JP4238598B2 - NOx purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a NOx purification device that purifies NOx in exhaust gas of an internal twist engine, and in particular, a NOx purification device for an internal combustion engine in which a device for spraying an exhaust gas reducing agent is disposed upstream of a reduction catalyst provided in an exhaust system. About.
[0002]
[Prior art]
The NOx in the exhaust gas discharged from the internal twisting engine is purified by the NOx purification device. In particular, the NOx purification device used in the diesel engine places a urea SCR catalyst (NOx catalyst) in its exhaust system, and its upstream side. There is known a device provided with a reducing agent supply means. This reducing agent supply means supplies urea water (urea water) to the exhaust system, and urea contained therein is hydrolyzed and thermally decomposed as shown in the following formula (1) to release NH3.
[0003]
(NH ₂ ) 2 CO + H ₂ O → 2NH ₃ + CO ₂ ... (1)
Ammonia (NH ₃ ) generated by hydrolysis is supplied as a reducing agent to the SCR catalyst (NOx catalyst). As a result, the SCR catalyst can purify NOx in an oxygen-excess atmosphere.
[0004]
[Problems to be solved by the invention]
By the way, in the above-described urea water addition type NOx purification device, if the exhaust gas temperature is equal to or higher than a certain value, urea water that has become steam is hydrolyzed on the catalyst, and ammonia (which is an effective reducing agent for NOx) NH ₃₎ is generated. However, when the exhaust gas temperature is low, ammonia generation due to the hydrolysis of urea water as in the formula (1) does not proceed sufficiently.
[0005]
In addition, ammonia is deposited on the reaction tube and the nozzle, and cyanuric acid, which is a reaction by-product, is generated, which may make it difficult to spray urea. When the reducing agent is not sufficiently supplied, the NOx purification on the SCR catalyst does not proceed and is completely discharged to the environment.
As described above, in the urea water addition type NOx purification device, a system capable of NOx purification even when the exhaust gas temperature is low has not been established.
[0006]
Based on the above problems, the present invention expands the hydrolysis region of urea water in a NOx purification device that performs urea water addition, and enables NOx purification by the NOx purification device in a wider temperature range. An object is to provide a purification device.
[0007]
[Means for Solving the Problems]
The first aspect of the present invention is a NOx catalyst that is provided in an exhaust system of an internal combustion engine and selectively reduces NOx in exhaust gas, and a first method of supplying urea water to the exhaust system upstream of the NOx catalyst via a urea water supply pipe. A reducing agent supply means; a second reducing agent supply means for supplying a reducing agent gas to the exhaust system upstream of the NOx catalyst via a reducing agent supply pipe; and the reducing agent is mixed in a predetermined ratio to heat urea water. A urea water tank equipped with a heater, a reducing agent gas circulation path for circulating the reducing agent gas in the urea water liquid surface area of the urea water tank with a circulation pump, and a reductant gas circulation path disposed in the middle of the reducing agent gas circulation path, And a reducing agent gas valve that can be switched to supply the reducing agent gas to the reducing agent supply pipe in a timely manner.
[0008]
In this way, urea water can be supplied to the exhaust system upstream of the NOx catalyst by the first reducing agent supply means and reducing agent gas can be supplied by the second reducing agent supply means. Instead, the reducing agent gas is supplied into the exhaust gas, and even when the temperature is low, NOx can be selectively reduced by the NOx catalyst that has received the reducing agent gas, and NOx purification can be performed in a wider operating range.
Preferably, the urea water supply pipe may be provided with opening / closing means that operates to supply the urea water of the urea water tank into the exhaust gas at an appropriate time. In this case, the urea water from the urea water supply pipe can be accurately supplied and discharged.
[0009]
According to a second aspect of the present invention, in the NOx purifying device for an internal combustion engine according to the first aspect, the reducing agent gas circulation path includes a reducing agent storage tank capable of communicating with the urea water surface area of the urea water tank. And
The reducing agent gas generated in the urea water tank can be stored in the reducing agent storage tank in advance, and the reducing agent gas can be supplied into the exhaust gas in a timely manner through the reducing agent supply pipe of the second reducing agent supply means. Even at low temperatures, NOx can be selectively reduced by the NOx catalyst that has received the reducing agent gas, and NOx purification can be performed in a wider operating range.
[0010]
According to a third aspect of the present invention, in the NOx purification device for an internal combustion engine according to the first aspect, the reducing agent gas circulation path has a downstream end opening of a discharge-side circulation of the circulation pump below the urea water level of the urea water tank. It is characterized by being located.
Thus, since the circulating gas from the downstream end opening of the discharge side circulation of the circulation pump blows out below the urea water level of the urea water tank, the function of generating the reducing agent gas from the urea water is improved.
[0011]
According to a fourth aspect of the present invention, in the NOx purification device for an internal combustion engine according to the first aspect, the urea water tank is a concentration sensor that detects the concentration of urea water, a high concentration urea water tank, and a high concentration urea water tank. And a control valve capable of supplying and stopping the high-concentration urea water, and a control means for controlling the control valve so as to maintain the urea water concentration at a predetermined value based on the urea water concentration information.
Thus, the high-concentration urea water in the high-concentration urea water tank can be supplied to the urea water tank in a timely manner, and the urea water concentration in the urea water tank can be kept constant at all times.
[0012]
According to a fifth aspect of the present invention, in the NOx purification device for an internal combustion engine according to the second aspect, the heater for heating the urea water is stopped according to the storage capacity of the reducing agent storage tank.
As described above, since the heater for heating the urea water is stopped in a state exceeding the storage capacity of the reducing agent storage tank, wasteful power consumption can be prevented.
[0013]
A sixth aspect of the present invention is the internal combustion engine NOx purification device according to the first aspect, further comprising an exhaust gas temperature sensor for detecting the exhaust gas temperature, and an open / close control means for the reducing agent gas valve. When the exhaust gas temperature falls below the temperature capable of hydrolyzing urea water, the reducing agent gas valve is switched so that the reducing agent gas flows into the reducing agent supply pipe.
In this way, when the exhaust gas temperature falls below the urea water hydrolyzable temperature, the reducing agent gas is supplied to the exhaust system upstream of the NOx catalyst via the reducing agent supply pipe. Further, NOx can be selectively reduced by the NOx catalyst, and NOx purification can be performed in a wider operating range.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An internal combustion engine NOx purification device as an embodiment of the present invention will be described below with reference to FIG. The internal combustion engine NOx purification device (hereinafter simply referred to as NOx purification device) is mounted on an exhaust system 2 of a multi-cylinder diesel engine (hereinafter simply referred to as engine) 1 mounted on a vehicle (not shown).
The engine 1 includes an engine ECU 3 as engine control means, and a NOx purification device 4 is provided in the exhaust system of the engine 1. The engine ECU 3 and the exhaust system ECU 5 that forms the control unit of the NOx purification device 4 are connected to each other so that they can communicate with each other.
[0015]
Exhaust gas flowing out from the engine 1 to the exhaust passage E passes through an exhaust pipe 7 equipped with a downstream NOx catalytic converter 6 and is released into the atmosphere through a muffler (not shown).
The NOx catalytic converter 6 includes a ceramic catalyst carrier 9 having a honeycomb structure (not shown) in a casing 8, and a catalytic metal (for example, vanadium) for functioning as an SCR catalyst 10 that is a NOx catalyst is supported on the carrier. In place of the ceramic catalyst carrier 9 carrying the SCR catalyst 10, the catalyst may be solidified and formed into a honeycomb shape.
The SCR catalyst 10 can selectively reduce NOx in the exhaust gas by adsorbing ammonia (NH 3), which is a reducing agent, from first and second reducing agent supply means 11 and 12 described later. Here, in the ammonia adsorption state, the SCR catalyst 10 mainly performs the reactions of the following formulas (2) and (3), and can promote the denitration reaction between NH3 and nitrogen oxides.
[0016]
4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O (2)
2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ O (3)
In the exhaust passage E of the exhaust pipe 7, a first reducing agent supply means 11 that supplies urea water to the position a upstream of the SCR catalyst 10 that is a NOx catalyst via the nozzle 131 of the urea water supply pipe 13; Further, a second reducing agent supply means 12 for supplying a reducing agent gas is connected to the upstream position b via a reducing agent supply pipe 14. Note that the second reducing agent supply means 12 may be disposed on the downstream side of the first reducing agent 11.
[0017]
The urea water supply pipe 13 of the first reducing agent supply means 11 is connected to the urea water tank 16 via the urea water pump 15. Here, the urea water pump 15 constitutes an opening / closing means that stops the supply of the urea water when it is not operated and operates to supply the urea water into the exhaust gas when it is operated. The urea water pump 15 has a motor 151 connected to the exhaust system ECU 5.
[0018]
The urea water tank 16 is a sealed container and stores urea water having a predetermined concentration therein. The urea water tank 16 is covered with a heater 17 and a heat insulating sheet (not shown) around the container main body 161 so that the urea water can be heated by the heater 17. The heater 17 is configured to be driven by the exhaust system ECU 5 in a timely manner. The urea water tank 16 has a urea water supply pipe 13 connected to the lower side portion of the container body 161, an end portion of the suction pipe 18 connected to the upper side portion, and a discharge pipe 20 and high-concentration urea on the upper side wall. A supply pipe 21 from the water tank 19 is connected.
[0019]
The high-concentration urea water tank 19 is disposed at a position above the urea water tank 16 and is provided in the supply pipe 21 to supply high-concentration urea water. Water can be supplied to the urea water tank 16 by its own weight. The open / close solenoid valve 22 is driven by an exhaust system ECU 5 as a control means.
The urea water tank 16 sucks the reducing agent gas filling the upper region e of the urea water level f with the circulation pump 23 on the tip side of the suction pipe 18, and reduces the reducing agent gas discharged by the circulation pump 23 on the discharge pipe 20. It is formed so as to be discharged to the lower part of the urea water level f of the urea water tank 16 through the agent storage tank 24 and the reducing agent gas valve 25.
[0020]
That is, since the downstream end opening m of the discharge pipe 20 on the discharge side of the circulation pump 23 is located below the urea water surface f of the urea water tank, the reducing agent gas flowing along the discharge pipe 20 is urea. By blowing out below the water level f, the function of generating the reducing agent gas from the urea water is improved.
[0021]
The reducing agent storage tank 24 is a sealed container having a predetermined capacity, and stores a known adsorbent that adsorbs the reducing agent gas flowing along the discharge pipe 20, and can store ammonia gas as the reducing agent gas by the adsorbent. . The adsorbent has a limit in the amount of ammonia stored, and ammonia exceeding the limit is returned to the urea water tank 16 along the discharge pipe 20. Thus, the continuous drive time of the circulation pump 23 required for the ammonia gas in the reducing agent storage tank 24 to reach the maximum storage amount is preset. The driving of the circulation pump 23 exceeding the continuous drive time is a waste of power consumption, and the drive of the circulation pump 23 is stopped with the continuous drive time as a limit.
The outer peripheral portion of the main body 241 of the reducing agent storage tank 24 is covered with a heater 26 and a heat insulating material (not shown).
[0022]
The reducing agent gas valve 25 is a four-way switching valve as shown in FIG. 2 and is driven to be switched to the exhaust system ECU 5. When this valve is OFF, the inflow and outflow ports g1 and g2 of the discharge pipe 20 are communicated as shown in FIG. 2A, and the addition port g4 on the atmosphere side port g3 and the reducing agent supply pipe 14 side is shut off. . At the time of ON, as shown in FIG. 2B, the atmosphere side port g3 and the outflow port g2 are communicated, and the inflow port g1 and the addition port g4 on the reducing agent supply pipe 14 side are communicated.
The atmosphere side port g3 is released to the atmosphere through a cleaner 27 with a reversely supported valve, thereby preventing negative pressure on the discharge pipe 20 and urea water tank 16 side.
[0023]
Here, the reducing agent gas for circulating the reducing agent gas in the upper region e of the urea water level f of the urea water tank 16 through the urea water tank 16, the suction pipe 18, the circulation pump 23, the reducing agent storage tank 24, and the discharge pipe 20. A circulation path R is formed.
A concentration sensor 28 for detecting the concentration of urea water is attached to the lower portion of the urea water tank 16, thereby outputting urea water concentration information to the exhaust system ECU 5.
[0024]
The exhaust system ECU 5 has a number of ports in its input / output circuit, is mounted on the exhaust pipe 7 and a concentration sensor 28 that detects the concentration D of urea water, and an exhaust gas temperature sensor that outputs the exhaust gas temperature Tg of the exhaust passage E. 29 detection signals can be input, and on that basis, the heaters 17 and 24, the urea water pump 15, the circulation pump 23, the reducing agent gas valve 25, and the open / close solenoid valve 22 function as control means.
Next, the NOx purification control processing of the NOx purification device of FIG. 1 will be described along the NOx purification processing routine of FIG.
[0025]
When the engine 1 of a vehicle (not shown) equipped with the NOx purification device is driven, the exhaust system ECU 5 repeats the NOx purification processing routine of FIG. 3 for each predetermined control cycle simultaneously with the turning on of the engine key. Here, key-on is confirmed in step s1, and when step s2 is reached, the exhaust gas temperature (catalyst temperature) Tg, urea water concentration D, and other data are taken to determine whether or not the values are appropriate. The failure indicator lamp that is not to be driven is driven, and normally proceeds to step s3.
In step s3, it is determined whether or not the exhaust gas temperature Tg is lower than the urea water hydrolyzable temperature Tα. If it is lower, the process proceeds to step s7, and if it is equal to or higher than the urea water hydrolyzable temperature Tα, the process proceeds to step s4.
[0026]
When step s4 is reached at the urea water hydrolyzable temperature Tα or higher, the urea water pump 15 is driven here to supply urea water to the exhaust passage E from the urea water supply pipe 13 of the first reducing agent supply means 11. In this case, the exhaust gas temperature Tg is relatively high, the hydrolysis reaction of the above-described formula (1) is quickly achieved, and ammonia (NH ₃ ) as a reducing agent gas is easily generated, whereby the SCR catalyst 10 Ammonia (NH 3) is adsorbed and the reaction according to the above formula (2) or (3) is performed, and NOx in the exhaust gas can be easily selectively reduced.
[0027]
Thereafter, when step s5 is reached, the reducing agent gas valve 25 is turned off and the circulation pump 23 is turned on here. In this state, the continuous drive time TIMEα is counted by a timer, and then stopped. By this processing, ammonia can be reliably stored in the reducing agent storage tank 24 in the maximum capacity state, and the circulation pump 23 can be prevented from being driven wastefully, and the ammonia gas in the reducing agent storage tank 24 is in a standby state. Retained.
[0028]
Next, when reaching steps s9 and s10, the concentration D of urea water is taken here, and it is determined whether or not this is less than a predetermined concentration value Dβ. The urea water tank 16 is replenished with high-concentration urea water, and the process returns to step s2 as it is.
[0029]
As described above, the NOx purification device of FIG. 1 can supply the high-concentration urea water in the high-concentration urea water tank 19 to the urea water tank in a timely manner by the processing in steps s9 and s10. Can be held constant. Note that the amount of water from a water tank (not shown) may be adjusted and supplied to the urea water tank 16 to keep the urea water concentration constant. Both the water tank (not shown) and the high concentration urea water tank 19 may be used. You may adjust the amount.
[0030]
Step s7 is reached assuming that the exhaust gas temperature Tg is lower than the urea water hydrolyzable temperature Tα in step s3. Here, the heaters 17 and 24 are both driven to promote the heating of the urea water tank 16 and the reducing agent storage tank 24, thereby promoting the generation of ammonia as a reducing agent gas. Next, in step s8, the reducing agent gas valve 25 is turned on to supply the ammonia gas in the reducing agent storage tank 24 to the exhaust path E from the reducing agent supply pipe 14. In this case, although the exhaust gas temperature is low, ammonia (NH ₃ ), which is a reducing agent gas, is directly supplied to the SCR catalyst 10, and the SCR catalyst 10 adsorbs ammonia (NH 3) to react in the above formula (3). Thus, NOx in the exhaust gas can be selectively reduced even at a low temperature.
[0031]
At the same time, the circulation pump 23 is turned on, the reducing agent gas is circulated through the reducing agent gas circulation path R, the reducing agent gas is supplied below the liquid level f of the urea water tank 16, and the reducing agent gas is generated. The ammonia gas can be stably supplied from the reducing agent supply pipe 14 to the exhaust passage E.
Thereafter, step s8 is reached to step s9, and the concentration D of urea water is taken in the same manner as described above. If this is less than the predetermined concentration value Dβ, the open / close solenoid valve 22 is driven for a certain period of time to convert a certain amount of high concentration urea water into urea. The water tank 16 is replenished to suppress the decrease in concentration, and the process returns to step s2.
[0032]
1 can supply urea water to the exhaust system upstream of the SCR catalyst 10 by the first reducing agent supply means 11 and can supply reducing agent gas by the second reducing agent supply means 12. In an operating area where water cannot be hydrolyzed, instead of urea water, a reducing agent gas is supplied into the exhaust gas, and even at low temperatures, NOx can be selectively reduced by the NOx catalyst that receives the reducing agent gas. NOx purification becomes possible in the region.
[0033]
【The invention's effect】
As described above, according to the present invention, urea water cannot be hydrolyzed because urea water can be supplied to the exhaust system upstream of the NOx catalyst by the first reducing agent supply means and reducing agent gas can be supplied by the second reducing agent supply means. In the operating region where the exhaust gas is at a low temperature, instead of urea water, a reducing agent gas is supplied into the exhaust gas, and even at low temperatures, NOx can be selectively reduced by the NOx catalyst that has received the reducing agent gas. NOx purification can be performed in the operating range.
[0034]
In the invention of claim 2, the reducing agent gas generated in the urea water tank can be stored in advance in the reducing agent storage tank, and the reducing agent gas is exhausted in a timely manner through the reducing agent supply pipe of the second reducing agent supply means. Even when the temperature is low, NOx can be selectively reduced by the NOx catalyst that has received the reducing agent gas, and NOx purification can be performed in a wider operating range.
[0035]
In the invention of claim 3, since the circulating gas from the downstream end opening of the discharge-side circulation of the circulation pump blows out below the urea water surface of the urea water tank, the function of generating the reducing agent gas from the urea water is improved.
[0036]
According to the invention of claim 4, the high concentration urea water in the high concentration urea water tank can be supplied to the urea water tank at an appropriate time, and the urea water concentration in the urea water tank can be kept constant at all times.
[0037]
According to the invention of claim 5, since the heater for heating the urea water is stopped in a state exceeding the storage capacity of the reducing agent storage tank, wasteful power consumption can be prevented.
[0038]
In the sixth aspect of the present invention, when the exhaust gas temperature falls below the urea water hydrolyzable temperature, the reducing agent gas is supplied to the exhaust system upstream of the NOx catalyst via the reducing agent supply pipe. NOx can be selectively reduced by the NOx catalyst that has received the gas, and NOx purification can be performed in a wider operating range.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a NOx purification device and an engine equipped with the same as an embodiment of the present invention.
FIGS. 2A and 2B are operation explanatory views of a reducing agent gas valve used in the NOx purification device of FIG. 1, wherein FIG. 2A is an off time and FIG. 2B is an on time;
FIG. 3 is a flowchart of a NOx purification processing routine used by the exhaust system ECU of FIG.
[Explanation of symbols]
1 Engine 2 Exhaust system 5 Exhaust system ECU
10 SCR catalyst (NOx catalyst)
11 First reducing agent supply means 12 Second reducing agent supply means 13 Urea water supply pipe 14 Reducing agent supply pipe 16 Urea water tank 17 Heater 23 Circulation pump 25 Reducing agent gas valve e Upper surface of urea water surface R Reducing agent gas circulation

Claims (6)

A NOx catalyst provided in an exhaust system of the internal combustion engine for selectively reducing NOx in the exhaust gas;
First reducing agent supply means for supplying urea water to the exhaust system upstream of the NOx catalyst via a urea water supply pipe;
A second reducing agent supply means for supplying a reducing agent gas to the exhaust system upstream of the NOx catalyst via a reducing agent supply pipe;
A urea water tank in which the reducing agent is mixed at a predetermined ratio and equipped with a heater for heating urea water;
A reducing agent gas circulation path for circulating the reducing agent gas above the urea water level of the urea water tank with a circulation pump;
A NOx purification device for an internal combustion engine, comprising: a reducing agent gas valve disposed in the middle of the reducing agent gas circulation path and switchable so as to supply the reducing agent gas to the reducing agent supply pipe in a timely manner. In the internal combustion engine NOx purification device according to claim 1,
The NOx purification device for an internal combustion engine, characterized in that the reducing agent gas circulation path includes a reducing agent storage tank capable of communicating with the upper surface of the urea water level of the urea water tank. In the internal combustion engine NOx purification device according to claim 1,
A NOx purification device for an internal combustion engine, wherein the reducing agent gas circulation path has a downstream end opening of a discharge side circulation of the circulation pump located below a urea water level of the urea water tank. In the internal combustion engine NOx purification device according to claim 1,
The urea water tank includes a concentration sensor for detecting the concentration of urea water, a high concentration urea water tank, a control valve capable of supplying and stopping the high concentration urea water of the high concentration urea water tank, and the urea water concentration information. And a control means for controlling the control valve so as to maintain the concentration of the urea water at a predetermined value. The NOx purification device for an internal combustion engine according to claim 2,
The NOx purification device for an internal combustion engine, wherein the heater for heating the urea water is stopped according to the storage capacity of the reducing agent storage tank. In the internal combustion engine NOx purification device according to claim 1,
An exhaust gas temperature sensor for detecting the exhaust gas temperature, and an open / close control means for the reducing agent gas valve,
The open / close control means switches the reducing agent gas valve so that the reducing agent gas flows into the reducing agent supply pipe when the exhaust gas temperature falls below the urea water hydrolyzable temperature.

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