JP3911917B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly to an exhaust gas purification apparatus for an internal combustion engine that is suitable for use in a lean combustion internal combustion engine having an occlusion-type NO _x catalyst.
[0002]
[Prior art]
Currently, a storage-type NO _X catalyst (trap type NO _X catalyst) has been developed that purifies NO _X in exhaust gas by storing NO _X on the catalyst in an oxygen-excess atmosphere in which oxygen in the exhaust gas becomes excessive. ing. In a lean combustion internal combustion engine, this NO _x catalyst is provided to purify NO _x during lean combustion.
[0003]
The storage-type NO _x catalyst oxidizes NO in exhaust gas to generate nitrates in an oxidizing atmosphere (oxygen concentration excess atmosphere), thereby storing NO _x , while in a reducing atmosphere (oxygen concentration decreasing atmosphere), Conversely, a function of releasing the occluded NO _X. Therefore, for example, in the disclosed in Japanese Patent No. 2586738 art, by a reducing atmosphere surrounding atmosphere of the NO _X catalyst at appropriate time intervals, to release NO _X occluding on the catalyst, thereby, NO _X The catalyst is prevented from being saturated to ensure NO _x storage performance. Then, release the NO _X, with its reduction function if (in the case of the three-way catalyst unitary) which NO _X catalyst has also reduced function, NO _X catalyst is provided downstream in the case having no reducing function three The original catalyst is used to purify together with HC and CO to prevent discharge into the atmosphere.
[0004]
[Problems to be solved by the invention]
By the way, the purification capacity of the catalyst greatly depends on the temperature, and when the catalyst temperature is lower than the activation temperature (for example, the temperature at which the purification efficiency reaches 50%), the sufficient purification capacity cannot be exhibited. Then, the activation temperature depends on the type of catalyst, for example, in the NO _X catalyst and the three way catalyst, towards the activation temperature of the three-way catalyst may be higher than the activation temperature of the NO _X catalyst. Even when towards the activation temperature of the three-way catalyst is lower than the activation temperature of the NO _X catalyst is provided downstream of the three-way catalyst NO _X catalyst when the NO _X catalyst and the three way catalyst is in a different type Therefore, the inflowing exhaust gas temperature is low, the catalyst temperature is relatively low, and the NO _x catalyst may reach the activation temperature first. Furthermore, even in the case of a three-way catalyst integrated NO _x catalyst, the activation temperature at which the reduction function is exhibited may be higher than the temperature at which NO _x release starts.
[0005]
Thus, in the prior art, before the difference in degree of increase in the activation temperature differences and catalyst temperature, the NO _X reduction capacity of the three-way catalyst (or NO _X catalyst having a reducing function) is exerted, NO _X may NO _X occluded from the catalyst is released, the released NO _X is without being purified, there is fear that is directly discharged to the atmosphere.
[0006]
The present invention was devised in view of such problems, and it is possible to prevent NO _x released from the NO _x catalyst from being discharged into the atmosphere without being reduced. An object is to provide an apparatus.
[0007]
[Means for Solving the Problems]
Therefore, in the exhaust purification system of an internal combustion engine of the present invention, a storage capacity of occluding NO _X in the exhaust gas when the oxygen concentration in the exhaust gas excess, NO _X in which acidity concentration in the exhaust gas is occluded when it lowered and NO _X catalyst device which exhaust gas flows constantly discharged from the internal combustion engine and having an emission ability to release NO _X released provided downstream of the NO _X catalyst device is the NO _X catalyst device NO _X the provided a chromatic cheat device and a reducing ability may be reduced at different temperatures in an exhaust passage of the internal combustion engine is a temperature capable of releasing, occlusion amount of the NO _X is in the exhaust gas by NO _X release means when the approaching saturation NO _x is released from the NO _x catalyst device by reducing the oxygen concentration, but prohibited if the temperature of the catalyst device detected or estimated by the temperature sensing means is lower than the temperature at which the catalyst device can exert its reducing ability To the means Further, the operation of the NO _x releasing means is prohibited.
[0008]
Incidentally, NO _X catalyst device and the catalyst device, reducing the occlusion-type NO _X catalyst the oxygen concentration in the exhaust gas to absorb NO _X when excessive, provided downstream of the occlusion-type NO _X catalyst toxic substances in the exhaust gas be composed of the to be three-way catalysts, or by absorbing the NO _X when the oxygen concentration is excessive in the exhaust gas, three-way is oxygen concentration in the exhaust gas for reducing the NO _X occluded when the drops A catalyst-integrated storage-type NO _x catalyst is preferable.
[0009]
Further, it is preferable to provide a temperature raising means for forcibly raising the temperature of the catalyst when the catalyst does not reach a temperature at which the reduction ability can be exhibited even after a predetermined time has elapsed since the start of the operation of the internal combustion engine. In the case of an internal combustion engine, it is preferable to use two-stage combustion in which additional injection is performed during the expansion stroke and burned to raise the temperature of the catalyst.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 to 4 show an exhaust purification device for an internal combustion engine as an embodiment of the present invention, and FIG. 1 shows a configuration of a lean combustion internal combustion engine to which the exhaust purification device for the internal combustion engine is applied. .
[0011]
As shown in FIG. 1, an engine (lean combustion internal combustion engine) 1 includes a high-pressure injection valve (injector) 7 at an upper side edge of a combustion chamber 2, and direct injection of fuel into the combustion chamber 2. It is comprised as an engine (cylinder injection engine). An ignition plug 5 is provided at the center of the combustion chamber 2 and an intake passage 3 and an exhaust passage 4 communicate with each other. The intake passage 3 is provided with an air cleaner and an electronically controlled throttle valve (ETV) 6 (not shown) sequentially from the upstream side, and the exhaust passage 4 is provided with an exhaust purification device 8 for exhaust gas purification and the illustrated one from the upstream side. There is no muffler. An EGR device 9 that recirculates the exhaust gas is provided between the exhaust passage 4 and the intake passage 3.
[0012]
The exhaust purification device 8 is a combination of an occlusion-type NO _x catalyst (hereinafter simply referred to as “NO _x catalyst”) 8A and a three-way catalyst 8B. The NO _x catalyst 8A is disposed on the upstream side, and the downstream side. A three-way catalyst 8B is arranged. In the exhaust gas purifying device 8, oxidizing the NO _X atmosphere (i.e., oxygen-rich atmosphere) occluded by NO _X catalyst 8A to function in, and released from the NO _X catalyst 8A in a reducing atmosphere, the NO _X catalyst 8A and the three-way catalyst 8B NO _x is reduced. However, since here it is a setting not so high NO _X reduction function of the NO _X catalyst 8A is mainly adapted to reduce NO _X by a three-way catalyst 8B. Further, under the stoichiometric air-fuel ratio is adapted to purify CO in the exhaust gas, HC, and NO _X by the function of the three-way catalyst 8B. In addition, a proximity catalyst (for example, a three-way catalyst) may be separately provided at a position close to the engine 1 upstream of the exhaust purification device 8 to mainly perform exhaust gas purification at the time of starting.
[0013]
Further, in order to control the engine 1, an electronic control unit (ECU) 20 and various sensors are provided. As sensors provided in the engine 1, first, on the intake passage 3 side, an air flow sensor 10 for detecting an intake air amount is provided in an air cleaner provided portion, and an ETV 6 is provided in an ETV 6 provided portion. There is provided a throttle sensor 11 for detecting the opening degree. Further, an O ₂ sensor 13 for detecting the oxygen concentration in the exhaust gas and a high temperature sensor 14 for detecting the exhaust temperature are provided on the exhaust passage 4 side in the upstream portion of the exhaust purification device 8. Further, as other sensors, a crank angle sensor 15 that outputs a signal in synchronization with rotation of a crankshaft (not shown) is provided.
[0014]
The function of the ECU 20 will be described. As shown in the functional block diagram of FIG. 2, the ECU 20 includes an injection control means 21, an ETV opening control means 22, an ignition timing control means 23, and an EGR opening control means 24 as its functional elements. The air-fuel ratio, the ignition timing, and the EGR opening are controlled according to the operating state. For example, in a low-load operation region, lean operation is performed by increasing the air-fuel ratio to improve fuel efficiency.
[0015]
By the way, if the NO _X catalyst 8A continues to occlude NO _x in the lean operation state, it eventually reaches a saturated state, and NO _x that cannot be occluded is released into the atmosphere. Therefore, in the ECU 20, the NO _x catalyst 8A is occluded by the NO _x catalyst 8A by the start determination means 25 and the NO _x release control means (NO _x release means) 26 which are functional elements before the NO _x catalyst 8A reaches the saturated state. release of NO _X, thereby performing the reduction. Hereinafter, the start / cancel determination of the NO _X released by the start determination unit 25, and the NO _X release control by the NO _X release controlling means 26 will be described.
[0016]
The start determination means 25 determines that NO _X release starts when both the following first condition and second condition are satisfied. First, the first condition is that the catalyst temperature Th of the three-way catalyst 8B estimated by the catalyst temperature estimation means (temperature detection means) 27 is a predetermined reference temperature [the activation temperature (for example, the temperature at which the purification efficiency reaches 50%), For example, about 250 to 300 ° C.] Th ₀ or more. This is a condition for prohibiting NO _X release in a state where the reduction function of the three-way catalyst 8B is not sufficiently exerted. Here, the start determining means 25 functions as a prohibiting means. In the catalyst temperature estimating means 27, the relationship between the exhaust gas temperature detected by the high temperature sensor 14 and the catalyst temperature of the three-way catalyst 8B is obtained in advance by experiment and stored in a map, and the detected exhaust gas temperature is corrected by the map. Thus, the catalyst temperature Th of the three-way catalyst 8B is estimated.
[0017]
The second condition is that the lean operation continuation time T _L reaches a predetermined time (for example, 30 seconds) T _L0 . If the lean operation time is too long, the catalyst temperature will decrease and the purification efficiency of HC will decrease, and if it is too short, the fuel consumption will deteriorate, but the time ratio occupied by the air-fuel ratio tailing part that does not contribute to NO _X release will increase. Set to an appropriate value that satisfies
[0018]
When the start determination unit 25 makes a start determination, the NO _X release control unit 26 starts releasing NO _X. Hereinafter, the NO _X release control will be specifically described. NO _X release control, as shown in FIG. 3, is composed of a rich mode and S-F / B (stoichiometric feedback) mode and two control modes, the NO _X release controlling means 26, NO _X released Along with the control start determination, first, NO _X release control in the rich mode is started.
[0019]
In the rich mode, by setting the air-fuel ratio to be richer than the stoichiometric air-fuel ratio (stoichio), the ambient atmosphere of the NO _x catalyst 8A is changed to a reducing atmosphere (that is, an oxygen-deficient state) to an excess CO atmosphere, and the NO _x catalyst 8A This is a control mode for releasing the stored NO _x and generating a large amount of HC and CO as reducing agents to promote the reduction of NO _x in the three-way catalyst 8B. For this reason, the NO _X release control means 26 sets the target air-fuel ratio to the rich side, and at the same time, sets the target air-fuel ratio to the throttle opening, ignition timing, and fuel injection timing according to the set target air-fuel ratio. 22, the ETV 6, the spark plug 5, and the injector 7 are appropriately controlled via the ignition timing control means 23 and the injection control means 21.
[0020]
However, torque fluctuation occurs when the air-fuel ratio is suddenly switched from lean to rich, so that the target air-fuel ratio is not set rich with start determination, but is gradually set to the rich side by tailing processing. It has become. When the target air-fuel ratio is enriched to a predetermined air-fuel ratio (for example, stoichiometric), in addition to fuel injection by normal rich release control, for a very short time (for example, about 0.1 to 1 second), Expansion stroke injection is performed.
[0021]
Release characteristics of the NO _X of the NO _X catalyst 8A, since has been initially most emission ambient atmosphere becomes a reducing atmosphere air-fuel ratio reaches the stoichiometric number, the properties such as decreasing gradually, NO _X A reducing agent for reducing NO is most needed at the beginning of NO _X release. Therefore, fuel injection is performed after the expansion stroke in which the fuel does not contribute to combustion (preferably from the latter stage of the expansion stroke to the exhaust stroke), thereby generating a large amount of unburned gas and supplying the reducing agent mainly to the three-way catalyst 8B. To do. In this expansion stroke injection, the target air-fuel ratio is set to a richer side than the target air-fuel ratio in the normal rich mode, the fuel injection time is set according to the set target air-fuel ratio, and the fuel injection timing is set to the expansion stroke. It is set to a predetermined time thereafter.
[0022]
After completion of the expansion stroke injection returns to the normal rich mode, NO _X emission control start of a predetermined time in the rich mode (e.g., about 1-5 seconds) T _R after, NO _X in the S-F / B mode Shift to release control. As described above, since the NO _X release from NO _X catalyst 8A gradually decreases after the start of NO _X release, increase in fuel consumption and also reduces the amount of reducing agent in accordance with the decrease in the discharge amount of the NO _X In order to prevent this, NO _x release is performed with the air-fuel ratio stoichiometric or slightly rich. Incidentally, changing suddenly air-fuel ratio at the time of transition from the rich mode to the S-F / B mode, the torque fluctuation is generated, so that gradually switched gradually stoichiometric side from a predetermined time before T _R elapsed . In S-F / B mode, NO _X release controlling means 26, with the feedback integral gain is the air-fuel ratio is set to a direction becomes slightly richer than stoichiometric or stoichiometric, so that the torque step of the rich mode does not occur The ignition timing and the throttle opening are set to the fuel injection timing, and the fuel injection timing is set according to the ignition timing and the throttle opening.
[0023]
The NO _X release control in the SF / B mode is performed until a predetermined time (SF / B release time) T _S elapses from the start. The SF / B release time T _S is a time K _A × T _{L obtained} by multiplying the lean operation duration T _L by a coefficient K _A determined by the intake air amount (ie, T _S = K _A × T _L). ), the longer the lean operation continuation time T _L time, also, the greater the intake air amount, and a long time so as to perform the NO _X release control in the S-F / B mode. Then, after the S-F / B release time T _S, it proceeds to the lean operation again, in order to prevent a torque variation due to abrupt change of the air-fuel ratio here, S-F / B release time T _S It gradually switches to the lean side from before the lapse of time.
[0024]
Further, the NO _x release control means 26 is configured to cancel the NO _x release control even in the middle of the control in a predetermined case. Release condition of the NO _X release control are among 3 below, is adapted to be released when satisfied even any one. First, the first condition, the catalyst temperature Th of the three-way catalyst 8B estimated by the catalyst temperature estimating means 26, is to be less than the reference temperature Th _0, NO _X is released from the NO _X catalyst 8A, three This is a condition for preventing the original catalyst 8B from being released into the atmosphere as it is inactive. The second condition is a case where the fuel is cut at the time of deceleration. In this case, since the fuel is not supplied, NO _x release cannot be performed inevitably. The third condition is when the engine is in a rich operation state, for example, when the driver depresses the accelerator. In this case, NO _X release is naturally performed without performing NO _X release control.
[0025]
An exhaust purification system of an internal combustion engine as an embodiment of the present invention, which is configured as described above, for example, as shown in the flowchart of FIG. 4 NO _X release control from the NO _X catalyst 8A is performed.
First, in step S100, it is determined whether or not the flag F _S is 1. When the flag F _S is 1, the routine proceeds to step S200 and the NO _X release control in the SF / B mode is started, and when the flag F _S is 0, the routine proceeds to step S110 and the start determination means 25. The NO _x release start determination is performed. That is, it is determined whether or not the catalyst temperature Th of the three-way catalyst 8B estimated by the catalyst temperature estimating means 27 is equal to or higher than a predetermined reference temperature (active temperature) Th ₀ (step S110), and the lean operation duration time T _L is predetermined. it is determined whether or not time T _L0 elapsed (step S120), step S110, S120 when the conditions are both satisfied of, NO _X catalyst 8A in the occluded NO _X desorbed in the subsequent stage three-way catalyst In order to purify at 8B, the NO _X release control means 26 starts NO _X release control (rich release control) (step S130).
[0026]
The reduction, together with the NO _X release control, the catalyst temperature Th of the three-way catalyst 8B estimated by the catalyst temperature estimating means 27 at step S140 as to whether or not less than a predetermined reference temperature Th _0, the fuel in step S150 is cut whether time, whether entered into enriched operation in step S160, step S170 in whether a predetermined time has elapsed T _R from the control start of the rich mode respectively determined. When any of the conditions of steps S140~S160 is satisfied cancels the NO _X release control (step S190), and the condition of step S170 does also not satisfied any of the conditions of step S140~S160 it is satisfied In that case, a flag F _s is set to shift to the SF / B mode (step S180).
[0027]
When the flag F _S is set (F _S = 1), the NO _X release control means 26 starts NO _X release control in the SF / B mode (step S200). The reduction, together with the NO _X release control, the catalyst temperature Th of the three-way catalyst 8B estimated by the catalyst temperature estimating means 27 at step S210 as to whether or not less than a predetermined reference temperature Th _0, the fuel in step S220 is cut In step S230, it is determined whether or not an enrichment operation has been started. In step S240, it is determined whether or not a predetermined time T _s has elapsed since the start of control. When any of the conditions of steps S210~S230 is satisfied cancels the NO _X release control (step S250), and the condition of step S240 does also not satisfied any of the conditions of step S210~S230 it is satisfied in this case, it terminates the NO _X release control in the S-F / B mode as NO _X emission is completed, and resets the flag F _s (step S260).
[0028]
Thus, according to the exhaust emission control device of the internal combustion engine, does not perform the NO _X release to a temperature where the reduction function of the three-way catalyst 8B is sufficiently exhibited, NO _X released from the NO _X catalyst There is an advantage that it can be prevented from being discharged into the atmosphere without being reduced.
Note that the estimation method of the catalyst temperature Th of the three-way catalyst 8B is not limited to the estimation method described in the above embodiment. For example, in addition to exhaust gas temperature, the relationship between parameters such as engine load, engine speed, vehicle speed, water temperature, elapsed time after start-up, and catalyst temperature of the three-way catalyst 8B is obtained by experiment and stored in a map. The catalyst temperature Th may be estimated from the parameters. Further, the relationship of the temperature of the three-way catalyst 8B with respect to the exhaust gas temperature detected by the high temperature sensor 14 may be estimated by the control logic to estimate the catalyst temperature Th.
[0029]
Further, in the above-described embodiment, the exhaust purification device 8 is provided with the three-way catalyst 8B downstream of the NO _x catalyst 8A, but the three-way catalyst integrated type NO _x catalyst having a high NO _x reduction function. You may have only. In this case, the catalyst temperature of the NO _X catalyst is estimated (or detected), and NO _X release is started when the catalyst temperature at which the reduction function is exerted is reached.
[0030]
Further, in the above-described embodiment, as one of the start conditions for the NO _X release control, the lean operation continuation time _TL is required to elapse for a predetermined time _TL0. The operation mode may be switched by (switching from lean operation to stoichiometric feedback operation). According to this condition, it is possible to reduce fuel consumption deterioration and torque fluctuation caused by switching the air-fuel ratio from lean to rich when the driver does not intend.
[0031]
Further, if the catalyst temperature Th does not reach the predetermined reference temperature Th ₀ even after a predetermined time has elapsed from the start of operation of the engine 1, the catalyst temperature Th may be forcibly increased. In particular, in the case of a direct injection internal combustion engine as in the above-described embodiment, it is possible to use two-stage combustion in which additional injection is performed during the expansion stroke and recombustion is performed in the combustion chamber 2 or the exhaust passage 4. This is effective in terms of cost and temperature rise effect.
[0032]
【The invention's effect】
As described above in detail, according to the exhaust gas purification apparatus for an internal combustion engine of the present invention, NO _x released from the catalyst device can be reliably prevented from being discharged into the atmosphere without being reduced. There are advantages.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of a lean combustion internal combustion engine according to an exhaust gas purification apparatus for an internal combustion engine as one embodiment of the present invention.
FIG. 2 is a functional block diagram according to the present embodiment.
FIG. 3 is a time chart of NO _X release control according to the present embodiment.
FIG. 4 is a flowchart of NO _X release control according to the present embodiment.
[Explanation of symbols]
1 engine (internal combustion engine)
8A occlusion-type NO _X catalyst (NO _X catalyst device)
8B Three-way catalyst (catalyst device)
25 Start determination means (prohibition means)
26 NO _X release control means (NO _X release means)
27 Catalyst temperature estimation means (temperature detection means)

Claims (4)

Provided in an exhaust passage of an internal combustion engine, release to release when the storage capacity of the oxygen concentration in the exhaust gas to occlude NO _X in the exhaust gas when excess, the occluded NO _X is acidity concentration in the exhaust gas was reduced and NO _X catalyst device which exhaust gas flows constantly discharged from the internal combustion engine, the NO _X released provided downstream of the NO _X catalyst device is the NO _X catalyst device capable of releasing NO _X which has a capability a catalyst device for perforated with a reducing ability may be reduced at different temperatures of the temperature,
Temperature detecting means for detecting or estimating the temperature of the catalyst device;
And NO _X emission means for reducing the oxygen concentration in the exhaust gas when releasing NO _X from the NO _X catalyst device,
And a prohibiting means for prohibiting the operation of the NO _x releasing means when the catalyst temperature detected or estimated by the temperature detecting means is lower than a temperature at which the reducing ability of the catalytic device can be exhibited. An exhaust purification device for an internal combustion engine. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the NO _x catalyst device is arranged upstream of the catalyst device and in the exhaust passage. If the temperature of the catalytic device detected by the temperature detecting means does not reach the temperature at which the catalytic device can reduce NO _x even after a predetermined time has elapsed from the start of operation of the internal combustion engine, the temperature of the catalytic device is set. an exhaust purification system of an internal combustion engine according to claim 1 or 2 forcibly warmed, characterized in that the catalytic converter is provided with a heating means allowed to a temperature capable of reducing NO _X.   The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the temperature raising means causes the internal combustion engine to burn in two stages to raise the temperature of the catalyst device.

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