Heat exchanger for SOx or NOx regeneration of catalyst

Details Heat exchanger for SOx or NOx regeneration of catalyst Heat exchanger for SOx or NOx regeneration of catalyst

2001-05-19

2003-06-03

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...

C060S285000, C060S295000, C060S320000, C060S297000, C123S295000, C123S430000

Reexamination Certificate

active

06571551

ABSTRACT:

BACKGROUND OF INVENTION
1. Technical Field
The present invention relates to a device for use with an internal combustion engine, particularly a lean burning engine. Furthermore, the invention is intended for use with the exhaust system of an internal combustion engine, with the exhaust system having a NO
X
absorber.
2. Background Information
Vehicles operating with an internal combustion engine must meet the general requirement of low level of harmful substances in the exhaust gas coming from the engine. These substances are predominantly compounds such as nitrogen oxides (NO
X
), hydrocarbons (HC) and carbon monoxide (CO). In the case of modern internal combustion engines, the exhaust gas is normally purified with the aid of a catalytic converter, which is part of the exhaust system and through which the exhaust gas is passed. Known “three-way catalysts” remove the predominant part of the above harmful compounds through catalytic reactions. In order that the catalyst gives the highest possible degree of purification with respect to NO
X
, HC and CO, the engine is run on a stoichiometric mixture of air and fuel, i.e., one with a lambda value (&lgr;) of 1 in most operating modes.
Although modern three-way catalysts enable a very high degree of purification thereby greatly reducing the emission of harmful compounds into the atmosphere, there is a continual need for further reductions in the emission of harmful substances. This is in part due to the increasingly rigorous legislation introduced in various countries, requiring an extremely low emission of NO
X
, CO and HC compounds.
Furthermore, it is a general requirement that vehicles have the lowest possible fuel consumption. This has recently led to the development of engines with a new type of combustion chamber for the cylinders of the engine, making it possible to run the engine on ever leaner fuel mixtures, i.e., on mixtures with a lambda value (&lgr;) greater than 1. Such engines are generally called “lean-burn” engines. A direct-injection (or “DI”) engine, which is a spark-ignition engine with direct injection, operates in a “stratified” manner. Here, fuel introduced into the engine's combustion chamber is concentrated to a great extent at the sparking plugs. When working in certain modes, such as under a low or medium engine loading, these engines are able to run on very lean air/fuel mixtures with a lambda value of the order of magnitude of 4. This considerably increases the fuel economy for these types of engines. A direct-injection engine can also run in a “homogeneous” manner in certain modes of operation, mainly when driven under a high engine loading. The homogeneous operation corresponds to a stoichiometric (i.e., a relatively rich) air/fuel mixture being fed into the engine.
Since, in certain modes of operation, a direct-injection engine works on a very lean air/fuel mixture, the exhaust gas mixture that consequently flows through the three-way catalyst is also lean. As such, the three-way catalyst cannot reduce the NO
X
compounds in the exhaust gas since it is designed to give an optimum purification in the case of a stoichiometric mixture. To remedy this, the ordinary three-way catalyst is combined with a nitrogen oxide adsorber, also called a NO
X
adsorber or NO
X
trap. The purpose of this arrangement is to adsorb the NO
X
compounds, e.g., from the exhaust gas of an internal combustion engine. These NO
X
adsorbers can thus be installed and used in conjunction with a conventional three-way catalyst.
A NO
X
adsorber may be arranged as a separate unit located upstream of the ordinary three-way catalyst, or alternatively it may be integrated with the three-way catalyst, i.e., combined with the catalytic material in the three-way catalyst. The NO
X
adsorber takes up (adsorbs) NO
X
compounds present in the exhaust gas of an engine when it is run on a lean air/fuel mixture, and gives off (desorbs) the NO
X
compounds when the engine is being run on a rich air/fuel mixture for a certain time.
Furthermore, the NO
X
adsorber can only adsorb the NO
X
compounds up to a certain limit, i.e., it gradually “gets full” with its adsorption capacity reaching its limit. When this happens, the NO
X
adsorber has to be regenerated, which means that it must be made to desorb and release accumulated NO
X
compounds. If there is a conventional three-way catalyst downstream of the NO
X
adsorber, these desorbed NO
X
compounds can be eliminated in the three-way catalyst, provided that the latter has reached its operating temperature.
The NO
X
adsorber can be regenerated by making the exhaust gas that passes through it relatively rich for a certain time. This is achieved by running the engine on a relatively rich air/fuel mixture for a short time, e.g., for a few seconds. In doing so, the NO
X
adsorber is depleted so that it can again adsorb NO
X
compounds for a certain time before another regeneration is needed. U.S. Pat. No. 5,461,857 describes achieving regeneration by controlling the concentration of air in the exhaust gas mixture flowing through the NO
X
adsorber.
A NO
X
adsorber is designed to operate at a certain temperature, which, in turn, depends on the mode of operation of the engine at the time. During stratified operation, i.e., in the case of a lean air/fuel mixture, the temperature of exhaust gas flowing through the NO
X
adsorber should be about 200 to about 500° C. if operation of the adsorber is to be optimized. Furthermore, it is a general requirement that the exhaust gas temperature should not exceed about 800° C. because the NO
X
adsorber may be damaged at higher temperatures.
A phenomenon that occurs in a NO
X
adsorber is that sulfur compounds, such as sulfur dioxide (SO
2
), present in exhaust gas flowing through the NO
X
adsorber, form a deposit on the active material of the NO
X
adsorber. Having this deposit on it prevents the NO
X
adsorber from adsorbing NO
X
compounds. The sulfur compounds come from the fuel and vary with the quality of the fuel. As a result of this a sulfur deposit, the adsorption capacity of the NO
X
adsorber progressively decreases with time.
To remedy the problem of this sulfur deposit, the NO
X
adsorber is regenerated at set intervals in order to free the absorber from these compounds. As known in the art, such sulfur regeneration can be carried out by running the engine for a certain time wherein a rich exhaust gas with a lambda value of less than 1 is produced while the exhaust gas has a relatively high temperature, specifically one in excess of about 650° C. In this manner, sulfur compounds are desorbed, i.e., they are discharged from the NO
X
adsorber. As known in the art, this sulfur regeneration is preferably done at intervals based upon how much of the NO
X
storage capacity of the NO
X
adsorber has been lost. The loss of storage capacity is estimated from the sulfur content of the fuel and the fuel consumption of the vehicle in question.
However, a problem that occurs in the prior art with the sulfur regeneration is that it is difficult to reconcile the desired exhaust gas temperature when running a lean fuel mixture. This desired gas temperature for a lean fuel mixture is about 200 to about 500° C. However, in order to carry out sulfur regeneration, it should be at least about 650° C. in the NO
X
adsorber. This problem can be conventionally solved by raising the exhaust gas temperature during sulfur regeneration, e.g., by delaying the ignition timing for the cylinders of the engine. However, this measure is not sufficient to raise the exhaust gas temperature to the required value if the vehicle in question is never run under a high engine loading, which happens with certain types of drivers and certain types of driving situations.
Accordingly, there is a need to resolve the conflicting requirements of a high temperature of at least about 650° C. needed for sulfur regeneration, and a relatively low temperature of about 200-500° C. that occurs when running a lean mixture and sulfur regeneration is needed. Further, the temperature must stay below abo

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