Patent Grant
8991154
The present invention relates generally to Selective Catalytic Reduction (SCR) catalysts and, more particularly, to methods and systems for controlling reductant levels in SCR catalysts.
Selective catalytic reduction is an important tool in efforts to meet increasingly strict engine emissions standards. Certain techniques for reducing CO emissions result in greater production of nitrogen oxides, also referred to as NOx. SCR is a means of converting NOx with the aid of a catalyst into diatomic nitrogen, N2, and water, H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is absorbed onto a catalyst. Carbon dioxide, CO2 is a reaction product when urea is used as the reductant.
A controlled level of NH3 storage buffer in the catalyst is desired in order to maintain high NOx conversion efficiency (μNOx), defined here by
where NOx(inlet) is the NOx level proximate an inlet of the SCR catalyst and NOx(outlet) is the NOx level proximate and outlet of the SCR catalyst. As seen in the schematic graph of
The known technique for controlling NH3 levels is not considered to produce acceptable results. By this technique, a device such as an electronic control unit (ECU) (various suitable devices are hereinafter referred to generically as a controller) estimates the amount of NH3 stored in the SCR catalyst by keeping track of how much NH3 has been added to the system via dosing and estimating how much NH3 has been consumed by reaction with NOx. The first component—addition of NH3—is quite simple because the amount of NH3 added is directly proportional to urea dosing because the urea decomposes to NH3 and CO2 under high temperature conditions with adequate humidity. The second component—consumption—can be somewhat more difficult because it uses an estimated exhaust mass flow in addition to NOx sensor measurements both before and after the SCR to estimate how much NOx is reduced. The technique assumes that the amount of NH3 that is used is directly proportional to the NOx that is reduced.
A problem with the known technique is that error accumulates over time in the stored NH3 calculation, which leads to reduced NOx conversion efficiency. The controller uses the modeled stored NH3 mass as a feedback to a controller that tries to maintain stored NH3 at the target. However with nothing to correct this model over time, there is a risk that the model will diverge from actual NH3 levels. In this case failure to properly control stored NH3 directly leads to reduced NOx conversion efficiency.
The only mechanism to keep the modeled stored NH3 from diverging from actual NH3 levels is to periodically start over by using all up of the NH3 in the SCR and then resetting the model. In addition to having a direct impact on emissions from the time the SCR begins to operate at low efficiency as the actual stored NH3 approaches zero, emissions control can be dramatically compromised if the model diverges from actual levels before the calibration is triggered.
It is desirable to provide a method and a system for controlling NH3 levels to better ensure NH3 levels in an SCR catalyst are kept within a desired range.
In accordance with an aspect of the present invention, a method of controlling reductant levels in an SCR catalyst comprises measuring a change of NOx conversion efficiency (dμNOx) across the SCR catalyst, measuring a change of reductant level (dB) in the SCR catalyst, comparing a measured ratio dμNOx/dB to a target ratio, and adjusting reductant injection to cause the measured ratio to approach the target ratio.
In accordance with another aspect of the present invention, a system for controlling reductant levels in an SCR catalyst comprises an injector for injecting reductant upstream of the SCR catalyst, and a controller arranged to measure a change of NOx conversion efficiency (dμNOx) across the SCR catalyst, measure a change of reductant level (dB) in the SCR catalyst, compare a measured ratio dμNOx/dB to a target ratio, and control the injector to adjust reductant injection to cause the measured ratio to approach the target ratio.
In accordance with another aspect of the present invention, a method of controlling reductant levels in an SCR catalyst comprises a) calculating a quantity of reductant in the SCR catalyst as a function of an amount of reductant injected over a first period of time minus an amount of NOx reduced over the first period of time, b) determining a first NOx conversion efficiency (μNOx1) at an end of the first period of time, c) changing reductant injection by a first change amount for a second period of time to a second injection rate different from an injection rate at the end of the first period of time, d) determining a second NOx conversion efficiency (μNOx2) at the end of the second period of time and, if μNOx2>μNOx1, maintaining the second injection rate, and if μNOx2<μNOx1, changing reductant injection by a second change amount
In accordance with another aspect of the present invention, a system for controlling reductant levels in an SCR catalyst comprises an injector for injecting reductant upstream of the SCR catalyst, and a controller arranged to calculate a quantity of reductant in the SCR catalyst as a function of an amount of reductant injected over a first period of time minus an amount of NOx reduced over the first period of time, determine a first NOx conversion efficiency (μNOx1) at an end of the first period of time, control the injector to change reductant injection by a first change amount for a second period of time to a second injection rate different from an injection rate at the end of the first period of time, determine a second NOx conversion efficiency (μNOx2) at the end of the second period of time and, if μNOx2>μNOx1, control the injector to maintain the second injection rate, and if μNOx2<μNOx1, control the injector to change reductant injection by a second change amount in a direction opposite a direction of the change amount.
The features and advantages of the present invention are well understood by reading the following detailed description in conjunction with the drawings in which like numerals indicate similar elements and in which:
The system 21 and SCR catalyst 23 are part of an exhaust aftertreatment system of a diesel engine 37 such as might be used as a vehicle engine or for other purposes. Typically, the system 21 and SCR catalyst 23 are arranged downstream of a diesel particulate filter 39 in the aftertreatment system. The aftertreatment system may include other features not illustrated.
The controller 35 can be arranged to determine NOx conversion efficiency (μNOx) by the equation
The controller 35 can also be arranged to measure an amount of reductant injected over time (Bi) and measure an amount of NOx reduced (NOxred) over time. The controller 35 can also be arranged to measure a change of reductant level (dB) in the SCR catalyst as a function of the amount of reductant (Bi) added over a period of time and the amount of NOx reduced (NOxred) over the period of time.
In an aspect of the invention referred to as “perturbation control”, the controller 35 can also be arranged to measure a change of NOx conversion efficiency (dμNOx) across the SCR catalyst 23 and to measure a change of reductant level (dB) in the SCR catalyst. The controller 35 can be arranged to compare a measured ratio
dμNOx/dB (2)
to a target ratio, usually “0” (zero) in the graph of
The controller 35 can further be arranged to compare a second measured ratio of the first measured ratio to the NOx conversion efficiency
to a second target ratio, usually “0” (zero) in the graph of
A method of controlling reductant levels in the SCR catalyst 23 will be further described in connection with the flow chart seen in
In step 103, an amount of reductant (Bi) injected over time is measured, and in step 105, an amount of NOx reduced (NOxred) over time is measured. In step 107, the change of reductant level (dB) in the SCR catalyst 23 is measured as a function of B and NOxred. Technically, the change of reductant level in the SCR catalyst 23 can only be estimated or modeled with the inputs of amount of reductant (B) injected over time and the amount of NOx reduced (NOxred) over time, however, for purposes of the present discussion, the change of reductant level (dB) in the SCR catalyst 23 shall be referred to as being measured using these inputs.
In a perturbation control aspect, in step 109, a change of NOx conversion efficiency (dμNOx) across the SCR catalyst 23 is determined. In step 111, the measured ratio
dμNOx/dB (2)
is compared to a target ratio. In step 113, reductant injection is adjusted, if necessary, to cause the measured ratio to approach the target ratio.
If desired (as reflected by dotted lines), in step 115, the second measured ratio
is compared to a second target ratio and, in step 117, reductant injection is adjusted, if necessary, to cause the second measured ratio to approach the second target ratio. Also, if desired (as reflected by dotted lines), in step 119, the third measured ratio
ƒ(dμNOx/dB) (4)
is compared to a third target ratio and, in step 121, reductant injection is adjusted, if necessary, to cause the third measured ratio to approach the third target ratio. ƒ(dμNOx/dB) is a function of dμNOx/dB that is defined such that it has a near constant negative slope across the buffer level (similar to the one shown in
A system 21 for controlling reductant levels in an SCR catalyst 23 according to another aspect of the present invention referred to as “storage correction” can be structurally similar to the system described above, but is arranged to operate differently. In the system according to this further aspect of the invention, an amount of NOx reduced over a first period of time is determined at step 201 and an amount of reductant required for the reduction is subtracted from an amount of reductant injected (Bi) over a first period of time determined at step 203, and, at step 205, the controller 35 is arranged to calculate a quantity of reductant (B) in the SCR catalyst as a function of the values determined at steps 201 and 203 according to a conventional technique for measuring (or, perhaps more accurately, estimating or modeling) reductant levels in an SCR.
The controller 35 is arranged to determine a first NOx conversion efficiency (μNOx1) at an end of the first period of time at step 207 while the injector 25 injects reductant at a rate R1. At step 209, the controller 35 is arranged to control the injector 25 to change reductant injection by a change amount X1 for a second period of time to a second injection rate R2 (R2=R1−X) different from the injection rate R1 at the end of the first period of time. At step 211, the controller 35 is arranged to determine a second NOx conversion efficiency (μNOx2) at the end of the second period of time.
At step 213, the controller 35 is arranged to compare μNOx2 and μNOx1. If μNOx2>μNOx1, at step 215, the controller 35 is arranged to control the injector 25 to maintain the second injection rate R2. If μNOx2≦μNOx1, at step 217, the controller 35 controls the injector to change reductant injection a second amount X2 in a direction opposite a direction of the change amount (i.e., if the change amount X1 was a reduction of injection rate, then the change amount X2 will be an increase of injection rate). If, by a comparison at step 219, the NOx conversion efficiency μNOx3 at this further dosing rate R3 is better than μNOx1, i.e., μNOx3>μNOx1, then, at step 221, dosing remains at this changed rate and if, at step 223, μNOx3≦μNOx1, then, at step 225, dosing will return to R1 and, ordinarily, the process will repeat to attempt to obtain increased NOx conversion efficiency. Typically, change amount X2 will be twice change amount X1. For example, if the injector 25 injects reductant at a rate of 1 unit reductant per unit time, the controller 35 might reduce the rate of reductant injection by 10%, or 0.1 units reductant per unit time, and, if NOx conversion efficiency decreases, the controller might then increase the rate of reductant injection by 0.2 units reductant per unit time.
The method for using the system 21 according to this aspect can be triggered to operate so as to change injection and, as appropriate, maintain injection at the changed level or change injection again in an opposite direction by any number of events, such as automatically after a predetermined period of operation or when NOx conversion efficiency falls below a target value. The method permits the conventional mass-based model of calculating reductant level in the SCR as shown in steps 201-205 to be substantially maintained, however, it provides for a correction that will permit the system to be operated for a substantially longer period of time than is typical in a conventional system without resetting the entire system.
It will be appreciated that perturbation control and storage control as described above are not mutually exclusive and can be run at the same time.
In the present application, the use of terms such as “including” is open-ended and is intended to have the same meaning as terms such as “comprising” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such.
While this invention has been illustrated and described in accordance with a preferred embodiment, it is recognized that variations and changes may be made therein without departing from the invention as set forth in the claims.
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| Entry |
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| English Machine Translation of JP2003-293743A to Kawai et al. |
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