JP2009085172A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of maintaining exhaust emission control efficiency of an ammonia selective reduction type NOx catalyst excellent by starting to feed urea into exhaust gas when exhaust gas temperature is lower than before. <P>SOLUTION: An ECU 46 controls feeding of urea from a urea water injector 40 into an exhaust passage according to the operating condition of an engine 1, thereby ammonia formed from the urea is fed to an ammonia selective reduction type NOx catalyst 36, and NOx in the exhaust gas is selectively reduced with the ammonia as a reducing agent. When accumulated quantity Qi within the exhaust passage of isocyanic acid which is intermediate product formed along with the ammonia when the ammonia is formed from the urea fed into the exhaust gas reaches the upper limit of isocyanic acid accumulated quantity Qimax, the ECU 46 inhibits feeding of the urea water from the urea water injector 40. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device, and more particularly to an exhaust emission control device including an ammonia selective reduction type NOx catalyst that selectively reduces NOx contained in engine exhaust using ammonia generated from urea as a reducing agent.

As an exhaust purification device for purifying exhaust by reducing NOx (nitrogen oxide), which is one of the pollutants contained in engine exhaust, an ammonia selective reduction type NOx catalyst (hereinafter referred to as SCR catalyst) is provided in the exhaust passage of the engine. And an exhaust gas purification device that reduces NOx in the exhaust gas by supplying ammonia to the SCR catalyst as a reducing agent.
In the exhaust aftertreatment device using the SCR catalyst, urea water is supplied to the upstream side of the SCR catalyst, and ammonia generated by the decomposition of the urea water by the heat of the exhaust is supplied to the SCR catalyst. The ammonia supplied to the SCR catalyst is once adsorbed by the SCR catalyst, and NOx reduction is performed by promoting the denitration reaction between this ammonia and NOx in the exhaust gas by the SCR catalyst.

The urea water supplied into the exhaust gas does not decompose well into ammonia when the exhaust temperature is below about 200 ° C., and solids precipitated from the urea water when such a state continues for a long time. As a result, the urea and intermediate products accumulate in the exhaust passage, causing problems in the operation of the nozzle for supplying urea water, and the function of the exhaust gas purification device being deteriorated.
In order to solve such a problem, control is performed such that the supply of urea water is stopped in an operation state in which the exhaust temperature is about 200 ° C. or lower. In this case, the exhaust temperature is about 200 Since ammonia is no longer supplied to the SCR catalyst in an operating state of less than or equal to ° C., NOx in the exhaust gas cannot be properly reduced, resulting in a problem that exhaust purification efficiency is reduced.

Therefore, a heater panel is provided in the exhaust passage where the urea water is supplied, and even in an operation state where the exhaust temperature is low, the urea water is efficiently injected by injecting the urea water toward the heater panel that generates heat by energization. Patent Document 1 proposes an exhaust gas purification device that is well decomposed into ammonia and thereby maintains good purification efficiency of the SCR catalyst.
Further, Patent Document 2 discloses an exhaust emission control device that prevents the occurrence of ammonia slip by prohibiting the supply of urea water into the exhaust when the amount of accumulated ammonia with respect to the SCR catalyst reaches a predetermined amount. Proposed.
JP 20062662 A JP 2005-226504 A

However, in the exhaust purification device of Patent Document 1, since a heater panel must be provided in the exhaust passage, the heater panel and circuit components for energizing the heater panel are additionally required, and the structure of the exhaust passage is complicated. The manufacturing cost also increases. Further, since electric energy is consumed by energizing the heater panel, the fuel consumption of the engine is also deteriorated.
Further, in the exhaust gas purification device disclosed in Patent Document 2, only the amount of ammonia accumulated in the SCR catalyst is monitored, and intermediate products and solid urea produced and accumulated from urea water when the exhaust gas temperature is low. Since it is not what was paid attention, the urea water cannot be properly supplied at a low exhaust temperature.

  The present invention has been made in view of such problems, and the object of the present invention is to supply urea into the exhaust gas from a lower exhaust temperature than before without requiring a complicated additional device. An object of the present invention is to provide an exhaust purification device capable of maintaining good exhaust purification efficiency of an SCR catalyst.

  In order to achieve the above object, an exhaust purification system of the present invention is provided in an exhaust passage of an engine, an ammonia selective reduction type NOx catalyst that selectively reduces NOx in exhaust using ammonia as a reducing agent, and an exhaust passage in the exhaust passage. A urea supply means for supplying ammonia generated from the urea to the ammonia selective reduction type NOx catalyst by supplying urea, and urea from the urea supply means into the exhaust passage according to the operating state of the engine When the amount of accumulation in the exhaust passage of the intermediate product generated when the ammonia is generated from the urea supplied in the exhaust passage reaches a predetermined upper limit intermediate product accumulation amount And a control means for prohibiting the supply of urea from the urea supply means into the exhaust passage (claim 1).

  According to the exhaust emission control device configured as described above, the control means controls the supply of urea from the urea supply means into the exhaust passage according to the operating state of the engine, whereby the ammonia generated from the urea is selected by the ammonia. The reduced NOx catalyst is supplied, and NOx in the exhaust is selectively reduced using ammonia as a reducing agent. When ammonia is generated from urea supplied into the exhaust gas, an intermediate product may be generated together with the ammonia. However, the amount of intermediate product accumulated in the exhaust passage is the upper limit intermediate product accumulation in the control means. When the amount is reached, the urea supply from the urea supply means is prohibited.

More specifically, in the exhaust emission control device, the control means includes a production amount of the intermediate product obtained from a thermal decomposition amount of urea supplied from the urea supply means, and a hydrolysis amount of the intermediate product. Based on the above, the accumulation amount of the intermediate product in the exhaust passage is obtained (claim 2).
More specifically, in the exhaust purification apparatus, the control means repeatedly calculates the accumulation amount of the intermediate product in the exhaust passage, and accumulates the intermediate product in the exhaust passage that has been obtained most recently. The hydrolysis amount of the intermediate product is determined based on the amount and the temperature of the ammonia selective reduction type NOx catalyst (claim 3).

Further, in the exhaust purification apparatus, the control means further includes a case where the amount of accumulated urea in the exhaust passage from the urea supply means reaches a predetermined upper limit urea accumulated amount. You may make it prohibit the supply of the urea from the said urea supply means in the said exhaust passage (Claim 4).
When urea is supplied into the exhaust passage from the urea supply means, a part of the supplied urea may accumulate in the exhaust passage. Therefore, according to the exhaust gas purification apparatus configured as described above, the control unit is configured such that when the accumulation amount of urea supplied in the exhaust passage from the urea supply unit reaches a predetermined upper limit urea accumulation amount. Also, the urea supply from the urea supply means into the exhaust passage is prohibited.

More specifically, in the exhaust purification apparatus, the control means is based on the urea supply amount from the urea supply means into the exhaust passage and the thermal decomposition amount of urea supplied in the exhaust passage. The amount of urea accumulated in the exhaust passage is obtained (claim 5).
More specifically, in the exhaust emission control device, the control means repeatedly calculates the accumulated amount of urea in the exhaust passage, and the accumulated amount of urea in the exhaust passage obtained most recently, Based on the temperature of the ammonia selective reduction type NOx catalyst, the thermal decomposition amount of urea supplied into the exhaust passage is obtained (claim 6).

In the exhaust gas purification apparatus, the control means further enters the exhaust passage from the urea supply means when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst reaches a predetermined upper limit ammonia accumulation amount. The urea supply may be prohibited (claim 7).
Ammonia generated from urea supplied from the urea supply means into the exhaust passage is once adsorbed by the ammonia selective reduction type NOx catalyst, and the denitration reaction between the adsorbed ammonia and NOx in the exhaust is the ammonia selective reduction type. NOx is reduced by being promoted by the catalyst. At this time, if the amount of ammonia supplied to the ammonia selective reduction catalyst is larger than the amount of ammonia consumed for the reduction of NOx, the amount of ammonia adsorbed and accumulated on the ammonia selective reduction catalyst will be There is a possibility that ammonia slip may occur exceeding the allowable accumulation amount of the ammonia selective reduction catalyst. Therefore, according to the exhaust gas purification apparatus configured as described above, the control means is configured so that the exhaust passage from the urea supply means also when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst reaches a predetermined upper limit ammonia accumulation amount. The supply of urea into the inside is prohibited.

  More specifically, in the exhaust gas purification apparatus, the control means includes a first generation of ammonia generated simultaneously with the intermediate product by thermal decomposition from urea supplied from the urea supply means into the exhaust passage. The amount of ammonia accumulated in the ammonia selective reduction type NOx catalyst is calculated based on the total amount of ammonia produced by adding the amount of ammonia and the second amount of ammonia produced by hydrolysis of the intermediate product. It is characterized (claim 8).

More specifically, in the exhaust purification apparatus, the control means is based on the total ammonia production amount and the consumption amount of ammonia in the ammonia selective reduction type NOx catalyst determined according to the operating state of the engine. The ammonia accumulation amount in the ammonia selective reduction type NOx catalyst is obtained (claim 9).
More specifically, in the exhaust gas purification apparatus, the control means obtains the first generation amount from the thermal decomposition amount of urea supplied from the urea supply means (claim 10). .

More specifically, in the exhaust purification apparatus, the control means includes an accumulation amount of urea supplied from the urea supply means into the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Based on the above, the heat decomposition amount of the urea is obtained (claim 11).
More specifically, in the exhaust purification apparatus, the control means obtains the second generation amount from the hydrolysis amount of the intermediate product (claim 12).

More specifically, in the exhaust emission control device, the control means adds the intermediate product based on the accumulated amount of the intermediate product in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. The decomposition amount is obtained (claim 13).
Alternatively, in order to achieve the above object, an exhaust purification apparatus of the present invention is provided in an exhaust passage of an engine, an ammonia selective reduction type NOx catalyst that selectively reduces NOx in exhaust using ammonia as a reducing agent, and an exhaust passage in the exhaust passage. By supplying urea to the urea selective means for supplying the ammonia generated from the urea to the ammonia selective reduction type NOx catalyst, and from the urea supply means to the exhaust passage according to the operating state of the engine. Urea supplied to the exhaust passage by controlling the supply of urea, the amount of urea supplied in the exhaust passage from the urea supply means being less than a predetermined upper limit urea storage amount, and The accumulated amount of the intermediate product produced when the ammonia is produced from the exhaust passage is less than a predetermined upper limit intermediate product accumulated amount, And a control means for allowing the urea supply from the urea supply means into the exhaust passage when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst is less than a predetermined upper limit ammonia accumulation amount. An exhaust emission control device (claim 14).

  According to the exhaust emission control device configured as described above, the control means controls the supply of urea from the urea supply means into the exhaust passage according to the operating state of the engine, whereby the ammonia generated from the urea is selected by the ammonia. The reduced NOx catalyst is supplied, and NOx in the exhaust is selectively reduced using ammonia as a reducing agent. As described above, when ammonia is generated from urea supplied into the exhaust gas, an intermediate product may be generated together with the ammonia, and part of the urea supplied into the exhaust gas is in the exhaust passage. May accumulate. Further, ammonia generated from urea is once adsorbed on the ammonia selective reduction type NOx catalyst, and the NOx reduction between the adsorbed ammonia and NOx in the exhaust is promoted by the ammonia selective reduction type catalyst, thereby reducing NOx. Is done. The control means is configured so that the amount of urea accumulated in the exhaust passage from the urea supply means is less than a predetermined urea accumulation amount and ammonia is generated from the urea supplied in the exhaust passage. When the accumulated amount of the produced intermediate product in the exhaust passage is less than the predetermined upper limit intermediate product accumulated amount, and the accumulated amount of ammonia in the ammonia selective reduction type NOx catalyst is less than the predetermined upper limit ammonia accumulated amount The urea supply from the urea supply means into the exhaust passage is allowed.

  According to the exhaust emission control device of the present invention according to claim 1, the control means prohibits the supply of urea from the urea supply means when the accumulation amount of the intermediate product in the exhaust passage reaches the upper limit intermediate product accumulation amount. Therefore, even in the case of a low exhaust temperature where the supply of urea has been prohibited in the past, the accumulated amount in the exhaust passage of the intermediate product generated from urea supplied in the exhaust is the upper limit intermediate product. Until the accumulation amount is reached, it is possible to supply ammonia generated from urea supplied into the exhaust gas from the urea supply means to the ammonia selective reduction catalyst.

  As a result, even when the exhaust temperature is low, ammonia is adsorbed on the ammonia selective reduction type NOx catalyst to perform selective reduction of NOx, and the ammonia is accumulated in the ammonia selective reduction type NOx catalyst, so that the exhaust temperature subsequently increases. In this case, a sufficient amount of ammonia can be secured, and the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst can be improved.

Further, when the accumulation amount of the intermediate product in the exhaust passage reaches the upper limit intermediate product accumulation amount, the control means prohibits the supply of urea from the urea supply means. Ammonia slip caused by hydrolysis to a large amount of ammonia can also be prevented accurately.
Further, the intermediate product is produced by the thermal decomposition of urea supplied into the exhaust passage with the heat of the exhaust, while the intermediate product thus produced is hydrolyzed by the heat of the exhaust and becomes ammonia and Become. Therefore, according to the exhaust gas purification apparatus of claim 2, the control means is based on the production amount of the intermediate product obtained from the thermal decomposition amount of urea supplied from the urea supply device and the hydrolysis amount of the intermediate product. Since the accumulation amount of the intermediate product in the exhaust passage is obtained, the accumulation amount of the intermediate product in the exhaust passage can be obtained accurately in accordance with the actual production state and decomposition state of the intermediate product. it can.

  Further, the hydrolysis amount of the intermediate product varies depending on the accumulation amount of the intermediate product in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Therefore, according to the exhaust gas purification apparatus of claim 3, the control means repeatedly calculates the accumulated amount of the intermediate product in the exhaust passage, and the intermediate product accumulated amount in the exhaust passage and the ammonia selection determined most recently. Since the hydrolysis amount of the intermediate product is obtained based on the temperature of the reduced NOx catalyst, the hydrolysis amount of the intermediate product can be obtained with high accuracy.

  According to the exhaust purification device of the fourth aspect, the control means can also be used when the accumulation amount of urea supplied in the exhaust passage from the urea supply means reaches a predetermined upper limit urea accumulation amount. Since the supply of urea from the supply means into the exhaust passage is prohibited, the intermediate product in the exhaust passage can be used even when the exhaust temperature is at a low temperature, which was conventionally prohibited from supplying urea. If the accumulated amount of urea does not reach the upper limit intermediate product accumulated amount, ammonia generated from the urea supplied into the exhaust gas from the urea supply means until the accumulated amount of urea in the exhaust passage reaches the upper limit urea accumulated amount is ammonia. It becomes possible to supply the selective catalytic reduction catalyst.

  As a result, even when the exhaust temperature is low, ammonia is adsorbed on the ammonia selective reduction type NOx catalyst to perform selective reduction of NOx, and the ammonia is accumulated in the ammonia selective reduction type NOx catalyst, so that the exhaust temperature subsequently increases. In this case, a sufficient amount of ammonia can be secured, and the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst can be improved.

Further, when the urea accumulation amount in the exhaust passage reaches the upper limit urea accumulation amount, the control means prohibits the urea supply from the urea supply means, so that the malfunction of the urea supply means due to the urea accumulation is accurately detected. It can also be prevented.
Accumulation of urea in the exhaust passage occurs when the amount of urea supplied from the urea supply means exceeds the amount of urea decomposed in the exhaust passage. Therefore, according to the exhaust purification device of claim 5, the control means is configured to control the exhaust passage based on the urea supply amount from the urea supply means into the exhaust passage and the thermal decomposition amount of urea supplied in the exhaust passage. Therefore, the urea accumulation amount in the exhaust passage can be obtained with high accuracy corresponding to the actual supply situation and decomposition state of urea in the exhaust passage.

  Furthermore, the thermal decomposition amount of urea varies depending on the amount of urea accumulated in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Therefore, according to the exhaust emission control device of claim 6, the control means repeatedly calculates the accumulated amount of urea in the exhaust passage, and the urea accumulated amount in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst that are obtained most recently. On the basis of the above, since the amount of thermal decomposition of urea supplied into the exhaust passage is obtained, the amount of thermal decomposition of urea can be obtained with high accuracy.

  According to the exhaust emission control device of the seventh aspect, the control means also enters the exhaust passage from the urea supply means even when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst reaches a predetermined upper limit ammonia accumulation amount. The amount of intermediate product accumulated in the exhaust passage is limited to the upper limit intermediate production even when the exhaust temperature is at a low temperature, which was conventionally prohibited from the urea supply. If the accumulated amount of the urea in the exhaust passage does not reach the upper limit urea accumulated amount, the accumulated amount of ammonia in the ammonia selective reduction type NOx catalyst is the upper limit ammonia accumulated. Until the amount reaches, it is possible to supply ammonia generated from urea supplied into the exhaust gas from the urea supply means to the ammonia selective reduction catalyst.

  As a result, even when the exhaust temperature is low, ammonia is adsorbed on the ammonia selective reduction type NOx catalyst to perform selective reduction of NOx, and the ammonia is accumulated in the ammonia selective reduction type NOx catalyst, so that the exhaust temperature subsequently increases. In this case, a sufficient amount of ammonia can be secured, and the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst can be improved.

Further, when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst reaches the upper limit ammonia accumulation amount, the control unit prohibits the supply of urea from the urea supply unit, so that the ammonia from the ammonia selective reduction type NOx catalyst The occurrence of slip can be accurately prevented.
Ammonia supplied to the ammonia selective reduction type NOx catalyst includes ammonia generated together with an intermediate product when urea supplied into the exhaust is thermally decomposed, and when this intermediate product is hydrolyzed by the heat of the exhaust. It consists of ammonia produced. Therefore, according to the exhaust emission control device of claim 8, the first production amount of ammonia produced simultaneously with the intermediate product from the urea supplied from the urea supply means into the exhaust passage by thermal decomposition, and the intermediate product are Since the control means obtains the accumulated amount of ammonia based on the total amount of ammonia produced by adding the second amount of ammonia produced by hydrolysis, the actual state of ammonia production in the exhaust passage Therefore, the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst can be obtained with high accuracy.

  Further, according to the exhaust emission control device of claim 9, the control means includes the total ammonia production amount obtained as described above, and the ammonia consumption amount in the ammonia selective reduction type NOx catalyst obtained according to the operating state of the engine. Based on the above, the amount of ammonia accumulated in the ammonia selective reduction type NOx catalyst is obtained, so that it corresponds to the actual supply and consumption status of ammonia in the ammonia selective reduction type NOx catalyst, and the ammonia selective reduction type more accurately. The amount of ammonia accumulated in the NOx catalyst can be determined.

  Further, when urea is thermally decomposed, one molecule of ammonia is generated for one molecule of urea. Therefore, according to the exhaust emission control device of the tenth aspect, the control means obtains the first generation amount from the thermal decomposition amount of urea supplied from the urea supply means. Corresponding to the actual production situation, the first production amount can be obtained with high precision, and the accurate ammonia accumulation amount in the ammonia selective reduction type NOx catalyst can be obtained.

  Furthermore, the thermal decomposition amount of urea varies depending on the amount of urea accumulated in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Therefore, according to the exhaust gas purification apparatus of the eleventh aspect, the control means is based on the accumulated amount of urea supplied in the exhaust passage from the urea supply means and the temperature of the ammonia selective reduction type NOx catalyst. Therefore, it is possible to accurately determine the amount of heat decomposition of urea, and as a result, to determine the exact amount of ammonia accumulated in the ammonia selective reduction type NOx catalyst.

According to the exhaust gas purification apparatus of the twelfth aspect, since the control means obtains the second production amount from the hydrolysis amount of the intermediate product, the actual production of ammonia by the hydrolysis of the intermediate product. Corresponding to the situation, the second generation amount can be obtained with high accuracy, and the accurate ammonia accumulation amount in the ammonia selective reduction type NOx catalyst can be obtained.
Further, the hydrolysis amount of the intermediate product varies depending on the accumulation amount of the intermediate product in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Therefore, according to the exhaust purification device of the thirteenth aspect, the control means obtains the hydrolysis amount of the intermediate product based on the accumulation amount of the intermediate product in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Therefore, the hydrolysis amount of the intermediate product can be obtained with high accuracy, and as a result, the accurate ammonia accumulation amount in the ammonia selective reduction type NOx catalyst can be obtained.

  According to the exhaust gas purification apparatus of the present invention according to claim 14, the control means is configured such that the accumulated amount of urea supplied in the exhaust passage from the urea supply means is less than a predetermined urea accumulated amount, and In the ammonia selective reduction-type NOx catalyst, the amount of accumulation in the exhaust passage of intermediate products produced when ammonia is produced from urea supplied into the exhaust passage is less than a predetermined upper limit intermediate product accumulation amount. When the ammonia accumulation amount is less than a predetermined upper limit ammonia accumulation amount, the urea supply from the urea supply means into the exhaust passage is permitted. As a result, even if the exhaust gas temperature is low as conventionally prohibiting the supply of urea, the accumulated amount of urea supplied into the exhaust gas in the exhaust passage has reached the upper limit urea accumulated amount. If the accumulated amount of the intermediate product in the exhaust passage does not reach the upper limit intermediate product accumulated amount and the accumulated amount of ammonia in the ammonia selective reduction type NOx catalyst does not reach the upper limit ammonia accumulated amount, Ammonia produced from urea supplied into the exhaust gas from the supply means can be supplied to the ammonia selective reduction catalyst.

  As a result, even when the exhaust temperature is low, ammonia is adsorbed on the ammonia selective reduction type NOx catalyst to perform selective reduction of NOx, and the ammonia is accumulated in the ammonia selective reduction type NOx catalyst, so that the exhaust temperature subsequently increases. In this case, a sufficient amount of ammonia can be secured, and the exhaust gas purification efficiency of the ammonia selective reduction type NOx catalyst can be improved.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram of an engine to which an exhaust emission control device according to an embodiment of the present invention is applied. The configuration of the exhaust emission control device will be described based on FIG.
A diesel engine (hereinafter referred to as an engine) 1 has a high-pressure accumulator chamber (hereinafter referred to as a common rail) 2 common to each cylinder, and high-pressure fuel supplied from a fuel injection pump (not shown) and stored in the common rail 2 is supplied to each cylinder. The fuel is supplied to the injectors 4 provided, and fuel is injected from the injectors 4 into the respective cylinders.

  The intake passage 6 is equipped with a turbocharger 8. The intake air drawn from an air cleaner (not shown) flows into the compressor 8a of the turbocharger 8 from the intake passage 6, and the intake air supercharged by the compressor 8a is intercooler. 10 to the intake manifold 12. Air introduced into the intake manifold 12 is taken into each cylinder of the engine 1 via an intake port (not shown). An intake air amount sensor 14 for detecting an intake air flow rate to the engine 1 is provided upstream of the compressor 8a in the intake passage 6.

On the other hand, an exhaust port (not shown) through which exhaust is discharged from each cylinder of the engine 1 is connected to an exhaust pipe 18 via an exhaust manifold 16. An EGR passage 22 is provided between the exhaust manifold 16 and the intake manifold 12 to communicate the exhaust manifold 16 and the intake manifold 12 via the EGR valve 20.
The exhaust pipe 18 is connected to the exhaust aftertreatment device 24 after passing through the turbine 8 b of the turbocharger 8. The rotating shaft of the turbine 8b is connected to the rotating shaft of the compressor 8a, and the turbine 8b receives the exhaust flowing in the exhaust pipe 18 to drive the compressor 8a.

  The exhaust aftertreatment device 24 includes an upstream casing 26 and a downstream casing 30 connected to the downstream side of the upstream casing 26 by a communication passage 28. The exhaust aftertreatment device 24 has an exhaust passage together with the exhaust pipe 18 connected to the front and rear thereof. Forming. A upstream oxidation catalyst 32 is accommodated in the upstream casing 26, and a particulate filter (hereinafter referred to as a filter) 34 is accommodated downstream of the upstream oxidation catalyst 32. The filter 34 is provided to purify the exhaust of the engine 1 by collecting particulates in the exhaust.

Since the front-stage oxidation catalyst 32 oxidizes NO in the exhaust gas to generate NO ₂ , by arranging the front-stage oxidation catalyst 32 and the filter 34 in this way, the particulates collected and deposited in the filter 34 are It reacts with NO ₂ supplied from the pre-stage oxidation catalyst 32 to oxidize, and the filter 34 is continuously regenerated.
On the other hand, an ammonia selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) that adsorbs ammonia in the exhaust gas in the downstream casing 30 and selectively reduces NOx in the exhaust gas by using the adsorbed ammonia as a reducing agent to purify the exhaust gas. 36 is accommodated. The SCR catalyst 36 can exhibit the highest purification efficiency when the ratio of NO and NO _{2 in} the exhaust gas is substantially equal to each other. A part of the NO ₂ generated by the pre-stage oxidation catalyst 32 is part of the filter 34. A high purification rate is maintained by flowing into the SCR catalyst 36 together with NO in the exhaust discharged from the engine 1 without contributing to the continuous regeneration.

A downstream oxidation catalyst 38 for oxidizing the ammonia flowing out from the SCR catalyst 36 to N ₂ is accommodated downstream of the SCR catalyst 36. The post-stage oxidation catalyst 38 also has a function of oxidizing CO (carbon monoxide) generated when the particulates are incinerated by forced regeneration of the filter 34, which will be described later, and discharging it to the atmosphere as CO ₂ (carbon dioxide). is doing.
The communication passage 28 is provided with a urea water injector (urea supply means) 40 for injecting and supplying urea water into the exhaust gas in the communication passage 28, and is not shown from the urea water tank 42 in which the urea water is stored. By supplying the urea water by the supply pump, the urea water is injected into the exhaust gas in the communication passage 28 from the urea water injector 40.

Urea contained in the urea water injected from the urea water injector 40 is thermally decomposed and hydrolyzed by the heat of the exhaust to become ammonia, and is supplied to the SCR catalyst 36 together with the exhaust. The SCR catalyst 36 adsorbs the supplied ammonia and promotes a denitration reaction between the adsorbed ammonia and NOx in the exhaust gas, thereby purifying the exhaust gas with NOx being harmless N ₂ .
At this time, when ammonia flows out of the SCR catalyst 36 without reacting with NOx, the ammonia is oxidized by the post-stage oxidation catalyst 38 and is released into the atmosphere as harmless N ₂ . .

An inlet side temperature sensor 44 that detects the exhaust temperature on the inlet side of the SCR catalyst 36 is provided on the upstream side of the SCR catalyst 36 in the downstream casing 30.
In order to perform comprehensive control including operation control of the engine 1 configured as described above, an ECU (control means) 46 is provided. The ECU 46 includes a CPU, a memory, a timer counter, and the like. The ECU 46 calculates various control amounts and controls various devices connected to the ECU 46 based on the control amounts.

  On the input side of the ECU 46, in addition to the intake flow rate sensor 14 and the inlet side temperature sensor 44 described above, in order to collect information necessary for various controls, a rotational speed sensor 48 for detecting the rotational speed of the engine 1 and an accelerator (not shown) Various sensors such as an accelerator opening sensor 50 for detecting the pedal depression amount are connected. Further, various devices such as the injector 4, the EGR valve 20, and the urea water injector 40 of each cylinder that are controlled based on the calculated control amount are connected to the output side of the ECU 46.

  The ECU 46 also performs calculation of the fuel supply amount to each cylinder of the engine 1 and control of fuel supply from the injector 4 based on the calculated fuel supply amount. The fuel supply amount (main injection amount) necessary for the operation of the engine 1 is stored in advance based on the rotational speed of the engine 1 detected by the rotational speed sensor 48 and the accelerator opening detected by the accelerator opening sensor 50. It is determined by reading from the map. The amount of fuel supplied to each cylinder is adjusted by the valve opening time of the injector 4, and each injector 4 is driven to open in a driving time corresponding to the determined fuel amount, and main injection is performed in each cylinder. As a result, an amount of fuel necessary for the operation of the engine 1 is supplied.

In addition to such fuel supply control to each cylinder, the ECU 46 also performs control for forcibly regenerating the filter 34 and urea water supply control by the urea water injector 40.
In the urea water supply control, the ECU 46 obtains a target supply amount of urea water necessary for the selective reduction of NOx in the exhaust gas by the SCR catalyst 36 based on the operating state of the engine 1, and the urea water is based on the target supply amount. By controlling the injector 40, urea water is supplied from the urea water injector 40 into the exhaust gas upstream of the SCR catalyst 36.

As described above, urea contained in the urea water injected from the urea water injector 40 is thermally decomposed and hydrolyzed by the heat of the exhaust gas to become ammonia, and is supplied to the SCR catalyst 36. The SCR catalyst 36 adsorbs the supplied ammonia and promotes a denitration reaction between the adsorbed ammonia and NOx in the exhaust, thereby reducing NOx to harmless N ₂ and purifying the exhaust.

Here, the thermal decomposition of urea is one in which one molecule of urea is decomposed into one molecule of ammonia and one molecule of isocyanic acid as an intermediate product by the heat of exhaust gas, and the exhaust temperature is relatively low (for example, 120 to 160 ° C.), urea supplied into the exhaust gas from the urea water injector 40 is decomposed into ammonia and isocyanic acid mainly by such thermal decomposition.
On the other hand, urea hydrolysis means that after urea is thermally decomposed into ammonia and isocyanic acid by exhaust heat as described above, one molecule of isocyanate is further converted into ammonia by exhaust heat and H ₂ O (water) in the exhaust. One molecule and one carbon dioxide molecule are decomposed. As a result, one molecule of urea is decomposed into two ammonia molecules and one carbon dioxide molecule. When the exhaust temperature is relatively high (for example, 160 ° C. or more), urea supplied from the urea water injector 40 into the exhaust gas is decomposed into ammonia and carbon dioxide by such thermal decomposition and hydrolysis.

  When urea water is supplied from the urea water injector 40 into the exhaust gas and ammonia generated from the urea is supplied to the SCR catalyst 36, a part of urea supplied into the exhaust gas and urea are used as described above. A portion of the isocyanic acid, which is an intermediate product generated in this way, may accumulate in various places in the exhaust passage, that is, in the exhaust aftertreatment device 24 constituting the exhaust passage. As described above, ammonia generated from urea is once adsorbed by the SCR catalyst 36 and then used for selective reduction of NOx.

Therefore, the ECU 46 calculates the accumulated amount of urea and isocyanate in the exhaust passage and the accumulated amount of ammonia adsorbed on the SCR catalyst 36, and controls the urea water injector 40 based on these accumulated amounts. Hereinafter, the control of the urea water injector 46 by the ECU 46 will be described with reference to FIGS.
FIG. 2 is a flowchart of the accumulation amount calculation control executed by the ECU 46 in order to obtain the accumulation amount of urea and isocyanic acid in the exhaust passage and the accumulation amount of ammonia in the SCR catalyst 36. This accumulated amount calculation control is performed at a predetermined time interval tc when the exhaust temperature on the inlet side of the SCR catalyst 36 detected by the inlet side temperature sensor 44 during operation of the engine 1 is a predetermined temperature (for example, 120 ° C. or more). Repeated every cycle. In this embodiment, when the exhaust temperature on the inlet side of the SCR catalyst 36 does not reach a predetermined temperature, it is difficult to decompose the urea supplied into the exhaust by the heat of the exhaust. The urea water from 40 is not supplied.

  In the accumulation amount calculation control, first, in step S1, the ECU 46 calculates the urea accumulation amount Qu (mol), which is the urea accumulation amount in the exhaust passage. In the exhaust passage, a part of the urea contained in the urea water injected from the urea water injector 40 becomes isocyanic acid and ammonia by thermal decomposition as described above, but the remainder is deposited in the exhaust passage as urea. Therefore, the ECU 46 determines the urea addition rate Rua (mol / h), which is the supply amount of urea supplied into the exhaust gas by the urea water injector 40 per unit time, and the thermal decomposition per unit time of the urea supplied into the exhaust gas. By calculating the difference from the urea decomposition rate Rur (mol / h), which is the amount, and multiplying this difference by the time interval tc of each control cycle, the inside of the exhaust passage from the previous control cycle to the current control cycle is obtained. The amount of accumulated urea is determined.

  More specifically, the ECU 46 obtains a target supply amount of urea water necessary for selectively reducing NOx in the exhaust gas by the SCR catalyst 36 based on the operating state of the engine 1. The urea addition rate Rua is obtained from the amount. On the other hand, for the urea decomposition rate Rur, the urea decomposition rate Rur obtained and stored in step S2 described later in the previous control cycle is used. In the case of the first control cycle, the urea decomposition rate Rur is 0 mol / h.

  In step S1, the ECU 46 calculates the urea accumulation amount Qu by integrating the accumulated amount of urea in the exhaust passage between the previous control cycle and the current control cycle thus obtained for each control cycle, and stores this. Then, the process proceeds to the next step S2. By obtaining the urea accumulation amount Qu in this manner, it is possible to accurately obtain the urea accumulation amount in the exhaust passage in accordance with the actual supply state and decomposition state of urea in the exhaust passage.

In the next step S2, the urea decomposition rate Rur corresponding to the urea decomposition rate map stored in advance is calculated based on the urea accumulation amount Qu stored in step S1 by the ECU 46 and the catalyst temperature Tc (° C.) of the SCR catalyst 36. Read and store.
The urea decomposition rate Rur changes according to the urea accumulation amount Qu in the exhaust passage and the catalyst temperature Tc of the SCR catalyst 36, increases as the urea accumulation amount Qu increases, and increases as the catalyst temperature Tc increases. To increase. Therefore, in this embodiment, the relationship between the urea accumulation amount Qu and the catalyst temperature Tc and the urea decomposition rate Rur is obtained in advance by experiments or the like, and this is stored in the ECU 46 as a urea decomposition rate map. FIG. 3 shows an example of such a urea decomposition rate map. In this embodiment, as the catalyst temperature Tc, the exhaust temperature on the inlet side of the SCR catalyst 36 detected by the inlet side temperature sensor 44 is used.

  By obtaining the urea decomposition rate Rur in this way, it is possible to accurately determine the amount of heat decomposition of urea per unit time in the exhaust passage, but the urea decomposition rate Rur obtained and stored in step S2 is the above-described value. As described above, in the next control cycle, the urea accumulation amount Qu used in the calculation of the urea accumulation amount Qu in step S1 is obtained, and as a result, the urea accumulation amount Qu obtained in step S1 is also highly accurate.

  When the process proceeds to the next step S3, the ECU 46 generates the isocyanate generation rate Rig (mol / h), which is the amount of isocyanate generated per unit time of isocyanic acid generated by thermal decomposition from urea supplied in the exhaust, In some cases, an ammonia production rate Rag1 (mol / h), which is a production amount (first production amount) of ammonia produced from urea together with isocyanic acid per unit time, is calculated. As described above, in the thermal decomposition of urea, one molecule of urea is decomposed into one molecule of isocyanate and one molecule of ammonia. Accordingly, in step S3, the ECU 46 stores the urea decomposition rate Rur obtained in step S2 as the isocyanic acid production rate Rig and the ammonia production rate Rag1, and proceeds to the next step S4. By obtaining the ammonia production rate Rag1 in this way, it is possible to accurately obtain the amount of ammonia produced per unit time generated by the thermal decomposition of urea corresponding to the actual production state of ammonia by the thermal decomposition of urea. .

  In step S4, the ECU 46 calculates an isocyanic acid accumulation amount Qi (mol) in the exhaust passage. A part of the isocyanic acid generated by the thermal decomposition of urea as described above is further decomposed into ammonia and carbon dioxide by hydrolysis, but the remainder is accumulated in the exhaust passage. Therefore, the ECU 46 generates the isocyanate generation rate Rig stored in step S3 and the isocyanate decomposition rate Rir (mol / h), which is the hydrolysis amount per unit time of isocyanic acid obtained in step S5 described later in the previous control cycle. ) And multiplying this difference by the time interval tc of each control cycle, the amount of isocyanate accumulated in the exhaust passage from the previous control cycle to the current control cycle is obtained. In the first control cycle, the isocyanic acid decomposition rate Rir is 0 mol / h.

  In step S4, the ECU 46 calculates the isocyanate accumulation amount Qi by accumulating the accumulated amount of isocyanate in the exhaust passage from the previous control cycle to the current control cycle thus obtained for each control cycle. Store it and proceed to the next step S5. By obtaining the isocyanic acid accumulation amount Qi in this way, it is possible to accurately obtain the isocyanic acid accumulation amount in the exhaust passage, corresponding to the actual production state and decomposition state of isocyanic acid in the exhaust passage.

When the processing proceeds to the next step S5, the ECU 46 corresponds to the corresponding isocyanic acid from the isocyanic acid decomposition rate map stored in advance based on the isocyanic acid accumulation amount Qi stored in step S4 and the catalyst temperature Tc of the SCR catalyst 36. The decomposition rate Rir is read and stored.
The isocyanic acid decomposition rate Rir changes according to the isocyanate accumulation amount Qi in the exhaust passage and the catalyst temperature Tc of the SCR catalyst 36, and increases as the isocyanate accumulation amount Qi increases, and increases the catalyst temperature Tc. It increases according to. Therefore, in this embodiment, the relationship between the isocyanate accumulation amount Qi and the catalyst temperature Tc and the isocyanate decomposition rate Rir is obtained in advance through experiments or the like, and this is stored in the ECU 46 as an isocyanate decomposition map. FIG. 4 shows an example of such an isocyanic acid decomposition rate map. In this embodiment, the catalyst temperature Tc is the exhaust temperature on the inlet side of the SCR catalyst 36 detected by the inlet side temperature sensor 44 as described above.

  By obtaining the isocyanate decomposition rate Rir in this way, the amount of isocyanic acid hydrolyzed per unit time in the exhaust passage can be obtained with high precision, but the isocyanate decomposition rate Rir obtained and stored in step S5. As described above, is used for calculating the isocyanate accumulation amount Qi in step S4 in the next control cycle, and as a result, the isocyanate accumulation amount Qi obtained in step S4 is also highly accurate.

  In the next step S6, the ECU 46 calculates an ammonia production rate Rag2 (mol / h) which is a production amount (second production amount) of ammonia produced by hydrolysis from isocyanic acid per unit time. As described above, in the hydrolysis of isocyanic acid, one molecule of isocyanic acid is decomposed into one molecule of ammonia and one molecule of carbon dioxide. Therefore, in step S6, the ECU 46 stores the isocyanic acid decomposition rate Rir obtained in step S5 as the ammonia production rate Rag2, and proceeds to the next step S7. By obtaining the ammonia production rate Rag2 in this way, the production amount per unit time of ammonia generated by hydrolysis of isocyanic acid can be accurately obtained corresponding to the actual production situation of ammonia by hydrolysis of isocyanic acid. Can do.

In step S7, the ECU 46 adds up the ammonia generation rate Rag1 stored in step S3 and the ammonia generation rate Rag2 stored in step S6, so that the total ammonia production amount per unit time in the exhaust passage is obtained. The generation rate Rat (mol / h) is obtained, stored, and the process proceeds to the next step S8.
Ammonia generated from urea supplied into the exhaust gas from the urea injector 40 includes ammonia generated together with isocyanic acid as an intermediate product when urea is thermally decomposed, and ammonia generated by hydrolysis of isocyanic acid. Therefore, by determining the total ammonia production rate Rat in this way, the production amount per unit time of ammonia produced from the urea supplied into the exhaust gas can be determined according to the actual production situation of ammonia. It can be obtained with high accuracy.

  In the next step S8, the ECU 46 calculates the ammonia accumulation amount Qa (mol) in the SCR catalyst 36. As described above, ammonia generated from urea in the exhaust is once adsorbed by the SCR catalyst 36 and then consumed in the selective reduction of NOx in the exhaust by the SCR catalyst 36. Therefore, the ECU 46 obtains the difference between the total ammonia production rate Rat stored in step S7 and the ammonia consumption rate Rac (mol / h), which is the consumption amount of ammonia per unit time in the SCR catalyst 36, and controls each difference to this difference. By multiplying the cycle time interval tc, the amount of ammonia accumulated in the SCR catalyst 36 from the previous control cycle to the current control cycle is obtained.

  Here, the ammonia consumption rate Rac indicates the amount of ammonia consumed for the selective reduction of NOx in the exhaust per unit time. Therefore, the ECU 46 detects the amount of fuel supplied to each cylinder or the intake air amount sensor 14. The NOx emission amount per unit time from the engine 1 is obtained from the operating state of the engine 1 such as the intake air amount and the rotation speed of the engine 1 detected by the rotation speed sensor 48, and the ammonia consumption rate Rac is obtained from this NOx emission amount. Seeking.

  In step S8, the ECU 46 obtains an ammonia accumulation amount Qa by accumulating the ammonia accumulation amount in the SCR catalyst 36 from the previous control cycle to the current control cycle thus obtained for each control cycle, and stores this. Then, the control cycle is ended, and the process is started again from step S1 as described above in the next control cycle. In this way, by obtaining the ammonia accumulation amount Qa corresponding to the actual ammonia production state and consumption state, it corresponds to the actual supply state and consumption state of ammonia in the SCR catalyst 36, and the ammonia amount in the SCR catalyst 36. Accumulated amount can be obtained accurately.

  By repeatedly executing the accumulated amount calculation control as described above, the accumulated amount Qu of urea in the exhaust passage, the accumulated amount Qi of isocyanate as an intermediate product, and the accumulated amount Qa of ammonia in the SCR catalyst 36 are obtained. It can be obtained with high accuracy corresponding to the actual production status of isocyanic acid and ammonia in the exhaust passage and the actual consumption status of ammonia in the SCR catalyst 36.

  Based on the accumulated amount Qu of urea and the accumulated amount Qi of isocyanic acid in the exhaust passage and the accumulated amount Qa of ammonia in the SCR catalyst 36, the ECU 46 performs a predetermined control cycle according to the flowchart of FIG. Then, the urea water supply control to the urea water injector 40 is performed. In the urea water supply control, the exhaust gas temperature on the inlet side of the SCR catalyst 36 detected by the inlet side temperature sensor 44 during the operation of the engine 1 is a predetermined temperature (for example, 120 ° C. or more), as in the above-described accumulation amount calculation control. ), And if the exhaust gas temperature on the inlet side of the SCR catalyst 36 does not reach a predetermined temperature, it is difficult for the ECU 46 to decompose the urea supplied into the exhaust gas by the heat of the exhaust gas. The urea water is not supplied from the water injector 40.

  In the urea water supply control, the ECU 46 first determines in step S11 whether the urea accumulation amount Qu is less than a predetermined upper limit urea accumulation amount Qumax. This upper limit urea accumulation amount Qumax is determined when the urea contained in the urea water supplied from the urea injector 40 into the exhaust passage accumulates in the exhaust passage, and the urea water injector 40 malfunctions or the exhaust aftertreatment device 24. Is set based on an upper limit value of an accumulation amount obtained in advance through experiments or the like as an accumulation amount of urea that does not cause problems such as functional degradation.

If it is determined in step S11 that the urea accumulation amount Qu has not reached the upper limit urea accumulation amount Qumax, the ECU 46 advances the process to step S12, where the isocyanate accumulation amount Qi is equal to the predetermined upper limit isocyanate accumulation amount (upper limit intermediate product accumulation). It is determined whether the amount is less than Qimax.
This upper limit isocyanic acid accumulation amount Qimax is determined when the isocyanic acid generated by the thermal decomposition of urea water supplied from the urea injector 40 into the exhaust passage is accumulated in the exhaust passage. It is set based on the upper limit value of the accumulated amount obtained in advance by experiments or the like as the accumulated amount of isocyanate that does not cause ammonia slip due to ammonia that is sometimes hydrolyzed.

If it is determined in step S12 that the isocyanic acid accumulation amount Qi has not reached the upper limit isocyanic acid accumulation amount Qimax, the ECU 46 proceeds to step S13, and determines whether or not the ammonia accumulation amount Qa is less than the predetermined upper limit ammonia accumulation amount Qamax. Determine whether.
This upper limit ammonia accumulation amount Qamax is obtained when ammonia generated from urea water supplied from the urea injector 40 into the exhaust passage is supplied to and adsorbed to the SCR catalyst 36, and the amount of ammonia accumulated in the SCR catalyst 36 increases. The ammonia accumulation amount at which ammonia slip from the SCR catalyst 36 does not occur is set based on the upper limit value of the accumulation amount obtained in advance through experiments or the like.

If it is determined in step S13 that the ammonia accumulation amount Qa has not reached the upper limit ammonia accumulation amount Qamax, the ECU 46 advances the process to step S14.
In step S14, the ECU 46 allows the urea water supply from the urea water injector 40, and corresponds to the NOx emission amount from the engine 1 determined based on the operating state of the engine 1 as described above. The urea water injector 40 is controlled so that an amount of urea water necessary for selective reduction of NOx is supplied into the exhaust gas from the urea water injector 40, and the control cycle ends. In the next control cycle, the ECU 46 performs the same process again from step S11.

  As described above, when the urea accumulation amount Qu, the isocyanate accumulation amount Qi, and the ammonia accumulation amount Qa have not reached the corresponding upper limit accumulation amounts Qumax, Qimax, and Qamax, the urea from the urea water injector 40 is used. Water supply is acceptable. Therefore, all of the urea accumulation amount Qu, the isocyanate accumulation amount Qi and the ammonia accumulation amount Qa are present even at a low exhaust temperature at which urea water supply from the urea water injector 40 is prohibited in the conventional exhaust purification device. As long as the upper limit accumulation amounts Qumax, Qimax and Qamax corresponding to each of them are not reached, it is possible to supply urea water from the urea water injector 40 without any trouble.

  Therefore, even when the exhaust gas temperature is low, ammonia generated from urea water is adsorbed on the SCR catalyst 36, and NOx is selectively reduced, and ammonia is accumulated in the SCR catalyst 36, so that the exhaust gas temperature subsequently increases. In some cases, a sufficient amount of ammonia can be secured to improve the exhaust purification efficiency of the SCR catalyst 36. Furthermore, in order to obtain such an effect, an additional mechanism such as a heating device is not required, so that the structure of the exhaust passage becomes complicated, the manufacturing cost increases, and extra power is required. There is no deterioration in fuel consumption.

On the other hand, if it is determined in step S11 that the urea accumulation amount Qu has reached the upper limit urea accumulation amount Qumax, the ECU 46 advances the process to step S15, prohibiting the supply of urea water from the urea water injector 40, and its control cycle. Exit. In the next control cycle, the ECU 46 performs the process again from step S11 as described above.
Therefore, when the urea accumulation amount Qu reaches the upper limit urea accumulation amount Qumax, the supply of urea water from the urea water injector 40 is prohibited, and further urea accumulation in the exhaust passage is prevented. As a result, it is possible to accurately prevent the malfunction of the urea water injector 40 and the deterioration of the function of the exhaust aftertreatment device 24 due to the accumulation of urea.

  Further, when the process proceeds to step S12 and it is determined that the isocyanic acid accumulation amount Qi has reached the upper limit isocyanic acid accumulation amount Qimax, the ECU 46 proceeds to the process to step S15, and urea water from the urea water injector 40 is processed. The supply is prohibited and the control cycle ends. In the next control cycle, the ECU 46 performs the process again from step S11 as described above.

  Therefore, even when the isocyanate accumulation amount Qi reaches the upper limit isocyanate accumulation amount Qimax, the supply of urea water from the urea water injector 40 is prohibited and further accumulation of isocyanate in the exhaust passage is prevented. As a result, it is possible to accurately prevent ammonia slip caused by excessively accumulated isocyanic acid being hydrolyzed at a high temperature to produce a large amount of ammonia.

  Further, the process proceeds to step S13, and even when it is determined that the ammonia accumulation amount Qa has reached the upper limit ammonia accumulation amount Qamax, the ECU 46 proceeds to the process to step S15 and supplies urea water from the urea water injector 40. Prohibit and end the control cycle. In the next control cycle, the ECU 46 performs the process again from step S11 as described above.

Therefore, even when the ammonia accumulation amount Qa reaches the upper limit ammonia accumulation amount Qamax, the supply of urea water from the urea water injector 40 is prohibited, the supply of further ammonia to the SCR catalyst 36 is stopped, and the SCR catalyst 36 Generation of ammonia slip can be accurately prevented.
Although the description of the exhaust emission control device according to one embodiment of the present invention is finished above, the present invention is not limited to the above embodiment.

  For example, in the urea water supply control of the above embodiment, the urea accumulation amount Qu, the isocyanic acid accumulation amount Qi, and the ammonia accumulation amount Qa do not reach the corresponding upper limit accumulation amounts Qumax, Qimax, and Qamax, respectively. However, the urea accumulation amount Qu and the ammonia accumulation amount Qa are calculated and their magnitude relations with the upper limit accumulation amounts Qumax and Qamax respectively corresponding to the urea accumulation amount Qu and the ammonia accumulation amount Qa. It is also possible to control the supply of urea water based only on the determination whether or not the isocyanic acid accumulation amount Qi has reached the upper limit accumulation amount Qimax.

  That is, in the flowchart of FIG. 5, the processing of steps S11 and S13 is omitted, and if it is determined in step S12 that the isocyanic acid accumulation amount Qi is less than the upper limit accumulation amount Qimax, the process proceeds to step S14 and the urea water injector 40 When it is determined in step S12 that the isocyanate accumulation amount Qi has reached the upper limit accumulation amount Qimax, the process proceeds to step S15 and the urea water supply from the urea water injector 40 is prohibited. You may do it.

  In this case, in such urea water supply control, malfunction of the urea water injector 40 due to urea accumulation in the exhaust passage, deterioration of the function of the exhaust aftertreatment device 24, and generation of ammonia slip from the SCR catalyst 36 are caused. Although it cannot be prevented, at least it is caused by excessive accumulation of isocyanic acid, which is an intermediate product when ammonia is produced from urea, and this isocyanic acid is hydrolyzed to a large amount of ammonia at high temperatures. Thus, ammonia slip caused by the above can be prevented accurately. Further, even in a low exhaust temperature at which urea water supply from the urea water injector 40 is prohibited in the conventional exhaust purification device, there is no problem as long as the isocyanate accumulation amount Qi does not reach the upper limit isocyanate accumulation amount Qimax. The urea water can be supplied from the urea water injector 40. Therefore, even when the exhaust gas temperature is low, ammonia generated from urea water is adsorbed on the SCR catalyst 36, and NOx is selectively reduced, and ammonia is accumulated in the SCR catalyst 36, so that the exhaust gas temperature subsequently increases. In some cases, a sufficient amount of ammonia can be secured to improve the exhaust purification efficiency of the SCR catalyst 36.

  Furthermore, only one of the determination of the magnitude relationship between the urea accumulation amount Qu and the upper limit urea accumulation amount Qumax and the determination of the magnitude relationship between the ammonia accumulation amount Qa and the upper limit ammonia accumulation amount Qamax, that is, step S11 and step S11 in the flowchart of FIG. Only one of the processes in S13 may be omitted. In this case, although the effect obtained by the process of the omitted step is lost, the effect of the process of the step S11 and S13 that is not omitted is obtained in addition to the effect of the process of step S12 as described above. It will be.

  In the above embodiment, the upstream oxidation catalyst 32 is disposed upstream of the upstream casing 26, the filter 34 is disposed downstream thereof, and the SCR catalyst 36 is disposed upstream of the downstream casing 30. In addition, the exhaust post-treatment device 24 is configured by installing the post-stage oxidation catalyst 38 on the downstream side thereof, but the configuration of the exhaust post-treatment device 24 is not limited thereto.

  That is, the present invention can be applied to any apparatus including the SCR catalyst 36 that selectively reduces NOx in the exhaust gas using ammonia generated from urea supplied in the exhaust gas as a reducing agent. The filter 34 and the post-stage oxidation catalyst 38 may be disposed as necessary, and an exhaust purification device other than the pre-stage oxidation catalyst 32, the filter 34, and the post-stage oxidation catalyst 38 may be provided. Furthermore, the exhaust aftertreatment device 24 may be configured by one casing without being divided into the upstream casing 26 and the downstream casing 30.

  In the above embodiment, when the exhaust temperature on the inlet side of the SCR catalyst 36 detected by the inlet side temperature sensor 44 is equal to or higher than a predetermined temperature (for example, 120 ° C.), the accumulation amount calculation control according to the flowcharts shown in FIGS. And the urea water supply control is performed, but isocyanic acid, which is an intermediate product when ammonia is produced from urea, is ammonia in the case where the exhaust temperature is in a relatively low temperature range (for example, 120 to 160 ° C.). Therefore, the accumulated amount calculation control and urea water supply control may be executed only when the exhaust temperature is within a predetermined temperature range (for example, 120 to 160 ° C.).

In the above embodiment, the engine 1 is a diesel engine. However, the type of the engine is not limited to this, and NOx in the exhaust gas is reduced using ammonia generated from urea supplied in the exhaust gas as a reducing agent. The present invention can be applied to any engine provided with the SCR catalyst 36 for selective reduction.
Further, in the above embodiment, the exhaust temperature at the inlet side of the SCR catalyst 36 detected by the inlet side temperature sensor 44 is used as it is as the temperature Tc of the SCR catalyst 36. However, the method for detecting the temperature Tc of the SCR catalyst 36 is this. It is not limited to. For example, correction based on the structure of the SCR catalyst 36 may be added to the detection value of the inlet side temperature sensor 44, detection by a temperature sensor in which the inlet side temperature sensor 44 is disposed inside the SCR catalyst 36, or the SCR catalyst 36. Detection of the exhaust gas temperature at the outlet side of the engine may be employed.

1 is an overall configuration diagram of an engine to which an exhaust emission control device according to an embodiment of the present invention is applied. It is a flowchart of the accumulation | storage amount calculation control performed with the exhaust gas purification apparatus of FIG. It is a urea decomposition rate map used with the flowchart of FIG. 3 is an isocyanic acid decomposition rate map used in the flowchart of FIG. 2. It is a flowchart of the urea water supply control performed with the exhaust gas purification apparatus of FIG.

Explanation of symbols

1 Engine 18 Exhaust pipe (exhaust passage)
24 Exhaust aftertreatment device (exhaust passage)
36 Ammonia selective reduction type NOx catalyst 40 Urea water injector (urea supply means)
46 ECU (control means)

Claims (14)

An ammonia selective reduction type NOx catalyst that is disposed in the exhaust passage of the engine and selectively reduces NOx in the exhaust gas using ammonia as a reducing agent;
Urea supply means for supplying ammonia to the ammonia selective reduction NOx catalyst by supplying urea into the exhaust passage;
An intermediate product that is generated when the ammonia is generated from the urea supplied into the exhaust passage by controlling the supply of urea from the urea supply means into the exhaust passage according to the operating state of the engine Control means for prohibiting the supply of urea from the urea supply means into the exhaust passage when the accumulated amount in the exhaust passage reaches a predetermined upper limit intermediate product accumulation amount. Purification equipment.   The control means is configured to generate the intermediate product in the exhaust passage based on the production amount of the intermediate product obtained from the thermal decomposition amount of urea supplied from the urea supply device and the hydrolysis amount of the intermediate product. The exhaust emission control device according to claim 1, wherein an accumulated amount of the matter is obtained.   The control means repeatedly calculates the amount of accumulation of the intermediate product in the exhaust passage, and the amount of accumulation of the intermediate product in the exhaust passage determined most recently and the temperature of the ammonia selective reduction type NOx catalyst are calculated. The exhaust purification device according to claim 2, wherein the hydrolysis amount of the intermediate product is obtained based on   The control means further includes the urea supply means to the exhaust passage even when the accumulation amount of urea supplied into the exhaust passage from the urea supply means reaches a predetermined upper limit urea accumulation amount. 2. The exhaust emission control device according to claim 1, wherein supply of urea into the inside is prohibited.   The control means determines the amount of urea accumulated in the exhaust passage based on the urea supply amount from the urea supply means into the exhaust passage and the thermal decomposition amount of urea supplied in the exhaust passage. The exhaust emission control device according to claim 4, wherein the exhaust purification device is obtained.   The control means repeatedly calculates the accumulated amount of urea in the exhaust passage, and based on the recently accumulated amount of urea in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst, 6. The exhaust emission control device according to claim 5, wherein a thermal decomposition amount of urea supplied into the exhaust passage is obtained.   The control means further inhibits the supply of urea from the urea supply means into the exhaust passage even when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst reaches a predetermined upper limit ammonia accumulation amount. The exhaust emission control device according to claim 1.   The control means includes a first generation amount of ammonia that is simultaneously generated by the thermal decomposition from urea supplied from the urea supply means into the exhaust passage, and the intermediate product is hydrolyzed. The exhaust gas purification apparatus according to claim 7, wherein an accumulated amount of ammonia in the ammonia selective reduction type NOx catalyst is obtained based on a total ammonia production amount obtained by adding together a second production amount of ammonia produced by the catalyst. .   The control means determines the amount of ammonia accumulated in the ammonia selective reduction type NOx catalyst based on the total ammonia production amount and the consumption amount of ammonia in the ammonia selective reduction type NOx catalyst determined according to the operating state of the engine. The exhaust emission control device according to claim 8, wherein the exhaust purification device is obtained.   9. The exhaust emission control device according to claim 8, wherein the control unit obtains the first generation amount from a thermal decomposition amount of urea supplied from the urea supply unit.   The control means obtains the thermal decomposition amount of the urea based on the accumulation amount of urea supplied from the urea supply means into the exhaust passage in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. The exhaust emission control device according to claim 10.   The exhaust emission control device according to claim 8, wherein the control means obtains the second production amount from the hydrolysis amount of the intermediate product.   The control means obtains the hydrolysis amount of the intermediate product based on the accumulated amount of the intermediate product in the exhaust passage and the temperature of the ammonia selective reduction type NOx catalyst. Exhaust gas purification device described in 1. An ammonia selective reduction type NOx catalyst that is disposed in the exhaust passage of the engine and selectively reduces NOx in the exhaust gas using ammonia as a reducing agent;
Urea supply means for supplying ammonia to the ammonia selective reduction NOx catalyst by supplying urea into the exhaust passage;
The urea supply from the urea supply means to the exhaust passage is controlled in accordance with the operating state of the engine, and the amount of accumulated urea in the exhaust passage from the urea supply means is predetermined. Less than the upper limit urea accumulation amount, and the accumulation amount in the exhaust passage of the intermediate product generated when the ammonia is generated from the urea supplied into the exhaust passage is predetermined upper limit intermediate product accumulation Control means for permitting urea supply from the urea supply means into the exhaust passage when the ammonia accumulation amount in the ammonia selective reduction type NOx catalyst is less than a predetermined upper limit ammonia accumulation amount. And an exhaust emission control device.

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