FR2814966A1 - Catalyst control of IC motor exhaust emissions uses a solid reduction agent which is heated to give a supply of molten agent as a dosed feed into the exhaust gas according to running conditions - Google Patents

Abstract

The exhaust gas system, for an internal combustion motor (1), has a catalyst (8) in the exhaust channel and a unit (11) to add a reduction agent to the catalyst with a collector (12) to gather the solid reduction agent (A). A heater heats and melts the agent for a collector (13) which gathers the molten agent. The molten reduction agent is fed (14) into the exhaust gas upstream of the catalyst. The exhaust gas system, for an IC motor, has a unit (UCE) to determine the dosage rate of molten reduction agent to be fed. A heating control operates the heater, to melt sufficient reduction agent to provide the dosed feed. The catalyst is an NOx type with a selective reduction (10), to reduce or degrade the NOx by the reduction agent derived from ammonia using urea. A monitor registers the level of the molten reduction agent. The dosing control calculates the feed rate together with the monitored available supply. The heater is set at a level which does not degrade the reduction agent, and maintains the molten agent in a liquid state. A thermal sensor monitors the exhaust gas temperature, to compute the dosage of the reduction agent and whether the gas temperature indicates the need for reduction, so that the use of a reduction agent is set according to the operation of the motor.

Description

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APPARATUS AND METHOD FOR CONTROLLING THE EXHAUST GASES OF A
INTERNAL COMBUSTION ENGINE
The present invention relates to an exhaust gas control apparatus and method for controlling exhaust gases discharged from an internal combustion engine.

 A NOx catalyst of the selective reduction type to reduce or decompose harmful NOx with a reducing agent in an atmosphere of excess oxygen is often employed in an exhaust control apparatus which mainly removes NOx from the exhaust gas discharged from an internal combustion engine capable of performing combustion at a l / fuel ratio (e.g. a diesel engine, a lean combustion gasoline engine).

 Reduction catalysts require a reduction agent. A technology which uses solid urea as reducing agent has been developed. An example of a means for discharging solid urea from an accumulator is the device for adding reducing agent described in the application for Japanese patent for public inspection number 2000-27 626.

 The device described in Japanese patent application to the public inspection number 2000-27 626 includes a spraying means formed by a spring element fixed to an upper wall of an accumulator and a mechanism for

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 spray attached to a lower distal end portion of the spring member. The spray mechanism includes a plurality of spray arms which extend in a radial direction from a vertical rod member in the vicinity of a side wall of the accumulator. A lower distal end portion of the vertical rod member is slidably guided in the up-down direction by a guide member which extends inwardly from a bottom wall of the accumulator. The spring means of the spraying means is designed so that when urea is supplied to the accumulator, the spring element will not be buried in the urea and as a result may expand and freely contract.

 Urea easily agglomerates in the accumulator.

However, if the spraying means as described above is arranged in the accumulator, the spraying means, slidingly guided vertically by the guide element, vibrates up and down via the spring element due vibrations of the vehicle, so that the spraying means atomizes lumps of urea in the entire accumulator. As a result, the urea sprayed into the accumulator can be continuously discharged.

 However, the previously mentioned apparatus described in the Japanese patent application: public inspection number 2000-27,826 requires the spring element and the spraying mechanism in order to discharge the solid reducing agent from the accumulation device. Consequently, the device for adding reducing agent is complex. The patent application to the public inspection does not describe any specific means for delivering a solid reducing agent discharged from the solid reducing agent accumulation device.

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 In particular, if a solid reducing agent is liquefied for use, the molten reducing agent, such as urea or the like may remain in a feed path. If such a case occurs the temperature decreases with the passage of time, the reducing agent remaining in the feed path solidifies again. Thus, there is a risk that the next feeding of the reducing agent may become impossible.

 Accordingly, it is desirable to provide a device capable of reliably supplying a predetermined amount of reducing agent to a position upstream of a NOx catalyst of the selective reduction type.

 The invention has been accomplished in view of the above-mentioned problems. It is an object of the invention to provide an exhaust gas control apparatus and a method capable of heating and melting a reducing agent in solid form and delivering a predetermined amount of molten reducing agent to a passage of the exhaust gases upstream of a NOx catalyst of the selective reduction type with a high control ability.

 To achieve the above-mentioned object, the invention provides an exhaust gas control apparatus comprising a catalyst arranged in an exhaust gas passage of an internal combustion engine and a device for adding a reducing agent. for adding the reducing agent to the catalyst, said reducing agent adding device including: solid reducing agent accumulating means for accumulating a solid reducing agent; heating / melting means for heating and melting the solid reducing agent; a molten reducing agent accumulating means for accumulating a molten reducing agent introduced from the accumulating means solid accumulating device; and means for supplying molten reducing agent to deliver the reducing agent.

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 tion melted in part of the exhaust gas passage upstream of the catalyst.

 In accordance with the exhaust gas control apparatus described above, the solid reducing agent is melted by heating and the molten reducing agent is delivered to the exhaust gas passage upstream of the catalyst. As a result, it is possible to overcome the disadvantage that the reducing agent remains in a solid form in a feeding passage, and consequently prevents the regular feeding of the reducing agent.

 The exhaust control apparatus of the invention may further include: means for determining the amount of molten reducing agent supply to determine an amount of molten reducing agent to be supplied by the means for supplying molten reducing agent; and reducing agent heating control means for controlling the heating / melting means based on the amount of the reducing agent to be added determined by the amount of the molten reducing agent supply determining means .

 The means for determining the amount of supply of molten reducing agent can be a means of calculating the amount of supply of molten reducing agent to calculate the amount of molten reducing agent.

 Furthermore, the exhaust gas control apparatus of the invention is applicable if the catalyst is a NOx catalyst of the selective reduction type for reducing or decomposing NOx in the presence of a reducing agent obtained from ammonia. The reducing agent can be, for example, urea.

 Furthermore, in the exhaust gas control apparatus of the invention, the means for accumulating molten reducing agent can be provided with a means for

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 detection for detecting a residual amount of molten reducing agent and the means for calculating the amount of molten reducing agent can be controlled so that the residual amount of molten reducing agent detected by the detecting means and the amount of reducing agent to be supplied calculated by the means for calculating the amount of supply of molten reducing agent.

 In accordance with this structure of the exhaust gas control apparatus, since the amount of molten solid reducing agent and the amount of molten reducing agent supplied are controlled, it is possible to prevent the when the residual molten reducing agent left unused solidifies in the apparatus and blocks a passage of molten reducing agent or the like in the apparatus.

 Furthermore, in the exhaust gas control apparatus of the invention, the heating temperature of the heating / melting means can be set at a temperature which does not cause a deterioration in the quality of the blowing agent. solid reduction.

 Still further, in the exhaust gas control apparatus of the invention, the heating temperature of the heating / melting means can be set at a temperature which allows the solid reducing agent to be melted. so that a reducing agent in the liquid state is retained.

 Still further, the exhaust gas control apparatus of the invention may further include temperature maintaining means for maintaining a temperature of the means for accumulating the molten reducing agent at a predetermined temperature .

 In addition, the exhaust control apparatus of the invention may further include means for determining

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 temperature sensing to detect an exhaust gas temperature that is used for. ordering an amount of molten reducing agent supplied from the means for supplying molten reducing agent. In the exhaust gas control apparatus, the molten reducing agent cannot be supplied from the molten reducing agent supply means if the temperature of the exhaust gases is at a predetermined value or less.

 It is preferred that the temperature for melting the solid reducing agent is in the range of about 133 ° C to 2000 ° C. Within this temperature range, deterioration in the quality of urea is unlikely.

 In accordance with the invention, since the amount of flow of the reducing agent in the liquid state is controlled, the apparatus can be of a reduced size and can be of simpler structure. In addition, due to the good control ability, the amount of reducing agent to be delivered can be controlled with high precision.

 Examples of the previously mentioned internal combustion engine include a direct injection type lean combustion gasoline engine, a diesel engine, etc.

 The previously mentioned selective reduction type NOx catalyst includes a catalyst formed by charging zeolite with a transition metal such as Cu or the like via ion exchange, a catalyst formed by charging zeolite or alumina with a precious metal, a titanium dioxide / vanadium catalyst, etc.

 If the aforementioned solid reducing agent is delivered, for example, in spherical form, the

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 solid reduction can be easily removed from the solid reducing agent accumulation means.

 In addition, the invention provides a method for controlling exhaust gases comprising the steps of: accumulating a solid reducing agent; determining a quantity of molten reducing agent to be supplied to a catalyst arranged in the exhaust gas passage of an internal combustion engine on the basis of the operating state of the internal combustion engine, acquiring a reducing agent melted by heating and melting the solid reducing agent so as to obtain the calculated amount of molten reducing agent to be delivered accumulating the molten reducing agent; and delivering the accumulated molten reducing agent, in a part of the exhaust gas passage upstream of the catalyst.

 In accordance with this control method, since the solid reducing agent is melted by heating and the molten reducing agent is delivered in the exhaust gas passage upstream of the catalyst, it is possible to overcome the disadvantage the reducing agent remains in a solid state in a feed passage and therefore prevents regular feeding of the reducing agent. Furthermore, since the amount of molten reducing agent to be supplied is determined on the basis of the operating state of the internal combustion engine and the solid reducing agent is melted by heating into a molten reducing agent so that the determined amount of molten reducing agent to be delivered is acquired, it is possible to accumulate an appropriate amount of molten reducing agent and to deliver the molten reducing agent

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 in the exhaust gas passage upstream of the catalyst.

 The amount of the molten reducing agent can be. calculated on the basis of the operating state of the internal combustion engine.

 This control method is also applicable if the catalyst is a NOx catalyst of the selective reduction type for reducing or decomposing NOx in the presence of a reducing agent obtained from ammonia such as urea.

 Further, in this ordering method, an order can be made so that a remaining amount of accumulated molten reducing agent and an amount of delivered reducing agent become equal.

 In accordance with this control method, it is possible to overcome the disadvantage that the unused residual molten reducing agent solidifies in the apparatus and blocks a passage of molten reducing agent or the like in the apparatus.

 Furthermore, in the control method, the amount of molten reducing agent to be supplied can be controlled based on the temperature of the exhaust gases.

 Still further, in the control method, it is also possible to adopt a structure in which if the temperature of the exhaust gases is a predetermined value or lower, the molten reducing agent is not delivered.

 The foregoing and other objects, features, advantages and technical and industrial meanings of this invention will be better understood by reading the following detailed description of the exemplary embodiments.

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, la figure 4 est un organigramme illustrant une opération consistant à délivrer un agent de réduction en confor- mité avec le premier mode de réalisation ; et la figure 5 est un organigramme illustrant l'opération consistant à délivrer un agent de réduction en conformité avec le second mode de réalisation. res of the invention, when read in conjunction with the accompanying drawings, among which: FIG. 1 is a simplified diagram of an apparatus "for controlling the exhaust gases of an engine in accordance with the invention; FIG. 2 is a diagram illustrating in a simplified manner a structure of a device for adding a reducing agent in a first embodiment of the invention; FIG. 3 is a diagram illustrating in a simplified manner a structure of a device for addition of reducing agent in a second embodiment of the invention

FIG. 4 is a flow chart illustrating an operation of delivering a reducing agent in accordance with the first embodiment; and Figure 5 is a flow diagram illustrating the operation of delivering a reducing agent in accordance with the second embodiment.

 In the following description and in the accompanying drawings, the present invention will be described in more detail in terms of preferred embodiments.

 In the embodiments described below, the invention is applied to a diesel engine driving a vehicle.

 In a diesel engine for vehicle 1, air is introduced into a combustion chamber 2 of each cylinder from an intake pipe 4 via an air filter 8. The fuel is injected into each combustion chamber 2 through from a fuel injection valve 5, and burns

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 a poor r / fuel ratio. In FIG. 1, the reference numeral 6 represents a piston.

 The exhaust gases from each combustion chamber 2 are discharged into the atmosphere via an exhaust pipe 7 downstream of each combustion chamber 2, an NOx catalytic converter 8 and an exhaust pipe 9 downstream of the NOx catalytic converter 8. The NOx catalytic converter 8 contains a NOx catalyst of the selective reduction type 10 of the zeolite / silica or TiN family to reduce or decompose NOx in the presence of a reducing agent .

 The presence of a reducing agent is necessary in order to remove the NOx in the exhaust gases through the use of the NOx catalyst of the selective reduction type 10. Consequently, in this control apparatus for the exhaust gas, a device for adding a reducing agent 11, that is to say an adding means for adding a reducing agent, is arranged in the exhaust pipe 7 upstream of the catalytic converter of the NOx 8.

 A first embodiment of the engine exhaust control device of the invention will now be described with reference to Figures 1 and 2.

 Figure 2 shows a device for adding reducing agent 11 in this embodiment. The reducing agent adding device 11 liquefies many spherical solid urea pieces A supplied as reducing agent and then delivers the liquefied urea into the exhaust pipe 7. The adding agent reduction 11 comprises an accumulation container 12 for accumulating the solid urea pieces A and a heating device 31 arranged on an internal lower face 12c of the accumulation container 12. The internal lower face 12c provided with the heating device 31 comprises a opening of

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 load 12d at the lowest position and is inclined downwards towards a central part.

 The solid urea pieces A can be prevented from adhering to each other to form larger lumps, by coating the solid urea pieces A with an adhesion preventing agent.

 In addition, since the solid urea pieces A have the property of easily absorbing moisture and adhering to each other, the device for adding reducing agent 11 can be provided with a dehumidification means. using silica gel or the like (not shown). In this case, the dehumidification means 15 can be a container which contains silica gel or the like and which is connected by a passage 15a to the accumulation container 12, to which the pieces of solid urea A are supplied (see at figure 1).

 The accumulation container 12 has in its upper part a reduction agent loading opening 12a which can be opened and closed via a cover 12b. A connection passage 12e is connected to the discharge opening 12d of the accumulation container 12. A liquid urea accumulation chamber 13 is connected to and arranged below the connection passage 12e. A molten reducing agent transport passage 33 extends from a lower part of the liquid urea accumulation chamber 13. The molten reducing agent transport passage 33 is connected to a liquid control valve. addition 14.

 As shown in FIG. 2, the inclined lower face 12c is provided with the heating device 31 for heating the surface of the internal lower face 12c.

The supply of electricity to the heating device 31 is controlled by an ECU 16.

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The liquid urea accumulation chamber 13 accumulates the liquid urea U delivered from the accumulation container 12 via the connection passage 12e. Liquid urea: U is added to the exhaust pipe 7 while the quantity of its flow is controlled by the addition control valve 14.

 In the add-on control valve 14, a valve body 14c having a shaft-like shape extending in a direction of its axis penetrates through a supply passage 14d. A distal end of the valve body 14c forms a needle valve 14a. The feed passage 14d is connected to the connection passage 12e extending from the liquid urea accumulation chamber 13. The valve body 14c is guided by a support 14e so as to be displaceable alternately. The needle valve 14c is opposite an addition opening 14g which is formed on a distal end to the left of the addition control valve 14 (see Figure 2), and is opened and closed by forward and backward movements of the valve body 14c. At a rear end of the addition control valve 14, a stop 14f fixed to a rear portion of the penetrating valve body 14c is provided.

The stop 14f is engaged with a rear end face of a body of the addition control valve 14a.

An electromagnet 87 is disposed at the rear of the stop 14f. When energized, the electromagnet. 87 pulls the stop 14f backwards, so that the addition opening 14g is open to introduce the liquid urea U into the exhaust pipe 7. An external peripheral surface of the support 14e for the valve body 14c is provided with a seal 14b to prevent the leakage of the liquid urea U delivered.

 In the reducing agent addition device 11 constructed as described above, the pieces of solid urea

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 A are heated until melting by the heating device 31. After the solid urea pieces A are melted, the liquid urea flows down towards the lower face 12c and descends continuously from the opening discharge 12d in the liquid urea accumulation chamber 13. The liquid urea U is added to the exhaust pipe 7 via the addition control valve 14 while the quantity of the flow is controlled by the addition control valve 14. That is to say that the opening / closing times of the addition control valve 14 have their duty cycle controlled by UCE 16 so as to control the quantity of flow of liquid urea U and the duration of addition of liquid urea U.

 More specifically, in accordance with an order from the ECU 16, the electromagnet 87 of the addition control valve 14 is energized and, consequently, magnetized so that the stop 14f is moved backwards (towards the right in Figure 2). As a result, the valve body 14c is moved rearward and the needle valve at the distal end 14a opens, so that a predetermined amount of urea (liquid urea U) is added from the addition opening 14g in the exhaust passage for a predetermined opening time of the addition control valve 14.

 When the excitation of the electromagnet 87 stops, the stop 14f is returned to the original position and the needle valve 14a is closed. Thus, the addition of urea (liquid urea U) ends.

 A pump (not shown) for pressurizing the liquid urea U and supplying it to a pressure regulating part and a pressure regulator 39 for pressurizing the liquid urea U to a certain pressure in the regulating part pressure are arranged between the

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 addition control valve 14 and the liquid urea accumulation chamber 13.

 Regarding the heating source, such as. the electric heating device 31 placed in the accumulation container 12 of the solid urea pieces, its heating state is controlled by the ECU 16 so as to obtain an optimal temperature (for example 133 C at 2300C) to liquefy the pieces of solid urea A and for accumulating liquid urea in the liquid urea accumulation chamber 13. If the pieces of solid urea A are heated to a temperature higher than the optimum temperature mentioned above, the pieces of solid urea A can be transformed into gas instead of being liquefied.

However, the temperature of the heating source can be set at a temperature greater than or equal to 2300C if disadvantages, such as deterioration in the quality of the solid urea parts or transformation into gas thereof does not occur.

 The accumulation container 12 is provided with a remaining quantity sensor 17 (FIG. 1) for detecting the quantity of the pieces of solid urea A remaining in it. The remaining quantity sensor 17 outputs to the ECU 16 an output signal proportional to the residual quantity detected of solid urea pieces A.

 When the ECU 16 enters a signal indicating a predetermined value of the residual quantity (which will be called hereinafter "residual warning value") from the remaining quantity sensor 17, the ECU 16 lights a warning lamp. alarm 23 in a measuring device panel 22 to indicate that the residual quantity of pieces of solid urea A becomes low.

 If the ECU 16 enters a signal indicating a lower limit value which is lower than the residual value

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 warning of the remaining quantity sensor 17, the ECU 16 stops the operation of the reducing agent adding device 11 and completely closes the adding control valve 14 to stop adding liquid urea U .

 The exhaust pipe 7 upstream of the NOx catalytic converter 8 is provided with an inlet gas temperature sensor 19 to detect the temperature of the exhaust gases flowing in the NOx catalytic converter 8. The sensor of incoming gas temperature 19 outputs to the ECU 16 an output signal proportional to the temperature of the detected incoming gas. The exhaust pipe 9 downstream of the NOx catalytic converter 8 is provided with a gas temperature sensor 36 for detecting the temperature of the exhaust gases leaving the NOx catalytic converter 8. The gas temperature sensor 36 exits to the ECU 16 an output signal proportional to the temperature of the gases detected.

 The ECU 16 is formed by a digital computer and includes a read only memory (MEM), a read memory (MEV), a central unit (central processor unit), an entry access point and an access point Release. The ECU 16 performs the basic controls, for example, a fuel injection amount control of the engine 1 and the like and also performs a control of the amount of liquid urea U added and a power supply control of the device heater 31 in this embodiment.

 For these commands, a signal from an air flow meter 20 is entered at the input access point of the ECU 16 via an A / D converter. The air flow meter sends an output signal to the ECU 16 proportional to the quantity of intake air. The ECU 16 calculates a quantity of intake air on the basis of the output signal from the air flow meter 20.

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The exhaust pipe 7 upstream of the NOx catalytic converter 8 is provided with a NOx sensor 21 to detect the amount of NOx flowing in the NOx catalytic converter '8. The NOx sensor 21 exits towards UCE 16 an output signal proportional to the quantity of NOx detected.

 On the basis of the quantity of NOx detected, the ECU 16 calculates a target quantity of liquid urea U to be added which is necessary for substantially removing the NOx. The ECU 16 then calculates a duration of electrical supply to the heating device 31 corresponding to the target quantity of addition and, consequently, electrically supplies the heating device 31. A duty cycle of the addition control valve 14 which obtains a quantity of flow corresponding to the target quantity of addition is calculated.

The addition control valve 14 is controlled accordingly on the basis of duty cycle. Thus, only a necessary quantity of solid urea piece A is liquefied and added. Therefore, it is possible to prevent an undesired case or an excess quantity of liquid urea U remaining in the device for adding reducing agent 11 and, in particular, in the chamber d accumulation of liquid urea 13 and solidifies to block the device.

 The operation of the exhaust control device of the internal combustion engine will now be described. As described above, the ECU 16 controls the power supply to the heater 31 and controls the duty cycle of the addition control valve 14 so as to add the correct amount of liquid urea U in the exhaust pipe 7, in accordance with the functional state of the diesel engine 1, that is to say, in accordance with the quantity of NOx discharged. The liquid urea U supplied in the exhaust pipe 7 is heated by the exhaust gases, so that

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 the liquid urea U immediately transforms into a reducing gas (ammonia gas) and flows into the catalytic converter of NOx 8 associated with the exhaust gases.

 The reducing gas reduces or decomposes the NOx present in the exhaust gases on the NOx catalyst of the selective reduction type 10. The exhaust gases after removal of the NOx are released into the atmosphere via the exhaust pipe 9.

The selective reduction type NOx catalyst 10 has the property that the NOx removal rate is low when the temperature of the exhaust gas is less than or equal to a certain temperature and that the NOx removal rate increases quickly when the exhaust gas temperature exceeds the certain temperature. Consequently, if the reducing gas is added when the exhaust gas temperature is low, the added reducing gas passes through the NOx catalytic converter 8 without being used for NOx reduction reactions, and is released into the atmosphere. Thus, in accordance with this embodiment, if the temperature of the incoming gases detected by the incoming gas temperature sensor 19 is less than or equal to the previously mentioned temperature, the ECU 16 controls the addition control valve 14 in a completely closed state, consequently stopping the addition of liquid urea U and preventing the escape or emission of the reducing gas. '
In the previously stated embodiment, the exhaust pipe 7 upstream of the NOx catalytic converter 8 is provided with a NOx sensor 21 to detect the concentration of NOx in the exhaust gases. The quantity of NOx discharged is calculated from the consumption of NOx detected by the NOx sensor 21 and the quantity of intake air detected by the air flow meter 20. However, this structure can be replaced by a structure in

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 which a relation between the operating state of the diesel engine 1 and the quantity of NOx discharged has been determined in the form of a correspondence table in which the quantity of NOx discharged in operating state of the actual engine is estimated in referring to the correspondence table.

 In addition, the amounts of reducing agent which can be accumulated in the liquid urea accumulation chamber 13, in the addition control valve 14 and in the molten reducing agent transport passage 33 s extending from the liquid urea accumulation chamber 13 to the addition control valve 14 and the reducing agent melting capacity of the heater 31 can be set so that the time required to deliver the entire amount of molten reducing agent accumulated in the liquid urea accumulation chamber 13 and in the addition control valve 14 in the exhaust pipe 7 becomes longer than the time required to heat and melt the solid reduction using the heater 31 and to transport the molten reduction agent to the addition control valve 14.

 Although the reducing agent can be added to the exhaust pipe 7 after the entire amount of solid reducing agent has been melted, it is also possible to always monitor the amount of molten reducing agent and the amount of reducing agent added and start melting and adding simultaneously and perform feedback control so that the amount of added reducing agent ultimately becomes equal to the calculated amount.

 We will now describe the flow diagram of FIG. 4 illustrating the supply of reducing agent accomplished by the device for supplying reducing agent. AT

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une étape 100, l'UCE 16 calcule une quantité cible d'urée liquide U qui doit être ajoutée qui est. nécessaire pour enlever nettement le NOx, sur la base de la quantité de NOx détectée.

a step 100, the ECU 16 calculates a target quantity of liquid urea U which must be added which is. necessary to remove NOx clearly, based on the amount of NOx detected.

 After the calculation, the program progresses to a step 101, in which the ECU 16 electrically supplies the heating device to heat the liquid urea in order to liquefy the solid urea pieces A, thereby producing the target quantity liquid urea U which must be added. Subsequently, in a step 102, the liquid urea produced is accumulated in the liquid urea accumulation chamber 13.

 Thereafter, in a step 103, the ECU 16 determines whether the time for adding the liquid urea U, that is to say the reducing agent, to the exhaust pipe 7 has passed. If the duration of addition has passed, the progress of the program advances to a step 104, in which the liquid urea U is added to the exhaust pipe 7. Conversely, if the duration of addition does not has not passed, the program progress returns to step 102 in which the liquid urea U is accumulated in the liquid urea accumulation chamber 13. Then, in step 103, it is again determined whether the adding time has passed. If the addition time has arrived, the liquid urea U is added.

 In the exhaust gas control apparatus of the invention, the solid urea pieces A are heated and melted into liquid urea U and a predetermined quantity of liquid urea U is added to the exhaust pipe 7 by the addition control valve 14, as described above. Since the amount of heat required to liquefy solid urea is less than the amount of heat required to turn solid urea into gas, a smaller heating source (electric heater) can be adopted in this mode of re

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 only in the case where carbonated urea is used. In addition, since the liquid urea U is. produced by directly heating the solid urea pieces A, the liquid urea 'U has a urea concentration of 100% and, from this, the quantity of liquid urea U which must be added must be ordered with good precision. Since the purpose of the control is not a gas but a liquid in some cases, the flow quantity of the urea can be controlled by the addition control valve 14 with sufficiently high precision.

 A second embodiment of the apparatus for controlling the exhaust gases of an internal combustion engine of the invention will now be described. FIG. 3 shows a device for adding reducing agent 30 in this embodiment. The device for adding reducing agent 30 is substantially identical to the device of the first embodiment in that the pieces of solid urea A, delivered as reducing agent are liquefied and the liquefied urea is delivered into the pipe d exhaust 7.

 An accumulation container 12 has in its upper face a reduction agent loading opening 12a. A discharge opening 12d is formed in a lower part of the accumulation container 12 and is connected to a liquid urea accumulation chamber 13 via a connection passage 12e. The discharge opening 12d is provided with a grid-shaped heating device 31, with the effect that the solid urea pieces A are heated and melted. The liquid urea thus produced U flows downward from the discharge opening 12d into the liquid urea accumulation chamber 13 and accumulated therein.

 A predetermined quantity of liquid urea U is added to the exhaust pipe 7 by a control valve 14. As in the first embodiment, the

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 opening / closing times of the addition control valve 14 have their duty cycle controlled by an ECU 16 so that the amount of liquid urea flow U 'and the duration of addition of liquid urea U are ordered.

 In the second embodiment, a liquid level sensor 32 for detecting the amount of molten reducing agent is connected to the liquid urea accumulation chamber 13 to accumulate the liquid urea U and the ECU 16 controls the heater 31 based on a signal from the liquid level sensor 32.

 The liquid level sensor 32 outputs an output signal to the ECU 16 indicating the amount of detected liquid urea U remaining in the liquid urea accumulation chamber 13. Upon receipt of an input signal from the liquid level sensor 32, that is to say of the molten reducing agent detection means, which indicates a predetermined residual quantity value, the ECU 16 electrically supplies the heating device 31 to melt the pieces of solid urea A into liquid urea U, so that the liquid urea U flows downwards into the liquid urea accumulation chamber 13.

 The value of the residual quantity, a threshold, is fixed at a value such that the liquid urea U will not run out (the liquid urea accumulation chamber 13 will not become empty) during the liquefaction process of the parts solid urea A and addition of liquid urea into the exhaust pipe 7 via the liquid urea accumulation chamber 13 and the addition control valve 14. Consequently, the quantity of liquid urea U corresponding to the aforementioned value is always accumulated in the liquid urea accumulation chamber 13. The accumulation of a large quantity of liquid duration U must be avoided since the liquid urea can solidify again due of the temperature drop occurring

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avec l'écoulement du temps. Toutefois, si la chambre d'accumulation d'urée liquide 13 devait devenir vide, la réduc- 1 1 1 tion du NOx obtenu par le catalyseur du NOx du type à ru- duction sélective 10 s'arrêtera, de sorte que le NOx dans les gaz d'échappement peut être libéré dans l'atmosphère.

with the passage of time. However, if the liquid urea accumulation chamber 13 should become empty, the reduction in NOx obtained by the selective reduction type NOx catalyst 10 will stop, so that the NOx in exhaust gas can be released into the atmosphere.

 Therefore, considering the time necessary to heat and melt the solid urea pieces A and add the liquid urea U into the exhaust pipe 7, the filling is carried out when the level of liquid urea U becomes equal at or below the threshold. As a result of this operation, the liquid urea U, that is to say the reducing agent, is added in the following cycle before the liquid urea U solidifies. Therefore, the liquid urea U will not remain in the liquid urea accumulation chamber 18 for a long time.

 The liquid level sensor 32 provided as a means for detecting the residual amount of liquid urea U can be, for example, a sensor which outputs a signal to the ECU 16 when a predetermined level is reached or a sensor which is capable always measure the quantity of liquid urea U remaining in the liquid urea accumulation chamber 13.

 For the liquefaction of the solid urea parts A, the temperature of the heating device 31 is controlled by the ECU 16 so as to reach an optimal temperature (within a range which avoids the deterioration in quality of the urea or the like, for example from 133 C to 2000C).

 In addition, the liquid urea accumulation chamber 13 is provided with a temperature sensor 18 for detecting the temperature of the liquid urea U. The temperature sensor 18 outputs an output signal proportional to the ECU 16 the detected liquid temperature of the liquid urea U.

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The exhaust pipe 7 upstream of the NOx catalytic converter 8 is provided with an entering gas temperature sensor 19 to detect the temperature of the exhaust gases' flowing in the NOx catalytic converter 8. The incoming gas temperature sensor 19 outputs to the ECU 16 an output signal proportional to the detected incoming gas temperature.

 In this embodiment, the ECU 16 controls the amount of liquid urea U which must be added and the electrical supply to the heating device 31. For these commands, an input signal from an air flow meter 20 is entered on the input access point of the ECU 16 via an A / D converter. Thus, the air flowmeter 20 issues an output signal to the ECU 16 proportional to the quantity of intake air and the ECU 16 calculates the quantity of intake air on the basis of the output signal from the air flow meter 20.

 In addition, the exhaust pipe 7 upstream of the NOx catalytic converter 8 is provided with a NOx sensor 21 to detect the amount of NOx flowing in the NOx catalytic converter 8. The NOx sensor 21 exits towards the ECU 16 an output signal proportional to the quantity of NOx detected.

 On the basis of the quantity of NOx detected, the ECU 16 calculates a target quantity of liquid urea U to be added which is necessary for substantially removing the NOx. The ECU 16 electrically supplies the heating device 31 to melt the solid urea until the quantity of liquid urea U detected by the liquid level sensor 32 becomes equal to the calculated target value. In addition, the ECU 16 calculates a duty cycle of the addition control valve 14 which obtains the flow quantity corresponding to the target quantity to be added and, consequently,

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 performs the duty cycle control of the add-on control valve 14.

 We will now describe the flow diagram of FIG. 5 illustrating the supply of reducing agent accomplished by the device described above. First of all, in a step 200, the ECU 16 calculates a target quantity of liquid urea U to be added which is necessary to remove the NOx clearly, on the basis of the quantity of NOx detected.

 Subsequently, in a step 201, the ECU 16 compares the quantity of liquid urea U accumulated in the liquid urea accumulation chamber 13 with a target quantity of liquid urea U. If the quantity of liquid urea U accumulated is less than the target quantity to be added, the progress of the program advances to a step 202, in which the ECU 16 electrically supplies the heating device 31 to liquefy the pieces of solid urea A and deliver the liquid urea U in the liquid urea accumulation chamber 13 until the quantity of liquid urea U accumulated becomes equal to the target quantity to be added.

 Conversely, if the quantity of liquid urea U accumulated is greater than the target quantity to be added, an additional supply of liquid urea U is not carried out.

 In a step 203, the ECU 16 determines whether the moment to add the liquid urea U, that is to say the reducing agent in the exhaust pipe 7 has arrived. If the time for addition has arrived, the progress of the program advances to a step 204 in which the liquid urea U is added to the exhaust pipe 7. Conversely, if the time for addition does not has not arrived, the program sequence jumps to a step 208 in which the liquid urea U is accumulated in the liquid urea accumulation chamber 13.

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After the liquid urea U is added in a step 204, the ECU 16 detects the residual amount of liquid urea U in the step 205. In a step 206, it is determined whether the residual amount of liquid urea U is less than or equal to a threshold. If the residual quantity is less than or equal to the threshold, the progress of the program advances to a step 207, in which the solid urea is heated and melted so as to increase the quantity of liquid urea U at or above the threshold . Subsequently, in step 208, the quantity of liquid urea U is accumulated.

 Conversely, if the quantity of liquid urea U is greater than the threshold, the progress of the program jumps to step 208, in which the liquid urea U is accumulated.

 In the exhaust gas control apparatus of the invention, a necessary amount of solid urea is melted and added on a base as appropriate in accordance with the state of operation. Consequently, it is possible to prevent the unwanted case where an excess amount of liquid urea U remaining in the device solidifies therein and clogs the device.

 It is also possible to prevent the case where before the solid urea is liquefied so that the urea can be added in the exhaust pipe 7, the liquid urea U in the device runs out, having for result in a lack of liquid reducing agent.

 Thus, in accordance with the exhaust gas control device of the invention, since the solid urea is accumulated and liquefied for use, the device can be of a reduced size and of a simpler structure and the amount of reducing agent delivered can be controlled with high precision.

 In addition, in accordance with the invention, a necessary quantity of molten reducing agent can be delivered in

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 compliance with the operating condition of the internal combustion engine. Therefore, among the solid reducing agent accumulated, an amount necessary for the NOx catalyst of the selective reduction type can be heated and melted on a base as appropriate. Thus, it is possible to prevent an unwanted case where an excess amount of molten reducing agent remaining in the exhaust control apparatus solidifies again and clogs the apparatus, thereby preventing operation. next supply of reducing agent. As a result, a predetermined amount of reducing agent can be reliably delivered to the exhaust gas upstream of the catalyst 10.

Claims (18)

CLAIMS 1. Exhaust gas control apparatus comprising a catalyst (8) disposed in an exhaust gas passage of an internal combustion engine (1) and a device for adding reducing agent ( 11; 30) for adding a reducing agent to the catalyst (8), characterized in that the reducing agent adding device (11 30) includes: a solid reducing agent accumulation means (12) to accumulate a solid reducing agent (A); heating / melting means (31) for heating and melting the solid reducing agent (A); a molten reducing agent accumulating means (13) for accumulating a molten reducing agent (U) introduced from the solid reducing agent accumulating means (12); and a molten reducing agent supply means (14) for delivering the molten reducing agent (U) to said part of the exhaust gas passage upstream of the catalyst (8).  2. An exhaust gas control apparatus according to claim 1, characterized in that it further comprises means for determining the quantity of supply of molten reducing agent (16) for determining a quantity of reducing agent. fade (U) to be delivered <Desc / Clms Page number 28>  by the means for supplying molten reducing agent (14); and reducing agent heating control means (7) for controlling the heating / melting means (31) based on the amount of reducing agent to be added determined by the supply amount determining means as a molten reducing agent (16).  3. Exhaust gas control apparatus according to claim 2, characterized in that the means for determining the quantity of supply of molten reducing agent (16) is a means of calculating the quantity of supply of reducing agent molten (16) to calculate the amount of molten reducing agent (U).  4. Exhaust gas control apparatus according to any one of claims 1 or 3, characterized in that the catalyst (8) is a NOx catalyst of the selective reduction type (10) for reducing or decomposing NOx into the presence of a reducing agent obtained from ammonia.  5. An exhaust gas control apparatus according to any one of claims 1 to 4, characterized in that the reducing agent is urea.  6. Exhaust gas control apparatus according to any one of claims 3 to 5, characterized in that the means for accumulating molten reducing agent (13) is provided with a detection means (32) for detecting a residual amount of molten reducing agent (U), and the means for calculating the supply of molten reducing agent (16) is controlled so that the amount <Desc / Clms Page number 29>  residual molten reducing agent detected by the detecting means (32) and the amount of reducing agent to be delivered calculated by the amount of the molten reducing agent supply calculating means (16) become equal.  7. Exhaust gas control apparatus according to any one of claims 4 to 6, characterized in that a heating temperature of the heating / melting means (31) is fixed at a temperature which does not cause deterioration quality of the solid reducing agent (A).  8. Exhaust gas control apparatus according to claim 7, characterized in that the heating temperature of the heating / melting means (31) is fixed at a temperature which allows the solid reducing agent (A) d be melted so that a reducing agent in the liquid state is retained.  9. An exhaust gas control apparatus according to any one of claims 4 to 8, characterized in that it further comprises a temperature maintaining means (16,31) for maintaining a temperature of the means d accumulation of molten reducing agent (13) at a predetermined temperature.  10. Exhaust gas control apparatus according to any one of claims 2 to 9, characterized in that it further comprises a temperature detection means (19) for detecting a temperature of the exhaust gases , wherein an amount of molten reducing agent (U) supplied from the molten reducing agent supply means (14) is controlled based on <Desc / Clms Page number 30>  the exhaust gas temperature detected by the temperature detection means (19).  11. Exhaust gas control apparatus according to claim 10, characterized in that if the temperature of the exhaust gases is a predetermined value or less, the molten reducing agent (U) is not delivered from the means for supplying molten reducing agent (14).  12. A method of controlling exhaust gases characterized in that it comprises the steps consisting in: accumulating a solid reducing agent (A); determining an amount of molten reducing agent (U) to be supplied to a catalyst (8) arranged in the exhaust gas passage of an internal combustion engine (1) on the basis of the operating state of an internal combustion engine (1); acquiring a molten reducing agent (U) by heating and melting the solid reducing agent (A) so as to obtain the determined quantity of molten reducing agent (U) to be delivered; accumulating the molten reducing agent (U); and delivering the molten reducing agent (U) accumulated in a part of the exhaust gas passage upstream of the catalyst (8).  13. An exhaust gas control method according to claim 12, characterized in that the quantity of molten reducing agent (U) is calculated on the basis of the operating state of the internal combustion engine (1). <Desc / Clms Page number 31> 14. An exhaust gas control method according to claim 12 or 13, characterized in that the catalyst (8) is a NOx catalyst of the selective reduction type (10) for reducing or decomposing NOx in the presence of a reducing agent obtained from ammonia.  15. An exhaust gas control method according to any one of claims 12 to 14, characterized in that the reducing agent is urea.  16. An exhaust gas control method according to any one of claims 12 to 15, characterized in that an order is made so that a remaining amount of molten reducing agent (U) accumulated and an amount of reducing agent issued become equal.  17. Method for controlling the exhaust gases according to any one of claims 12 to 16, characterized in that the quantity of molten reducing agent (U) which must be supplied is controlled on the basis of the temperature of the gases exhaust.  18. A method of controlling the exhaust gases according to claim 17, characterized in that if the temperature of the exhaust gases is a predetermined value or less, the molten reducing agent (U) is not delivered.