catalyst  14  is disposed in rear of the DPF  13 D and the DPF  13 D supports thereon the second oxidation catalyst  15 . 
     An injection valve  18  that is an electromagnetic valve is provided upstream of the first oxidation catalyst layer  12  in the cylindrical portion  11 C of the casing  11 . The injection valve  18  forms the urea water supply device of the present invention. The injection valve  18  is connected to a urea water tank  19  provided in a vehicle (not shown) and operable to inject urea water upstream of the first oxidation catalyst layer  12  (SCR catalyst  14 ) in the casing  11 . The injection valve  18  is electrically connected to a dosing control unit (DCU)  20  that controls the opening and closing operation of the injection valve  18 . The urea water tank  19  has an electric pump for supplying urea water to the injection valve  18 . The electric pump is electrically connected to the DCU  20  and the pump operation is controlled by the DCU  20 . The DCU  20  may be provided separately or formed integrally with an ECU for the vehicle. 
     A cylindrically-shaped mixer  17  is provided on the upstream end face  13 A of the DPF  13  with catalyst for distributing substances in exhaust gas uniformly over the end face  13 A. The mixer  17  has a structure that is similar to that disclosed in Published Japanese Translation H06-509020 of PCT international publication or Japanese Patent Application Publication 2006-9608. The mixer disclosed in Published Japanese Translation H06-509020 is made in the form of a lattice that divides the gas passage into plural cells so as to cause the gas flowing through each cell to flow spirally and also to flow toward the adjacent cell. This helps the substances in exhaust gas to be dispersed uniformly in the whole passage. On the other hand, the mixer disclosed in Japanese Patent Application Publication 2006-9608 has plural plates each extending perpendicularly to the direction of gas flow, which provides serpentine gas passage serving to distribute the substances in the gas uniformly. 
     An exhaust gas temperature sensor  31  is provided downstream of the upstream end face  11 A of the casing  11  for detecting the temperature of exhaust gas. The exhaust gas temperature sensor  31  is electrically connected to the DCU  20  and sends detected temperature information to the DCU  20 . As described above, the exhaust gas purification apparatus  101  includes the SCR catalyst  14  and the DPF  13 D in a manner that they are integrally formed and is fixed to the engine assembly  10  and disposed adjacent to the engine  1  (refer to  FIG. 1 ). 
     The following will describe the operation of the exhaust gas purification apparatus  101  according to the embodiment and its associated components with reference to  FIGS. 1 and 2 . Referring to  FIG. 1 , while the engine  1  is running, outside air is flowed into the compressor housing  8 A of the turbocharger  8  through the intake pipe  2 . The air is pumped by a compressor wheel (not shown) in the compressor housing  8 A and flowed to the engine intake pipe  3  under an increased pressure. The air is flowed into a cylinder  1 A in the engine  1  through the engine intake pipe  3  and the intake manifold  4 . Then, the air in the cylinder  1 A is mixed with fuel (light oil) supplied into the cylinder  1 A and the fuel is ignited spontaneously for combustion. 
     Exhaust gas produced by the combustion is discharged into the exhaust manifold  5  through a plurality of exhaust ports  1 C to be collected by the exhaust manifold  5  and then flows into the turbine housing  8 B of the turbocharger  8 . The exhaust gas flowing through the turbine housing  8 B increases the rotation speed of the turbine wheel (not shown) in the turbine housing  8 B and the compressor wheel connected to the turbine wheel and then is discharged into the exhaust gas purification apparatus  101 . After flowing through the exhaust gas purification apparatus  101 , the exhaust gas flows through the exhaust pipe  6  and the muffler  7  and then is discharged outside the vehicle (not shown). 
     Referring to  FIG. 2 , all the exhaust gas flowed into the exhaust gas purification apparatus  101  flows firstly through the first oxidation catalyst layer  12 . While the exhaust gas passes through the first oxidation catalyst layer  12 , hydrocarbons and carbon monoxide in the exhaust gas are oxidized into carbon dioxide and water, and part of NO is oxidized into NO2 that can be reduced easily. After flowing through the first oxidation catalyst layer  12 , the exhaust gas flows through the mixer  17  and then into the DPF  13  with catalyst. PM in the exhaust gas is captured by the DPF  13 D of the DPF  13  with catalyst. 
     Meanwhile, the DCU  20  performs either one of the following two operations described under (1) and (2) based on the temperature information sent by the exhaust gas temperature sensor  31 .
     (1) When the temperature T of exhaust gas detected by the exhaust gas temperature sensor  31  is below the predetermined temperature Tp° C. at which the second oxidation catalyst  15  is activated:   

     It is noted the temperature T of exhaust gas detected by the exhaust gas temperature sensor  31  may be regarded as the temperature of the second oxidation catalyst  15  of the DPF  13  with catalyst. Therefore, when the temperature T of exhaust gas detected by the exhaust gas temperature sensor  31  shows Tp ° C. or higher, it may be considered that the second oxidation catalyst  15  is under a temperature environment where the second oxidation catalyst  15  can oxidize and decompose ammonia. 
     The following description will be made with the assumption that the predetermined temperature Tp° C. is 250° C. 
     When the temperature of exhaust gas is below 250° C., the DCU  20  operates the electric pump in the urea water tank  19  and also opens the injection valve  18 . Then, urea water is injected from the injection valve  18  upstream of the first oxidation catalyst layer  12  in the casing  11 . 
     The injected urea water is carried by the exhaust gas and flows through the first oxidation catalyst layer  12 . The first oxidation catalyst layer  12  has therein the heat due to the exhaust gas flowing therethrough and also the reaction heat due to the oxidation of NO and other substances in exhaust gas. Therefore, most of the urea water flowing through the first oxidation catalyst layer  12  is hydrolyzed into ammonia and carbon dioxide by the heat that the first oxidation catalyst layer  12  has and the heat of the exhaust gas flowing through the first oxidation catalyst layer  12 . 
     After flowing through the first oxidation catalyst layer  12 , the exhaust gas containing urea water and ammonia flows through the space  16  and then to the mixer  17 . Urea water and ammonia are flowed through the mixer  17  while being dispersed and then into the DPF  13  with catalyst. Meanwhile, urea water in the exhaust gas that is not hydrolyzed in the first oxidation catalyst layer  12  is hydrolyzed into ammonia due to the heat of the exhaust gas before reaching the DPF  13  with catalyst that is integrally formed with the SCR catalyst  14 . The time during which the urea water stays in the first oxidation catalyst layer  12 , the space  16  and the mixer  17  while flowing therethrough before reaching the SCR catalyst  14  satisfies the reaction time required for the hydrolysis of urea water. Thus, the hydrolysis of urea water is accomplished with a high efficiency. 
     As described above, since urea water injected from the injection valve  18  is hydrolyzed not only in the first oxidation catalyst layer  12  but also in the space  16  through which urea water flows before reaching the DPF  13  with catalyst, urea water is hydrolyzed into ammonia with a high efficiency. Therefore, the distance between the first oxidation catalyst layer  12  and the DPF  13  with catalyst in the exhaust gas purification apparatus  101 , i.e., the distance of the space  16  can be shortened, thus making it possible to construct the exhaust gas purification apparatus  101  small. 
     Ammonia contained in exhaust gas flowing into the DPF  13  with catalyst performs either one of the following two operations (1A) and (1B) depending on the temperature condition of the SCR catalyst  14  of the DPF  13  with catalyst. The temperature of the SCR catalyst  14  is equivalent to the temperature of the second oxidation catalyst  15  and, therefore, the temperature T of the exhaust gas detected by the exhaust gas temperature sensor  31  is regarded as the temperature of the SCR catalyst  14 .
     (1A) When the temperature of the SCR catalyst  14  is below temperature Ts° C. at which the SCR catalyst  14  is activated:   

     The following description will be made with the assumption that the predetermined temperature Ts° C. is 150° C. at which catalyst is generally activated. When the temperature of the SCR catalyst  14  is below 150° C., the SCR catalyst  14  is not activated and, therefore, ammonia contained in the exhaust gas flowing into the DPF  13  with catalyst does not reduce NOx (including NO and NO2) contained in the exhaust gas by the catalytic reaction of the SCR catalyst  14  but is adsorbed on the SCR catalyst  14 . After flowing through the DPF  13  with catalyst, the exhaust gas from which harmful ammonia is removed is discharged from the exhaust gas purification apparatus  101 . Therefore, the use of an SCR catalyst with high ammonia adsorption property is preferable for preventing harmful ammonia from being discharged outside the vehicle (not shown).
     (1B) When the temperature of the SCR catalyst  14  is the temperature Ts° C. (150° C.) at which the SCR catalyst  14  is activated, or higher   

     Ammonia that is contained in exhaust gas flowing into the DPF  13  with catalyst reduces NOx in the exhaust gas into N2 by the catalytic reaction of the SCR catalyst  14 . Residual ammonia that is not used in the reduction of NOx is adsorbed on the SCR catalyst  14 . Thus, the exhaust gas having reduced its NOx content and removed harmful ammonia therefrom while flowing through the DPF  13  with catalyst is discharged from the exhaust gas purification apparatus  101 . The lower the temperature Ts° C. at which the SCR catalyst  14  is activated, the larger the temperature range in which the SCR catalyst  14  can reduce NOx in exhaust gas. Therefore, the temperature Ts° C. at which the SCR catalyst  14  is activated should preferably be low. 
     In either case (1A) or (1B), the catalytic temperature of the second oxidation catalyst  15  of the DPF  13  with catalyst is substantially equivalent to the temperature of the exhaust gas detected by the exhaust gas temperature sensor  31 , which is below 250° C. that does not cause catalyst to oxidize and decompose ammonia. Therefore, the second oxidation catalyst  15  neither oxidizes nor decomposes ammonia in exhaust gas flowing through the DPF  13  with catalyst. 
     Thus, ammonia generated on the hydrolysis of urea water injected by the injection valve  18  is neither oxidized nor decomposed by the second oxidation catalyst  15 , but used for reducing NOx in exhaust gas or adsorbed on the SCR catalyst  14  with a high efficiency.
     (2) When the temperature T of the exhaust gas detected by the exhaust gas temperature sensor  31  is the predetermined temperature Tp° C. (250° C.), at which the second oxidation catalyst  15  is activated, or higher:   

     The DCU  20  stops the operation of the electric pump in the urea water tank  19  and also closes the injection valve  18  thereby to stop the injection of urea water from the injection valve  18 . Therefore, exhaust gas introduced into the casing  11  containing neither urea water nor ammonia generated by the hydrolysis of urea water flows through the first oxidation catalyst layer  12  and the mixer  17  and then into the DPF  13  with catalyst. 
     Meanwhile, the SCR catalyst  14  of the DPF  13  with catalyst has a lot of ammonia that is generated and adsorbed on the SCR catalyst  14  when the temperature T of the exhaust gas is below 250° C. The SCR catalyst  14  is activated when the temperature T of the exhaust gas is 250° C. or higher. Therefore, NOx in exhaust gas flowing into the DPF  13  with catalyst is reduced by ammonia that is adsorbed on the SCR catalyst  14  under the catalytic reaction of the SCR catalyst  14 . Thus, the exhaust gas having reduced its NOx content and removed harmful ammonia therefrom while flowing through the DPF  13  with catalyst is discharged from the exhaust gas purification apparatus  101 . 
     The temperature at which the second oxidation catalyst  15  of the DPF  13  with catalyst is activated is 250° C. at which the second oxidation catalyst  15  can oxidize and decompose ammonia, or higher. However, exhaust gas flowing through the DPF  13  with catalyst contains no ammonia, but ammonia is only adsorbed on the SCR catalyst  14 . Since the ammonia that is adsorbed on the SCR catalyst  14  is used for reducing NOx as described before, the ammonia is neither oxidized nor decomposed by the second oxidation catalyst  15 . Thus, when the exhaust gas temperature is 250° C. or higher, ammonia generated by the hydrolysis of urea water is used for reducing NOx in the exhaust gas without being oxidized and decomposed by the second oxidation catalyst  15 . 
     Referring to  FIG. 1 , the exhaust gas purification apparatus  101  is disposed adjacent to the engine  1  and, therefore, hot exhaust gas immediately after being emitted from the engine  1  flows into the exhaust gas purification apparatus  101  through the turbocharger  8 . Furthermore, the heat generated by the engine  1  is imparted to the exhaust gas purification apparatus  101  located adjacent to the engine  1  and transmitted inward through outer wall of the casing  11 . 
     Referring to  FIG. 2 , the first oxidation catalyst layer  12  and the DPF  13  with catalyst both disposed inside the casing  11  are subject to the heat of the hot exhaust gas and the heat imparted from the engine  1  and, therefore, the temperature of the respective components tends to increase. The temperature increasing rate of the respective components, i.e., the oxidation catalyst of the first oxidation catalyst layer  12  and the SCR catalyst  14  of the DPF  13  with catalyst, in the exhaust gas purification apparatus  101  during a cold start of the engine  1  is improved and the time required for activating each catalyst during such cold start of the engine  1  is shortened. Eventually, the performance of NOx reduction is improved. 
     Thus, the exhaust gas purification apparatus  101  according to the present invention includes the first oxidation catalyst layer  12  provided in exhaust gas passage, the DPF  13 D provided downstream of the first oxidation catalyst layer  12 , the SCR catalyst  14  that is integrally formed with the DPF  13 D and can adsorb ammonia, the second oxidation catalyst  15  that is integrally formed with the DPF  13 D and oxidizes ammonia at a predetermined temperature or higher and the injection valve  18  provided upstream of the SCR catalyst  14  for supplying urea water only when the temperature of the second oxidation catalyst  15  is below a predetermined temperature. 
     The exhaust gas purification apparatus  101 , in which the SCR catalyst  14  and the second oxidation catalyst  15  are integrally formed with the DPF  13 D, can be made small. Ammonia is generated on the hydrolysis of urea water supplied by the injection valve  18  with the aid of the first oxidation catalyst layer  12  and the like. However, since urea water is supplied below a predetermined temperature at which the second oxidation catalyst  15  is not activated for the oxidation, no generated ammonia is oxidized and decomposed by the second oxidation catalyst  15 . In other words, when the temperature of the second oxidation catalyst  15  is below the predetermined temperature at which the second oxidation catalyst  15  is not activated for the oxidation, NOx in exhaust gas is reduced under the catalytic reaction of the SCR catalyst  14  by ammonia that is generated on the hydrolysis of urea water and contained in exhaust gas. Residual ammonia that is not used for reducing NOx is adsorbed on the SCR catalyst  14 . On the other hand, when the temperature of the second oxidation catalyst  15  is the predetermined temperature or higher, no urea water is supplied and NOx in exhaust gas is reduced by ammonia that is adsorbed on the SCR catalyst  14  under the catalytic reaction of the SCR catalyst  14 . Thus, ammonia generated on the hydrolysis of urea water is used efficiently for reducing NOx, thereby improving the efficiency of NOx reduction for the urea water usage. 
     The exhaust gas temperature sensor  31  is provided in the exhaust gas purification apparatus  101  for detecting the temperature of exhaust gas flowing through the second oxidation catalyst  15 . Regarding the temperature detected by the exhaust gas temperature sensor  31  as the temperature of the second oxidation catalyst  15 , controlling of the urea water supply from the injection valve  18  can be made easily by using the temperature of the second oxidation catalyst  15 . 
     By supplying urea water from the injection valve  18  provided upstream of the first oxidation catalyst layer  12 , urea water flowing through the first oxidation catalyst layer  12  can make use of the heat that the first oxidation catalyst layer  12  has therein such as the reaction heat due to the oxidation of nitrogen monoxide in exhaust gas into nitrogen dioxide and the heat that the exhaust gas has therein. Therefore, urea water flowing through the first oxidation catalyst layer  12  can be hydrolyzed at a high efficiency. Furthermore, by supplying urea water upstream of the first oxidation catalyst layer  12 , the time during which the supplied urea water stays upstream of the SCR catalyst  14  before reaching the SCR catalyst  14  is lengthened and. therefore, urea water can be hydrolyzed efficiently before reaching the SCR catalyst  14 , with the result that NOx reduction performance of the exhaust gas purification apparatus  101  is improved. Accordingly, the distance between the first oxidation catalyst layer  12  and the DPF  13  with catalyst including the SCR catalyst  14  can be shortened, thereby making the exhaust gas purification apparatus  101  small. 
     Since the second oxidation catalyst  15  reduces the combustion temperature of the DPF 13  with catalyst during burning of PM captured by the DPF  13 D, the influence of the heat caused by burning PM for regenerating the DPF  13 D on the SCR catalyst  14  can be reduced. Therefore, the catalytic function of the SCR catalyst  14  for reducing NOx in the exhaust gas purification apparatus  101 , i.e., the durability of the SCR catalyst  14 , can be improved. 
     Furthermore, since the first oxidation catalyst layer  12 , the DPF  13  with catalyst (the DPF  13 D, the SCR catalyst  14  and the second oxidation catalyst  15 ) and the injection valve  18  are all housed in the single casing  11 , the exhaust gas purification apparatus  101  can be made still smaller. 
     The exhaust gas purification apparatus  101  is mounted to the engine assembly  10  and the hot exhaust gas emitted from the engine assembly  10  is introduced into the exhaust gas purification apparatus  101 . The heat generated by the engine assembly  10  in operation is transmitted inside the casing  11  of the exhaust gas purification apparatus  101 . Therefore, the time for the temperature of the exhaust gas purification apparatus  101  to be increased to the level required for the hydrolysis of urea water and also for the temperature of the SCR catalyst  14  to the level required for activating the SCR catalyst  14  during a cold start of the engine can be shortened, with the result that the NOx reduction performance can be improved. 
     The exhaust gas purification apparatus  101  in the embodiment is provided in the engine assembly  10  having the turbocharger  8 , but the present invention is not limited to this structure. When the engine assembly  10  dispenses with the turbocharger  8 , the exhaust gas purification apparatus  101  may be directly connected to the outlet  5 A of the exhaust manifold  5 . The exhaust gas purification apparatus  101  may be provided spaced apart from the engine assembly  10 . 
     The injection valve  18  is provided upstream of the first oxidation catalyst layer  12  so as to supply urea water upstream of the first oxidation catalyst layer  12  in the exhaust gas purification apparatus  101  according to the embodiment, but the present invention is not limited to this structure. The injection valve  18  may be so arranged that urea water is supplied toward the downstream side of the first oxidation catalyst layer  12 . This structure prevents ammonia generated on the hydrolysis of urea water from being oxidized and decomposed by the first oxidation catalyst layer  12 . 
     The casing  11  of the exhaust gas purification apparatuses  101  according to the embodiment is cylindrically-shaped, but the casing  11  is not limited to this shape. The casing  11  may be formed with a cross-section including a prism such as quadratic prism, a sphere or an ellipsoid. 
     The exhaust gas purification apparatus  101  according to the embodiment includes the exhaust gas temperature sensor  31  provided upstream of the end face  11 A of the casing  11 , but the structure is not limited to this structure. The exhaust gas temperature sensor  31  may be provided at a position that is adjacent to and immediately upstream or downstream of the second oxidation catalyst  15 . The mixer  17  of the exhaust gas purification apparatus  101  may be dispensed with in the embodiment.