JP2010223178A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain the generation of an ammonia slip while improving reduction efficiency, in a selective reduction catalyst system for reducing NOx by adding urea water. <P>SOLUTION: This exhaust emission control device has a catalyst 50 arranged in an exhaust passage 10 of an internal combustion engine 1 and selectively reducing the NOx included in exhaust gas with ammonia generated by hydrolyzing the urea water as a reducing agent, and a means 100 for controlling a urea water adding valve 62 for adding the urea water from the catalyst upstream side of the exhaust passage and an injection quantity of the urea water 63 according to a catalyst state. The control means 100 improves the dispersiveness of the reducing agent in the catalyst 50 by temporarily injecting urea exceeding a predetermined quantity when a state of being smaller than a predetermined value in the injection quantity of the urea water 63 is continued for a predetermined time. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  A urea selective reduction exhaust purification device is known as an exhaust purification device that purifies nitrogen oxides (NOx) contained in exhaust gas discharged from an internal combustion engine (see, for example, Patent Document 1). This exhaust purification device includes a catalytic converter including a selective reduction catalyst (SCR) in an exhaust passage, and a urea water addition valve provided upstream thereof. The selective reduction catalyst carries a catalyst metal such as vanadium oxide on its catalyst carrier. When urea water is added to the exhaust passage, the urea water is hydrolyzed and ammonia is generated by the heat of the exhaust gas, and this ammonia and NOx undergo a denitration reaction with NOx in the selective reduction catalyst to generate nitrogen and water.

JP 2003-301737 A

  By the way, conventionally, the urea addition amount to the selective reduction catalyst is, for example, the injection amount according to the purification efficiency of NOx introduced into the selective reduction catalyst and the target adsorption amount of ammonia to be adsorbed on the selective reduction catalyst. The injection amount for compensating for the shortage is calculated, and the total amount is set as the final injection amount.

  However, even if the adsorption amount of ammonia on the selective reduction catalyst is sufficient, the dispersibility to the catalyst varies depending on the injection amount. For example, it has been found from experimental results that if a small amount of injection is continued, the dispersibility of urea spray is poor, the adsorption and reaction to the catalyst become partial, and the purification efficiency of the selective reduction catalyst as a whole decreases. On the other hand, it is known that when the urea water injection amount is increased more than necessary, so-called ammonia slip occurs. This is particularly noticeable in situations such as sudden acceleration of the vehicle.

  An object of the present invention is to provide an exhaust gas purification apparatus for an internal combustion engine that can suppress the generation of ammonia slip while improving the purification efficiency.

  An exhaust gas purification apparatus for an internal combustion engine according to the present invention is provided in an exhaust passage of an internal combustion engine, and selectively reduces NOx contained in exhaust gas selectively using ammonia produced by hydrolysis of urea water as a reducing agent. A catalyst, a urea water addition valve for adding urea water toward the selective reduction catalyst from the upstream side of the selective reduction catalyst in the exhaust passage, and a urea water addition valve depending on the state of the selective reduction catalyst. Control means for controlling the injection amount of the reducing agent, and when the state where the injection amount of the reducing agent is less than a predetermined amount continues for a predetermined time, the control means temporarily An amount of the reducing agent exceeding a predetermined amount is injected.

  In the above configuration, the control means temporarily estimates the injection amount of urea water immediately after the amount of the reducing agent exceeding the predetermined amount is injected, and the estimated adsorption of the reducing agent of the selective reduction catalyst immediately after the injection. It can be corrected based on the amount.

  According to the present invention, by temporarily increasing the injection amount of the reducing agent, the dispersibility of the reducing agent with respect to the selective reduction catalyst can be improved, and as a result, the purification efficiency can be increased. Further, since the injection amount of the reducing agent is increased only temporarily, the occurrence of so-called ammonia slip can be suppressed.

1 is a schematic diagram illustrating a configuration of an exhaust gas purification apparatus for an internal combustion engine according to an embodiment of the present invention. It is a functional block diagram which shows an example of the process in ECU. It is a figure which shows an example of an instantaneous maximum injection amount map. It is a figure which shows an example of an instantaneous maximum injection amount injection time map. It is a figure which shows an example of a dispersibility improvement correction coefficient map. It is a figure which shows the other example of an instantaneous maximum injection amount map.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of an exhaust emission control device for an internal combustion engine according to an embodiment of the present invention.
In FIG. 1, the internal combustion engine 1 is, for example, a diesel engine. In the exhaust passage 10 of the internal combustion engine 1, an oxidation catalyst converter 30, a diesel particulate filter (DPF) 40, a urea water addition valve 62, and a selective reduction catalyst converter 50 are provided from the upstream side. The exhaust passage 10 is provided with NOx sensors 70A and 70B for detecting the concentration of nitrogen oxides (NOx) in the exhaust gas EG. The NOx sensor 70 </ b> A is provided downstream of the urea water addition valve 62 and upstream of the selective catalytic reduction converter 50. The NOx sensor 70B is provided on the downstream side of the selective catalytic reduction converter 50.

  The oxidation catalyst converter 30 carries an oxidation catalyst made of a catalyst metal or the like that oxidizes unburned fuel or the like in the exhaust gas EG in order to raise the temperature of the exhaust gas EG supplied to the DPF 40 or the like at the subsequent stage.

  The DPF 40 is a filter that collects particulate matter (PM) contained in the exhaust gas EG. As is well known, the structure of the DPF 40 is composed of, for example, a honeycomb body made of metal or ceramics. The DPF 40 needs to be regenerated when a predetermined amount of PM is deposited. Specifically, for example, the oxidation catalyst converter 30 is activated by raising the temperature, and the exhaust gas EG heated by the oxidation action of the oxidation catalyst converter 30 is supplied to the DPF 40. As a result, the collected PM is burned and the filter function is regenerated. The DPF 40 may be configured to carry an oxidation catalyst made of a catalyst metal.

  The urea water addition valve 62 is connected to the urea tank 60 and adds a predetermined concentration of urea water 63 supplied from the urea tank 60 toward the selective reduction catalytic converter 50 to the exhaust passage 10. The urea water addition valve 62 is configured to add an addition amount according to a control command from the electronic control unit (ECU) 100. The urea water added to the exhaust passage 10 is hydrolyzed by the heat of the exhaust gas EG to generate ammonia.

The selective reduction catalytic converter 50 selectively reduces NOx contained in the exhaust gas EG into nitrogen gas and water using ammonia generated from the urea water added from the urea water addition valve 62 as a reducing agent. . This selective reduction catalytic converter 50 has a well-known structure, for example, a material composed of zeolite containing Si, O, and Al as main components and containing Fe ions, or a surface of a base material made of aluminum oxide alumina, for example. In addition, a catalyst on which a catalyst metal such as a vanadium catalyst (V ₂ O ₅ ) is supported can be used, but it is not particularly limited thereto.

  The ECU 100 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), a backup memory such as an EEPROM (Electronically Erasable and Programmable Read Only Memory), an A / D converter, a buffer, and the like. It comprises hardware including an output interface circuit including an input interface circuit, a drive circuit, etc., and necessary software. The ECU 100 receives detection signals from the NOx sensors 70A and 70B and outputs a control signal to the urea water addition valve 62. The processing content of the ECU 100 will be described later.

Next, an example of processing of the ECU 100 will be described with reference to FIG.
FIG. 2 shows an example of the determination process of the urea water injection amount at the normal time.
First, the concentration of NOx in the exhaust gas discharged from the internal combustion engine 1 is acquired from the NOx sensor 70A (S1). The amount of NOx introduced into the selective catalytic reduction converter 50 is calculated from the NOx sensor 70A and the intake air amount D1 taken from the intake system of the internal combustion engine 1 (S2). Next, the amount of NH ₃ necessary for NOx purification is calculated using the NOx / NH ₃ conversion map D2 for converting the amount of NOx and the amount of ammonia (NH ₃ ) (S3).

Next, the amount of consumed NH ₃ is calculated using an SCR purification efficiency map D3 that defines the purification efficiency of the selective catalytic reduction converter 50 according to the state of the selective catalytic reduction converter 50 (S4).

Next, the target adsorption amount D4, which is the amount of NH ₃ to be adsorbed by the selective catalytic reduction converter 50, is added to the consumed NH ₃ amount calculated in S4 above, and this is set as the NH ₃ addition amount to be added (S5).

Next, the NH ₃ addition amount calculated in S5 is feedback corrected (S6). Feedback correction, for example, to correct the NH ₃ amount calculated in step S5 on the basis of a deviation between the amount of actual NH ₃ with respect to the command value.

Next, the corrected NH ₃ addition amount is converted into a urea water amount (S7), and the obtained urea water amount is output to the urea water addition valve 62 as a final command injection amount.

Next, an example of processing performed by the ECU 100 when the urea water injection from the urea water addition valve 62 is continued at a predetermined injection amount or less will be described.
Here, the command injection amount of urea water is Qv (mg / s), the predetermined injection amount is QL (mg / s), the predetermined injection time is TL (sec), the catalyst bed temperature is TSCR (° C.), and the air amount is Ga. (G / sec).
As an example, QL = 10 mg / s and TL = 10 sec.

  When the state where the command injection amount Qv falls below the predetermined injection amount QL continues for the predetermined injection time TL or longer, the ECU 100 sets the command injection amount Qv to the instantaneous maximum injection amount Qmax map shown in FIG. It is determined based on a map of the maximum injection amount injection time tmax. The map shown in FIG. 3 defines the instantaneous maximum injection amount Qmax according to the catalyst bed temperature TSCR and the air amount Ga, and the map shown in FIG. 4 is instantaneous according to the catalyst bed temperature TSCR and the air amount Ga. The maximum injection amount Qmax is defined, and the map shown in FIG. 5 defines the instantaneous maximum injection amount injection time tmax according to the catalyst bed temperature TSCR and the air amount Ga. The urea water of the instantaneous maximum injection amount Qmax determined according to these maps is injected during the instantaneous maximum injection amount injection time tmax. The instantaneous maximum injection amount Qmax is a value larger than the predetermined injection amount QL. Further, when the state where the command injection amount Qv falls below the predetermined injection amount QL continues for the predetermined injection time TL, the case where there is no injection of urea water is also included. Needless to say, the injection amount increase control is not performed when the catalyst bed temperature TSCR is not activated and is equal to or lower than the allowable injection temperature.

  Further, from the viewpoint of improving the dispersibility of urea water (ammonia) in the selective reduction catalyst and suppressing ammonia slip, it is preferable that the instantaneous maximum injection amount Qmax is large and the instantaneous maximum injection amount injection time tmax is short. Suitable values are, for example, QL = 25 mg / sec and TL = 0.5 sec.

Next, an example of processing after injecting urea water having the instantaneous maximum injection amount Qmax for the instantaneous maximum injection amount injection time tmax will be described.
After the urea aqueous solution having the instantaneous maximum injection amount Qmax is injected, an error occurs in the estimated adsorption amount of ammonia of the selective reduction catalyst.
Here, when the corrected estimated adsorption amount is Lm (mg), the estimated adsorption amount is La (mg), and the correction coefficient is Kb, the corrected estimated adsorption amount Lm is defined by the following equation (1). The correction coefficient Kb can be specified from a map as shown in FIG.
Lm = La + Kb × Qmax × tmax (1)

Further, when the corrected injection amount is Qm (mg / s) and the drive pulse interval of the urea addition valve 62 is Tfr, the corrected injection amount Qm is defined by the following equation (2).
Qm = Qv−Qmax × tmax / Tfr (2)

  It should be noted that the corrected injection amount Qm after injecting the urea water having the instantaneous maximum injection amount Qmax is basically assumed to be zero.

  The cause of the decrease in the adsorption amount of ammonia in the selective reduction catalyst is firstly that ammonia reacts with the introduced NOx. Secondly, when the catalyst bed temperature is high, ammonia adsorbed on the selective reduction catalyst is spontaneously released, which causes the amount of adsorption to decrease.

Here, an example of urea water injection amount control in consideration of the above two causes will be described.
For example, as shown in FIG. 6, a map in which the maximum adsorption amount Ldmax is defined in accordance with the air amount Ga and the catalyst bed temperature TSCR is held in advance.

  In normal times, the saturated adsorption amount is set as the target adsorption amount Ld and the maximum adsorption amount Ldmax, and an injection amount that is not adsorbed beyond this is set.

The current estimated adsorption amount at (t1) is La1, and the corrected estimated adsorption amount after acceleration (t2) is La2. If the catalyst bed temperature TSCR increases due to the running state, the adsorbable amount decreases, leading to the occurrence of ammonia slip. For this reason, the corrected estimated adsorption amount La2 is obtained by the following equation (3).
La2 = La1 + ΔLd = 2La1-Ldmax (3)

  Then, the corrected estimated adsorption amount La2 obtained by the equation (3) is set as a corrected target adsorption amount. Thus, the next injection of urea water can start calculating the adsorption amount based on the corrected estimated adsorption amount La2.

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 10 ... Exhaust passage 30 ... Oxidation catalytic converter 40 ... Diesel particulate filter (DPF)
50 ... Selective reduction catalytic converter 62 ... Urea water addition valve 70A, 70B ... NOx sensor

Claims (2)

A selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine and selectively reduces NOx contained in the exhaust gas by hydrolyzing urea water and using ammonia as a reducing agent;
A urea water addition valve for adding urea water toward the selective reduction catalyst from the upstream side of the selective reduction catalyst in the exhaust passage;
Control means for controlling the injection amount of the reducing agent from the urea water addition valve in accordance with the state of the selective reduction catalyst,
When the state in which the injection amount of the reducing agent is less than a predetermined amount is continued for a predetermined time, the control means temporarily injects the reducing agent in an amount exceeding the predetermined amount. An exhaust purification device for an internal combustion engine.   The control means temporarily determines the injection amount of urea water immediately after injecting the reducing agent in an amount exceeding the predetermined amount based on the estimated adsorption amount of the reducing agent of the selective reduction catalyst immediately after the injection. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the correction is performed.

Images