Power plants – Internal combustion engine with treatment or handling of... – Having means analyzing composition of exhaust gas
Reexamination Certificate
2000-03-07
2001-11-06
Vrablik, John J. (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
Having means analyzing composition of exhaust gas
C060S277000, C060S286000, C060S301000
Reexamination Certificate
active
06311480
ABSTRACT:
The disclosure of Japanese Patent Application No. HEI
11-86237
filed on Mar. 27, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an emission control apparatus for an internal combustion engine and, more particularly, to an internal combustion engine emission control apparatus having an NOx sensor for detecting an NOx concentration in exhaust gas.
2. Description of the Related Art
An emission control apparatus having an NOx sensor for detecting a concentration of oxides of nitrogen (NOx) in exhaust gas from an internal combustion engine is described in, for example, Japanese Patent Application Laid-Open No. HEI 7-166851. In the emission control apparatus, an NOx-absorbing and reducing catalyst is disposed in an exhaust passage of the engine, and the NOx sensor is disposed in a portion of the exhaust passage downstream of the catalyst so as to detect the NOx concentration in exhaust gas that has passed through the NOx-absorbing and reducing catalyst. The NOx-absorbing and reducing catalyst absorbs NOx from exhaust gas when the air-fuel ratio of exhaust gas flowing into the catalyst is on a lean side of the theoretical air-fuel ratio. The catalyst releases NOx when the oxygen concentration in exhaust gas flowing into the catalyst decreases. Therefore, the NOx-absorbing and reducing catalyst is used as an NOx-lessening catalyst.
The NOx-absorbing capability of NOx-absorbing and reducing catalysts decreases with an increase in the amount of NOx absorbed therein. Therefore, as the amount of NOx absorbed in an NOx-absorbing and reducing catalyst increases, the amount of NOx that passes through the NOx-absorbing and reducing catalyst without being adsorbed therein increases, so that the amount of NOx remaining in the exhaust gas from the exhaust system increases.
Therefore, when the NOx concentration in exhaust gas detected by the NOx sensor disposed in a portion of the exhaust passage downstream of the NOx-absorbing and reducing catalyst increases to a predetermined value, the aforementioned apparatus causes the engine to operate at a fuel-rich air-fuel ratio for a short time so as to change the condition of exhaust gas flowing into the NOx-absorbing and reducing catalyst (in this case, change the exhaust gas air-fuel ratio to a rich side). Due to this operation, NOx is released from the NOx-absorbing and reducing catalyst, so that the NOx-absorbing capability of the NOx-absorbing and reducing catalyst is reactivated.
When the air-fuel ratio of exhaust gas from the engine shifts to the rich side, the oxygen concentration in the exhaust gas decreases and the quantity of components that function as reducers, such as unburned hydrocarbons (HC), carbon monoxide (CO) and the like, increases. When the oxygen concentration in the exhaust gas decreases, NOx is released from the NOx-absorbing and reducing catalyst. That is, the amount of NOx stored in the NOx-absorbing and reducing catalyst decreases. An amount of NOx released from the NOx-absorbing and reducing catalyst reacts with reducer components in exhaust gas on the catalyst, thereby lessening the amount of NOx present in exhaust gas.
However, if the release of NOx from the NOx-absorbing and reducing catalyst and the reduction thereof are controlled on the basis of the exhaust NOx concentration detected by the NOx sensor, a problem may arise if the NOx sensor has deteriorated so that the NOx sensor output drifts. In this case, an NOx concentration indicated by an output of the NOx sensor may deviate from the actual NOx concentration.
If such a sensor output deviation occurs, it becomes difficult to properly perform the release and reduction of NOx (reactivation of the NOx-absorbing and reducing catalyst). Unnecessary performance of the reactivating process may result in degradation of the engine fuel consumption, release of unburned HC and CO into the atmosphere, and an outflow of NOx from the NOx-absorbing and reducing catalyst downstream due to an increased amount of NOx stored in the NOx-absorbing and reducing catalyst, and the like.
Problems substantially the same as those stated above also occur in an apparatus that employs, as an NOx-lessening catalyst, a selective reducing catalyst that is able to selectively reduce NOx present in a lean-air-fuel ratio exhaust gas.
A selective reducing catalyst reduces NOx by using HC and CO components of exhaust gas. If the amounts of HC and CO components present on the selective reducing catalyst become insufficient, the amount of NOx that flows downstream from the catalyst without being reduced increases. For example, in an apparatus in which the amount of NOx that flows out of a selective reducing catalyst without being reduced thereby is detected by an NOx sensor disposed downstream of the catalyst and, in accordance with the amount of unreduced NOx, the engine operating air-fuel ratio is changed to adjust characteristics of exhaust gas (in this case, the amounts of HC and CO components) flowing into the catalyst, a deviation of the output of the NOx sensor causes emission of unreacted HC and CO components into the atmosphere, emission of unreduced NOx into the atmosphere, or other problems.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an internal combustion engine emission control apparatus capable of appropriately lessening the amount of NOx present in exhaust gas on the basis of the Nox sensor output even if the output of the NOx sensor deviates during adjustment of characteristics of exhaust gas flowing into an NOx-lessening catalyst.
To achieve the aforementioned and other objects, the invention provides an emission control apparatus for an internal combustion engine including an NOx-lessening catalyst that is disposed in an exhaust passage of the internal combustion engine and that lessens an amount of NOx present in exhaust gas in accordance with a condition of exhaust gas flowing therein, an NOx sensor disposed in the exhaust passage downstream of the NOx-lessening catalyst, an output of the Nox sensor corresponding to an NOx concentration in exhaust gas flowing out of the NOx-lessening catalyst, and a controller. The controller determines an operational condition of the internal combustion engine, and determines a deviation of the output value of the NOx sensor from a predetermined reference value when the engine is being operated under a predetermined operational condition. Based on the deviation of the output value from the predetermined reference value, the controller corrects the output value of the NOx sensor.
The predetermined operational condition of the internal combustion engine may be a condition where the NOx concentration is normal, and where NOx discharged from the internal combustion engine is small in quantity (e.g., during idling, low-load operation, or fuel cut operation). In such an operational condition of the engine, the NOx concentration in exhaust gas reaching the NOx sensor becomes substantially zero, so that the output value of the NOx sensor should correspond to zero NOx concentration unless the NOx sensor output has drifted. An emission control apparatus according to the invention calculates an extent of drift of the present output value of the NOx sensor based on the reference output value (e.g., an NOx sensor output value corresponding to zero NOx concentration) and the value outputted from the NOx sensor under the aforementioned condition, and corrects the NOx sensor output in accordance with the calculated extent of drift.
Therefore, the emission control apparatus of the invention is able to accurately detect the NOx concentration in exhaust gas even if the NOx sensor output deviates due to deterioration of the sensor or the like.
REFERENCES:
patent: 5201802 (1993-04-01), Hirota et al.
patent: 5209061 (1993-05-01), Takeshima
patent: 5412946 (1995-05-01), Oshima et al.
patent: 5426934 (1995-06-01), Hunt et al.
patent: 5437153 (1995-08-01), Takeshima et al.
patent: 55
Katoh Kenji
Suzuki Naoto
Kenyon & Kenyon
Toyota Jidosha & Kabushiki Kaisha
Tran Diem
Vrablik John J.
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