Recently, the global warming phenomenon by carbon dioxide has become an issue, and to reduce the carbon dioxide emission amount has become an assignment. In automobile as well, to reduce the carbon dioxide amount in the exhaust gas has become an assignment, and a lean-burn engine which can thinly burn a fuel in an excessive oxygen atmosphere has been developed. By this lean-burn engine, since mileage is improved, it is possible to suppress the emission amount of carbon dioxide.
By the way, in a case where harmful components in an exhaust gas from a lean-burn engine are reduced, since the exhaust gas is put in an excessive oxygen atmosphere, reduction reactions become less likely to occur, and the reduction and purification of NOx become difficult. Hence, in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652, an NOx storage-and-reduction type catalyst is disclosed on which an NOx storage component, being selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements, is loaded along with a noble metal. By using this NOx storage-and-reduction type catalyst and controlling an air-fuel ratio so that it becomes from a fuel-stoichiometric to rich atmosphere in a pulsating manner in the midway of a fuel-lean atmosphere, it is possible to efficiently progress the oxidation of HC and CO as well as the reduction of NOx, and accordingly it is possible to acquire high purifying performance.
Namely, an exhaust gas which is burned in a fuel-lean atmosphere becomes a reduction components lean atmosphere, in the reduction components lean atmosphere, NO in the exhaust gas is oxidized to become NOx and is stored into the NOx storage component, and accordingly the emission of NOx is suppressed. Then, when it is controlled from a fuel-stoichiometric to rich air-fuel ratio in a pulsating manner, the exhaust gas becomes from a reduction components stoichiometric to rich atmosphere. Therefore, NOx are released from the NOx storage component, and they react with the reduction components, such as HC, which exist in the exhaust gas, so that they are reduced, and accordingly the emission of NOx is suppressed. Therefore, it is possible to suppress the emission of NOx in all of the atmospheres from fuel-rich to fuel-lean.
To control an air-fuel ratio in a pulsating manner so as to become from a fuel-stoichiometric to rich atmosphere is referred to as rich spiking, and the extent of making a fuel-rich atmosphere by the rich spiking is expressed by deep or shallow. Namely, by rich spiking, making a heavy degree of fuel-rich atmosphere is referred to as “charging rich spiking deeply,” and making a light degree of fuel-stoichiometric to rich atmosphere is referred to as “charging rich spiking shallowly.” Then, in Japanese Unexamined Patent Publication (KOKAI) No. 11-107,810, and the like, there is set forth to appropriately control the extent of the rich spiking and the timing of charging it.
Then, it has been required to purify NOx, which are emitted from an engine being operated under a variety of fuel-lean conditions, by using an NOx storage-and-reduction type catalyst while always sustaining a purifying rate as high as 90% or more. However, in conventional NOx storage-and-reduction type catalysts, when the NOx amount, which is emitted from an engine in a unit period of time, is large, the rich spiking should be charged at intervals of from a couple of seconds to 60 seconds. However, in the excessive fuel component to be supplied as the rich spiking, there exists, in addition to an amount to be used in the reduction of NOx, an amount to be used in controlling the combustion state of an engine to a fuel-rich air-fuel ratio. Therefore, in a case where the rich spiking is charged frequently, there arises the deterioration of mileage. In particular, in a case where being driven ordinarily at a high speed, the NOx amount, which is emitted from an engine, enlarges remarkably. Therefore, in order to sustain a high NOx purifying rate, the rich spiking should be kept being charged at very short intervals of 10 seconds or less, and accordingly there is a problem in that the mileage is lowered considerably.
The present invention has been done in view of such circumstances, and its main object is to improve mileage by prolonging the intervals of the rich spiking as well as to improve the reduction-and-purification efficiency of NOx.
The characteristics of an exhaust gas purifying apparatus for an internal combustion engine, which solves the aforementioned assignments, lie in that it is used for an internal combustion engine, which can select an operation at a fuel-lean air-fuel ratio and an operation at a fuel-stoichiometric or rich air-fuel ratio and comprises: an NOx storage-and-reduction type catalyst disposed in an exhaust gas flow passage and exhibiting a saturated NOx storage amount of 5 g or more as NO2 with respect to 1 liter of a catalyst volume at 500° C.; NOx storage amount estimating means for estimating an actual NOx storage amount of the NOx storage-and-reduction type catalyst; air-fuel ratio adjusting means for adjusting an exhaust gas atmosphere to reduction components lean or reduction components rich; and a controlling device for controlling the air-fuel ratio adjusting means based on an estimated value estimated by the NOx storage amount estimating means so that the actual NOx storage amount becomes 50% or less of the saturated NOx storage amount.
Moreover, the characteristics of an exhaust gas purifying process for an internal combustion engine of the present invention lie in that, in an exhaust gas purifying process for an internal combustion engine, in which an NOx storage-and-reduction type catalyst including an NOx storage component is contacted with an exhaust gas from an internal combustion engine, which can select an operation at a fuel-lean air-fuel ratio and an operation at a fuel-stoichiometric or rich air-fuel ratio, thereby storing NOx contained in the exhaust gas into the NOx storage component in a reduction components lean atmosphere, and reducing NOx released from the NOx storage component by making a reduction components stoichiometric to rich atmosphere by rich spiking, an NOx storage-and-reduction type catalyst exhibiting a saturated NOx storage amount of 5 g or more as NO2 with respect to 1 liter of a catalyst volume at 500° C. is used, and in that the rich spiking is controlled so that an actual NOx storage amount of the NOx storage-and-reduction type catalyst becomes 50% or less of the saturated NOx storage amount.
The “fuel-lean air-fuel ratio” refers to an air-fuel ratio which makes an exhaust gas into an atmosphere in which oxygen exists in a concentration which exceeds an oxygen equivalent ratio required for oxidizing all of reduction components, such as CO, THC and H2. In the case of an air-fuel ratio (A/F: ratio of air to fuel by weight), the equivalent (stoichiometric) point is around 14.6, and, in the present invention, an atmosphere whose A/F exceeds 14.6 is called a fuel-lean air-fuel ratio. Moreover, the “fuel-rich air-fuel ratio” refers, contrary to the “fuel-lean air-fuel ratio,” to an air-fuel ratio which makes an exhaust gas into an atmosphere in which oxygen exists in a concentration which does not reach an oxygen equivalent ratio required for oxidizing all of the reduction components, an air-fuel ratio in which A/F does not reach 14.6 is called a fuel-rich air-fuel ratio.