JP5553630B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

The present invention relates to an exhaust purifying apparatus for an internal combustion engine, and more particularly relates to an exhaust purifying apparatus for an internal combustion engine provided with a selective reduction catalyst for reducing NO _x in the exhaust passage in the presence of a reducing agent.

2. Description of the Related Art Conventionally, as one of exhaust purification apparatuses that purify NO _x in exhaust gas, an apparatus has been proposed in which a selective reduction catalyst that selectively reduces NO _x in exhaust gas by adding a reducing agent is provided in an exhaust passage. Yes. For example, in a urea addition type selective reduction catalyst using urea water as a reducing agent, the added urea is hydrolyzed to generate ammonia (NH ₃ ), and NO _x in the exhaust gas is selectively reduced by this ammonia. .

In such a selective reduction catalyst, if less than the injection amount is optimum amount of the reducing agent, the reduction rate of the NO _x by NH ₃ to be consumed in the reduction of the NO _x is insufficient is lowered, the optimum When the amount is larger than the required amount, NH _{3 that} is excessive for the reduction of NO _x is discharged. For this reason, it is important to appropriately control the injection amount of the reducing agent in the exhaust emission control device including the selective reduction catalyst.

  Patent Document 1 discloses an exhaust gas purification system for a diesel engine using a selective reduction catalyst. This exhaust gas purification system stops the injection of urea water that hits the reducing agent when the engine is under a low load, and injects urea water when the engine is under a high load.

JP 2004-270565 A

However, even during low-load operation, it is possible to selectively reduce NO _x in the exhaust by NH ₃ generated from urea water by raising the temperature of the catalyst. The system does not support this.

Furthermore, during low-load operation, NO _x emissions from the engine is small, especially in idling operation, urea water required for reducing the amount of NO _x being discharged becomes small amount, the injection of the urea water injector There is a problem that the injection amount is less than the amount accuracy, that is, the injection amount becomes uncontrollable.

Therefore, the present invention provides an internal combustion engine exhaust gas purification apparatus equipped with a selective reduction catalyst, supplying a reducing agent within a range in which supply (injection) accuracy can be maintained as required even during low-load operation. The object is to selectively reduce NO _x .

The present invention provides an exhaust emission control device for an internal combustion engine. The exhaust purification device is provided in an exhaust passage of an internal combustion engine, captures NH ₃ as a reducing agent, and uses the captured NH ₃ to reduce NO _x flowing through the exhaust passage, According to the reducing agent supply means for supplying the reducing agent to the upstream side of the selective reduction catalyst, means for calculating the amount of NH ₃ in the selective reduction catalyst, the calculated amount of NH ₃ and the operating state of the internal combustion engine, A reducing agent supply amount setting means for setting a reducing agent supply amount by the reducing agent supply means, and the reducing agent supply amount setting means is used when the reducing agent supply amount is equal to or lower than a lower limit supply amount that can ensure supply accuracy. The supply amount is set to zero.

According to the present invention, it is possible to stop the supply of the reducing agent within a range where the supply accuracy cannot be maintained. As a result, it becomes possible to reliably grasp the amount of reducing agent supplied, and hence the amount of NH ₃ in the selective reduction catalyst.

According to one aspect of the present invention, the reducing agent supply amount setting means sets the reducing agent supply amount to a predetermined amount that is equal to or greater than the lower limit supply amount when the calculated NH ₃ amount becomes equal to or lower than the lower limit set value.

According to one aspect of the present invention, the amount of NH ₃ in the selective reduction catalyst can be maintained at a predetermined amount (lower limit set value) or more while supplying the reducing agent within a range in which supply accuracy can be maintained. As a result, it is possible to prevent a reduction in the reduction rate of NO _x due to insufficient supply of the reducing agent.

  According to one aspect of the present invention, the reducing agent supply amount setting means sets a predetermined amount equal to or greater than the lower limit supply amount when the rotational speed of the internal combustion engine is equal to or less than a predetermined value.

According to the present invention, the reducing agent can be supplied within a range in which the supply (injection) accuracy can be maintained as required even during low-load operation. As a result, it is possible to prevent a reduction in NO _x reduction rate due to insufficient supply of the reducing agent even during low-load operation.

According to one aspect of the present invention, the reducing agent supply amount setting means sets the reducing agent supply amount to zero when the calculated NH ₃ amount becomes equal to or higher than the upper limit set value.

According to the present invention, the amount of NH ₃ in the reduction catalyst can be maintained within a predetermined range. As a result, it is possible to prevent the NH _{3 that} is excessive in the reduction of NO _x from being discharged.

It is a mimetic diagram showing composition of an engine and its exhaust gas purification device according to one example of the present invention. It shows the concentration of NO _x in the selective reduction catalyst, ammonia concentration, the relationship between the storage amount of ammonia. The rotation speed Ne of the engine, the NO _x emissions, and the injection amount of urea water is a diagram showing the relationship between the adsorbed NH ₃ amount in the selective reduction catalyst. It is a figure which shows the processing flow of the injection quantity control of urea water. NH ₃ is a diagram showing a flow for calculating the estimated value of the adsorption amount.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust purification device 2 according to an embodiment of the present invention. The engine 1 is a gasoline engine or a diesel engine, and is mounted on a vehicle (not shown).

The exhaust purification device 2 includes an oxidation catalyst 21 provided in the exhaust passage 11 of the engine 1 and a nitrogen oxide (hereinafter referred to as “hereinafter referred to as“ nitrogen oxide ”) provided in the exhaust passage 11 provided downstream of the oxidation catalyst 21 in the exhaust passage 11. NO _x ”) in the presence of a reducing agent, a selective reduction catalyst 23, a urea water injection device 25 for supplying urea water as a reducing agent to the upstream side of the selective reduction catalyst 23 in the exhaust passage 11, and electronic control A unit (hereinafter referred to as “ECU”) 3 is included.

  The ECU 3 is a computer that includes a central processing unit (CPU) and a memory. The memory can store a computer program for realizing various controls of the vehicle and data, tables, and maps necessary for executing the program. As will be described later, the ECU 3 receives data sent from each part (sensor or the like) of the vehicle, performs calculation, and generates a control signal for controlling each part of the vehicle.

  The urea water injection device 25 includes a tank 251 and an injector 25. The tank 251 stores urea water, and is connected to the injector 253 via the urea water supply path 254 and a pump (not shown). The tank 251 is provided with a urea water level sensor 255. This sensor 255 detects the water level of the urea water in the tank 251 and outputs a detection signal corresponding to this water level to the ECU 3. The injector 253 is connected to the ECU 3 and injects urea water into the exhaust passage 11 in accordance with a control signal from the ECU 3. That is, the injector 253 injects a predetermined amount of urea water into the exhaust passage 11 at an injection time (seconds / shot) set by the ECU 3 and an injection interval (seconds, period (Hz)).

The oxidation catalyst 21 is provided on the upstream side of the selective reduction catalyst 23 and the injector 253 in the exhaust passage 11, and converts NO occupying most of the NO _{x in} the exhaust gas into NO ₂ , thereby the selective reduction catalyst. to promote the reduction of the NO _x in 23.

The selective reduction catalyst 23 includes a first selective reduction catalyst 231 and a second selective reduction catalyst 232 provided downstream of the first selective reduction catalyst 231 in the exhaust passage 11. The first and second selective reduction catalysts 231 and 232 selectively reduce NO _x in the exhaust gas in an atmosphere in which ammonia as a reducing agent exists. Specifically, when urea water is injected by the urea water injection device 25, ammonia (hereinafter referred to as “NH ₃ ”) is generated from urea by hydrolysis, and the NO _x in the exhaust gas in the selective reduction catalyst 23 is generated by this NH _3. (NO and NO ₂ ) are selectively reduced. The selective reduction catalyst 23 also has a function of storing NH _3, NO _x is reduced and purified by NH ₃ that is stored.

In addition to the NH ₃ sensor 26, the catalyst temperature sensor 27, and the NO _x sensor 28, the ECU 3 is connected with a crank angle position sensor 14, an accelerator opening sensor 15, and a urea water remaining amount warning lamp 16.

The NH ₃ sensor 26 detects the concentration of ammonia in the exhaust gas between the first selective reduction catalyst 231 and the second selective reduction catalyst 232 in the exhaust passage 11 (hereinafter referred to as “NH ₃ concentration”), and detects the detected NH A detection signal corresponding to the _three concentrations is sent to the ECU 3.

The catalyst temperature sensor 27 detects the temperature of the first selective reduction catalyst 231 (hereinafter referred to as “catalyst temperature”), and sends a detection signal corresponding to the detected catalyst temperature to the ECU 3. The NO _x sensor 28 detects the concentration of NO _{x in} the exhaust gas flowing into the first selective reduction catalyst 231 and sends a detection signal corresponding to the detected NO _x to the ECU 3.

  The crank angle position sensor 14 detects the rotation angle of the crankshaft of the engine 1, generates a pulse at every crank angle, and sends the pulse signal to the ECU 3. The ECU 3 calculates the rotational speed NE of the engine 1 based on this pulse signal. The crank angle position sensor 14 further generates a cylinder identification pulse at a predetermined crank angle position of the specific cylinder and sends it to the ECU 3.

  The accelerator opening sensor 15 detects the amount of depression of an accelerator pedal (not shown) of the vehicle (hereinafter referred to as “accelerator opening”), and sends a detection signal corresponding to the detected accelerator opening to the ECU 3. In the ECU 3, the required torque of the engine 1 is calculated according to the accelerator opening and the rotational speed.

  The urea water remaining amount warning lamp 16 is provided, for example, on a meter panel of the vehicle, and lights up when the remaining amount of urea water in the tank 251 is less than a predetermined remaining amount. As a result, the driver is warned that the remaining amount of urea water in the tank 251 has decreased.

FIG. 2 is a diagram illustrating the relationship among the NO _x concentration, the NH ₃ concentration, and the storage amount of NH ₃ in the selective reduction catalyst 23 of FIG. Graphs 30 and 31 in FIG. 2 indicate that as NO _x is reduced and the NO _x concentration decreases, NH ₃ of the selective reduction catalysts 231 and 232 is consumed and the NH ₃ concentration also decreases.

In FIG. 2, NH ₃ is almost saturated in the first selective reduction catalyst 231, that is, NH ₃ is fully captured (adsorbed) (reference numeral 32). In the second selective reduction catalyst 232, NH ₃ leaking (through) the first selective reduction catalyst 231 is stored (adsorbed) by an amount indicated by reference numeral 33. A broken line 34 indicates that this storage amount varies depending on the catalyst temperature. The amount of storage tends to increase at low temperatures and decrease with increasing temperatures.

The amount indicated by reference numeral 35 in the NH ₃ storage amount of the first selective reduction catalyst 231 is the minimum NH ₃ storage amount (lower limit) required to reduce NO _x in a low load (low rotation) operation state such as when idling. Storage amount). Therefore, in a state where NH ₃ is almost saturated in the first selective reduction catalyst 231, the injection of urea water from the injector 253 can be stopped (to zero), but at least the amount of NH ₃ indicated by reference numeral 35 is reduced. It is necessary to ensure in the first selective reduction catalyst 231 or the second selective reduction catalyst 232. That is, it is necessary to restart the injection of urea water from the injector 253 so that the NH ₃ storage amount does not become smaller than the amount indicated by the reference numeral 35.

3 shows the rotational speed Ne of the engine 1 of FIG. 1, the NO _x emission amount, the injection amount of urea water from the injector 253, and the selective reduction catalyst 23 (the first selective reduction catalyst 231 or the second selective reduction catalyst). NH ₃ is a diagram showing the relationship between adsorption amount at 232). (A) is a conventional example shown for comparison, and (b) is an embodiment of the present invention. The control of the urea water injection amount in (b) is executed by the ECU 3 in FIG. 1 sending a control signal to the injector 253. First, the conventional example (a) will be described.

3 (a), the also reduced NO _x emissions from the engine as the rotational speed Ne of the engine is low. The T / P out of the NO _x emissions of the graph, which means NO _x emissions of the exhaust pipe on the downstream side of the selective reduction catalyst (outlet side). In this conventional example, the NO _x emission amount at the T / P out varies relatively even in the idle region where the engine speed Ne is low (1). The reason is that since the command injection amount of urea water in the idle region is smaller than the lower limit value that can ensure the injection accuracy, the actual injection amount (actual injection amount) cannot be sufficiently controlled and fluctuates as shown in the figure. (2). As a result, the amount of NH ₃ adsorbed on the selective reduction catalyst also fluctuates greatly as shown in the figure (3), and the NO _x purification rate (reduction rate) decreases, and the amount of adsorption possible on the selective reduction catalyst There will be a case in which excessive NH ₃ exceeding the gas flows out from the exhaust pipe on the downstream side of the selective reduction catalyst. The present invention aims to overcome these two cases.

In FIG. 3B, a predetermined threshold value is provided for the engine speed Ne, and when the engine speed Ne is equal to or lower than the threshold value, the instructed injection amount of urea water from the injector 253 is set to zero. At this time, since the NH3 adsorption amount at the selective reduction catalyst 23 satisfies the adsorption target amount, NOx is continuously reduced. The NH3 adsorption amount is consumed for NOx reduction and gradually decreases to reach the adsorption lower limit target amount (3). The lower limit target amount of adsorption corresponds to the minimum amount of NH3 storage required to reduce NOx in a low-load operation state such as idling indicated by reference numeral 35 in FIG. At this time, the command injection amount of the urea water from the injector 253 is set equal to the lower limit value that can ensure the injection accuracy. As a result, the actual injection amount of urea water can be brought close to the commanded injection amount, and the injection amount can be controlled reliably (2) .

The injection of the urea water from the injector 253, NH ₃ is its adsorption amount at the same time is consumed in _the NO _x reduction is increased to reach the adsorption target amount gradually in the selective reduction catalyst 23 (3). At this time, the command injection amount of urea water from the injector 253 is set to zero again. Thereby, the injection of urea water from the injector 253 is stopped. Thus, the NH ₃ adsorption amount in the selective reduction catalyst 23 is controlled by controlling the instructed injection amount of urea water from the injector 253 between zero and a predetermined amount (for example, a lower limit value that can ensure the injection accuracy). Can fall within a predetermined range (for example, between the adsorption lower limit target amount and the adsorption target amount) (3). As a result, the NO _x emission amount at the T / P out can be kept within a relatively small fluctuation range even in the idle region where the engine speed Ne is low (1). Furthermore, the NO _x purification rate (reduction rate), which is a problem of the prior art of (a), is reduced, and excessive NH ₃ exceeding the amount that can be adsorbed by the selective reduction catalyst is reduced by the selective reduction catalyst. It is possible to prevent going out from the exhaust pipe on the downstream side.

  FIG. 4 shows an example of a control flow of the injection amount of urea water from the injector 253 executed by the CPU of the ECU 3 according to one embodiment of the present invention. This injection amount control flow sets the injection amount of urea water and is executed every predetermined control cycle.

  In step S1, it is determined whether or not the failure flag of the urea water injection device 25 is “1”. This failure flag is set to “1” when it is determined in the determination process (not shown) that the urea water injection device has failed, and is set to “0” otherwise. When this determination is Yes, the process proceeds to step S16, and the urea water injection amount is set to “0”, and then this process ends. If this determination is No, the process proceeds to step S2.

  In step S2, it is determined whether or not the deterioration flag of the selective reduction catalyst is “1”. This catalyst deterioration flag is set to “1” when it is determined in the determination process (not shown) that one of the first selective reduction catalyst 231 and the second selective reduction catalyst 232 in FIG. 1 has failed, and otherwise. Set to “0”. When this determination is Yes, the process proceeds to step S16, and the urea water injection amount is set to “0”, and then this process ends. If this determination is No, the process proceeds to step S3.

  In step S3, it is determined whether or not the urea water remaining amount is less than a predetermined value. The remaining amount of urea water indicates the remaining amount of urea water in the urea water tank 251 in FIG. 1, and is calculated based on the output of the level sensor. If this determination is Yes, the process moves to step S4, and if No, the process moves to step S5.

  In step S4, a warning lamp for the remaining amount of urea water is turned on, the process proceeds to step S16, and after the urea water injection amount is set to “0”, this process ends.

  In step S5, it is determined whether or not the warm-up timer value of the oxidation catalyst 21 in FIG. 1 is greater than a predetermined value. This catalyst warm-up timer value measures the warm-up time of the oxidation catalyst 21 after the engine is started. If this determination is Yes, the process proceeds to step S6. When this determination is No, the process proceeds to step S16, and the urea water injection amount is set to “0”, and then this process ends.

In step S6, it is determined whether or not the sensor failure flag is “0”. This sensor failure flag is set to “1” when it is determined that at least one of the NH ₃ sensor 26 and the catalyst temperature sensor 27 in FIG. 1 has failed in a determination process (not shown), and “0” otherwise. "Is set. If this determination is Yes, the process proceeds to step S7. When this determination is No, the process proceeds to step S16, and the urea water injection amount is set to “0”, and then this process ends.

In step S7, it is determined whether or not the activation flag of the NH ₃ sensor 26 is 1. The NH ₃ sensor activation flag is set to “1” when it is determined in the determination process (not shown) that the NH ₃ sensor 26 has reached the active state, and is set to “0” otherwise. If this determination is Yes, the process proceeds to step S8. When this determination is No, the process proceeds to step S16, and the urea water injection amount is set to “0”, and then this process ends.

  In step S8, it is determined whether or not the temperature of the selective reduction catalyst 23 is higher than a predetermined temperature. The predetermined temperature is, for example, about 250 ° C., which is a temperature at which the purification performance of the selective reduction catalyst can be maximized. If this determination is Yes, it is determined that the selective reduction catalyst 23 has been activated, and the routine goes to Step S9. When this determination is No, it is determined that the selective reduction catalyst 23 has not yet been activated and the urea water injection should be stopped, and the process proceeds to step S16, where the urea water injection amount is set to “0”. After setting to, this process ends.

  In step S9, calculation of the urea water injection amount is started. First, in step S10, it is determined whether or not the rotational speed Ne of the engine 1 is equal to or greater than a predetermined threshold value. When this determination is Yes, it is determined that the engine 1 is in a high-load operation state, and the process proceeds to step S11. If this determination is No, it is determined that the engine 1 is in a low-load operation state, and the process proceeds to step S12.

  In step S11, the urea water injection amount is set to “normal injection amount”, and then this process ends. The normal injection amount is a preset injection amount corresponding to the high load operation state.

In step S12, the NH ₃ adsorption amount in the first selective reduction catalyst 231 is calculated as an estimated amount. FIG. 5 is a diagram showing a flow for calculating an estimated value of the NH ₃ adsorption amount. The estimated value V of the NH ₃ adsorption amount is based on the supplied NH ₃ amount V1, the NH ₃ amount V2 consumed in the NO _x reduction reaction, and the NH ₃ amount V3 slipping the first selective reduction catalyst 231. It is calculated by the following formula. The symbol Σ in the following equation means that the estimated value Vs is obtained as an integrated value per unit time. In FIG. 5, the loop 37 of the block “1 / Z” means that this integration is performed.

Vs = Σ [V1-V2-V3]

The supplied NH ₃ amount V1 is calculated from the amount of urea water injected from the injector 253 using the hydrolysis reaction equation shown in FIG. The NH ₃ amount V2 consumed in the NO _x reduction reaction is determined as follows. It is converted into the amount of NO and NO ₂ amount using the NO _x conversion map the amount of NO _x detected by the NO _x sensor 28 is stored in advance in ECU3 memory. Similarly, purification (reduction) rates F1 and F2 for NO and NO ₂ are obtained from the NO _x purification rate map stored in the memory of the ECU 3. F1, F2 and NO amount, by multiplying each of the NO ₂ amount, and calculates the reduced amount of NO and reducing NO ₂ amount. For each of the three reduction reaction equations (1) to (3) in FIG. 5, the amount of NH ₃ corresponding to the amount of reduced NO and the amount of reduced NO ₂ is determined. The total amount of NH ₃ obtained for each formula is the NH ₃ amount V2 consumed in the NO _x reduction reaction.

The NH ₃ amount V3 slipping the first selective reduction catalyst 231 is obtained as the NH ₃ weight by multiplying the NH ₃ concentration detected by the NH ₃ sensor 26 by a parameter such as the volume of the selective reduction catalyst obtained in advance.

In step S13, it is determined whether or not the calculated NH ₃ adsorption amount is equal to or less than the adsorption lower limit target amount. The adsorption lower limit target amount is the adsorption lower limit target amount shown in FIG. 3B, and is the minimum requirement for reducing NO _x in the low load operation state such as idling indicated by reference numeral 35 in FIG. This corresponds to the amount of NH ₃ storage. If this determination is Yes, the process proceeds to step S14. If this determination is No, this process ends.

In step S14, it is determined whether or not the calculated NH ₃ adsorption amount is equal to or less than the adsorption target amount. The adsorption target amount is the NH ₃ adsorption target amount shown in FIG. If this determination is Yes, the process proceeds to step S15. When this determination is No, the process proceeds to step S16, and the urea water injection amount is set to “0”, and then this process ends.

In step S15, the urea water injection amount is set to the “injection accuracy lower limit injection amount”, and then this process ends. The injection accuracy lower limit injection amount corresponds to the lower limit value that can ensure the injection accuracy of urea water from the injector 253 already described in FIG .

  The normal injection amount set in step S11, the injection accuracy lower limit injection amount set in step S15, or the zero injection amount set in step S16 is sent as a part of the control signal from the ECU 3 to the injector 253, respectively. The injector 253 injects urea water into the exhaust passage 11 at the timing and the injection amount indicated by the control signal from the ECU 3.

The above-described embodiment is an example, and the present invention is not limited to this. The present invention is applicable to an engine having an arbitrary number of cylinders. The present invention can also be applied to an engine such as a direct injection gasoline engine. Furthermore, the present invention can be applied not only to the urea water supply that is the above-described embodiment but also to the case where gaseous NH ₃ is directly supplied as the reducing agent supply. For example, when using the embodiment of FIG. 1, NH ₃ gas can be directly supplied by using an NH ₃ gas supply device instead of the urea water injection device 25.

Claims (1)

An exhaust purification device for an internal combustion engine,
A selective reduction catalyst provided in an exhaust passage of the internal combustion engine for capturing NH ₃ as a reducing agent and reducing NO _X flowing through the exhaust passage using the captured NH ₃ ;
Reducing agent supply means for supplying the reducing agent to the upstream side of the selective reduction catalyst in the exhaust passage;
Means for calculating the amount of NH ₃ in the selective reduction catalyst;
Reducing agent supply amount setting means for setting the supply amount of the reducing agent by the reducing agent supply means according to the calculated NH ₃ amount and the operating state of the internal combustion engine,
In the case where the rotational speed of the internal combustion engine is not more than a threshold value,
The reducing agent supply amount setting means sets the reducing agent supply amount to zero and sets the reducing agent supply amount to zero when the calculated NH ₃ amount is equal to or larger than an adsorption target amount smaller than the NH ₃ adsorption possible amount. Thereafter, when the calculated NH ₃ amount reaches the minimum NH ₃ adsorption lower limit target amount necessary for reducing NO _X in the idling operation state, the supply accuracy of the reducing agent can be ensured. An exhaust purification device for an internal combustion engine, which is set to an injection accuracy lower limit injection amount corresponding to a limit amount .

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