JP2019073984A - Exhaust emission control device - Google Patents

Abstract

To provide an exhaust emission control device which can effectively suppress the urea corrosion of a urea water addition valve.SOLUTION: An exhaust emission control device comprises: a urea water addition valve 15 for adding urea water into exhaust emission; a catalyst device 20 for purifying NOx in the exhaust emission by selective catalyst reduction with ammonia produced by the thermal decomposition of the urea water as a reductant; and an addition control part 23 for calculating a cooling addition amount as an addition amount of the urea water necessary for cooling the urea water addition valve 15, and controlling the addition amount of the urea water of the urea water addition valve 15 so as to reach an amount not smaller than the cooling addition amount. The exhaust emission control device also comprises a urea concentration sensor 19 for detecting a urea concentration of the urea water, and the addition control part 23 is constituted so that an amount which is larger than that when the urea concentration is low is calculated as the cooling addition amount when the urea concentration is high.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a urea water addition valve for adding urea water, and a catalyst device for purifying nitrogen oxides (NOx) in exhaust gas by selective catalytic reduction using ammonia generated by thermal decomposition of urea water as a reducing agent. And an exhaust gas purification apparatus.

  An apparatus described in Patent Document 1 is known as an exhaust gas purification apparatus adopting the so-called urea SCR (Selective Catalytic Reduction) system as described above. In the exhaust gas purification device described in the same document, the NOx emission amount of the engine is determined from the operating state of the engine, and the NOx purification rate is set to a target value (target purification rate) with respect to the emission amount. The amount of urea water to be added to the urea water addition valve is determined so that the necessary amount of ammonia is supplied to the catalyst device.

  In the exhaust gas purification apparatus described in the same document, the determination of the addition amount of urea water is performed in consideration of the urea concentration of urea water to be added to the exhaust gas by the urea water addition valve. Even if the amount of urea water added is the same, the higher the concentration of urea, the greater the amount of ammonia supplied to the catalytic converter. Therefore, when the urea concentration is high, the amount of urea water addition necessary for achieving the target purification rate is smaller than when the urea concentration is low.

JP, 2014-005745, A

  By the way, when the temperature of the urea water addition valve is increased by being exposed to high temperature exhaust during operation of the engine, the corrosion of the urea water addition valve by the urea in the urea water tends to progress. The corrosion rate of the urea water addition valve by urea under such a high temperature environment is particularly high when the urea concentration of the urea water is high.

  If the amount of urea added is sufficiently large, replacement of urea water in the urea water addition valve increases and cooling of the urea water addition valve by urea water proceeds, so the temperature rise of the urea water addition valve Is reduced. On the other hand, when the amount of urea water addition necessary for NOx purification is determined in consideration of the urea concentration as in the exhaust gas purification device of the above-mentioned document, the urea concentration is high and corrosion is likely to proceed. As the amount of water added decreases and it becomes difficult to suppress the temperature rise of the urea water addition valve, urea corrosion particularly easily progresses.

  The present invention has been made in view of these circumstances, and an object of the present invention is to provide an exhaust purification system capable of effectively suppressing urea corrosion of a urea water addition valve.

  The exhaust gas purification apparatus to solve the above problems is a urea water addition valve installed in the exhaust passage of the engine and adding urea water into the exhaust, and a portion downstream of the urea water addition valve in the exhaust passage. A catalytic device for purifying nitrogen oxides in exhaust gas by selective catalytic reduction using ammonia generated by thermal decomposition of water as a reducing agent, and for cooling, which is the addition amount of urea water necessary for cooling the urea water addition valve And an addition control unit that calculates the addition amount and controls the addition amount of urea water of the urea water addition valve so as to be equal to or more than the addition amount for cooling.

  In the urea water addition valve of such an exhaust gas purification device, regardless of the urea water addition amount necessary for purification of NOx, urea water addition of an amount equal to or more than the cooling addition amount is performed. Therefore, if an appropriate amount is set as the cooling addition amount, the temperature of the urea water addition valve can be suppressed to a temperature that can suppress corrosion or less.

  The higher the urea concentration of the urea water added to the exhaust gas by the urea water addition valve, the more the urea corrosion of the urea water addition valve progresses at a lower temperature. On the other hand, the above-mentioned exhaust purification system is provided with a urea concentration detection part which detects urea concentration of urea water. Then, the addition control unit calculates the cooling addition amount so that when the urea concentration is high, the amount becomes larger than when the urea concentration is low. Therefore, when the urea concentration is high, the temperature of the aqueous urea solution addition valve is maintained at a lower temperature than when the urea concentration is low. Therefore, according to the exhaust gas purification apparatus, it is possible to effectively suppress urea corrosion of the urea water addition valve.

The schematic diagram which shows the structure of one Embodiment of an exhaust gas purification device. The block diagram which shows the control structure of the addition control part provided in the same exhaust gas purification device. The graph which shows the relation between the concentration amendment coefficient and the urea concentration which the above-mentioned addition control part computes when controlling the amount of urea water addition of a urea water addition valve.

Hereinafter, one embodiment of the exhaust gas purification apparatus will be described in detail with reference to FIGS. 1 to 3.
As shown in FIG. 1, an air flow meter 12 for detecting the flow rate of intake air flowing through the intake passage 11 is provided in the intake passage 11 of the on-vehicle engine 10 to which the exhaust gas purification device of the present embodiment is applied. . A throttle valve 13, which is a valve for adjusting the intake flow rate, is provided at a portion of the intake passage 11 downstream of the air flow meter 12.

  The exhaust system according to the present embodiment includes a urea water addition valve 15 provided in the exhaust passage 14 of the engine 10 and a selective catalytic reduction type provided downstream of the urea water addition valve 15 in the exhaust passage 14. And the catalyst device 20 of The urea aqueous solution addition valve 15 injects the urea aqueous solution sent from the urea aqueous solution tank 16 by the urea aqueous solution pump 17 to add urea aqueous solution into the exhaust gas flowing through the exhaust passage 14. The urea aqueous solution tank 16 is provided with a urea aqueous solution temperature sensor 18 for detecting the temperature of the urea aqueous solution stored therein and a urea concentration sensor 19 for detecting the urea concentration of the urea aqueous solution. The catalytic converter 20 purifies nitrogen oxides (NOx) in the exhaust gas by selective catalytic reduction. The selective catalytic reduction of NOx by the catalytic converter 20 is performed using ammonia generated by thermal decomposition of urea water added by the urea water addition valve 15 as a reducing agent. An exhaust temperature sensor 21 for detecting the temperature of exhaust flowing through the exhaust passage 14 and an NOx concentration sensor 22 for detecting the concentration of NOx in the exhaust are provided in a portion on the upstream side of the urea water addition valve 15 in the exhaust passage 14 , Is provided.

  Furthermore, the exhaust purification system of the present embodiment includes the addition control unit 23 that controls addition of urea water to the exhaust gas by the urea water addition valve 15. The addition control unit 23 receives detection results of the air flow meter 12, the urea water temperature sensor 18, the urea concentration sensor 19, the exhaust temperature sensor 21, and the NOx concentration sensor 22. Further, the addition control unit 23 includes a crank angle sensor 25 that detects the rotational phase of the crankshaft 24, which is an output shaft of the engine 10, and a vehicle speed sensor that detects the traveling speed (vehicle speed) of the vehicle on which the engine 10 is mounted. 26 detection results are input. Actually, the addition control unit 23 is not an independent control unit specialized in addition control of urea water, but an electronic control unit for engine control performs processing as the addition control unit 23 as a part of engine control. .

  The addition control unit 23 obtains the rotational speed of the crankshaft 24, that is, the engine speed from the detection result of the crank angle sensor 25. Further, the addition control unit 23 obtains the flow rate (exhaust flow rate) of the exhaust flowing through the exhaust passage 14 based on the intake flow rate and the engine rotational speed detected by the air flow meter 12.

  As shown in FIG. 2, the addition control unit 23 includes a purification addition amount calculation process P100, a tip temperature calculation process P110, a cooling addition amount calculation process P120, a concentration correction process P130, an addition amount command value calculation process P140, and addition. The addition control of the urea water of the urea water addition valve 15 is performed through the valve drive process P150. The addition of urea water to the exhaust gas is performed by the urea water addition valve 15 injecting urea water at each cycle set in accordance with the operating state of the engine 10. The addition amount of urea water in the following description represents the product of the total amount of urea water injected by the urea water addition valve 15 per unit time, that is, the injection amount of urea water per injection multiplied by the number of injections per unit time. ing.

  In the purification addition amount calculation process P100, calculation of the purification addition amount, which is the addition amount of urea water necessary for purification of NOx in the catalyst device 20, is performed. In calculating the addition amount for purification, first, the flow rate of NOx flowing into the catalyst device 20 is obtained based on the exhaust flow rate and the NOx concentration. Then, the addition amount of urea water capable of supplying an amount of ammonia necessary for the purification of NOx corresponding to the flow rate is calculated as the value of the purification addition amount.

  In the tip temperature calculation process P110, calculation of the temperature (tip temperature) of the tip portion of the urea water addition valve 15 exposed to the exhaust is performed. When calculating the tip temperature, first, the amount of decrease in exhaust temperature due to cooling of the traveling wind in the section from the installation position of the exhaust temperature sensor 21 in the exhaust passage 14 to the installation position of the urea water addition valve 15 is determined based on the vehicle speed. . Subsequently, a difference obtained by subtracting the calculated amount of decrease from the detected value of the exhaust gas temperature of the exhaust gas temperature sensor 21 is obtained as the temperature of the exhaust gas at the installation position of the urea water addition valve 15 in the exhaust gas passage 14. Further, based on the exhaust flow rate and the exhaust temperature at the installation position of the urea water addition valve 15, the heat reception amount of the urea water addition valve 15 from the exhaust is discharged from the urea water addition valve 15 to the urea water based on the urea water temperature. The amount of heat is determined respectively. Then, the tip temperature of the urea water addition valve 15 is calculated from the heat reception amount and the heat release amount.

  In the cooling addition amount calculation process P120, calculation of a cooling addition amount which is an addition amount of urea water necessary for cooling the urea water addition valve 15 is performed. The addition amount for cooling at this time is calculated based on the tip temperature of the urea water addition valve 15 calculated in the tip temperature calculation process P110 so that the amount increases as the tip temperature increases. In addition, in the cooling addition amount calculation process P120, the addition amount of urea water necessary to make the tip temperature of the urea water addition valve 15 equal to or lower than the predetermined reference temperature is calculated as the value of the cooling addition amount. The reference temperature here is the tip temperature of the urea water addition valve 15 capable of suppressing the progress of the corrosion of urea water due to urea when the urea concentration of the urea water added by the urea water addition valve 15 is a predetermined reference concentration. Is the upper limit value of. That is, in the cooling addition amount calculation process P120, the cooling addition amount is calculated on the premise that the urea concentration of the urea water is the reference concentration.

  In the concentration correction process P130, correction (concentration correction) based on the urea concentration of the urea water detected by the urea concentration sensor 19 is performed on the cooling addition amount calculated by the cooling addition amount calculation process P120. In the concentration correction process P130, first, the value of the concentration correction coefficient is calculated based on the urea concentration. Then, the density correction is performed by using the product of the value of the cooling addition amount calculated by the cooling addition amount calculation process P120 multiplied by the value of the concentration correction coefficient as the value of the cooling addition amount after the concentration correction. . The calculation of the concentration correction coefficient is performed with reference to the calculation map M1 stored in advance in the addition control unit 23.

  As shown in FIG. 3, the calculation map M1 is set such that the value of the concentration correction coefficient when the urea concentration is the reference concentration is "1". Then, in the calculation map M 1, when the urea concentration is changed from the reference concentration to the high concentration side, the value of the concentration correction coefficient gradually increases from “1”, and the urea concentration from the reference concentration to the low concentration side The value of the density correction coefficient is set to gradually decrease from "1" when changing.

  On the other hand, in the addition amount command value calculation processing P140, 2 of the value of the addition amount for purification calculated in the addition amount calculation processing for purification P100 and the value of the addition amount for cooling after concentration correction in the concentration correction processing P130. Of the two values, the larger one is calculated as the addition amount command value. And in addition valve drive processing P150, drive control of urea water addition valve 15 is performed so that addition of the amount of urea water for the quantity which the value of addition amount command value shows is performed in unit time.

  In addition, in the addition amount command value calculation process P140, when the value of the cooling addition amount after concentration correction is smaller than the value of the purification addition amount, the value of the purification addition amount is calculated as the value of the addition amount command value. . On the other hand, when the value of the cooling addition amount after concentration correction is larger than the value of the purification addition amount, the value of the cooling addition amount after concentration correction is calculated as the value of the addition amount command value. As described above, in any case, a value greater than or equal to the value of the cooling addition amount after concentration correction is set as the addition amount command value.

(Operation and effect of the present embodiment)
The operation and effects of the exhaust gas purification apparatus of the present embodiment configured as described above will be described.
A fixed amount of urea water is always present inside the urea water addition valve 15 provided in the exhaust gas purification device as described above. When the temperature of the aqueous urea solution inside the aqueous urea solution addition valve 15 rises, the corrosion of the aqueous urea solution addition valve 15 by the urea of the aqueous urea solution proceeds. On the other hand, when the urea aqueous solution addition valve 15 injects urea aqueous solution, an amount of urea aqueous solution in the injected aqueous urea solution is replaced with low temperature urea aqueous solution sent from the urea aqueous solution tank 16. Therefore, the increase in the tip temperature of the urea water addition valve 15 can be suppressed as the urea water addition amount increases.

  On the other hand, in the exhaust purification system of the present embodiment, the amount of addition of urea water necessary to maintain the tip temperature of the urea water addition valve 15 in a temperature range that can suppress the progress of corrosion is the addition amount for cooling. It is computing. Then, by setting the addition amount of the urea water of the urea water addition valve 15 to an amount of the cooling addition amount or more, the progress of the corrosion is suppressed.

  The higher the concentration of urea in the aqueous urea solution, the higher its corrosiveness. Therefore, when the concentration of urea is high, the upper limit of the tip temperature of the aqueous urea solution addition valve 15 that can suppress the progress of corrosion decreases. In that respect, in the exhaust purification system of the present embodiment, through the concentration correction by the concentration correction processing P130, when the urea concentration is high, the addition amount for cooling is calculated so as to be a larger amount than when the urea concentration is low. . Therefore, when the urea concentration is high, the tip temperature of the urea water addition valve 15 is maintained at a lower temperature than when the urea concentration is low. Therefore, according to the exhaust purification system of the present embodiment, urea corrosion of the urea water addition valve 15 can be effectively suppressed.

The above embodiment can be modified as follows.
In the above embodiment, in the concentration correction process P130, the concentration correction coefficient is calculated as a value that changes linearly with respect to a change in urea concentration. The relationship between the urea concentration and the value of the concentration correction coefficient is appropriately set according to the relationship between the tip temperature of the urea water addition valve 15 in the exhaust gas purification apparatus to be applied, the rate of urea corrosion, and the addition amount. In some cases it may be desirable to set the density correction factor to be a value that changes non-linearly with changes in density. Alternatively, the concentration correction coefficient may be set as a value that changes discontinuously with respect to a change in urea concentration, that is, a value that changes stepwise.

  In the above embodiment, when the urea concentration is high, the cooling addition amount is calculated so as to be a larger amount than when the urea concentration is low through the two processes of the cooling addition amount calculation process P120 and the concentration correction process P130. Was. That is, when the urea concentration is high, the addition amount for cooling when the urea concentration is the reference concentration is first calculated, and the calculated value is corrected by the concentration correction coefficient calculated according to the urea concentration. The amount of cooling addition was calculated so that the amount would be larger than when the concentration was low. Such calculation of the cooling addition amount may be performed in a single process. That is, the concentration correction process P130 is omitted, and the cooling addition amount is calculated based on the tip temperature and the urea concentration in the cooling addition amount calculation process P120. Also in this case, if the calculation of the addition amount for cooling in the addition amount calculation process for cooling P120 is performed so as to be a larger amount when the urea concentration is high than when the urea concentration is low. The urea corrosion of the urea water addition valve 15 can be effectively suppressed.

  DESCRIPTION OF SYMBOLS 10 ... Engine 11, 11 ... intake passage, 12 ... Air flow meter, 13 ... Throttle valve, 14 ... Exhaust passage, 15 ... Urea water addition valve, 16 ... Urea water tank, 17 ... Urea water pump, 18 ... Urea water temperature sensor, 19: Urea concentration sensor (urea concentration detection unit), 20: catalytic device, 21: exhaust temperature sensor, 22: NOx concentration sensor, 23: addition control unit, 24: crankshaft, 25: crank angle sensor, 26: vehicle speed sensor .

Claims (1)

Urea water addition valve installed in the exhaust passage of the engine and adding urea water into the exhaust, and installed in a portion on the downstream side of the urea water addition valve in the exhaust passage and generated by thermal decomposition of the urea water A catalyst device for purifying nitrogen oxides in exhaust gas by selective catalytic reduction using ammonia as a reducing agent;
While calculating the addition amount for cooling which is the addition amount of the urea water necessary for cooling the urea water addition valve, the urea water addition amount of the urea water addition valve is set to be equal to or more than the cooling addition amount. An addition control unit that controls
A urea concentration detection unit that detects the urea concentration of the urea water;
And the addition control unit calculates the amount of cooling addition so as to be larger when the urea concentration is high than when the urea concentration is low.