Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2001-10-24
2003-05-20
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...
C060S274000
Reexamination Certificate
active
06564544
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an engine exhaust purification arrangement provided with a catalyst and more specifically to a control arrangement for an exhaust gas purification arrangement which maintains the air-fuel ratio in a catalytic converter at stoichiometric using specific oxygen adsorption/release characteristics of the catalyst.
BACKGROUND OF THE INVENTION
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein the amount of oxygen stored in a three-way catalyst (hereafter, “oxygen storage amount”) is computed based on an engine intake air amount and an air-fuel ratio of an exhaust flowing into the catalyst, and wherein engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is maintained essentially constant.
In order to maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of a three-way catalyst at a maximum or optimal level, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. By maintaining the oxygen storage amount of the catalyst constant, oxygen in the exhaust is stored in the catalyst when the air-fuel ratio of the exhaust flowing into the catalyst shifts to lean, and oxygen stored in the catalyst is released when the air-fuel ratio of the exhaust flowing into the catalyst shifts to rich, so the catalyst atmosphere can be assuredly maintained at the stoichiometric air-fuel ratio.
In an exhaust purification arrangement which performs this control, the conversion efficiency of the catalyst depends on the oxygen storage amount of the catalyst. Therefore, to control the oxygen storage amount to the desired constant level and to maintain the conversion efficiency of the catalyst at a high/optimal level, the oxygen storage amount must be precisely computed.
However, this system suffers from the drawback that it has proven difficult to precisely compute the oxygen storage amount of the catalyst using computational methods which have thus far been developed.
SUMMARY OF THE INVENTION
It is believed that this drawback results from the fact that the oxygen is stored and released rapidly by noble metal (platinum Pt, for example) contained in the catalyst, while stored and released slowly by an oxygen storage material such as cerium oxide, and that the amount of oxygen which is stored has not, in the prior art, been computed in a manner which takes this factor into account.
Therefore, as one approach to solving this problem, it is proposed to improve the precision with which the oxygen storage amount is determined by separately computing a high speed component/amount and a low speed component/amount and thus more accurately match the actual adsorption/release characteristics. Additionally, it is proposed to maintain the catalyst atmosphere based primarily on the high speed component and in a manner which maintains the catalyst conversion efficiency at a high-level. This is achieved by controlling the air-fuel ratio of the engine so that the high speed component is kept essentially constant at a target level.
However, if the air-fuel ratio of the exhaust gases, which are emitted from the engine and supplied to the catalyst, is strongly shifted into the lean region when the storage amount of the high speed component is effectively zero, or the engine air-fuel ratio is largely varied to rich when the storage amount of the high speed component has reached its maximum capacity, i.e., if the air-fuel ratio is varied without any restriction to maintain the high speed component constant, drivability and fuel economy decrease. On the other hand, when the oxygen storage capacity of the catalyst is a maximum, the NOx discharge amount tends to increase, and at such times, the oxygen storage amount must be rapidly decreased to the optimum amount.
This invention is therefore directed to controlling the amount of oxygen stored in a catalyst to an appropriate amount with good response so that the air-fuel ratio can be controlled to the degree that the oxygen storage amount of the catalyst is essentially constant while obviating the decrease of drivability and fuel-cost performance due to excessively large variations in the air-fuel ratio.
In order to achieve the above, a first aspect of the invention provides an exhaust purification arrangement for an engine which includes a catalyst provided in an exhaust passage of the engine, a front sensor for sensing the air fuel ratio of the exhaust gases which are flowing to the catalyst and a processor (e.g. microprocessor) arrangement which is responsive to the detected air/fuel ratio. In this arrangement the catalyst contains a material (or materials) which stores oxygen in the form of a high speed component wherein the oxygen is rapidly adsorbed and released from the material and which stores oxygen in the form of a low speed component wherein the oxygen is slowly adsorbed and released from the material. The microprocessor is programmed to compute the high speed oxygen storage amount of the material based on the detected exhaust air-fuel ratio, and to compute the target air-fuel ratio to be supplied to the engine (
1
) so that the amount of oxygen in the exhaust gas maintains the high speed oxygen storage amount in the material at a predetermined target value which is selected to be able to absorb or releases oxygen as required during transient periods and enables the atmosphere about the catalyst to be maintained at an air-fuel ratio which promotes efficient conversion of noxious compounds.
Another aspect of the invention resides in a method of controlling the air-fuel ratio of an atmosphere in an catalytic converter which is operatively connected with an internal combustion engine, comprising the steps of: storing oxygen on a first material in the catalytic converter which adsorbs and releases oxygen rapidly; storing oxygen on a second material in the catalytic converter which adsorbs and releases oxygen more slowly than the first material; and controlling the air-fuel ratio of the exhaust gas entering the catalytic converter to control the amount of oxygen which is adsorbed on the first material to a predetermined target amount which is less than the maximum amount of oxygen which can be adsorbed onto the first material.
In this method the step of controlling the air-fuel ratio of the exhaust gases can include controlling the air-fuel ratio of the exhaust gases to within upper and lower air-fuel ratio limits; determining if the first material is saturated with oxygen; and temporarily reducing the lower air-fuel ratio limit to enrich the air-fuel mixture to rapidly lower the amount of oxygen stored in the first material toward the target value which, as mentioned above, is selected to be between maximum and minimum amounts of oxygen which can be stored by the first material and thus allow oxygen to be rapidly stored or released from the first material in a manner which enables quick response to fluctuations in the air-fuel ratio of the incoming gases.
A further aspect of the invention resides in an arrangement for controlling the air-fuel ratio of an atmosphere in an catalytic converter which is operatively connected with an internal combustion engine, comprising: a first material disposed in the catalytic converter which adsorbs and releases oxygen rapidly; a second material disposed in the catalytic converter which adsorbs and releases oxygen more slowly than the first material; a control arrangement for controlling the air-fuel ratio of the exhaust gas entering the catalytic converter to adjust the amount of oxygen which is adsorbed on the first material to a target amount which is approximately half of the maximum amount of oxygen which can be adsorbed onto the first material.
The control arrangement in this case is also capable of executing the steps set forth above, so that should the first material become saturated with oxygen due to a temporary leaning of the air-fuel ratio such as results from no fuel being supplied to selected cylinders of the engine during so called “fuel cut
Kakizaki Shigeaki
Kakuyama Masatomo
Matsuno Osamu
Denion Thomas
Foley & Lardner
Nissan Motor Co,. Ltd.
Tran Diem
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