Patent Application
20130323130
The present invention relates to an exhaust purification system of an internal combustion engine.
Known in the art is a catalyst device which uses alumina as a carrier coat material and which carries silver oxide (for example, refer to Japanese Patent No. 2801423, Japanese Patent Publication No. 2008-240568, and Japanese Patent Publication No. 2009-160548). Such a silver-alumina-based catalyst device absorbs the NO2 in the exhaust gas when the air-fuel ratio of the exhaust gas is leaner than a stoichiometric air-fuel ratio and releases the NO2 which was adsorbed when it becomes a set temperature. When the silver-alumina-based catalyst device becomes the set temperature, the NOX catalyst device which was arranged at the downstream side is activated and can purify by reduction the NO2 which flows out from the silver-alumina-based catalyst device.
The above-mentioned silver-alumina-based catalyst device not only adsorbs the NO2 in the exhaust gas, but also adsorbs the NO in the exhaust gas. However, the adsorbed NO ends up being released at a temperature lower than the set temperature. At this time, the NOX catalyst device which is arranged at the downstream side is not activated, so the NO which flows out from the silver-alumina-based catalyst device ends up being discharged into the atmosphere without being purified by reduction at the NOX catalyst device.
Therefore, an object of the present invention is to provide an exhaust purification system of an internal combustion engine with a silver-alumina-based catalyst device wherein when the temperature is lower than the set temperature at which NO2 is released, the amount of NO which flows out from the silver-alumina-based catalyst device is reduced.
The exhaust purification system of an internal combustion engine as set forth in claim 1 of the present invention is characterized in that the system is provided with a silver-alumina-based catalyst device which is arranged in an engine exhaust system, when an air-fuel ratio of exhaust gas is leaner than a stoichiometric air-fuel ratio, the silver-alumina-based catalyst device releases adsorbed NO2 at a first set temperature and releases adsorbed NO at a second set temperature which is lower than the first set temperature, when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio and the silver-alumina-based catalyst device is a third set temperature which is lower than the second set temperature, a temperature rise of the silver-alumina-based catalyst device is suppressed to maintain the silver-alumina-based catalyst device near the third set temperature, and at least part of the NO which is adsorbed at the silver-alumina-based catalyst device is oxidized to NO2 and adsorbed at the silver-alumina-based catalyst device, then the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted.
The exhaust purification system of an internal combustion engine as set forth in claim 2 of the present invention is characterized in that in the exhaust purification system of an internal combustion engine as set forth in claim 1, when it is judged that there is no longer any NO which is adsorbed at the silver-alumina-based catalyst device, the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted.
The exhaust purification system of an internal combustion engine as set forth in claim 3 is characterized in that in the exhaust purification system of an internal combustion engine as set forth in claim 1 or 2, an NO reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted or after the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted, if it is judged that the NO reduction catalyst device is activated and it is judged that the NO2 amount which is adsorbed at the silver-alumina-based catalyst device has reached a set amount, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio.
The exhaust purification system of an internal combustion engine as set forth in claim 4 of the present invention is characterised in that in the exhaust purification system of an internal combustion engine as set forth in claim 1 or 2, an NO reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted or after the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted, if the silver-alumina-based catalyst device has been raised in temperature to the first set temperature or more, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio.
The exhaust purification system of an internal combustion engine as set forth in claim 5 of the present invention is characterised in that in the exhaust purification system of an internal combustion engine as set forth in claim 1, an NO reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when it is judged that the silver-alumina-based catalyst device is activated if lifting the suppression of the temperature rise of the silver-alumina-based catalyst device and it is judged that the NO2 amount which is adsorbed at the silver-alumina-based catalyst device has reached a set amount, the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted and the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio.
The exhaust purification system of an internal combustion engine as set forth in claim 6 of the present invention is characterized in that in the exhaust purification system of an internal combustion engine as set forth in any one of claims 3 to 5, when making the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made a set rich air-fuel ratio for exactly a set time, an NO2 release rate is set from the silver-alumina-based catalyst device at that time, and the NOX reduction catalyst device is raised in temperature so that the NOX reduction rate of the NOX reduction catalyst device becomes the NO2 release rate or more.
The exhaust purification system of an internal combustion engine as set forth in claim 7 of the present invention is characterized in that in the exhaust purification system of an internal combustion engine as set forth in claim 1, an NOX reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when it is judged that the NOX reduction catalyst device is activated if lifting the suppression of the temperature rise of the silver-alumina-based catalyst device, the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted and the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio.
According to the exhaust purification system of an internal combustion engine according to the present invention as set forth in claim 1, the system is provided with a silver-alumina-based catalyst device which is arranged in an engine exhaust system, when an air-fuel ratio of exhaust gas is leaner than a stoichiometric air-fuel ratio, the silver-alumina-based catalyst device releases adsorbed NO2 at a first set temperature and releases adsorbed NO at a second set temperature which is lower than the first set temperature, when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio and the silver-alumina-based catalyst device is a third set temperature which is lower than the second set temperature, a temperature rise of the silver-alumina-based catalyst device is suppressed to maintain the silver-alumina-based catalyst device near the third set temperature, and at least part of the NO which is adsorbed at the silver-alumina-based catalyst device is oxidized to NO2 and adsorbed at the silver-alumina-based catalyst device, then the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted. Due to this, it is possible to reduce the amount of NO which is adsorbed at the silver-alumina-based catalyst device. Even when suppression of the temperature rise of the silver-alumina-based catalyst device is lifted and the silver-alumina-based catalyst device becomes the second set temperature whereby the silver-alumina-based catalyst device releases NO, the amount of NO which flows out from the silver-alumina-based catalyst device can be reduced.
According to the exhaust purification system of an internal combustion engine as set forth in claim 2 of the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1, when it is judged that there is no longer any NO which is adsorbed at the silver-alumina-based catalyst device, the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted. Due to this, even when the silver-alumina-based catalyst device becomes the second set temperature, the silver-alumina-based catalyst device releases almost no NO.
According to the exhaust purification system of an internal combustion engine as set forth in claim 3 of the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1 or 2, an NOX reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted or after the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted, if it is judged that the NOX reduction catalyst device is activated and it is judged that the NO2 amount which is adsorbed at the silver-alumina-based catalyst device has reached a set amount, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio. Due to this, before the silver-alumina-based catalyst device can no longer adsorb NO2, it is possible to make the air-fuel ratio of the exhaust gas rich so as to make the silver-alumina-based catalyst device release NO2 even at a temperature lower than the first set temperature. The thus released NO2 can be purified by reduction at the activated NOX reduction catalyst device which is arranged at the downstream side of the silver-alumina-based catalyst device using the reducing substances in the rich air-fuel ratio exhaust gas.
According to the exhaust purification system of an internal combustion engine as set forth in claim 4 of the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1 or 2, an NOX reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted or after the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted, if the silver-alumina-based catalyst device has been raised in temperature to the first set temperature or more, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio. Due to this, the silver-alumina-based catalyst device reaching the first set temperature releases NO2. The thus released NO2 can be purified by reduction at the activated NOX reduction catalyst device which is arranged at the downstream side of the silver-alumina-based catalyst device using the reducing substances in the rich air-fuel ratio exhaust gas.
According to the exhaust purification system of an internal combustion engine as set forth in claim 5 of the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1, an NOX reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when it is judged that the silver-alumina-based catalyst device is activated if lifting the suppression of the temperature rise of the silver-alumina-based catalyst device and it is judged that the NO2 amount which is adsorbed at the silver-alumina-based catalyst device has reached a set amount, the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted and the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio. Due to this, before the silver-alumina-based catalyst device can no longer adsorb NO2, it is possible to lift the suppression of the temperature rise of the silver-alumina-based catalyst device and make the air-fuel ratio of the exhaust gas rich so as to enable the silver-alumina-based catalyst device to release NO2 even at a temperature lower than the first set temperature. The thus released NO2 can be purified by reduction at the activated NO reduction catalyst device which is arranged at the downstream side of the silver-alumina-based catalyst device using the reducing substances in the rich air-fuel ratio exhaust gas.
According to the exhaust purification system of an internal combustion engine as set forth in claim 6 of the present invention, in the exhaust purification system of an internal combustion engine as set forth in any one of claims 3 to 5, when making the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made a set rich air-fuel ratio for exactly a set time, an NO2 release rate is set from the silver-alumina-based catalyst device at that time, and the NO reduction catalyst device is raised in temperature so that the NOX reduction rate of the NO reduction catalyst device becomes the NO2 release rate or more. Due to this, almost all of the NO2 which is released from the silver-alumina-based catalyst device can be purified by reduction at the NO reduction catalyst device.
According to the exhaust purification system of an internal combustion engine as set forth in claim 7 of the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1, an NOX reduction catalyst device is arranged at a downstream side of the silver-alumina-based catalyst device, and, when it is judged that the NOX reduction catalyst device is activated if lifting the suppression of the temperature rise of the silver-alumina-based catalyst device, the suppression of the temperature rise of the silver-alumina-based catalyst device is lifted and the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device is made richer than the stoichiometric air-fuel ratio. Due to this, even if suppression of the temperature rise of the silver-alumina-based catalyst device is lifted, the silver-alumina-based catalyst device becomes the second set temperature or more, and NO is released, this can be purified by reduction at the activated NOX reduction catalyst device which is arranged at the downstream side of the silver-alumina-based catalyst device using the reducing substances in the rich air-fuel ratio exhaust gas.
The silver-alumina-based catalyst device 2 uses alumina as a carrier coating material and carries silver oxide. The NO2 in the exhaust gas can be adsorbed as silver nitrate and, when the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device 2 is leaner than the stoichiometric air-fuel ratio, if the first set temperature (about 300° C.) is reached, the adsorbed NO2 is released. At this time, the NOX reduction catalyst device 3 is sufficiently activated, and the released NO2 can be purified by reduction at the NOX reduction catalyst device 3 by a reducing agent which is fed from the reducing agent feed device 4. In this way, by arranging the silver-alumina-based catalyst device 2 at the upstream side of the NOX reduction catalyst device 3, before the NOX reduction catalyst device 3 is activated, it is possible to sufficiently keep the NO2 in the exhaust gas from being released into the atmosphere.
The silver-alumina-based catalyst device 2, for example, comprises a honeycomb-shaped base material on which an alumina Al2O3 carrier coat layer is formed and carrying silver oxide Ag2O on the alumina carrier coat layer in a ratio of silver 0.2 mol to alumina 200 g (for improving the heat resistance, lanthanum La may also be mixed in).
As the method of preparation of such a catalyst, for example, alumina MI386 (La/Al2O3) powder 1600 g, a binder A520 710.4 g, and water 3600 g are stirred by an atriter for 20 minutes, then the mixture is coated on the base material at 200 g/liter per unit volume. Next, this is fired in the atmosphere at 250° C. for 30 minutes, then fired at 500° C. for 1 hour to form an alumina carrier coat layer on the base material.
On the other hand, ion exchanged water is added to silver nitrate 236.2 g to make it dissolve to give a volume of 1700 cc to thereby prepare a silver nitrate aqueous solution with an Ag concentration of 0.82 mol/liter.
The above-mentioned alumina carrier coat layer is immersed in such a silver nitrate aqueous solution for 30 minutes to cause Ag to be carried at 0.2 mol/liter per unit volume by absorption. Next, a blower type dryer is operated to dry the catalyst for 20 minutes, the catalyst is fired in the atmosphere at 550° C. for 3 hours, then the catalyst is fired at 500° C. for 3 hours while running nitrogen containing 5% of hydrogen at a rate of 7 liters a minute.
In a catalyst which is prepared in this way, the silver oxide Ag2O is exposed from the alumina Al2O3 carrier coat layer. After the NO in the exhaust gas is oxidized to NO2, this can be held well as silver nitrate AgNO3.
The NOX reduction catalyst device 3 can be made a three-way catalyst device. In this case, from the reducing agent feed device 4, as the reducing agent, for example, the fuel is fed. The air-fuel ratio of the exhaust gas in the three-way catalyst device is made rich to purify the NOX by reduction.
Further, the NOX reduction catalyst device 3 can be made a selective reduction type NOX catalyst device which uses ammonia NH3 to selectively purify NOX by reduction. In this case, the reducing agent feed device 4 feeds urea as a reducing agent, for example. At the selective reduction type NOX catalyst device, the urea is hydrolyzed to generate ammonia to purify the NOX by reduction.
Further, if the NOX reduction catalyst device 3 is comprised of an upstream side oxidation catalyst device (carrying platinum Pt, silver Ag, copper Cu, etc.) which can partially oxidize the hydrocarbons in the exhaust gas and a downstream side catalyst device (carrying platinum Pt or rhodium Rh) which purify nitrogen-containing hydrocarbon compounds (amine compounds, isocyanate compounds, and nitroso compounds) generated in the oxidation catalyst device from the partially oxidized hydrocarbons and NOX in the exhaust gas, hydrocarbons (fuel) are fed as a reducing agent from the reducing agent feed device 4 for purifying the NOX. In particular, if the fed hydrocarbons are used to make the air-fuel ratio of the exhaust gas a 15.5 or less lean air-fuel ratio, the nitrogen-containing hydrocarbon compounds become easier to generate in the oxidation catalyst device and substantially all of the NOX in the exhaust gas can be purified.
As explained above, until the silver-alumina-based catalyst device 2 becomes the first set temperature T1, the NO2 in the exhaust gas which is adsorbed as silver nitrate AgNO3 is released when the silver-alumina-based catalyst device 2 becomes the first set temperature T1. At this time, the downstream side NOX reduction catalyst device 3 becomes the activation temperature (about 200° C.) or more, and the released NO2 can be purified by reduction by the NO reduction catalyst device 3.
On the other hand, the NO in the exhaust gas is adsorbed at the silver-alumina-based catalyst device 2, but the NO is adsorbed at the silver-alumina-based catalyst device 2 as silver nitrite AgNO2. The NO which is adsorbed as silver nitrite ends up being released from the silver-alumina-based catalyst device 2 at a second set temperature T2 (about 150° C.) which is lower than the first set temperature T1. At this time, the downstream side NO reduction catalyst device 3 is not sufficiently activated, and the NO which flows out from the silver-alumina-based catalyst device 2 ends up being released into the atmosphere without being purified by reduction at the NOX reduction catalyst device 3.
Due to this, at the second set temperature T2 which is lower than the first set temperature T1 at which NO2 is released, it is desirable to reduce the amount of NO which flows out from the silver-alumina-based catalyst device.
To realize this, the exhaust purification system of an internal combustion engine of the present embodiment is controlled by an electronic control unit (not shown) in accordance with the flow of the first flow chart shown in
First, at step 101, the current temperature TU of the silver-alumina-based catalyst device 2 is estimated. For example, a temperature sensor may be arranged at the immediate downstream side of the silver-alumina-based catalyst device 2 to measure the temperature of the exhaust gas which flows from the silver-alumina-based catalyst device 2 and the measured temperature may be used as the current temperature TU of the silver-alumina-based catalyst device 2. Further, the temperature of the exhaust gas which flows into the silver-alumina-based catalyst device 2 which is estimated based on the current engine operating state may be used as the basis to estimate the current temperature TU of the silver-alumina-based catalyst device 2. Further, the temperature of the silver-alumina-based catalyst device 2 may be measured directly.
Next, at step 102, the NO amount a1 which is discharged from each cylinder per unit time (interval of execution of flow chart) and is newly adsorbed as silver nitrite AgNO2 at the silver-alumina-based catalyst device 2 is estimated based on the current engine operating state. For example, such an NO amount a1 may be mapped for each engine operating state based on experiments and such a map may be utilized for the estimation of step 102.
Next, at step 103, the NO2 amount a2 which is discharged from each cylinder per unit time (interval of execution of flow chart) and is newly adsorbed as silver nitrate AgNO3 at the silver-alumina-based catalyst device 2 is estimated based on the current engine operating state. For example, such an NO2 amount a2 may be mapped for each engine operating state based on experiments and such a map may be utilized for the estimation of step 103.
Next, at step 104, the oxidation amount a3 of the NO adsorbed at the silver-alumina-based catalyst device 2 as silver nitrite AgNO2 to be oxidized and adsorbed as silver nitrate AgNO3 per unit time (interval of execution of flow chart) is estimated on the basis of the current temperature TU of the silver-alumina-based catalyst device 2 which is estimated at step 101 and the NO amount A1 which is adsorbed at the silver-alumina-based catalyst device 2 as silver nitrite AgNO2 (calculated at step 105). For example, such an oxidation amount a3 becomes greater the higher the current temperature TU of the silver-alumina-based catalyst device 2. Further, it becomes greater the greater the NO amount A1 which is adsorbed at the silver-alumina-based catalyst device 2 as silver nitrite AgNO2. The oxidation amount a3 can be mapped by experiments and such a map may be utilized for the estimation of step 104.
Next, at step 105, the NO amount a1 which was estimated at step 102 is added and the oxidation amount a3 which was estimated at step 104 is subtracted so as to calculate the current NO adsorption amount A1 at the silver-alumina-based catalyst device 2. At step 106, the NO2 amount a2 which was estimated at step 103 is added and the oxidation amount a3 which was estimated at step 104 is also added so as to calculate the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2.
Next, at step 107, it is judged if the flag F is “2” or not. The flag F is reset to “0” simultaneously with the engine being stopped. First, the judgment at step 107 is negative and the routine proceeds to step 108. At step 108, it is judged if the flag F is “1”. At first, this judgment is also negative and the routine proceeds to step 109.
At step 109, it is judged if the current temperature TU of the silver-alumina-based catalyst device 2 which was estimated at step 101 has reached the third set temperature T3. The third set temperature T3 is a temperature slightly lower than the second set temperature T2 at which the silver-alumina-based catalyst device 2 releases NO which is adsorbed as silver nitrite (for example, about 130° C.). If the silver-alumina-based catalyst device 2 is the third set temperature T3 or less, the silver-alumina-based catalyst device 2 will not release almost any NO. The third set temperature T3 is preferably made the upper limit temperature at which the silver-alumina-based catalyst device 2 does not release almost any NO. When the judgment at step 109 is negative, the routine ends as is.
Along with the rise of the exhaust gas temperature, the temperature TU of the silver-alumina-based catalyst device 2 gradually rises. When reaching the third set temperature T3, the judgment at step 109 is affirmative. Then at step 110, the secondary air feed device 6 feeds secondary air, the temperature rise of the silver-alumina-based catalyst device 2 is suppressed, and the temperature TU of the silver-alumina-based catalyst device 2 is maintained near the third set temperature T3. The secondary air feed device 6, for example, feeds air which is sucked in from the downstream side of the air cleaner of the intake system (preferably at the upstream side from the air flow meter) to the side immediately upstream of the silver-alumina-based catalyst device 2 of the exhaust system. By feeding a larger amount of air the higher the temperature of the exhaust gas which flows into the silver-alumina-based catalyst device 2, the temperature of the silver-alumina-based catalyst device 2 is maintained near the third set temperature T3.
Even while the secondary air feed device 6 is being used so that the secondary air is fed to the silver-alumina-based catalyst device 2 in this way, at step 101, the temperature TU of the silver-alumina-based catalyst device 2 is estimated, at step 105, the current NO adsorption amount A1 at the silver-alumina-based catalyst device 2 is calculated, and, at step 106, the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2 is calculated.
While the silver-alumina-based catalyst device 2 is low in temperature, the amount a1 of NO which is newly adsorbed at the silver-alumina-based catalyst device 2 as silver nitrite AgNO2 is larger than the amount a3 of oxidation of the NO adsorbed at the silver-alumina-based catalyst device 2 to be oxidized and adsorbed as silver nitrate AgNO3, so the current NO adsorption amount A1 at the silver-alumina-based catalyst device 2 increases. However, if the temperature of the silver-alumina-based catalyst device 2 becomes near the third set temperature T3, the amount a3 of oxidation of the NO adsorbed at the silver-alumina-based catalyst device 2 being oxidized and adsorbed as silver nitrate AgNO3 becomes larger than the amount a1 of NO which is newly adsorbed at the silver-alumina-based catalyst device 2 as silver nitrite AgNO2. The current NO adsorption amount A1 at the silver-alumina-based catalyst device 2 is reduced.
Due to this, by maintaining the silver-alumina-based catalyst device 2 near the third set temperature T3, the current NO adsorption amount A1 at the silver-alumina-based catalyst device 2 gradually decreases. Finally, it becomes substantially 0 and the judgment at step 201 is affirmative. If the judgment at step 201 is affirmative, at step 202, the flag F is set to “1”.
If in this way the flag F is set to “1”, the judgment at step 108 of the first flow chart of
In the second flow chart of
On the other hand, when the temperature TD of the NOX reduction catalyst device 3 is the activation temperature TA or more, the judgment at step 301 is affirmative. At step 302, it is judged if the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2 has reached the set amount AL, for example, the NO2 adsorption limit amount at the silver-alumina-based catalyst device 2. When this judgment is negative, the routine ends as is, while when this judgment is affirmative, at step 302, the flag F is made “2”.
When the temperature TU of the silver-alumina-based catalyst device 2 is the second set temperature T2 or more, NO is not newly adsorbed at the silver-alumina-based catalyst device 2. Further, NO is not adsorbed at the silver-alumina-based catalyst device 2, so is not oxidized to NO2 either. The NO amount a1 at step 102 of the first flow chart and the oxidation amount a3 at step 104 both become zero.
However, even while the NO2 in the exhaust gas is adsorbed at the silver-alumina-based catalyst device 2, the NO in the exhaust gas is oxidized to NO2 and adsorbed. While the flag F is “1”, at step 103 of the first flow chart, the NO2 amount a2 is estimated and the current NO2 adsorption amount A2 of the silver-alumina-based catalyst device 2 which is calculated at step 106 gradually increases and finally reaches the set amount AL.
If the flag F is set to “2” in this way, the judgment of step 107 of the first flow chart of
Next, at step 114, the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2 is reset to “0” and the flag F is set to “1”. Due to this, the flow of the fourth flow chart is repeated. If the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2 becomes the set amount AL, the flag F is again made “2”, rich control is performed, the silver-alumina-based catalyst device 2 is made to release the adsorbed NO2, and this is purified by reduction at the NOX reduction catalyst device 3. In this way, the frequency of rich control is minimized to suppress consumption of the fuel. In the third flow chart, at the instant in the second flow chart where the flag F is made “1” and the suppression of the temperature rise of the silver-alumina-based catalyst device 2 is lifted, the judgment at steps 301 and 302 is affirmative, the flag F is set to “2”, and rich control of step 113 of the first flow chart is sometimes performed.
Due to this, at step 113 of the first flow chart, the above-mentioned rich control is executed and the NO2 which is released from the silver-alumina-based catalyst device 2 when the first set temperature T1 is reached is purified by reduction at the NOX reduction catalyst device 3 which is inevitably activated at this time using the reducing substances in the exhaust gas in the rich control. At this time, the NO2 adsorption amount A2 of the silver-alumina-based catalyst device 2 does not reach the set amount AL, but if becoming the first set temperature T1, the NO2 ends up being released from the silver-alumina-based catalyst device 2, so to prevent release into the atmosphere, rich control is performed to purify this by reduction at the NOX reduction catalyst device 3.
Next, at step 114, the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2 is reset to “0” and the flag F is made “1”. Due to this, the flow of the fifth flow chart is repeated. When the temperature TU of the silver-alumina-based catalyst device 2 becomes the first set temperature T1, the flag F is again made “2”, rich control is performed, and the NO2 which is released from the silver-alumina-based catalyst device 2 is purified by reduction at the NO reduction catalyst device 3. In the fourth flow chart, at the instant in the second flow chart that the flag F is made “1” and the suppression of the temperature rise of the silver-alumina-based catalyst device 2 is lifted, the judgment at step 401 is affirmative, the flag F is set to 2, and rich control of step 113 of the first flow chart is executed.
On the other hand, if lifting the suppression of the temperature rise of the silver-alumina-based catalyst device 2, when the NOX reduction catalyst device 3 becomes the activation temperature TA or more, the judgment at step 501 is affirmative and, at step 502, it is judged if the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2 has reached the set amount AL (for example, the NO2 adsorption limit amount at the silver-alumina-based catalyst device 2) or not. When this judgment is negative, the routine ends as is, but when this judgment is affirmative, at step 503, the feed of the secondary air by the secondary air feed device 6 is stopped, while at the step 504, the flag F is made “2”.
Due to this, at step 113 of the first flow chart, the above-mentioned rich control is executed. Thus the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device 2 is made richer than the stoichiometric air-fuel ratio and even if the temperature of the silver-alumina-based catalyst device 2 is not the first set temperature T1, the silver-alumina-based catalyst device 2 is made to release NO2. In this way, before the silver-alumina-based catalyst device 2 can no longer adsorb NO2, the silver-alumina-based catalyst device is made to release the NO2. The thus released NO2 can be purified by reduction at the NOX reduction catalyst device 3 activated which is arranged at the downstream side of the silver-alumina-based catalyst device 2 using the reducing substances in the rich air-fuel ratio exhaust gas. Further, when lifting the suppression of the temperature rise of the silver-alumina-based catalyst device 2, the NO adsorption amount A1 at the silver-alumina-based catalyst device 2 is not zero, so the silver-alumina-based catalyst device 2 also releases NO, but the released NO can also be purified by reduction at the NOX reduction catalyst device 3 using the reducing substances in the rich air-fuel ratio exhaust gas.
In the fifth flow chart, if lifting the suppression of the temperature rise of the silver-alumina-based catalyst device 2, when the NOX reduction catalyst device 3 becomes the activation temperature TA or more, the judgment at step 501 is affirmative. At this time, regardless of the current NO2 adsorption amount A2 at the silver-alumina-based catalyst device 2, it is possible to stop the feed of the secondary air by the secondary air feed device 6 and set the flag F at “2”.
Due to this, at step 113 of the first flow chart, the above-mentioned rich control is executed, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device 2 is made richer than the stoichiometric air-fuel ratio, and, even if the temperature of the silver-alumina-based catalyst device 2 is not the first set temperature T1, NO2 is made to be released from the silver-alumina-based catalyst device 2. The thus released NO2 can be purified by reduction at the activated NOX reduction catalyst device 3 which is arranged at the downstream side of the silver-alumina-based catalyst device 2 by using the reducing substances in the exhaust gas of the rich air-fuel ratio. Further, NO is also released from the silver-alumina-based catalyst device 2, but the released NO can also be purified by reduction at the NOX reduction catalyst device 3 using the reducing substances in the exhaust gas of the rich air-fuel ratio.
As explained above, the silver-alumina-based catalyst device 2 releases the NO2 when the temperature becomes the first set temperature T1 if the air-fuel ratio of the inflowing exhaust gas is leaner than the stoichiometric air-fuel ratio, releases NO2 also when the temperature becomes lower than the first set temperature T1 if the air-fuel ratio of the inflowing exhaust gas is richer than the stoichiometric air-fuel ratio, and, further, releases NO2 at a lower temperature the smaller the air-fuel ratio of the exhaust gas. That is, at the silver-alumina-based catalyst device 2, it is possible to set an upper limit air-fuel ratio for causing release of NO2 for each temperature. If making the air-fuel ratio smaller than the upper limit air-fuel ratio at each temperature, the rate of release of NO2 becomes faster the smaller the air-fuel ratio.
In this regard, in the rich control of step 113 of the first flow chart, making the NO2 be released from the silver-alumina-based catalyst device 2 as slowly as possible enables easier removal by reduction at the downstream side NOX reduction catalyst device 3. For this reason, the rich air-fuel ratio of the rich control is preferably made the upper limit air-fuel ratio of each temperature TU. However, when the temperature TU of the silver-alumina-based catalyst device 2 is the first set temperature T1, the upper limit air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio, but with this, it is not possible to purify the NO2 by reduction at the downstream side NOX reduction catalyst device 3, so the air-fuel ratio is made a rich one slightly smaller than the stoichiometric air-fuel ratio. Further, the time during which the rich control is performed is preferably made longer the greater the NO2 which is released from the silver-alumina-based catalyst device 2, that is, the greater the NO2 adsorption amount A2 of the silver-alumina-based catalyst device 2, so as to sufficiently purify by reduction the NO2 which is released from the silver-alumina-based catalyst device 2 in the NOX reduction catalyst device 3.
At the rich control, when making the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device 2 richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the exhaust gas which flows into the silver-alumina-based catalyst device 2 is made the set rich air-fuel ratio for exactly the set time. The closer the set rich air-fuel ratio to the upper limit air-fuel ratio at each temperature of the silver-alumina-based catalyst device, the more it is possible to slow the NO2 release rate from the silver-alumina-based catalyst device. In such rich control, the NO2 release rate from the silver-alumina-based catalyst device can be estimated on the basis of the set rich air-fuel ratio.
In this way, in the rich control, NO2 is released from the silver-alumina-based catalyst device 2 by the estimated NO2 release rate. The NO2 is purified by reduction by the NOX reduction catalyst device 3 which is activated at this time, but if making the NOX reduction rate of the NOX reduction catalyst device 3 the NO2 release rate or more, it is possible to purify by reduction at the NOX reduction catalyst device 3 almost all of the NO2 which is released from the silver-alumina-based catalyst device 2.
The NOX reduction rate of the NOX reduction catalyst device 3 becomes faster the smaller the air-fuel ratio of the exhaust gas and becomes faster the higher the temperature TD of the NOX reduction catalyst device 3. The air-fuel ratio of the exhaust gas is the set rich air-fuel ratio of the rich control. Due to this, it is possible to estimate the target temperature TDt of the NOX reduction catalyst device 3 for giving rise to an NOX reduction rate which is equal to the NO2 release rate of the silver-alumina-based catalyst device 2.
Due to this, in the rich control, when the current temperature TD of the NOX reduction catalyst device 3 is lower than the target temperature TDt, it is preferable to operate the electric heater 5 to raise the NOX reduction catalyst device 3 in temperature to the target temperature TDt or more.
Further, at the time of the rich control, when making the silver-alumina-based catalyst device 2 release the NO as well, it is preferable to make the target temperature TD1 higher so as to also purify by reduction the NO at the NOX reduction catalyst device 3 so as to make the NOX reduction rate of the NOX reduction catalyst device 3 higher.
As the reducing agent feed device 4, it is also possible to use the fuel injector which injects fuel into a cylinder. The fuel injector may for example be used to inject additional fuel into the cylinder in the latter half of the exhaust stroke or the expansion stroke so as to realize the set rich air-fuel ratio of the rich control.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/054214 | 2/18/2011 | WO | 00 | 12/20/2011 |
