Patent Publication Number: US-2011052454-A1

Title: Exhaust gas purification apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust gas purification apparatus and more particularly to an exhaust gas purification apparatus that purifies exhaust gas by removing nitrogen oxides (NO x ) contained in the exhaust gas of a diesel engine with the aid of a urea selective catalytic reduction (SCR) catalyst. 
     A urea SCR system has been developed to purify exhaust gas by removing NO x  contained in the exhaust gas of a diesel engine. The urea SCR system uses an SCR catalyst as a selective reduction catalyst to convert NO x  into nitrogen (N 2 ) and water (H 2 O) by the chemical reaction between NO x  and ammonia (NH 3 ) produced by hydrolyzing urea water. 
     The SCR catalyst of the urea SCR system is provided in the exhaust gas passage formed between an engine and a muffler that is located downstream of the engine with respect to the flow of exhaust gas. An oxidation catalyst is provided in the exhaust gas passage at a position upstream of the SCR catalyst with respect to the flow of exhaust gas for promoting oxidization of hydrocarbons (HC) and carbon monoxide (CO) in exhaust gas to water (H 2 O) and carbon dioxide (CO 2 ) and also for promoting oxidization of nitrogen monoxide (NO) in exhaust gas to nitrogen dioxide (NO 2 ). An injection valve is also provided upstream of the SCR catalyst for injecting urea water into exhaust gas. Additionally, a diesel particulate filter (DPF) is provided in the exhaust gas passage for reducing particulate matter (PM), such as carbon contained in exhaust gas. 
     Japanese Patent Application Publication 2006-274986 discloses an exhaust gas aftertreatment apparatus including an NO x  storage catalyst activated under a high temperature, a DPF located downstream of the NO x  storage catalyst with respect to the flow of exhaust gas and having a urea SCR catalyst supported therein and activated under a low temperature, and a urea water injector located between the NO x  storage catalyst and the DPF, all of which are housed in one case of the exhaust gas aftertreatment apparatus. In this exhaust gas aftertreatment apparatus, when NO x  storage catalyst is under a low temperature below 400 degrees centigrade (° C.), urea water is injected into exhaust gas by the urea water injector and hydrolyzed thereby to produce ammonia, which reduces and removes NO x  contained in exhaust gas by the aid of the urea SCR catalyst. When NO x  storage catalyst is under a high temperature that is 400° C. or higher, NO x  contained in exhaust gas is stored by the NO x  storage catalyst and exhaust gas is purified, accordingly. 
     In the exhaust gas aftertreatment apparatus, however, all of the ammonia produced by hydrolyzing urea water injected by the urea water injector is not used to reduce NO x  contained in exhaust gas by the aid of the urea SCR catalyst. For example, if the amount of ammonia produced from urea water is large relative to the amount of NO x  contained in exhaust gas due to a large amount of urea water injected by the urea water injector, a surplus amount of ammonia is produced and discharged from the urea SCR catalyst. Such surplus ammonia deteriorates the efficiency of exhaust gas purification relative to urea water usage. Consequently, the above-described exhaust gas aftertreatment apparatus accomplishes purification of exhaust gas by removal of NO x  only with a low efficiency relative to urea water usage. 
     The present invention, which has been made in view of the above problems, is directed to an exhaust gas purification apparatus that improves the efficiency of removing NO x  contained in exhaust gas relative to the use of urea water. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, the exhaust gas purification apparatus includes an oxidation catalyst, a first selective catalytic reduction catalyst, a second selective catalytic reduction catalyst and a urea water supply device. The oxidation catalyst is provided in a passage through which exhaust gas flows. The first selective catalytic reduction catalyst is located in the passage downstream of the oxidation catalyst. The second selective catalytic reduction catalyst is located in the passage downstream of the first selective catalytic reduction catalyst and operable to adsorb more ammonia than the first selective catalytic reduction catalyst. The urea water supply device is provided for supplying urea water to the passage upstream of the first selective catalytic reduction catalyst. 
     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 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 showing an exhaust gas purification apparatus according to a first embodiment of the present invention and its peripheral equipment; 
         FIG. 2  is a longitudinal sectional view showing the exhaust gas purification apparatus of  FIG. 1 ; 
         FIG. 3  is a cross sectional view showing the exhaust gas purification apparatus as taken along the lines  3 A- 3 A of  FIG. 2 ; 
         FIG. 4  is a longitudinal sectional view showing an exhaust gas purification apparatus according to a second embodiment of the present invention; and 
         FIG. 5  is a longitudinal sectional view showing an exhaust gas purification apparatus according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe the embodiments of the present invention with reference to the accompanying drawings. An exhaust gas purification apparatus  101  according to the first embodiment of the present invention and its peripheral equipment will be described with reference to  FIGS. 1 and 2 . In the following embodiments, the exhaust gas purification apparatus is used for a diesel engine for a vehicle. 
     Referring to  FIG. 1  showing the exhaust gas purification apparatus  101  and its peripheral equipment in schematic view, an engine  1  has a plurality of engine cylinders  1 A each having a plurality of intake ports  1 B and a plurality of exhaust ports  1 C. An intake manifold  4  is connected to the intake ports  1 B of the engine cylinders  1 A for distributing intake air into the respective engine cylinders  1 A. The intake manifold  4  has an inlet  4 A through which air is drawn in. An engine intake pipe  3  has two opposite ends one of which is connected to the inlet  4 A of the intake manifold  4  and the other of which is connected to a compressor housing  8 A of the turbocharger  8 . An intake pipe  2  is connected to the compressor housing  8 A, through which ambient air is drawn in. 
     An exhaust manifold  5  is connected to the exhaust ports  1 C of the engine cylinders  1 A for collecting exhaust gas discharged from the exhaust ports  1 C. The exhaust manifold  5  has an outlet  5 A through which exhaust gas is discharged. A turbine housing  8 B of the turbocharger  8  is connected to the outlet  5 A of the exhaust manifold  5 . The exhaust gas purification apparatus  101  having substantially a cylindrical form is connected to the turbine housing  8 B and located on lateral side of the engine  1  at a position adjacent thereto. An exhaust pipe  6  is connected to the exhaust gas purification apparatus  101 . A muffler  7  is connected to the downstream end of the exhaust pipe  6 . Thus, the intake pipe  2 , the turbocharger  8 , the engine intake pipe  3  and the intake manifold  4  cooperate to form the inlet system in the vehicle (not shown). The exhaust manifold  5 , the turbocharger  8 , the exhaust gas purification apparatus  101 , the exhaust pipe  6  and the muffler  7  cooperate to form the outlet system in the vehicle (not shown). It is noted that the engine  1 , the engine intake pipe  3 , the intake manifold  4 , the exhaust manifold  5  and the turbocharger  8  cooperate to form an engine assembly  10 . 
     Referring to  FIG. 2  showing the exhaust gas purification apparatus  101  in longitudinal sectional view, it has a substantially cylindrical casing  11 . The casing  11  has an upstream end portion  11 A, a downstream end portion  11 B and a cylindrical intermediate portion  11 C formed between the upstream end portion  11 A and the downstream end portion  11 B. The turbine housing  8 B of the turbocharger  8  has an outlet  8 B 2  that is connected to the upstream end portion  11 A of the casing  11 . The exhaust pipe  6  has an upstream end  6 A that is connected to the downstream end portion  11 B of the casing  11 . Thus, the interior of the casing  11  communicates with the interior of the turbine housing  8 B and the interior of the exhaust pipe  6 . 
     The casing  11  has therein an oxidation catalyst layer  12 , a diesel particulate filter (DPF) body  14  and a second SCR catalyst layer  16 , which are located in this order along the flow of exhaust gas. The oxidation catalyst layer  12  supports therein the oxidation catalyst of the present invention. The second SCR catalyst layer  16  supports therein a second SCR catalyst  168 . It is noted that the DPF body  14  serves as the particulate matter collecting device of the present invention. The oxidation catalyst layer  12 , the DPF body  14  and the second SCR catalyst layer  16  have such a cylindrical form extending perpendicularly to the axis of the cylindrical portion  11 C of the casing  11  that closes the interior of the cylindrical portion  11 C as shown in  FIG. 2 . The oxidation catalyst layer  12  and the DPF body  14  are spaced away from each other and have therebetween a space  17 A. The DPF body  14  and the second SCR catalyst layer  16  are also spaced away from each other and have therebetween a space  17 B. 
     The oxidation catalyst layer  12  is formed by a layer in which the oxidation catalyst for promoting the oxidation of hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gas to water (H 2 O) and carbon dioxide (CO 2 ) and also for promoting the oxidation of nitrogen oxide (NO) contained in exhaust gas to nitrogen dioxide (NO 2 ) is supported by a substrate (not shown). For example, platinum (Pt), palladium (Pd), rhodium (Rh), silver (Ag), iron (Fe), copper (Cu), nickel (Ni), gold (Au), an alloy of two or more kinds of these catalyst materials and so forth are preferably used as the oxidation catalyst of the oxidation catalyst layer  12 . 
     The DPF body  14  is made of a porous material such as ceramic and used for collecting particulate matter (PM) contained in exhaust gas. 
     A first SCR catalyst  15 S as a selective reduction catalyst is supported by and throughout the DPF body  14  by using any suitable means such as coating. The DPF body  14  and the first SCR catalyst  15 S are integrated into a DPF  13  with catalyst. The first SCR catalyst  15 S may be supported partially by the DPF body  14 . Selective reduction catalyst promotes chemical reaction selectively between specific substances. Urea SCR catalyst (hereinafter referred to merely as SCR catalyst) of the selective reduction catalyst promotes chemical reaction specifically between nitrogen oxide (NO x ) and ammonia (NH 3 ) as a reducing agent, thereby reducing NO x  to N 2  (nitrogen) and water. 
     An SCR catalyst having a low ammonia adsorption property is used for the first SCR catalyst  15 S. The ammonia adsorption property may be represented by weight of ammonia adsorption capacity per unit volume of the substrate which supports therein catalyst. Specifically, the low ammonia adsorption property of the first SCR catalyst  15 S should preferably be such that, when 180 gram (g) of the first SCR catalyst  15 S is supported by one liter of substrate, ammonia of not more than 100 mg (milligram) per liter of substrate may be adsorbed under a temperature of 200° C. of the first SCR catalyst  15 S. Additionally, the ammonia adsorption capacity of the first SCR catalyst  15 S, which tends to decrease with an increase of the temperature of the first SCR catalyst  15 S, should preferably be such that the rate of decrease of the ammonia adsorption capacity is low, that is, the dependence of the ammonia adsorption capacity on temperature is low. The first SCR catalyst  15 S should preferably be made of an oxide of substance such as zirconium (Zr), titanium (Ti), silicon (Si), cerium (Ce) or tungsten (W), any complex of these oxides, or ZSM-5 zeolite which is partially replaced by metal such as iron (Fe) or copper (Cu) which is thermally treated under a high temperature that is 650° C. or higher. 
     The first SCR catalyst  15 S has a property of activating reduction when its temperature is at a predetermined level or higher, generally 150° C. or higher. “Activating reduction” means rapidly increasing the rate of reduction of NO x  by ammonia. The first SCR catalyst  15 S should preferably have low ammonia adsorption capacity when its temperature is 150° C., at which reduction is activated, or higher. The above-mentioned catalyst materials have such property. 
     The second SCR catalyst layer  16  is formed such that the second SCR catalyst  16 S is supported in a substrate (not shown) by any suitable means such as coating. The second SCR catalyst  16 S uses an SCR catalyst whose ammonia adsorption property is higher than that of the first SCR catalyst  15 S. 
     Specifically, the ammonia adsorption property of the second SCR catalyst  16 S should preferably be such that, when 180 g of the second SCR catalyst  16 S per liter of the substrate is supported, ammonia not less than 250 mg per liter (per unit volume) of the substrate may be adsorbed under the temperature of 200° C. of the second SCR catalyst  16 S. ZSM-5 zeolite which is partially replaced by metal such as iron (Fe) or copper (Cu) which is thermally treated under a temperature lower than 650° C. is preferably used as the second SCR catalyst  16 S. In addition, the ammonia adsorption capacity of the second SCR catalyst  16 S tends to decrease with an increase of the temperature of the second SCR catalyst  16 S. The SCR catalyst having the first SCR catalyst  15 S and the second SCR catalyst  16 S and also having a higher ammonia adsorption capacity has such a characteristic that the rate of decrease of the ammonia adsorption capacity becomes higher with an increase of the temperature of the SCR catalyst, that is, dependence of the ammonia adsorption capacity on temperature becomes higher. Therefore, the second SCR catalyst  16 S has higher dependence of the ammonia adsorption capacity on temperature than the first SCR catalyst  15 S. 
     The second SCR catalyst  16 S using the above-mentioned catalyst materials has a property of activating reduction when its temperature is 150° C. or higher. The ammonia adsorption capacity of the entire second SCR catalyst layer  16  formed as described above is greater than that of the entire DPF  13  having the first SCR catalyst  15 S. 
     The cylindrical portion  11 C of the casing  11  is provided with an injection valve  19 . The injection valve  19  is provided by an electromagnetic valve and serves as the urea water supply device of the present invention. In addition, the injection valve  19  is connected to a urea water tank  20  mounted on the vehicle (not shown) for injecting urea water supplied from the urea water tank  20  into the space  17 A in the casing  11  that is upstream of the first SCR catalyst  15 S. As shown in  FIG. 2 , the injection valve  19  is located at a position that is closer to the oxidation catalyst layer  12  than to the DPF  13  between the oxidation catalyst layer  12  and the DPF  13 . Further, the injection valve  19  is electrically connected to a dosing control unit (DCU)  30  which controls the opening and closing operation of the injection valve  19 . The urea water tank  20  is provided with a motor pump (not shown) for supplying urea water in the urea water tank  20  to the injection valve  19 . The motor pump is electrically connected to the DCU  30 , which also controls the operation of the motor pump. The DCU  30  may be provided separately for each of the injection valve  19  and the motor pump of the urea water tank  20 . Alternatively, the DCU  30  may be integrated with an ECU of the vehicle (not shown). 
     The DPF  13  has an upstream end face  13 A on which a cylindrical mixer  18  is provided for distributing substances contained in exhaust gas throughout the upstream end face  13 A evenly. The mixer disclosed by publication such as Japanese Patent Application Publication No. 6-509020T or No. 2006-9608 may be used as the mixer  18  of the present embodiment. The mixer disclosed by Japanese Patent Application Publication No. 6-509020T is made in the form of a lattice having a number of cells as the gas passage which causes the exhaust gas to swirl in the cells and also to flow toward their adjacent cells, thereby distributing the substances contained in the exhaust gas throughout the gas passage. The mixer disclosed by Japanese Patent Application Publication No. 2006-9608 is provided with a plurality of dispersion plates located perpendicularly to the direction of gas passage for causing the exhaust gas to meander, thereby distributing the substances contained in the exhaust gas throughout the gas passage. 
     An exhaust-gas temperature sensor  51  is provided in the upstream end portion  11 A of the casing  11  for detecting the temperature of exhaust gas. The exhaust-gas temperature sensor  51  is electrically connected to the DCU  30  for sending the detected temperature information to the DCU  30 . An NO x  sensor  52  is provided in the upstream end portion  11 A of the casing  11  at a position downstream of the exhaust-gas temperature sensor  51  for detecting the concentration of NO x . The NO x  sensor  52  is electrically connected to the DCU  30  for sending the detected concentration information to the DCU  30 . As is now apparent from the forgoing, the exhaust gas purification apparatus  101  has an exhaust gas purification mechanism having an SCR catalyst and an exhaust gas purification mechanism having a DPF integrated together and mounted to the engine assembly  10  adjacently to the engine  1 , as shown in  FIG. 1 . 
     The following will describe the operation of the exhaust gas purification apparatus  101  and its peripheral equipment with reference to  FIGS. 1 through 3 . Referring firstly to  FIG. 1 , when the engine  1  is operated, ambient air is drawn 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 sent to the engine intake pipe  3 . The air in the engine intake pipe  3  flows into the engine cylinder  1 A of the engine  1  via the intake manifold  4 . The air in the engine cylinder  1 A is mixed with the fuel (light oil) injected into the engine cylinder  1 A, and air-fuel mixture in the engine cylinder  1 A is ignited spontaneously. 
     Exhaust gas resulting from the combustion of the air-fuel mixture is discharged through the exhaust ports  1 C to the exhaust manifold  5  to be colleted therein. The exhaust gas then flows into the turbine housing  8 B of the turbocharger  8 . The exhaust gas in the turbine housing  8 B is flowed into the exhaust gas purification apparatus  101  while speeding up the turbine wheel (not shown) in the turbine housing  8 B and the compressor wheel connected to the turbine wheel. After flowing through the exhaust gas purification apparatus  101 , the exhaust gas is discharged out from the vehicle (no shown) via the exhaust pipe  6  and the muffler  7 . 
     Referring to  FIG. 2 , all the exhaust gas which has flowed into the exhaust gas purification apparatus  101  passes firstly through the oxidation catalyst layer  12 . When exhaust gas flows through the oxidation catalyst layer  12 , hydrocarbons and carbon monoxide contained in the exhaust gas are oxidized to carbon dioxide and water, while part of nitrogen monoxide contained in the exhaust gas is oxidized to nitrogen dioxide which is reduced more easily than nitrogen monoxide. The exhaust gas which has flowed through the oxidation catalyst layer  12  passes through the space  17 A and the mixer  18  and then flows into the DPF  13 . The DPF body  14  of the DPF  13  collects PM contained in exhaust gas flowing through the DPF  13 . 
     Simultaneously, the DCU  30  operates the motor pump of the urea water tank  20  and opens the injection valve  19 , so that urea water from the urea water tank  20  is injected by the injection valve  19  into the space  17 A. The injected urea water is hydrolyzed under the influence of the heat of the exhaust gas flowing through the space  17 A thereby to produce ammonia and carbon dioxide. Providing the injection valve  19  in the space  17 A at a position adjacent to the oxidation catalyst layer  12 , the time for the injected urea water to stay upstream of the DPF  13  before reaching the first SCR catalyst  15 S of the DPF  13  is increased. Thus, the reaction time that is taken to hydrolyze the urea water to the ammonia is increased, so that the efficiency of hydrolysis of the urea water is improved. For these reasons, the injection valve  19  should preferably be located at such a position upstream of the DPF  13  and adjacent to the oxidation catalyst layer  12  that the distance from the DPF  13  is as long as possible. Urea water is injected from downstream of the oxidation catalyst layer  12  and hydrolyzed to ammonia, so that no ammonia is oxidized under the influence of the oxidation catalyst layer  12 . 
     Ammonia produced by hydrolyzing urea water in the space  17 A passes through the mixer  18  with exhaust gas and is dispersed by the mixer  18  and then flowed into the DPF  13 . Ammonia flowed into the DPF  13  with exhaust gas reduces NO x  contained in exhaust gas to N 2  to purify exhaust gas by the aid of the first SCR catalyst  15 S of the DPF  13 . 
     Although ammonia contained in exhaust gas and passing through the mixer  18  is dispersed by the mixer  18 , distribution of ammonia is still uneven in a plane of the exhaust gas purification apparatus  101  that corresponds to the upstream end face  13 A of the DPF  13  taken along the line  3 A- 3 A of  FIG. 2  and is perpendicular to the center axis of the cylindrical portion  11 C of the casing  11 . Especially, when the distance L between the outlet of the injection valve  19  and the DPF  13  is short, the rate at which injected urea water is hydrolyzed to ammonia before reaching the DPF  13  decreases, so that the unevenness of distribution of ammonia in the above cross-section plane taken along the line  3 A- 3 A of  FIG. 2  is increased. 
     Referring to  FIG. 3  showing the cross sectional view of the exhaust gas purification apparatus  101  as taken along the line  3 A- 3 A of  FIG. 2 , the cross section has a first region P, a second region Q and a third region R. The first region P which is the closest to the injection valve  19  of the three regions has only a little distribution amount of ammonia due to the short distance between the first region P and the injection valve  19 . The third region R which is the farthest from the injection valve  19  has more distribution amount of ammonia due to the longer distance between the third region R and the injection valve  19 . Thus, NO x  content of exhaust gas flowing through the first region P is higher than ammonia content. NO x  content of exhaust gas flowing through the third region R is lower than ammonia content. NO x  content of exhaust gas flowing through the second region Q is substantially the same as ammonia content. 
     Referring back to  FIG. 2 , ammonia contained in exhaust gas flowing downstream of the first region P (refer to  FIG. 3 ) is all used for reduction of NO x , while part of NO x  unreacted with ammonia remains in exhaust gas. Thus, exhaust gas which contains such NO x  but contains no ammonia flows into the space  17 B out of the DPF  13 . On the other hand, NO x  contained in exhaust gas flowing downstream of the second region Q (refer to  FIG. 3 ) is all reduced by ammonia. Thus, exhaust gas containing no NO x  and ammonia flows into the space  17 B out of the DPF  13 . 
     NO x  contained in exhaust gas flowing downstream of the third region R (refer to  FIG. 3 ) is all reduced by ammonia, while part of ammonia unreacted with NO x  remains in exhaust gas. Thus, exhaust gas which contains such ammonia but contains no NO x  flows into the space  17 B out of the DPF  13 . It is noted that remaining ammonia flows into the space  17 B without being significantly adsorbed by the first SCR catalyst  15 S having a low ammonia adsorption property. Therefore, exhaust gas flowing out of the DPF  13  contains ammonia and NO x . The exhaust gas flowing out of the DPF  13  also contains urea water which is not hydrolyzed in flowing through the space  17 A and the DPF  13 . 
     The exhaust gas which has flowed out of the DPF  13  then flows into the second SCR catalyst layer  16  via the space  17 B. When the exhaust gas flows through the space  17 B, hydrolytic action of urea water remaining in the exhaust gas is promoted by the aid of the heat of exhaust gas and hence urea water is hydrolyzed to ammonia. Thus, almost all urea water remaining in exhaust gas is hydrolyzed to ammonia. Therefore, the urea water injected by the injection valve  19  is hydrolyzed with a high efficiency to ammonia before reaching the second SCR catalyst layer  16 . 
     The flow of exhaust gas passing through the DPF  13  is regulated by the DPF  13 . Exhaust gas flowing out of the downstream end face  13 B of the DPF  13  contains NO x  or ammonia depending on parts of the downstream end face  13 B. However, NO x  and ammonia contained in exhaust gas and flowing through the space  17 B are dispersed by the regulated flow of exhaust gas. Thus, distribution of NO x  and ammonia contained in exhaust gas flowing into the second SCR catalyst layer  16  is uniformed in the aforementioned plane corresponding to the cross section of the exhaust gas purification apparatus  101  as taken along the line  3 B- 3 B of  FIG. 2 . It is noted that the cross section of the exhaust gas purification apparatus  101  along the line  3 B- 3 B of  FIG. 2  is parallel to that taken along the line  3 A- 3 A of  FIG. 2 . 
     Ammonia contained in exhaust gas flowing into the second SCR catalyst layer  16  then reduces NO x  contained in the same exhaust gas by the aid of the second SCR catalyst layer  16 . Since NO x  and ammonia contained in exhaust gas are distributed uniformly, they react with each other at a high rate or with a high efficiency. 
     When the amount of ammonia contained in exhaust gas in the second SCR catalyst layer  16  is greater than the amount of ammonia that is necessary for reducing NO x  contained in the exhaust gas, the surplus ammonia is adsorbed by the second SCR catalyst  16 S having a high ammonia adsorption property. Therefore, ammonia contained in exhaust gas flowing into the second SCR catalyst layer  16  is used for reducing NO x  and adsorbed by the second SCR catalyst  16 S, so that ammonia is all removed from exhaust gas. 
     When the amount of NO x  contained in exhaust gas in the second SCR catalyst layer  16  is greater than the amount of NO x  that is reducible by ammonia contained in the exhaust gas, on the other hand, the surplus NO x  which is not reduced by such ammonia is reduced by the ammonia adsorbed by the second SCR catalyst  16 S. Therefore, NO x  contained in exhaust gas flowing into the second SCR catalyst layer  16  is removed from exhaust gas. Thus, ammonia flowing into the second SCR catalyst layer  16  with exhaust gas is used at a high rate for reducing NO x  without flowing out of the second SCR catalyst layer  16  and, therefore, efficiency in the use of ammonia is enhanced. 
     Exhaust gas having its NO x  content reduced and ammonia removed in the second SCR catalyst layer  16  is discharged from the casing  11  or the exhaust gas purification apparatus  101  into the exhaust pipe  6  and then discharged out of the vehicle (not shown) via the exhaust pipe  6  and the muffler  7 . 
     Injection of urea water by the injection valve  19  is performed under a temperature at which the first SCR catalyst  15 S and the second SCR catalyst  16 S are activated or higher. For example, such activating temperature is 150° C. or higher, as mentioned above. Since the temperature of the first SCR catalyst  15 S and the second SCR catalyst  16 S may be regarded as the temperature of exhaust gas flowing therethrough, the DCU  30  opens the injection valve  19  when the temperature detected by the exhaust-gas temperature sensor  51  is 150° C. or higher. The DCU  30  closes the injection valve  19  when the temperature detected by the exhaust-gas temperature sensor  51  is lower than 150° C. Whether or not the NO x  reduction should be performed is thus controlled. 
     The DCU  30  is operable to calculate the flow of NO x  per a predetermined period of time from the value of NO x  concentration detected by the NO x  sensor  52  and also to calculate the amount of ammonia necessary for reducing NO x  from the calculated flow of NO x  and further to calculate the amount of urea water necessary for producing the calculated amount of ammonia. The DCU  30  causes the injection valve  19  to inject the calculated amount of urea water per the predetermined period of time. Thus, the amount of ammonia produced from urea water is controlled so as not to be supplied excessively relative to the amount of NO x  contained in exhaust gas flowing through the casing  11 . 
     If the amount of ammonia contained in exhaust gas flowing into the second SCR catalyst layer  16  is temporarily varied, the second SCR catalyst  16 S adsorbs the remaining ammonia or supplements the deficient ammonia with the adsorbed ammonia, thereby suppressing the variation of the amount of ammonia relative to the amount of NO x  to be reduced. Thus, ammonia is consumed efficiently by the second SCR catalyst layer  16 . In addition, supplying an appropriate amount of urea water that is not excessive for reduction of NO x  contained in exhaust gas flowing into the second SCR catalyst layer  16 , but can just reduce NO x  contained in the exhaust gas, as described above, no excessive amount of ammonia is adsorbed and stored by the second SCR catalyst  168  per a predetermined period of time. Therefore, no ammonia adsorbed by the second SCR catalyst  16 S is released therefrom and flows out of the exhaust gas purification apparatus  101 , or no surplus amount of ammonia contained in exhaust gas flows out of the exhaust gas purification apparatus  101  without being adsorbed by the second SCR catalyst  16 S. 
     Since the first SCR catalyst  15 S has a low ammonia adsorption property, exhaust gas containing NO x  and ammonia, the content ratio of which is close to the content ratio of NO x  and ammonia that is just necessary for reducing the NO x  and calculated by the DCU  30  in accordance with the NO x  concentration detected by the NO x  sensor  52  flows into the second SCR catalyst  16 S. Therefore, reduction reaction between NO x  and ammonia contained in exhaust gas flowing through the second SCR catalyst layer  16  is accomplished efficiently. In addition, since the first SCR catalyst  15 S has a low ammonia adsorption property, it is easy to predict the amount of ammonia contained in exhaust gas flowing through the second SCR catalyst layer  16  and also easy to control the amount of ammonia adsorbed by the second SCR catalyst layer  16 . Thus, the DCU  30  controls the amount of ammonia to be adsorbed by the second SCR catalyst layer  16 , thereby meeting the change of the amount of NO x  contained in exhaust gas caused by the change of operating conditions of the vehicle (not shown). 
     If the first SCR catalyst  15 S has a higher ammonia adsorption property than the second SCR catalyst  16 S, or if the DPF  13  having the first SCR catalyst  15 S has a higher ammonia adsorption capacity than the second SCR catalyst layer  16 , on the other hand, a great amount of ammonia contained in exhaust gas is adsorbed by the first SCR catalyst  15 S of the DPF  13 , so that the amount of ammonia contained in exhaust gas flowing into the second SCR catalyst layer  16  is deficient relative to the amount of NO x  contained in the same exhaust gas. As a result, the second SCR catalyst layer  16  tends to have a shortage of ammonia in exhaust gas flowing thereinto, and hence the second SCR catalyst layer  16  adsorbs no ammonia. Therefore, the efficiency of purification of exhaust gas by removal of NO x  by the second SCR catalyst layer  16  is deficient, with the result that the efficiency of purification of exhaust gas by removal of NO x  by the exhaust gas purification apparatus  101  deteriorates. 
     If the ammonia adsorption capacity of the first SCR catalyst  15 S has high dependence on temperature, when the temperature of exhaust gas is increased, that is, when the temperature of the first SCR catalyst  15 S is increased, ammonia adsorbed by the first SCR catalyst  15 S tends to be released therefrom and hence the amount of ammonia released from the first SCR catalyst  15 S is increased with an increase of its temperature. Therefore, the amount of ammonia contained in exhaust gas flowing into the second SCR catalyst layer  16  becomes excessive relative to the amount of NO x  contained in the same exhaust gas. Thus, the second SCR catalyst  16 S becomes unable to remove all such excessive ammonia by adsorption, so that there is a fear that ammonia is discharged out of the exhaust gas purification apparatus  101  or eventually out of the vehicle (not shown). 
     Therefore, it is preferable that the first SCR catalyst  15 S should have a lower ammonia adsorption property than the second SCR catalyst  16 S, that the ammonia adsorption capacity of the entire DPF  13  is lower than that of the entire second SCR catalyst layer  16 , and also that the ammonia adsorption capacity of the first SCR catalyst  15 S has low dependence on temperature. Urea water is supplied, especially, under a temperature at which the first SCR catalyst  15 S and the second SCR catalyst  16 S are activated, or higher, for removing NO x  contained in exhaust gas thereby to purify exhaust gas, so that it is preferable that the exhaust gas purification apparatus  101  should have the characteristics described just above under such an activation temperature. 
     The second SCR catalyst  16 S uses ammonia adsorbed by itself when it has a shortage of ammonia due to the adsorption and use by the first SCR catalyst  15 S. In addition, the second SCR catalyst  16 S adsorbs excess ammonia when such excess ammonia flows through the second SCR catalyst  16 S due to the release from the first SCR catalyst  15 S. To suppress the change of the amount of ammonia flowing through the second SCR catalyst layer  16 , the second SCR catalyst  16 S should preferably have a high ammonia adsorption property. In addition, the second SCR catalyst layer  16  should preferably have a high ammonia adsorption capacity therethroughout. 
     Referring to  FIG. 1 , exhaust gas which is discharged directly from the turbocharger  8  (or from the engine  1 ) and the temperature of which is decreased only little flows into the exhaust gas purification apparatus  101 . Heat of the operating engine  1  is transmitted to the exterior of the casing  11  (refer to  FIG. 2 ) of the exhaust gas purification apparatus  101  located immediately adjacent to the engine  1  and then transmitted further to the interior of the casing  11 . Referring to  FIG. 2 , the interior of the casing  11  is heated by the heat of the exhaust gas discharged directly from the turbocharger  8 , as well as by the heat transmitted from the engine  1 , so that the interior of the casing  11  tends to be heated easily. Thus, during a cold start of the engine  1 , the time for urea water in the casing  11  to reach its hydrolyzing temperature and the time for the first SCR catalyst  15 S and the second SCR catalyst  16 S to reach their activating temperature are shortened. Therefore, the exhaust gas purification apparatus  101  can start its exhaust gas purifying operation to remove NO x  in a short time after the cold start of the engine  1 . Consequently, the efficiency of removing NO x  is improved. 
     As described above, the exhaust gas purification apparatus  101  of the present embodiment includes the oxidation catalyst layer  12  provided in a passage through which exhaust gas flows, the first SCR catalyst  15 S located in the passage downstream of the oxidation catalyst layer  12 , the second SCR catalyst  16 S located in the passage downstream of the first SCR catalyst  15 S and the injection valve  19  provided for supplying urea water to the passage upstream of the first SCR catalyst  15 S. The second SCR catalyst  16 S has a high ammonia adsorption capacity than the first SCR catalyst  15 S. 
     Since the second SCR catalyst  16 S adsorbs a greater amount of ammonia than the first SCR catalyst  15 S, surplus ammonia out of the ammonia produced by the hydrolysis of urea water supplied by the injection valve  19  which has been neither adsorbed by the first SCR catalyst  15 S nor used for removing NO x  from exhaust gas is adsorbed by the second SCR catalyst  16 S. Thus, NO x  contained in exhaust gas is removed by the first SCR catalyst  15 S and the second SCR catalyst  16 S. For example, when the amount of ammonia contained in exhaust gas flowing out of the first SCR catalyst  15 S and into the second SCR catalyst  16 S is greater than the amount of ammonia necessary for reducing NO x  contained in the same exhaust gas, the resulting surplus ammonia is adsorbed by the second SCR catalyst  16 S. When the amount of ammonia contained in exhaust gas flowing out of the first SCR catalyst  15 S and into the second SCR catalyst  16 S is less than the amount of ammonia necessary for reducing NO x  contained in the same exhaust gas, on the other hand, NO x  is reduced by the ammonia adsorbed in the second SCR catalyst layer  16 . Thus, ammonia produced from urea water is used at a high rate for reducing NO x  without being discharged out of the exhaust gas purification apparatus  101 . Therefore, the exhaust gas purification apparatus  101  improves the efficiency of exhaust gas purification, or the ratio of urea water used for removing NO x  contained in exhaust gas relative to urea water usage. 
     Since the ammonia adsorption capacity per unit volume of the second SCR catalyst  16 S is greater than that of the first SCR catalyst  15 S, the second SCR catalyst  16 S tends to adsorb more ammonia than the first SCR catalyst  15 S. Thus, the amount of ammonia adsorbed by the first SCR catalyst  15 S or the DPF  13  and the amount of ammonia released from the ammonia adsorbed are small, so that the influence of the first SCR catalyst  15 S on the change of the amount of ammonia supplied to the second SCR catalyst layer  16  is also small. Therefore, disposal of ammonia by the second SCR catalyst layer  16  is stabilized and the efficiency in the use of ammonia for removing NO x  is enhanced. 
     The DPF  13  having the first SCR catalyst  15 S and the second SCR catalyst layer  16  are spaced away from each other, so that urea water which has not been hydrolyzed is hydrolyzed between the DPF  13  and the second SCR catalyst layer  16  thereby to produce ammonia. Therefore, the efficiency of converting urea water to ammonia is improved and hence the efficiency of purification of exhaust gas by removing NO x  contained in exhaust gas relative to urea water usage is also improved. The DPF body  14  is integrated with the first SCR catalyst  15 S, so that the exhaust gas purification apparatus  101  can be made in compact. In addition, since the oxidation catalyst layer  12 , the first SCR catalyst  15 S integrated with the DPF body  14 , the second SCR catalyst  16 S and the injection valve  19  are provided in one casing  11 , the exhaust gas purification apparatus  101  can be made further in compact. In the arrangement wherein the exhaust gas purification apparatus  101  is mounted to the engine assembly  10 , high-temperature exhaust gas which is discharged directly from the engine assembly  10  and the temperature of which is decreased only little is flowed into the exhaust gas purification apparatus  101 . In addition, the heat generated by the operating engine  1  is transmitted to the interior of the casing  11  of the exhaust gas purification apparatus  101 . Thus, during a cold start of the engine  1 , the time for urea water in the casing  11  to reach its hydrolyzing temperature and also the time for the first SCR catalyst  15 S and the second SCR catalyst  16 S to reach their activating temperature are shortened. Therefore, the exhaust gas purification apparatus  101  can start its exhaust gas purifying operation to remove NO x  in a short time after the cold start of the engine  1 . Consequently, purification efficiency of exhaust gas by removal of NO x  is improved. 
     The following will describe the second embodiment of the present invention with reference to  FIG. 4 . The exhaust gas purification apparatus  102  according to the second embodiment of the present invention differs from the exhaust gas purification apparatus  101  of the first embodiment in that the DPF body  14  and the first SCR catalyst  15 S of the exhaust gas purification apparatus  101  according to the first embodiment are provided separately. Specifically, as shown in  FIG. 4 , the DPF body  24  corresponding to the DPF body  14  of the exhaust gas purification apparatus  101  of the first embodiment is provided upstream of the first SCR catalyst  25 S of the first SCR catalyst layer  25  corresponding to the first SCR catalyst  15 S of the exhaust gas purification apparatus  101  of the first embodiment. For the sake of convenience of explanation, like or same parts or elements will be referred to by the same reference numerals as those which have been used in the first embodiment, and the description thereof will be omitted. 
     Referring to  FIG. 4  showing the longitudinal sectional view of the exhaust gas purification apparatus  102  according to the second embodiment, as in the case of the first embodiment, the casing  11  of the exhaust gas purification apparatus  102  has therein the oxidation catalyst layer  12 , the DPF body  24 , the first SCR catalyst layer  25  and the second SCR catalyst layer  16  which are located in this order along the direction of exhaust gas flow. The oxidation catalyst layer  12  and the DPF body  24  adjoin each other, the DPF body  24  and the first SCR catalyst layer  25  are spaced away from each other via a space  27 A, and the first SCR catalyst layer  25  and the second SCR catalyst layer  16  are also spaced away from each other via a space  27 B. The first SCR catalyst layer  25  is formed of the first SCR catalyst  25 S corresponding to the first SCR catalyst  15 S of the first embodiment and supported in a substrate (not shown) by any suitable means such as coating as in the case of the second SCR catalyst layer  16  of the first embodiment. Ammonia adsorption capacity of the entire second SCR catalyst layer  16  is higher than that of the entire first SCR catalyst layer  25 . 
     The first SCR catalyst layer  25  has an upstream end face  25 A on which the mixer  18  is provided. The injection valve  29  is provided at a position that is closer to the DPF body  24  than to the first SCR catalyst layer  25  between the DPF body  24  and the first SCR catalyst layer  25  for injecting urea water supplied from the urea water tank  20  into the space  27 A in the casing  11  that is upstream of the first SCR catalyst  25 S. 
     Exhaust gas introduced into the casing  11  of the exhaust gas purification apparatus  102  flows through the oxidation catalyst layer  12  and then into the DPF body  24  as it is, in which PM contained in exhaust gas is collected. After flowing out of the DPF body  24 , the exhaust gas is flowed through the space  27 A into which urea water has been injected, the first SCR catalyst layer  25 , the space  27 B and the second SCR catalyst layer  16  and then discharged out of the exhaust gas purification apparatus  102 . Chemical reactions such as oxidation, reduction and hydrolysis taking place for the components and substances in exhaust gas flowing through the first SCR catalyst layer  25 , the space  27 B and the second SCR catalyst layer  16  in the second embodiment are substantially the same as in the case of the first embodiment. 
     Although the combustion of the PM collected in the DPF body  24  is regularly performed, the first SCR catalyst layer  25  is not directly affected by the heat due to such combustion. The rest of the structure of the second embodiment is substantially the same as that of the first embodiment, and the description thereof will be omitted. 
     The exhaust gas purification apparatus  102  of the second embodiment offers substantially the same effects as that of the first embodiment. The casing  11  of the exhaust gas purification apparatus  102  has therein the DPF body  24  which is located upstream of the first SCR catalyst layer  25 . Since the DPF body  24  and the first SCR catalyst layer  25  are provided separately, the first SCR catalyst  25 S is prevented from being directly affected by the heat due to the combustion of the PM collected in the DPF body  24 . Therefore, the durability of the first SCR catalyst  25 S is improved. The DPF body  24 , which is located upstream of the injection valve  29  in the second embodiment, may be located downstream of the injection valve  29 , or located between the injection valve  29  and the first SCR catalyst layer  25 . 
     The following will describe the third embodiment of the present invention with reference to  FIG. 5 . The exhaust gas purification apparatus  103  according to the third embodiment of the present invention differs from the exhaust gas purification apparatus  101  of the first embodiment in that the space  17 B between the DPF  13  and the second SCR catalyst layer  16  of the exhaust gas purification apparatus  101  of the first embodiment is omitted. Specifically, the DPF body  34  which supports therein the first SCR catalyst  35 S and the second SCR catalyst layer  36  adjoin each other to be integrated together, as shown in  FIG. 5 . 
     Referring to  FIG. 5  showing the longitudinal sectional view of the exhaust gas purification apparatus  103  according to the third embodiment, as in the case of the first embodiment, the casing  11  of the exhaust gas purification apparatus  103  has therein the oxidation catalyst layer  12 , the DPF body  34  and the second SCR catalyst layer  36  which are located in this order along the direction of exhaust gas flow. The oxidation catalyst layer  12  and the DPF body  34  are spaced away from each other via a space  37 , and the DPF body  34  and the second SCR catalyst layer  36  adjoin each other. The first SCR catalyst  35 S corresponding to the first SCR catalyst  15 S of the first embodiment is supported in the DPF body  34  by any suitable means such as coating. In addition, the second SCR catalyst layer  36  is formed of the second SCR catalyst  36 S supported in a substrate (not shown) by any suitable means such as coating as in the case of the second SCR catalyst layer  16  of the first embodiment. Ammonia adsorption capacity of the entire second SCR catalyst layer  36  is higher than that of the entire DPF body  34  supporting therein the first SCR catalyst  35 S. 
     The DPF body  34  supporting therein the first SCR catalyst  35 S and the second SCR catalyst layer  36  are integrated together thereby to form a DPF  33  with catalyst. The DPF  33  has an upstream end face  33 A on which the mixer  18  is provided. The injection valve  39  is provided at a position that is closer to the oxidation catalyst layer  12  than to the DPF  33  between the oxidation catalyst layer  12  and the DPF  33  for injecting urea water supplied from the urea water tank  20  into the space  37  in the casing  11  that is upstream of the first SCR catalyst  35 S. 
     Exhaust gas introduced into the casing  11  of the exhaust gas purification apparatus  103  flows through the oxidation catalyst layer  12  into the space  37 , in which urea water is added, and then flows into the DPF  33 . PM contained in exhaust gas flowed into the DPF  33  through the DPF body  34  is collected by the DPF body  34 . In addition, NO x  contained in exhaust gas is removed by ammonia contained in the same exhaust gas under the action of the first SCR catalyst  35 S supported in the DPF body  34 . The exhaust gas flowed out of the DPF body  34  then flows into the second SCR catalyst layer  36  as it is, in which NO x  contained in exhaust gas is removed by ammonia contained in the same exhaust gas under the action of the second SCR catalyst  36 S. The exhaust gas flowed out of the second SCR catalyst layer  36  is discharged out of the exhaust gas purification apparatus  103 . Chemical reactions such as oxidation, reduction and hydrolysis taking place for the components and substances in exhaust gas flowing through the DPF body  34  supporting therein the first SCR catalyst  35 S and the second SCR catalyst layer  36  in the third embodiment are substantially the same as in the case of the first embodiment. 
     Any urea water which has not been hydrolyzed in exhaust gas flowing through the DPF body  34  of the DPF  33  flows as it is into the second SCR catalyst layer  36 . Since all the urea water is not hydrolyzed, there is a fear that the amount of ammonia contained in exhaust gas flowing into the second SCR catalyst layer  36  may be deficient relative to the amount of NO x  contained in the same exhaust gas. In order to prevent such deficiency of ammonia, ammonia adsorption property of the second SCR catalyst  36 S and ammonia adsorption capacity of the second SCR catalyst layer  36  should preferably be higher than those of the second SCR catalyst  16 S and the second SCR catalyst layer  16  of the first embodiment. The rest of the structure of the third embodiment is substantially the same as that of the first embodiment, and the description thereof will be omitted. 
     Thus, the exhaust gas purification apparatus  103  of the third embodiment offers substantially the same effects as that of the first embodiment. The casing  11  of the exhaust gas purification apparatus  103  has therein the DPF body  34 , which has the first SCR catalyst  35 S and is adjacently integrated with the second SCR catalyst  36 S. Therefore, the casing  11  may be made in compact and the exhaust gas purification apparatus  103  may be made in compact, accordingly. 
     Although in the first through third embodiments each of the exhaust gas purification apparatuses  101 - 103  is mounted to the engine assembly  10  having the turbocharger  8 , the present invention is not limited to such structure. When the engine assembly dispenses with the turbocharger  8 , each of the exhaust gas purification apparatuses  101 - 103  may be directly connected to the outlet  5 A of the exhaust manifold  5 . Each of the exhaust gas purification apparatuses  101 - 103  may be provided at a position distant from the engine assembly  10 . 
     Although in the first through third embodiments the casing  11  of the exhaust gas purification apparatuses  101 - 103  is formed of a cylindrical shape, it may be formed of a prism shape such as a quadratic prism, a spherical shape or an ellipsoidal shape. The exhaust gas purification apparatuses  101 - 103  of the first through third embodiments may dispense with the mixer  18 . Although in the first and second embodiments the first SCR catalysts  15 S,  25 S and the second SCR catalyst  16 S are provided in one casing  11 , the present invention is not limited to such structure. Any of the first SCR catalysts  15 S,  25 S and the second SCR catalyst  16 S may be provided out of the casing  11 . The distance between the first SCR catalysts  15 S,  25 S and the second SCR catalyst  16 S should preferably be set so that activation of the second SCR catalyst  16 S is not decreased by a drop in temperature of the exhaust gas flowing through the first SCR catalysts  15 S,  25 S and the second SCR catalyst  16 S. Although in the first through third embodiments the second SCR catalysts  16 S,  36 S have higher ammonia adsorption property than the first SCR catalysts  15 S,  25 S,  35 S, this is not an essential requirement of the present invention. It may be so arranged that ammonia adsorption capacity of the entire second SCR catalyst layer which supports therein the second SCR catalysts  16 S,  36 S is higher than that of the entire DPF body or the entire first SCR catalyst layer which supports therein the first SCR catalysts  15 S,  25 S,  35 S.