Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2001-02-23
2003-01-07
Denion, Thomas (Department: 3748)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C060S276000, C060S277000, C060S301000
Reexamination Certificate
active
06502388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an engine exhaust gas cleaning system.
2. Description of the Prior Art
It is a well known technology in the art that an air-fuel ratio is set to a value excess in air concentration (hereinafter referred to as lean air-fuel ratio) more than the theoretical air-fuel ratio (hereinafter referred to as stoichiometric air-fuel ratio) to perform lean combustion in order to improve fuel economy of an engine.
For example, there are known a technology that lean combustion under an air-fuel ratio of 20 to 25 is attained in an engine of a type in which fuel is injected at a position near an intake port of an intake pipe portion (port injection type), and a technology that very lean combustion under an air-fuel ratio of 40 to 50 is attained by forming a stratified mixed gas in an engine of a type in which fuel is directly injected into a cylinder (in-cylinder injection type). In these technologies, fuel economy can be improved by lean combustion, that is, by increasing an amount of intake air to decrease pumping loss and heat loss.
In the case of stoichiometric combustion, exhaust gas can be cleaned by oxidizing and reducing HC, CO and NOx in the exhaust gas at a time using a three way catalyst. However, in the case of lean combustion, it is difficult to reduce NOx because the exhaust gas is in an oxygen excess state. In order to solve the problem, there is known an exhaust gas cleaning system for an engine. The exhaust gas cleaning system is that an NOx trapping agent which traps NOx in the exhaust gas by absorption or adsorption when the air-fuel ratio of the exhaust gas is lean and releases NOx to reduce or contact-reduce the NOx when the air-fuel ratio of the exhaust gas is rich (fuel excess) is disposed in the exhaust gas passage, and the air-fuel ratio of the exhaust gas is temporarily changed from a lean air-fuel ratio to the stoichiometric air-fuel ratio or a rich air-fuel ratio in an appropriate cycle to release or to reduce the NOx trapped to the NOx trapping agent in order to recover the NOx trapping ability (hereinafter, referred to as “purge”).
In such an exhaust gas cleaning system, it is preferable from the viewpoint of the fuel economy and reduction of the exhaust gas components such as HC in the exhaust gas to temporarily change the air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio only for a time period which is commensurate with an amount of the trapped NOx.
A technology for judging completion of release of NOx when the air-fuel ratio is temporarily changed to the stoichiometric air-fuel ratio or the rich air-fuel ratio is proposed in Japanese Patent No. 2692380 (WO94/17291). The completion of release of NOx after temporarily changing the air-fuel ratio from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio is judged from the time when an air-fuel ratio detected by an air-fuel ratio sensor disposed downstream of an NOx trapping agent is switched from a lean state to a rich state. This is based on the fact that the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the NOx absorbent is shown to be slightly lean because HC and CO in the exhaust gas are consumed in reduction of the NOx until the NOx absorbed to the NOx absorbent is released and reduced even if the air-fuel ratio upstream of the NOx absorbent becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, and consequently the air-fuel ratio detected by the air-fuel ratio sensor becomes rich after completion of release and reduction of the NOx absorbed to the NOx absorbent.
As a similar technology, Japanese Patent Application Laid-Open No. 10-128058 (U.S. Pat. No. 5,771,685) discloses a technology that the performance of an NOx trapping agent is monitored by estimating an amount of trapped NOx from a time interval between the time when the air-fuel ratio is switched from a lean air-fuel ratio to the stoichiometric air-fuel ratio or a rich air-fuel ratio and the time when the air-fuel ratio detected by the air-fuel ratio sensor disposed downstream of the NOx trapping agent is switched from a lean state to a rich state.
Further, Japanese Patent Application Laid-Open No. 8-260949 discloses a technology that the NOx absorbing performance and the oxygen storage capacity of the NOx absoebent are separated from each other because an output of the air-fuel ratio sensor is influenced by the NOx absorbent and the oxygen storage capacity possessed by a catalyst disposed at an upstream or downstream position close to the air-fuel ratio sensor when the performance of the NOx absorbent is monitored based on the air-fuel ratio detected by the air-fuel ratio sensor disposed at the position downstream of the NOx absorbent. In this technology, the oxygen storage capacity is detected by an output of the air-fuel ratio sensor disposed at the position downstream of the NOx absorbent when the amount of the absorbed NOx is nearly zero.
However, the output waveform of the air-fuel ratio sensor disposed downstream of the NOx trapping agent is affected an amount of the reducers such as HC, CO and so on flowing into the NOx trapping agent even if the amount of the NOx trapped to the NOx trapping agent is constant. Although the conventional technology discloses a method of estimating the amount of the reducers from an air-fuel ratio, the conventional technology does not take into consideration variations in controlled air-fuel ratio and what percentage of the reducers such as HC, CO and so on flowing into the NOx absoebent is actually used for reducing the absorbed NOx. Actually, the amount of the reducers exhausted from an engine is not totally used for the reduction of the NOx trapped to the NOx trapping agent, but part of the reducer is oxidized in the NOx trapping agent and in the catalyst arranged at a position upstream of the NOx trapping agent. Therefore, the percentage of the reducers used for the reduction of the NOx trapped to the NOx trapping agent to the total amount of the reducers exhausted from the engine is influenced by variations in operating condition and in the catalyst performance due to deterioration because the percentage differs depending on the performance of the NOx trapping agent and the catalyst performance (also influenced by the oxygen storage capacity) of the catalyst arranged upstream of the NOx trapping agent.
Further, because the performance of the NOx trapping agent is strongly influenced by temperature of the exhaust gas (temperature of the NOx trapping agent itself), the amount of trapped NOx is reduced, for example, when the exhaust gas temperature is too high even if the NOx itself is not deteriorated. Therefore, it may be erroneously judged that the NOx trapping agent is deteriorated. Furthermore, since the lean combustion is performed by delicate control of fuel injection timing and enhancement of intake air flow in the case of the in-cylinder injection type engine, the exhaust gas temperature sometimes fluctuates even if the engine is in a similar operating condition (rotating speed, load). However, the conventional technologies described above do not take the variations in the temperature into consideration. By mounting a temperature sensor and directly measuring the temperature of the NOx trapping agent to correct the temperature effect, the effect of the above-mentioned variations can be reduced. However, the cost is increased because it is necessary to use a comparatively highly accurate temperature sensor. In addition, it is also necessary to diagnose the temperature sensor itself, and accordingly the system becomes complex and high in cost.
Further, the output waveform of the air-fuel ratio sensor arranged downstream of the NOx trapping agent is influenced by the NOx trapping agent itself and/or the oxygen storage capacity of the catalysts closely arranged upstream and downstream of the NOx trapping agent even if the amount of NOx trapped to the NOx trapping agent is an equal value.
For example, if the NOx
Fujii Yoshihisa
Ishii Toshio
Kawamoto Shigeru
Nakagawa Shinji
Oosuga Minoru
Crowell & Moring LLP
Hitachi , Ltd.
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