Exhaust gas purification system and exhaust gas purification method

Abstract

An exhaust gas purification system includes a previous-stage oxidation catalyst device, an ammonia-based solution feeder, a Diesel Particulate Filter (DPF) device, a turbine of a turbocharger, and a NOx selective reduction catalyst device in the exhaust system of an internal combustion engine in this order from an exhaust port side. The NH.sub.3 production rate is improved, thereby improving the NOx removal rate; the temperature of the DPF device is kept high to increase the time and frequency of continuous regeneration, thus decreasing the frequency of forced regeneration of the DPF device and the amount of discharge of CO.sub.2 produced during the forced regeneration; and also corrosion of the turbine of the turbocharger by SOx is suppressed.

Claims

1. An exhaust gas purification system for removing nitrogen oxides in an exhaust gas of an exhaust system of an internal combustion engine, comprising: an ammonia-based solution feeder; an NOx selective reduction catalyst device disposed downstream of the ammonia-based solution feeder; wherein an oxidation catalyst device, the ammonia-based solution feeder, a diesel particulate filter device, a turbine of a turbocharger, and the NOx selective reduction catalyst device are disposed in the exhaust system in this order from an exhaust port side of the engine; and a controller configured to find, from an equivalence ratio of a chemical equation, an amount of the ammonia-based solution which enables reduction of an amount of NOx discharged from the engine, calculate a first ammonia-based solution amount larger than the amount enabling the reduction, calculate a second ammonia-based solution amount from a difference between an NOx target discharge amount from the engine, and an amount of NOx measured downstream of the NOx selective reduction catalyst device, set an amount of the ammonia-based solution to be fed to the exhaust system based on a sum of the first ammonia-based solution amount and the second ammonia-based solution amount, and feed the set amount of the ammonia-based solution from the ammonia-based solution feeder, perform a continuous regeneration control of the diesel particulate filter device, when a differential pressure between upstream and downstream sides of the diesel particulate filter device is equal to or higher than a continuous regeneration determination differential pressure, but is equal to or lower than an automatic forced regeneration determination differential pressure, and perform an automatic forced regeneration control of the diesel particulate filter device, when the differential pressure between the upstream and downstream sides of the diesel particulate filter device is equal to or higher than the continuous regeneration determination differential pressure, but is higher than the automatic forced regeneration determination differential pressure.

2. The exhaust gas purification system according to claim 1, further comprising a hydrocarbon feed controller configured to feed hydrocarbon into the exhaust gas upstream of the oxidation catalyst device by post injection via injection inside a cylinder of the engine or by exhaust pipe fuel injection, when the differential pressure between the upstream and downstream sides of the diesel particulate filter device is equal to or higher than the continuous regeneration determination differential pressure, but is equal to or lower than the automatic forced regeneration determination differential pressure, and when a temperature of the exhaust gas at an inlet of the diesel particulate filter device is equal to or lower than a continuous regeneration control start temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view of an exhaust gas purification system of an embodiment of the present invention, illustrating a configuration in which previous-stage oxidation catalyst devices are provided at a single stage.

(2) FIG. 2 is a view of an exhaust gas purification system of an embodiment of the present invention, illustrating a configuration in which previous-stage oxidation catalyst devices are provided at two stages.

(3) FIG. 3 is a chart illustrating one exemplary control flow of hydrocarbon feed control of the present invention.

(4) FIG. 4 is one exemplary control flow of urea feed control of the present invention.

(5) FIG. 5 is a graph illustrating the relationship between the proportion of decrease in the diameter of a DPF device and DPF pressure loss in Example and Conventional Example.

(6) FIG. 6 is a graph illustrating the relationship between the proportion of decrease in the diameter of the DPF device and exhaust manifold pressure in Example and Conventional Example.

(7) FIG. 7 is a graph illustrating the relationship between the proportion of decrease in the diameter of the DPF device and engine torque in Example and Conventional Example.

(8) FIG. 8 is a graph illustrating the time for which the DPF device is heated in Example and Conventional Example.

(9) FIG. 9 is a graph illustrating DPF inlet temperature in the JE05 mode in Example and Conventional Example.

(10) FIG. 10 is a graph illustrating the relationship between SCR device inlet temperature and the rate of production of NH.sub.3 from urea in Example and Conventional Example.

(11) FIG. 11 is a graph illustrating the relationship between turbine inlet temperature and the NOx removal rate in Example and Conventional Example.

(12) FIG. 12 is a graph illustrating an average DPF regeneration interval in Example and Conventional Example.

(13) FIG. 13 is a graph illustrating the ratio of the amount of discharge of CO.sub.2 between Example and Conventional Example.

(14) FIG. 14 is a graph illustrating an average NOx removal rate in Example and Conventional Example.

(15) FIG. 15 is a view illustrating an exemplary configuration of an exhaust gas purification system of a conventional technique.

DETAILED DESCRIPTION

(16) Hereinbelow, exhaust gas purification system and exhaust gas purification method of embodiments according to the present invention will be described with reference to the drawings. Here, an example will be given in which a NOx selective reduction catalyst is a urea NOx selective reduction catalyst, and an ammonia-based solution is urea. However, the present invention is not limited to this example and may employ an HC-selective reduction catalyst or the like.

(17) As illustrated in FIG. 1, an exhaust gas purification system 1 of an embodiment according to the present invention is an exhaust gas purification system for removing PMs and NOx (nitrogen oxides) in an exhaust gas G of an internal combustion engine (hereinafter referred to as the engine) 10 such as a diesel engine. The exhaust gas purification system 1 is configured such that a previous-stage oxidation catalyst device (DOC) 21, a urea injection nozzle 24 as an ammonia-based solution feeder, a diesel particulate filter device (hereinafter referred to as the DPF device) 22 supporting a urea NOx selective reduction catalyst (SCR catalyst), a turbine 14 of a turbocharger, and a NOx selective reduction catalyst device (hereinafter referred as the SCR device) 23 are disposed in the exhaust system of the engine 10 in this order from an exhaust port side connected to an engine body 11.

(18) As illustrated in FIG. 1, the previous-stage oxidation catalyst device 21 is disposed for each exhaust port in an exhaust manifold 12 so that each previous-stage oxidation catalyst device 21 can contact the exhaust gas G as long as possible and thereby increase the time for which the supported oxidation catalyst is at or above its activation temperature. Moreover, if necessary, a subsequent-stage oxidation catalyst device (R-DOC: not illustrated) is disposed downstream of the SCR device 23 for NH.sub.3 (ammonia) slip, so as to decompose NH.sub.3 flowing out of the SCR device 23.

(19) Each previous-stage oxidation catalyst device 21 is formed by disposing catalyst layers containing a metal catalyst which is excellent in removal of CO (carbon monoxide) and a catalyst in which an oxide with an oxygen storage capacity (OSC) and an oxide semiconductor exist in a mixed state. As the oxide with an oxygen storage capacity, an oxide containing Ce (cerium) is available. As the oxide semiconductor, TiO.sub.2 (titanium dioxide), ZnO (zinc oxide), Y.sub.2O.sub.3 (yttrium oxide), and the like are available. Moreover, a precious metal is supported on the oxide with an oxygen storage capacity.

(20) Meanwhile, depending on the exhaust temperature, HC (hydrocarbon) concentration, and CO concentration, the exhaust gas purification system 1 may employ a configuration in which the previous-stage oxidation catalyst devices 21 are provided at a single stage as illustrated in FIG. 1. However, in a case where the HC concentration and CO concentration in the exhaust gas are high, the previous-stage oxidation catalyst devices 21 are preferably divided and disposed as a first oxidation catalyst device (DOC-1) 21a and a second oxidation catalyst device (DOC-2) 21b so that a catalyst configuration excellent in low-temperature activation can be obtained. In this case, like an exhaust gas purification system 1A illustrated in FIG. 2, the first oxidation catalyst device 21a is disposed for each cylinder and exhaust port in the exhaust manifold 12, while the second oxidation catalyst device 21b is disposed downstream of the outlet of the exhaust manifold 12.

(21) In each first oxidation catalyst device 21a, catalyst layers are disposed which contain a metal catalyst which is excellent in CO removal, a catalyst in which an oxide with an OSC such as an oxide containing cerium (Ce), and an oxide semiconductor such as TiO.sub.2, ZnO, or Y.sub.2O.sub.3 exist in a mixed state. Moreover, a precious metal is supported on the oxide with an OSC. On the other hand, in the second oxidation catalyst device 21b, catalyst layers are disposed which contain a catalyst of a precious metal such as platinum (Pt) excellent in HC removal, or a catalyst in which a HC adsorbing material and a precious metal catalyst exist in a mixed state. With these, a catalyst configuration excellent in low-temperature activation can be obtained.

(22) The DPF device 22 is a DPF of a continuous regeneration type which captures and removes PM. This DPF device 22 is preferably formed by providing a coating layer of an SCR catalyst which is made of a catalyst with high NOx removal performance at high temperatures, e.g. a catalyst containing a rare earth composite oxide (such as a a CeZrO-based composite oxide). Moreover, the DPF device 22 needs such specifications as to be resistant to increase in pressure loss after the coating of the SCR catalyst, and therefore has a structure with optimized porosity, pore diameter, and wall thickness that allow equivalent purification characteristics and also a smaller pressure loss.

(23) While there is the SCR catalyst supported in the DPF device 22 disposed upstream of the turbine 14, the SCR device 23 is disposed downstream of the turbine 14. Thus, a temperature decrease of about 100 C. is expected as illustrated in FIG. 9, for example. For this reason, it is preferable to form the SCR device 23 by supporting a zeolite catalyst that has a function of adsorbing urea-derived substances and NOx at low temperatures. It is also preferable to use a small SCR device using a characteristic catalyst support (monolithic catalyst) or the like to increase the amount of the catalyst per specific volume so that a 50% or more decrease in size can be achieved as compared to conventional cases.

(24) The urea injection nozzle 24 is positioned downstream of the previous-stage oxidation catalyst devices 21 but upstream of the DPF device 22. In this way, since the DPF device 22 is present upstream of the turbine 14, urea L is injected upstream of the turbine 14. Thus, the urea L injected into the exhaust gas G is agitated and dispersed inside the turbine 14. Accordingly, the hydrolysis and pyrolysis of the urea L are accelerated. Further, the dispersion of the spray in an exhaust passage 13 after passing through the turbine 14 becomes uniform. Thus, the distance from the urea injection nozzle 24 to the SCR device 23 can be shortened, and closer arrangement can therefore be obtained.

(25) Moreover, a high pressure-exhaust gas recirculation (HP-EGR) passage 15 and a low pressure-exhaust gas recirculation (LP-EGR) passage 16 for performing EGR to decrease NOx are provided. The HP-EGR passage 15 separates an EGR gas Ge to be recirculated for HP (high pressure)-EGR after it passes the previous-stage oxidation catalyst devices 21 (or the first oxidation catalyst devices 21a in FIG. 2) at the exhaust passage 13 upstream of the urea injection nozzle 24. In this way, the EGR gas Ge after passing through the previous-stage oxidation catalyst devices 21 is recirculated to the HP-EGR passage 15. Thus, the SOF (soluble organic fraction) in the EGR gas Ge in the HP-EGR passage 15 can be decreased. Accordingly, it is possible to suppress the influence of the SOF such as clogging of an EGR cooler (not illustrated) and an EGR valve (not illustrated) of the HP-EGR passage 15.

(26) Further, the EGR gas Ge to be recirculated for LP (low-pressure)-EGR is separated downstream of the SCR catalyst 23. In this way, the EGR gas Ge after passing through the previous-stage oxidation catalyst devices 21 (or the first oxidation catalyst devices 21a and the second oxidation catalyst device 21b in FIG. 2), the DPF device 22 and the SCR device 23 is recirculated to the LP-EGR passage 16. Thus, the SOF, PMs, and NH.sub.3 in the EGR gas Ge in the LP-EGR passage 16 can be decreased. Accordingly, it is possible to suppress closing, corrosion, and the like of an EGR cooler (not illustrated) and an EGR valve (not illustrated) of the LP-EGR passage 16.

(27) The exhaust gas purification system 1, 1A further include a temperature sensor 31 which measures a DPF inlet temperature T, which is the temperature of the exhaust gas at the inlet of the DPF device 22, a differential pressure sensor 32 which measures a differential pressure P between upstream and downstream sides of the DPF device 22, and a NOx concentration sensor 33 which measures the concentration of NOx downstream of the SCR device 23. The exhaust gas purification system 1, 1A further include a control device (not illustrated) including an HC feed controller for inputting the measurement values of the temperature sensor 31 and the differential pressure sensor 32 and feeding HC (hydrocarbons) as a fuel into the previous-stage oxidation catalyst devices 21 by post-injection inside the cylinders, and a urea feed controller (ammonia-based solution feed controller) for feeding the urea L for producing NH3 for NOx reduction in the DPF device 22 and the SCR device 23, from the urea injection nozzle 24 into the exhaust gas G. Usually, a control device (not illustrated) called an ECU (engine control unit) which controls the entire operation of the engine 10 functions also as the above control device. In other words, the HC feed controller and the urea feed controller are incorporated in the control device (ECU).

(28) FIGS. 5 to 7 illustrate Example A in which the DPF device 22 is disposed upstream of the turbine 14 and Conventional Example B in which the DPF device 22 is disposed downstream of the turbine. In Example A, the DPF pressure loss is decreased, the pressure inside the exhaust manifold 12 is decreased, and the torque is increased, as compared to Conventional Example B. In other words, since there is no influence of the turbine expansion ratio, the influence of increase in DPF pressure loss on the exhaust manifold internal pressure and the torque is relatively smaller in Example A than in Conventional Example B.

(29) As can be seen from these FIGS. 5 to 7, provided that the torque, the exhaust manifold internal pressure, and the like that influence the engine performance are substantially the same and that the length of the DPF device 22 is the same, the diameter of the DPF device 22 can be decreased by about 40% in Example A as compared to Conventional Example B. As a result, as illustrated in FIG. 8, the DPF device 22 can be heated within a shorter time in Example A than in Conventional Example B, and therefore the time for heating to a predetermined temperature can be shortened.

(30) Moreover, in the present invention, since the DPF device 22 is positioned upstream of the turbine 14, the DPF device 22 can be disposed closer to the engine body 11 than in the conventional technique. As a result, as illustrated in FIG. 9, the DPF inlet temperature T can be kept higher by 100 C. or more.

(31) Further, since the urea injection nozzle 24 can also be disposed closer to the engine body 11 than in the conventional technique, the temperature at the urea injection position can be kept higher by 100 C. or more than in Conventional Example B, like the DPF inlet temperature T illustrated in FIG. 9. As a result, as illustrated in FIG. 10, the rate of production of NH.sub.3 from urea with respect to the temperature at the inlet of the SCR device 23 improves significantly in Example A as compared to Conventional Example B. Thus, as illustrated in FIG. 11, the NOx removal rate with respect to the turbine outlet temperature improves as well. In particular, for the DPF device 22 supporting the SCR catalyst disposed near the urea injection nozzle 24, raising the temperature of the exhaust gas that flows into the DPF device 22 can increase the NH.sub.3 production rate as much as possible and thereby increase the effect of causing a reaction of NOx and NH.sub.3 at the surface of the SCR catalyst of the DPF device 22. Thus, the removal rate can be improved.

(32) Next, HC (hydrocarbon) feed control in the above-described exhaust gas purification systems 1, 1A will be described. In the present invention, in view of the superiority of the above-described configuration, HC feed to each previous-stage oxidation catalyst device 21 is controlled such that HC adsorption and oxidation in the previous-stage oxidation catalyst device 21 raise the temperature of the exhaust gas G that flows into the DPF device 22, to thereby set the DPF inlet temperature T, which is the temperature of the exhaust gas at the inlet of the DPF device 22, to a temperature (250 to 500 C.) that allows continuous regeneration. In this way, the frequency and duration at and for which continuous regeneration can be performed are increased.

(33) This HC feed control can be performed through a control flow exemplarily illustrated in FIG. 3. The control flow in FIG. 3 is illustrated as a control flow which is repeatedly called and executed by a higher control flow which is actuated upon start of operation of the engine 10; the control flow is discontinued and returns to the higher control flow upon stop of operation of the engine 10, and stops as the higher control flow stops.

(34) When the control flow in FIG. 3 is called by the higher control flow and starts, in step S11, the DPF inlet temperature T is inputted from the temperature sensor 31, and the DPF upstream-downstream differential pressure P, which is the differential pressure between the upstream and downstream sides of the DPF device 22, is inputted from the differential pressure sensor 32. In the next step S12, it is determined whether or not the DPF upstream-downstream differential pressure P is equal to or higher than a continuous regeneration determination differential pressure PL. If the DPF upstream-downstream differential pressure P is equal to or higher than the continuous regeneration determination differential pressure PL (YES), it is determined in the next step S13 whether or not the DPF upstream-downstream differential pressure P is equal to or lower than an automatic forced regeneration determination differential pressure PH. If the DPF upstream-downstream differential pressure P is equal to or lower than the automatic forced regeneration determination differential pressure PH (YES), the flow proceeds to step S14.

(35) Note that if it is determined in step S12 that the DPF upstream-downstream differential pressure P is lower than the continuous regeneration determination differential pressure PL (NO), the flow returns to step S11. Moreover, if it is determined in step S13 that the DPF upstream-downstream differential pressure P is higher than the automatic forced regeneration determination differential pressure PH (NO), the flow proceeds to step S20, in which automatic forced regeneration control is performed to forcibly regenerate the DPF device 22. Then, the flow returns to the higher control flow, and the control flow in FIG. 3 is called again by the higher control flow and is repeated.

(36) In step S14, it is determined whether or not the DPF inlet temperature T is equal to or lower than a continuous regeneration control start temperature TL. If the DPF inlet temperature T is equal to or lower than the continuous regeneration control start temperature TL (YES), HC feed is performed in step S15 in which HC is fed to the previous-stage oxidation catalyst devices 21 by post-injection for a predetermined time t1 (a time set in advance according to the intervals of the determination of the DPF upstream-downstream differential pressure P and the determination of the DPF inlet temperature T). Thereafter, the flow returns to step S14. If the DPF inlet temperature T is higher than the continuous regeneration control start temperature TL (NO) in step S14, the flow proceeds to step S16.

(37) In step S16, since the DPF inlet temperature T is higher than the continuous regeneration control start temperature TL, the flow waits for a predetermined time t2, during which continuous regeneration of the DPF device 22 is performed. Thereafter, the flow proceeds to step S17, in which, if HC feed is being performed, the HC feed is stopped, and if no HC feed is being performed, HC feed is kept stopped, and the flow proceeds to step S18.

(38) In step S18, it is determined whether or not the DPF upstream-downstream differential pressure P is equal to or lower than the continuous regeneration determination differential pressure PL. If the DPF upstream-downstream differential pressure P is neither equal to nor lower than the continuous regeneration determination differential pressure PL (NO), the flow returns to step S14 to continue the continuous regeneration. If the DPF upstream-downstream differential pressure P is equal to or lower than the continuous regeneration determination differential pressure PL (YES), the continuous regeneration is considered to have been completed and to be unrequired. Thus, the flow returns to the higher control flow, and the control flow in FIG. 3 is called again by the higher control flow, and starts and is repeated again.

(39) It is determined in steps S11 to S13 whether or not to feed HC for heating the exhaust gas for continuous regeneration and steps S14, S15 are repeated, to thereby raise the DPF inlet temperature T until it exceeds the continuous regeneration control start temperature TL. Then, continuous regeneration is performed in step S16, and the HC feed is stopped in step S17 to prevent unnecessary HC consumption. Thereafter, it is determined in step S18 whether or not to end the continuous regeneration.

(40) By performing the control flow in FIG. 3, it is possible to perform control in which HC is fed into the exhaust gas G upstream of the previous-stage oxidation catalyst devices 21 by post-injection via injection inside the cylinders in the case where the DPF upstream-downstream differential pressure P of the DPF device 22 is equal to or higher than the continuous regeneration determination differential pressure PL but equal to or lower than the automatic forced regeneration determination differential pressure PH and also the DPF inlet temperature T of the DPF device 22 is equal to or lower than the continuous regeneration control start temperature TL. Note that instead of the post-injection, exhaust pipe fuel injection may be employed in which fuel is injected directly into the exhaust pipes upstream of the previous-stage oxidation catalyst devices 21.

(41) As illustrated in FIG. 12, by this HC feed control, the interval of automatic forced regeneration of the DPF device 22 can be greatly extended in Example A of the present invention as compared to Conventional Example B of the conventional technique. Further, as illustrated in FIG. 13, the amount of discharge of CO.sub.2 during DPF regeneration can be significantly decreased in Example A as compared to Conventional Example B. Note that a material capable of adsorption of a large amount of CO, such as CeO.sub.2 (cerium oxide) or ZrO.sub.2 (zirconium dioxide), may be used for the oxidation catalyst of each previous-stage oxidation catalyst device 21. In this way, the amount of heat production of the previous-stage oxidation catalyst device 21 can further be increased.

(42) By this HC feed control, the effect of HC adsorption and oxidation of the oxidation catalyst in each previous-stage oxidation catalyst device 21 upstream of the DPF device 22 can be exhibited effectively. Thus, when continuous regeneration of the DPF device 22 is needed, the temperature T of the exhaust gas that flows into the DPF device 22 (temperature of the exhaust gas at the inlet) can be raised above the temperature TL above which continuous regeneration is possible. In this way, the interval of automatic forced regeneration control for the DPF device 22 can be extended. Accordingly, the amount of discharge of CO.sub.2 during regeneration of the DPF device 22 can further be decreased.

(43) Next, urea feed control in the above-described exhaust gas purification systems 1, 1A will be described. In the present invention, in view of the superiority of the above-described configuration, the urea feed from the urea injection nozzle 24 to the DPF device 22 supporting SCR catalysts and SCR device 23 is controlled such that NOx in the exhaust gas is reduced in the DPF device 22 supporting SCR catalysts and SCR device 23 with NH.sub.3 produced from the urea L.

(44) This urea feed control can be performed through a control flow exemplarily illustrated in FIG. 4. The control flow in FIG. 4 is illustrated as a control flow which is called and executed by a higher control flow that is actuated upon start of operation of the engine 10; the control flow is discontinued by an interruption of step S40 upon stop of operation of the engine 10, returns to the higher control flow, and stops as the higher control flow stops.

(45) When the control flow in FIG. 4 is called by the higher control flow and starts, in step S31, a first NOx discharge amount Win is measured or calculated. The first NOx discharge amount Win is the amount of discharge of NOx representing the NOx (NO, NO.sub.2) from the engine body 11 converted into NO (the amount of discharge of the engine-out NOx). It is found from a measured concentration of NOx in the exhaust gas G and a calculated amount of exhaust gas, or calculated based, for example, on referring to preset map data based on the operating state of the engine 10.

(46) Further, in step S31, a first urea feed amount Wumol for the first NOx discharge amount Win is calculated. The first urea feed amount Wumol is obtained by calculating the amount of NH.sub.3 for the first NOx discharge amount that is necessary for NO reduction at an equivalence ratio of NH.sub.3 to NO of 1 to 1.3 (a value found and set in advance through a test or the like), and setting the amount of urea for producing this amount of NH.sub.3 as the first urea feed amount Wumol. Then, the counting of a urea feed elapsed time t is started. Also, a second urea feed amount Wuplas to be used later is set to zero.

(47) Then, in step 32, it is determined whether or not the urea feed elapsed time t that is being counted has reached a preset determination time t1. The determination time t1 is set to a time long enough for the exhaust gas G to reach the NOx concentration sensor 33 downstream of the SCR device 23, the exhaust gas G containing the urea L which has been fed thereinto upstream of the DPF device 22 from the urea injection nozzle 24. This time can be set based on an experimental value or a value calculated from the flow rate of the exhaust gas or the like.

(48) If the urea feed elapsed time t has reached the determination time t1 (YES) in step S32, the flow proceeds to step S33. Alternatively, if the urea feed elapsed time t has not reached the determination time t1 (NO), the flow proceeds to step S34, in which the urea L of a first urea feed amount Wumol is fed into the exhaust gas G upstream of the DPF device 22 from the urea injection nozzle 24 for a preset time (a time based on the time interval of the determination in step S32) t1. Thereafter, the flow returns to step S31.

(49) In step S33, the measurement value of the NOx concentration sensor 33 downstream of the SCR device 23 is inputted, and a measured discharge amount Wout is calculated from the NOx concentration thus inputted and the amount of exhaust gas. Note that the amount of exhaust gas can be calculated from the operating state of the engine 10 or the amount of intake measured with an intake sensor (mass air flow (MAF) sensor: not illustrated) and the amount of fuel injection.

(50) The measured discharge amount Wout and a target discharge amount WT, which is a target value for decreasing NOx discharge, are compared. If the measured discharge amount Wout is equal to or smaller than the target discharge amount WT (YES), the first urea feed amount Wumol is determined as a sufficient amount of urea, and the flow proceeds to step S34, in which the urea L of the first urea feed amount Wumol is fed for the preset time t1. Thereafter, the flow returns to step S31.

(51) On the other hand, if it is determined in step S33 that the measured discharge amount Wout is larger than the target discharge amount WT (NO), the first urea feed amount Wumol is determined as an insufficient amount of urea, and the flow proceeds to step S35.

(52) In step S35, the measured discharge amount Wout is newly calculated, and a discharge amount difference Wdef being the difference between the target discharge amount WT and the measured discharge amount Wout is calculated (Wdef=WTWout). Moreover, the amount of NH.sub.3 is calculated for the discharge amount difference Wdef which is necessary for reduction of NOx of an amount equal to the discharge amount difference Wdef, and the second urea feed amount Wuplas is calculated by using a urea amount Wud for producing this amount of NH.sub.3; in other words, Wuplas=Wuplas+Wud. Thus, the second urea feed amount Wuplas taking the discharge amount difference Wdef into account can be calculated. Furthermore, a total urea feed amount Wut being the sum of the first urea feed amount Wumol and the second urea feed amount Wuplas is calculated (Wut=Wumol+Wuplas).

(53) In the next step S36, the urea L of the total urea feed amount Wut is fed for a preset time (a time based on the time interval of update of the measured value of NOx concentration in step S35) t2. Thereafter the flow then returns to step S35. These steps S35 to S36 are repeated to feed the urea L of the total urea feed amount Wut into the exhaust gas G upstream of the DPF device 22. When an interruption of step S40 occurs upon stop of the engine 10, the flow returns to the higher control flow, and the control flow in FIG. 4 ends along with the higher control flow.

(54) According to the control described above, the urea L of the first urea feed amount Wumol can be fed in steps S31 to S34 in the case where the urea feed elapsed time t has not reached the predetermined determination time t1 (NO), or the measured discharge amount Wout is equal to or smaller than the target discharge amount WT (YES); on the other hand, the urea L of the total urea feed amount Wut, which is the sum of the first urea feed amount Wumol and the second urea feed amount Wuplas, can be fed in steps S35 and S36 in the case where the urea feed elapsed time t has reached the predetermined determination time t1 (YES), and the measured discharge amount Wout is larger than the target discharge amount WT (NO).

(55) In other words, urea injection control can be performed which includes: considering an amount of feed of urea, which is the amount of urea expected to be consumed by the DPF device 22 with the coated SCR catalyst upstream of the turbine 14, as the first urea feed amount Wumol which allows an equivalence ratio of the amount of ammonia (NH.sub.3) as urea to the amount of the engine-out NOx of 1 or greater; further, estimating the measured discharge amount Wout of NOx downstream of the SCR device 23 from the measured NOx concentration; calculating the discharge amount difference Wdef estimated as an insufficient amount for decreasing the NOx to the target discharge amount WT; calculating the second urea feed amount Wuplas to be consumed by the SCR device 23 downstream of the turbine 14; and feeding the urea L of the total urea feed amount Wut being obtained by adding the first urea feed amount Wumol and the second urea feed amount Wuplas.

(56) As a result, as illustrated in FIGS. 10, 11, and 14, high NOx removal performance can be obtained in wide ranges from low to high temperatures in Example A of the present invention as compared to Conventional Example B of the conventional technique. In particular, the NOx removal rate is improved by 30% or more in terms of JE05 mode average.

(57) Next, description will be given of an advantage for corrosion by SOx (sulfur oxides) achieved by disposing the urea injection nozzle 24 upstream of the DPF device 22. The urea L sprayed into the exhaust gas G from the urea injection nozzle 24 produces NH.sub.3 (ammonia) mainly through a pyrolysis reaction of the urea (NH.sub.2).sub.2CO.fwdarw.NH.sub.3+HNCO and a hydrolysis reaction of the isocyanic acid produced by the pyrolysis reaction HNCO+H.sub.2O.fwdarw.NH.sub.3+CO.sub.2. The NH.sub.3 produced from the urea undergoes a reaction of 2NH.sub.3+SO.sub.4.fwdarw.(NH.sub.4).sub.2SO.sub.4 with SOx in the exhaust gas, thereby producing (NH.sub.4).sub.2SO.sub.4 (ammonium sulfate).

(58) Further, the (NH.sub.4).sub.2SO.sub.4 undergoes a reaction of (NH.sub.4).sub.2SO.sub.4+CaCO.sub.3.fwdarw.(NH.sub.4).sub.2SO.sub.3+CaSO.sub.4 with CaCO.sub.3 (calcium carbonate) which is an ash component produced after the combustion of PMs in the downstream (subsequent-stage) DPF device 22. The (NH.sub.4).sub.2CO.sub.3 (ammonium carbonate) thus produced decomposes at 58 C. or higher through a pyrolysis reaction (NH.sub.4).sub.2CO.sub.3.fwdarw.2NH.sub.3+H.sub.2O+CO.sub.2. The NH.sub.3 produced by this reaction is captured by the SCR device 23 downstream of the DPF device 22 and used for a NOx removal reaction.

(59) The (NH.sub.4).sub.2SO.sub.4 produced by the reaction of NH.sub.3 and SO.sub.4 or the like is a neutralized product and thus has no corrosive properties. This solves the problem of corrosion of the turbine 14 and the exhaust passage 13 downstream of the DPF device 22 by SOx, and also solves the problem of corrosion of the EGR passage 16, the EGR valve (not illustrated) and the EGR cooler (not illustrated) for the LP (low pressure)-EGR in which the exhaust gas after the reaction of NH.sub.3 and SO.sub.4 or the like is used as the EGR gas Ge.

(60) Thus, according to the exhaust gas purification systems 1, 1A and the exhaust gas purification method described above, the urea injection nozzle 24 is disposed upstream of the DPF device 22, and therefore the position of the urea injection nozzle 24 can be closer to the engine body 11. Thus, the temperature of the exhaust gas G to be fed with the urea L can be kept high, and therefore the rate of production of NH.sub.3 (ammonia) from the urea L can be improved.

(61) Further, since the DPF device 22 is disposed upstream of the turbine 14, the position of the DPF device 22 is close to the exhaust ports, and therefore the temperature of the DPF device 22 can be kept high. This makes it possible to increase the frequency of continuous regeneration and decrease the size. The decrease in the size of the DPF device 22 can shorten the heating time during regeneration. Thus, it is possible to decrease the amount of discharge of CO.sub.2 during regeneration of the DPF device 22 and also to increase the degree of freedom in layout.

(62) In addition, since the urea injection nozzle 24, the DPF device 22, and the turbine 14 are disposed in this order from the upstream side, SOx produced by combustion in the cylinders can be changed to CaSO.sub.4, which has low corrosive properties. Thus, it is possible to suppress corrosion of the turbine by SOx. Further, since the DPF device 22 is disposed in such a way as not to be influenced by ash originating from the oil of the turbine 14, it is possible to avoid the influence of the ash on clogging of the DPF device 22.

(63) Further, by performing the hydrocarbon feed control, the temperature of the exhaust gas flowing into the DPF device 22 can be raised to temperatures that allow continuous regeneration of the DPF device 22, when such continuous regeneration is being needed. Thus, the interval of the automatic forced regeneration control for the DPF device 22 can be extended. Accordingly, the amount of discharge of CO.sub.2 during regeneration of the DPF device 22 can further be decreased.

(64) Furthermore, by the urea feed control, the urea L can be fed to the DPF device 22 and the SCR device 23 as a more appropriate amount of ammonia-based solution. Accordingly, it is possible to efficiently remove NOx in wide ranges from low to high temperatures and to high flow rates.

(65) Thus, by combining the arrangement of the exhaust gas purification units, the hydrocarbon feed control, and the ammonia-based solution feed control of the present invention, it is possible to improve the NOx removal rate in wide ranges from low temperatures and rates to high temperatures and flow rates.

(66) According to the exhaust gas purification system and the exhaust gas purification method of the present invention, the ammonia-based solution feeder is positioned close to the exhaust ports of the engine body to keep the temperature of the exhaust gas to be fed with urea high. Thus, the ammonia (NH.sub.3) production rate can be improved, thereby improving the NOx removal rate. Also, the DPF device is also disposed close to the exhaust ports of the engine body to keep the temperature of the DPF device high. Thus, the time and frequency of continuous regeneration of the DPF device are increased, thereby decreasing the frequency of forced regeneration of the DPF device and the amount of discharge of CO.sub.2 produced during the forced regeneration. Further, the ammonia-based solution feeder, the DPF, and the turbine are disposed in this order so that sulfur oxides (SOx) in the exhaust gas can react with the ash component of the DPF, thereby changing to calcium sulfate which has low corrosive properties, and then flow into the turbine. Thus, corrosion of the turbine of the turbocharger by the sulfur oxides can be suppressed. Accordingly, the exhaust gas purification system and the exhaust gas purification method of the present invention can be utilized as an exhaust gas purification system and an exhaust gas purification method for internal combustion engines mounted on automobiles and the like.