Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2001-03-01
2003-05-27
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, C060S276000, C060S288000, C060S320000
Reexamination Certificate
active
06568179
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus and method for vehicle emissions control and more particularly, to an apparatus and method for controlling the temperature of the exhaust gas stream exiting the exhaust manifold and entering an underfloor catalytic converter containing a multi-functional catalyst, e.g., a three-way conversion catalyst and a nitrogen oxides (“NO
x
”) trap.
BACKGROUND OF THE INVENTION
Conventional lean-burn engine control systems include an air/fuel controller that delivers fuel to the engine intake manifold proportional to measured air mass to maintain a desire air/fuel ratio, lean of stoichiometric. Emissions of nitrogen oxides (“NO
x
”) from lean-burn engines (described below) must be reduced in order to meet emission regulation standards. Conventional three-way conversion (“TWC”) automotive catalysts are suitable for abating NO
x
carbon monoxide (“CO”) and hydrocarbon (“HC”) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel that results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (“A/F”) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH
1.88
. The symbol &lgr; is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that &lgr;=1 is a stoichiometric mixture, &lgr;>1 is a fuel-lean mixture and &lgr;<1 is a fuel-rich mixture.
Engines, especially gasoline-fueled engines to be used for passenger automobiles and the like, are now designed to operate under lean conditions as a fuel economy measure. Such engines are referred to as “lean-burn engines”. That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are “lean”, i.e., the exhaust gases are relatively high in oxygen content.
Although lean-burn engines provide enhanced fuel economy, they have the disadvantage that conventional TWC catalysts are not effective for reducing NO
x
emissions from such engines because of excessive oxygen in the exhaust. The prior art discloses attempts to overcome this problem by operating lean-burn engines with brief periods of fuel-rich operation. (Engines which operate in this fashion are sometimes referred to as “partial lean-burn engines”.)
The typical TWC catalyst provided in the exhaust passage as a “close-coupled” catalytic converter does not convert the NO
x
produced when the engine is running lean, i.e., when &lgr;>1. In order to reduce the NO
x
emission to the atmosphere, it is known to use an underfloor catalytic converter located downstream of the medium-coupled or close-coupled catalytic converter. “Close-coupled” catalytic converters are known in the prior art and are generally defined as located in or near the engine compartment, typically less than one foot, more typically less than six inches from, and preferably immediately adjacent to, i.e., attached directly to, the outlet of the exhaust manifold. “Underfloor” catalytic converters are also known in the prior art and are located (downstream of any close-coupled catalysts)under the floor of the vehicle adjacent to or in combination with the vehicle's muffler.
It is known to treat the exhaust of such engines with an underfloor catalytic converter containing a multi-functional catalyst, e.g., a TWC catalyst/NO
x
trap which stores NO
x
during periods of lean (oxygen-rich) operation, and releases the stored NO
x
during the rich (fuel-rich) periods of operation. A typical NO
x
trap utilizes alkali metal or alkaline earth metal oxides in combination with the precious metal catalyst component in order to store or occlude NO
x
under lean operating conditions. The mechanism for NO
x
storage is believed to involve the oxidation of NO to NO
2
over the precious metal component of the TWC catalyst followed by the subsequent formation of a nitrate complex with the alkali metal or alkaline earth metal oxide. Under engine operation rich of stoichiometric (&lgr;<1), the nitrate complexes are thermodynamically unstable, and the stored NO
x
is released and catalytically reduced by the excess of CO, HCs and H
2
in the exhaust. Periodically, the lean-burn engine is switched to a relatively rich air/fuel ratio to purge the NO
x
trap.
It is also known that exposure of the NO
x
trap to excessive temperatures, e.g. 750° C. and higher, during the operation of the engine will result in a significant diminution of the capability of the NO
x
trap to absorb the NO
x
in the exhaust gas stream Therefore, it would be desirable to provide some means of insuring that the temperature of the exhaust gas stream entering the underfloor catalytic converter containing the NO
x
trap during the operation of the engine does not exceed the temperature at which the capability of the trap to absorb the NO
x
in the exhaust gas stream starts to fall off.
Lean-burn engines are designed for fuel economy. In such engines, operations alternate depending on speed and load. At the lean (&lgr;>1) condition, the NO
x
trap in the underfloor catalytic converter absorbs NO
x
, then a fuel-rich (&lgr;<1) spike is applied which results in NO
x
desorption from the trap and catalytic reduction of the NO
x
to N
2,
then a lean condition occurs followed by a rich spike, etc. Rich conditions are required from time to time at higher speeds and loads in order to maintain the temperature of the exhaust gas flowing into the underfloor catalytic converter at a temperature below that which would result in deterioration of the NO
x
trap. At stoichiometric or richer than stoichiometric conditions, i.e. &lgr;≦1, the multi-functional catalyst in the underfloor catalytic converter has the capability of reducing the NO
x
to N
2
without absorption of the NO
x
by the trap.
Typically, the lean-burn engine is periodically switched to a relatively rich air/fuel ratio to purge the NO
x
trap. The NO
x
trap must be exposed to minimum threshold temperatures at specific engine speeds and loads before it will perform efficiently and accordingly, a minimum exhaust temperature must be established before a lean-burn mode of engine operation is established. There are also upper or maximum temperatures within certain speeds and loads, above which the trap will cease operating effectively for trapping NO
x.
At such higher temperatures with specific speeds and loads, the engine operation will switch from lean to stoichiometric (or rich) conditions. The catalyst in the underfloor catalytic converter will act as a TWC catalyst such that NO
x,
CO and HCs are effectively removed. Since fuel economy is significantly improved by operating the engine at lean conditions, an apparatus and method are required for sensing the upper temperature limit of the lean operation, thus permitting the temperature of the exhaust gas stream entering the NO
x
trap to be lowered, thereby allowing the lean operation of the engine to be extended to cover high speeds and loads which would otherwise be required to occur at stoichiometric conditions. Such extension of the lean operation of the engine would result in dramatically improved fuel savings, while concurrently lowering the formation of the greenhouse gas CO
2
.
It is also known that at certain higher speeds and loads, the temperature of the exhaust gas stream entering the NO
x
trap may exceed the temperatures at which the trap starts to deteriorate. At such extreme conditions of speed and load, the exhaust gas temperature is usually lowered by using fuel enrichment (&lgr;<1) in order to prevent deterioration of the trap. This practice has a severe adverse impact on fuel economy and also defeats the purpose of fuel savings associated with the lean burn engines. Accordingly, a
Denion Thomas
Engelhard Corporation
Negin Richard A.
Nguyen Tu M.
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