Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
2000-07-13
2002-08-06
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, C060S295000, C060S301000
Reexamination Certificate
active
06427439
ABSTRACT:
TECHNICAL FIELD
The invention relates to a system and method for controlling reductant injection upstream of a selective reduction catalyst for use with an internal combustion engine.
BACKGROUND ART
In order to meet emission regulations, selective catalytic reduction (SCR) systems using externally added reducing agents are used. In such a system, regulated emissions, such as certain nitrogen oxides, or NO
X
, can be reduced in an oxygen-rich environment to nitrogen over a catalyst when a reducing agent or reductant, such as urea water solution (urea) or ammonia (NH3), is added.
Current SCR systems are very effective in NO
X
reduction at temperatures above 250° C. and have potential to substantially reduce NO
X
emission from diesel vehicles. However, the NO
X
reduction efficiency of SCR systems using urea or ammonia as reductant is rather low at and below 200° C., which limits its overall NO
X
reduction efficiency for many diesel operation conditions.
Various systems for controlling ammonia injection are proposed in the prior art. These prior art systems inject urea in proportion to the NO
X
concentration level above certain temperatures. One approach to increase the SCR NO
X
reduction efficiency is to inject excess amount of reductants at low temperature. However, tests show that the addition of excess ammonia or urea increases the NO
X
reduction efficiency only slightly and may cause a large ammonia emission.
DISCLOSURE OF INVENTION
We have discovered that urea injected into a SCR catalyst is much more effective, if it is injected when the engine-out NO
X
concentration is below a threshold value, which is in the range of 25-75 ppm. The main effect is to significantly enhance the reductant storage on SCR catalysts under these conditions. The storage of reductant is only partially effective when the engine-out NO
X
is above that level. High NO
X
levels inhibit the adsorption of the reductant on active sites adjacent to NO
X
to promote NO
X
reduction especially at temperatures below 250° C.
The present invention proposes a new method of enhancing the SCR NO
X
reduction efficiency at low temperature. It involves injecting relatively larger amounts of reductant into the catalyst while the engine produces low NO
X
emission during idling or deceleration. This allows the reductant to be stored in the SCR catalyst before the catalyst is exposed to the high concentration of NO
X
. The reductant injection is stopped when the catalyst storage capacity is nearly full. The resultant NO
X
reduction is much greater than without such storage. For example, excess urea was stored in SCR catalyst in engine deceleration condition, and upon subsequent acceleration, a largely enhanced NO
X
reduction efficiency was observed at low temperature. No reductant was injected into the system during the acceleration, so the only reductant available for NO
X
reduction was the urea (ammonia) pre-stored in the SCR catalyst. The NO
X
conversion at 200° C. was 90% compared to 60% for a similar test without such storage. The SCR NO
X
conversion light off (50% conversion) temperature was reduced about 60° C. from 210° C. to 150° C. The method of the present invention can greatly improve the SCR NO
X
reduction efficiency at low temperature for diesel vehicle applications.
In practice, a method of injecting urea when engine-out NO
X
is low may sometimes inject urea at the same point as a method of injecting urea when the catalyst temperature is low. The engine-out NO
X
level may be low at idle, for example. After running at idle for some time, the catalyst temperature may also be low. Therefore, both strategies would inject at idle. However, the strategies are different on decelerations. The method of the present invention being based on engine-out NO
X
would begin urea injection immediately on deceleration because the engine-out NO
X
level drops quickly. However, a method based on the catalyst temperature would not begin urea injection until the catalyst cooled down, which could take 20 to 60 seconds or perhaps never happen if the driver accelerates. In addition, the two strategies will respond differently to high EGR levels, which may provide low engine-out NO
X
levels while maintaining high catalyst temperatures. The method of the present invention provides more reductant to the active sites in the SCR catalyst to overcome NO
X
inhibition, and this increases NO
X
reduction efficiency.
In accordance with the present invention, a system and method for controlling ammonia injection upstream of a selective reduction catalyst (SCR) is disclosed that overcomes NO
X
inhibiting effects by injecting excess reductant into the SCR catalyst when engine-out NO
X
concentration is below a preset value such as when the engine is idling or decelerating. During these operations, the NO
X
adsorbed at the active sites is purged out by the low NO
X
exhaust flow, and the NO
X
inhibiting effects is minimized, thus the reductant can freely access the active sites to conduct a high NO
X
reduction process.
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Hammerle Robert Henry
Wu Ching-Hsong George
Xu Lifeng
Denion Thomas
Drouillard Jerome R.
Ford Global Technologies Inc.
Tran Diem
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