WO2014070248A1

WO2014070248A1 - Exhaust system after-treatment

Info

Publication number: WO2014070248A1
Application number: PCT/US2013/042835
Authority: WO
Inventors: Navtej Singh; Randall R. ROEPKE; Vijayasarathy SUBRAMANIAN
Original assignee: International Engine Intellectual Property Company, Llc
Priority date: 2012-11-05
Filing date: 2013-05-28
Publication date: 2014-05-08

Abstract

A processor processes data including data indicative of a quantity of NOx in exhaust upstream of where ammonia is introduced into an exhaust system, data indicative of a quantity of NOx in exhaust downstream of an SCR catalyst, and data indicative of exhaust flow rate, in accordance with an algorithm to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂O.

Description

EXHAUST SYSTEM AFTER-TREATMENT

Technical Field

[0001] This disclosure relates generally to after-treatment of exhaust gas by Selective Catalytic Reduction (SCR) to reduce oxides of Nitrogen (NOx) to Nitrogen (N₂) gas and water (H₂0) vapor, and more particularly to a system and method having improved accuracy in estimating a quantity of ammonia (NH₃) consumed by SCR conversion of NOx.

Background

[0002] One technology for after-treatment of engine exhaust utilizes SCR to enable certain chemical reactions to occur between NOx in the exhaust and ammonia introduced into an engine exhaust system upstream of an SCR catalyst to entrain with exhaust flowing toward the catalyst. Those reactions convert NOx into Nitrogen (N₂) and water (H₂0), two constituents found in abundance in earth's

atmosphere. NOx and NH₃ are the only reactants in certain of those reactions while Oxygen (0₂), which may be present in the exhaust, is a third reactant in other reactions.

[0003] Ammonia is introduced in sufficient quantity to maintain a

presence of ammonia on surfaces of the SCR catalyst where the chemical reactions which reduce NOx take place. Summary

[0004] Exhaust flow through an exhaust system of an internal combustion engine varies with how the engine is operating. The rate at which ammonia is introduced into an exhaust after-treatment system which has an SCR catalyst for converting NOx in the exhaust into N₂ and H₂0 by catalytic chemical reactions with the ammonia is controlled in a manner which at least at times is a function of the mass flow rate of exhaust.

[0005] The disclosed subject matter of the present patent application relates to a system and method in which the data from the two sensors is processed in a way which further improves accuracy of the estimate of ammonia actually consumed to reduce NOx into N₂ and H₂0.

[0006] One general aspect of the disclosed subject matter relates to an internal combustion engine comprising combustion chambers within which fuel is combusted to operate the engine, an intake system through which air for supporting combustion is introduced into the combustion chambers, an exhaust system through which exhaust resulting from combustion in the combustion chambers passes to atmosphere and which comprises an SCR catalyst, and an ammonia delivery system for introducing ammonia into the exhaust system upstream of the SCR catalyst for entrainment with exhaust flow toward the SCR catalyst.

[0007] A processor processes data according to an algorithm, including data indicative of exhaust flow rate, data indicative of a quantity of NOx present at a first time in exhaust at a location upstream of where ammonia is introduced into the exhaust system, and data from the downstream NOx sensor at a second time which is later than the first time, to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂0.

[0008] The algorithm performs a calculation of transit time for exhaust to travel to the downstream NOx sensor from the location upstream of where ammonia is introduced into the exhaust system, and uses the calculated transit time as a length of time by which the second time is delayed from the first time.

[0009] Another general aspect of the disclosed subject matter relates to a method for estimating a quantity of ammonia being consumed by chemical reactions which are enabled by an SCR catalyst within an exhaust system of an internal combustion engine to reduce NOx in exhaust passing through the exhaust system to N₂ and H₂0.

[0010] The method comprises: at a first time obtaining data indicative of a quantity of NOx in exhaust at a location upstream of where ammonia is introduced into the exhaust system; at a second time, which is later than the first time, obtaining data from a downstream NOx sensor indicative of a quantity of NOx in exhaust downstream of the SCR catalyst; calculating transit time for exhaust to travel to the downstream NOx sensor from the location upstream of where ammonia is introduced into the exhaust system; using the calculated transit time as a length of time by which the second time is delayed from the first time; and processing the data obtained at the first time and the data obtained at the second time in accordance with an algorithm to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂0.

[0011] Another general aspect of the disclosed subject matter relates to a system for estimating a quantity of ammonia consumed by selective catalytic reduction (SCR) of NOx in exhaust flow through an exhaust system having: an SCR catalyst; an ammonia supply; an ammonia outlet through which ammonia from the ammonia supply is introduced into the exhaust flow; a downstream NOx sensor providing data indicative of a quantity of NOx present in exhaust flow downstream of the SCR catalyst; a data source providing data indicative of a quantity of NOx present in exhaust flow at a location upstream of where ammonia is introduced into the exhaust system; and a processor for processing, according to an algorithm, data including data indicative of exhaust flow rate, data provided by the data source at a first time, and data provided by the downstream NOx sensor at a second time which is later than the first time, to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂0.

[0012] The algorithm performs a calculation of transit time for exhaust to travel to the downstream NOx sensor from the location upstream of where ammonia is introduced into the exhaust system, and uses the calculated transit time as a length of time by which the second time is delayed from the first time.

[0013] The foregoing summary is accompanied by further detail of the disclosure presented in the Detailed Description below with reference to the following drawings which are part of the disclosure. Brief Description of the Drawings

[0014] Figure 1 is a general schematic diagram of an internal combustion engine having an exhaust after-treatment system which utilizes SCR to reduce NOx in engine exhaust by chemical reaction with ammonia introduced into the exhaust.

[0015] Figure 2 is a diagram illustrating detail of a NOx reduction system present in Figure 1.

[0016] Figure 3 is a diagram of an algorithm for estimating a quantity of reductant (NH₃) consumed by chemical reaction with NOx in the NOx reduction system of Figure 2.

[0017] Figure 4 is a flow diagram of steps of the presently disclosed system and method.

[0018] Figures 5A and 5B are respective traces for illustrating the improvement provided by the presently disclosed system and method.

Detailed Description

[0019] Figure 1 shows a representative internal combustion engine 10 which can be used in stationary or mobile applications. For example, engine 10 may be a diesel engine which comprises structure forming a number of engine cylinders 12 into which fuel is injected by fuel injectors 14 to combust with air which has entered combustion chamber spaces of engine cylinders 12 through an intake system 16 when cylinder intake valves 18 for controlling admission of air from an intake manifold 20 into respective engine cylinders 12 are open. Other components which may be present in intake systems of contemporary diesel engines, such as a turbocharger compressor and charge air cooler, are not shown.

[0020] Engine 10 also comprises an exhaust system 22 through which engine exhaust created by combustion of injected fuel in the combustion chamber spaces to operate engine 10 is conveyed to atmosphere. Cylinder exhaust valves 24 control admission of exhaust from respective engine cylinders 12 into an exhaust manifold 26 for further conveyance through exhaust system 22.

[0021] Exhaust system 22 includes an exhaust after-treatment system 28 for treating exhaust prior to entry into the atmosphere. Other components which may be present in exhaust systems of contemporary diesel engines, such as a turbocharger turbine, are not shown.

[0022] A processor-based engine control module (ECM) 30 controls various aspects of engine operation, such as fueling of engine cylinders 12 by fuel injectors 14. Control is accomplished by processing various input data, indicated generally by reference numeral 32, to develop control data for control of functions being performed by various devices.

[0023] Exhaust after-treatment system 28 is shown in detail in Figure 2 to comprise structure through which exhaust is constrained to pass. The particular structure shown comprises a generally cylindrical housing 34 having an axial length, an exhaust entrance 36 at an upstream axial end, and an exhaust exit 38 at a downstream axial end. Arrows 40 indicate a direction of exhaust flow into, through and out of interior space of housing 34. [0024] Within its interior space, housing 34 contains a diesel oxidation catalyst (DOC) 42 downstream of exhaust entrance 36 and an SCR catalyst 44 downstream of DOC 42. Exhaust which enters the interior space of housing 34 through exhaust entrance 36 is forced to pass first through DOC 42 and subsequently through SCR catalyst 44 before exiting through exhaust exit 38. DOC 42 treats engine exhaust by removing certain entrained matter, such as the soluble organic fraction of diesel particulate matter. SCR catalyst 44 treats engine exhaust by reducing NOx according to chemical reactions such as:

[0025] 4 NO+ 4NH3+ 02 → 4N2 + 6H20

[0026] NO + N02 + 2NH3 → 2N2 + 3H20

[0027] 6 N02 + 8NH3 → 7N2 + 12H20

[0028] SCR catalyst 44 may be a type which not only reduces NOx but also traps entrained particulate matter (soot) and therefore requires occasional regeneration.

[0029] An ammonia supply 46 stores ammonia which is used for NOx reduction. Ammonia from supply 46 is introduced into the interior of housing 34 through a conduit 48 having an ammonia outlet 50 disposed at a location downstream of DOC 42 and upstream of SCR catalyst 44. Figure 2 is intended to portray good distribution of ammonia within the exhaust flow without reference to specific structural details of how that is accomplished so that a continuing presence of ammonia can be assured over as large a surface area of SCR catalyst 44 as possible.

[0030] A processor-based ammonia dosing controller 52 controls the introduction of ammonia from ammonia supply 46 into housing 34 by processing various data, including data from ECM 30 with which it has communication. Controller 52 also processes data according to an algorithm for estimating a quantity of ammonia consumed by conversion of NOx in exhaust passing through exhaust system 22. That algorithm 54 will be explained with reference to Figures 3 and 4.

[0031] The algorithm processes data indicating a quantity of NOx as measured at a location upstream of outlet 50 at a first time Tland data indicating a quantity of NOx as measured at a location downstream of SCR catalyst 44 at a second time Tl + ΔΤ. These data are provided respectively by an upstream NOx sensor 56 upstream of DOC 42 and a downstream NOx sensor 58, both shown in Figure 2. Assuming that NOx sensor 58 is insensitive to ammonia, it should provide a reasonably accurate measurement of NOx in the presence of any ammonia slip. NOx sensor 56 may be replaced by a sufficiently accurate virtual NOx sensor. In this way the downstream measurement is delayed from the upstream measurement by the length of time ΔΤ, the transit time required for exhaust present at upstream NOx sensor 56 at time Tl to travel to downstream NOx sensor 58.

[0032] The transit time ΔΤ is a function of the physical configuration of the exhaust system, typically a constant, and of mass flow rate of exhaust, a variable which depends on how engine is operating. Hence, Figure 4 shows the use of exhaust flow rate in the calculation of ΔΤ.

[0033] Figure 3 shows that controller 52 processes data from NOx sensor 56 indicating a quantity of NOx in untreated, i.e. "engine out", exhaust at time Tl and data from NOx sensor 58 at time Tl + ΔΤ indicating a quantity of NOx in exhaust which has been treated by SCR using ammonia stored on the surface of SCR 44. The processing performs a first calculation 60 which yields a fraction equal to the quantity of NOx indicated by downstream NOx sensor 58 divided by the quantity of NOx indicated by upstream NOx sensor 56. A second calculation 62 subtracts the calculated fraction from unity, leaving a resulting fraction representing the fraction of NOx which has been reduced.

Using the three chemical reactions given above and the result of calculation 62, algorithm 54 performs a calculation 64 of a quantity of ammonia which would have been consumed in order to reduce NOx to the resulting fraction calculated by step 62. Because the NOx sensors do not distinguish between NO and N0₂, the calculation may utilize a look-up table containing relative proportions of NO and N0₂ in the exhaust as a function of engine operation obtained by known techniques. The look-up table has been derived from previous mapping of engine exhaust at various engine operating conditions during engine development. Hence, algorithm 54 is repeatedly calculating the quantity of ammonia consumed to reduce NO and the quantity of ammonia consumed to reduce N0₂ over each interval of time between successive calculations as engine 10 operates. The calculation of the total quantity of ammonia consumed from an initial time to a present time is obtained by adding the respective quantities consumed to reduce NO and N0₂ during each interval between calculations. Because the calculation prior to a calculation 66 is based in parts per million (ppm), it must be converted from ppm to a flow rate [mass/time]. An estimate of the actual consumption is therefore calculated by calculation 66 which multiplies the result of calculation 64 by exhaust flow as measured or estimated in any suitably appropriate way. Depending on the unit of measurement of exhaust flow, a calculation 68 may be needed to convert the result of calculation 66 to desired units of ammonia consumption.

[0035] The processing which is provided by the presently disclosed system and method is illustrated in Figure 4. By taking into account exhaust mass flow rate, as measured or estimated in any suitably appropriate way, its potential influence on the estimate of actual ammonia consumption and/or NOx conversion efficiency is essentially eliminated.

[0036] The processing performed by controller 52 comprises a step 70 of measuring NOx upstream of outlet 50 at a time Tl using upstream NOx sensor 56, or alternately a virtual NOx sensor. Step 70 is followed by a step 72 of processing exhaust mass flow rate data to calculate the transit time ΔΤ required for exhaust present at upstream NOx sensor 56 at time Tl to travel to downstream NOx sensor 58.

[0037] Step 72 is followed by a step 74 of measuring NOx at downstream NOx sensor 58 at time Tl + ΔΤ. Step 74 is followed by a step 76 which comprises the processing of Figure 3 using the upstream NOx measurement at time Tl and the downstream NOx measurement at time T1+ ΔΤ to calculate an estimate of actual ammonia consumption and/or NOx conversion efficiency.

[0038] Transit time ΔΤ may be determined in various ways. If rate of change of exhaust mass flow rate is a second order effect which need not be taken into account, then controller 52 can comprise a look-up table containing transit time values each of which is correlated with a particular mass flow rate. When a measurement of upstream NOx is taken, the processing performed by step 72 includes using the flow rate measurement to select the corresponding transit time from the look-up table and then using that transit time to determine when the downstream NOx measurement is to be taken.

[0039] The effect of taking mass flow rate into account is graphically portrayed by comparing Figure 5A (mass flow rate not taken into account) and Figure 5B (mass flow rate taken into account).

[0040] Figure 5A shows a real time trace 80 representative changing actual upstream NOx as a function of time. Figure 5A shows a real time trace 82 representative of changing actual downstream NOx as a function of time when NOx is not being converted. (The sinusoidal shapes of the traces are solely for illustrative convenience.) The traces 80, 82 are not aligned with each other.

[0041] Figure 5B shows trace 82 having been brought into alignment with trace 80 by using the processing of Figure 4 so that downstream NOx is measured in the same exhaust as that in which the upstream NOx was measured.

Claims

WHAT IS CLAIMED IS:

1. An internal combustion engine comprising:

combustion chambers within which fuel is combusted to operate the engine; an intake system through which air for supporting combustion is introduced into the combustion chambers;

an exhaust system through which exhaust resulting from combustion in the combustion chambers passes to atmosphere and which comprises an SCR catalyst; an ammonia delivery system for introducing ammonia into the exhaust system upstream of the SCR catalyst for entrainment with exhaust flow toward the SCR catalyst;

a downstream NOx sensor providing data indicative of a quantity of NOx present in exhaust downstream of the SCR catalyst; and

a processor for processing data according to an algorithm, including data indicative of exhaust flow rate, data indicative of a quantity of NOx present at a first time in exhaust at a location upstream of where ammonia is introduced into the exhaust system, and data from the downstream NOx sensor at a second time which is later than the first time, to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and ¾0,

the algorithm performing a calculation of transit time for exhaust to travel to the downstream NOx sensor from the location upstream of where ammonia is introduced into the exhaust system, and using the calculated transit time as a length of time by which the second time is delayed from the first time.

2. The engine set forth in Claim 1 including an upstream NOx sensor providing the data indicative of a quantity of NOx present at the first time in exhaust at a location upstream of where ammonia is introduced into the exhaust system.

3. The engine set forth in Claim 2 including a diesel oxidation catalyst in the exhaust system upstream of the SCR catalyst, and in which the upstream NOx sensor is disposed upstream of the diesel oxidation catalyst.

4. The engine set forth in Claim 1 comprising a look-up table containing transit times, each correlated with a corresponding exhaust flow rate, and the algorithm using a determination of current exhaust flow rate to select a

corresponding transit time from the look-up table as the calculated transit time.

5. A method for estimating a quantity of ammonia being consumed by chemical reactions which are enabled by an SCR catalyst within an exhaust system of an internal combustion engine to reduce NOx in exhaust passing through the exhaust system to N₂ and H₂0, the method comprising:

at a first time obtaining data indicative of a quantity of NOx in exhaust at a location upstream of where ammonia is introduced into the exhaust system, at a second time, which is later than the first time, obtaining data from a downstream NOx sensor indicative of a quantity of NOx in exhaust downstream of the SCR catalyst, calculating transit time for exhaust to travel to the downstream NOx sensor from the location upstream of where ammonia is introduced into the exhaust system, using the calculated transit time as a length of time by which the second time is delayed from the first time, and processing the data obtained at the first time and the data obtained at the second time in accordance with an algorithm to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂0.

6. The method set forth in Claim 5 comprising using an upstream NOx sensor to provide the data indicative of a quantity of NOx in exhaust upstream of where ammonia is introduced into the exhaust system.

7. The method set forth in Claim 5 comprising using an upstream NOx sensor to provide the data indicative of a quantity of NOx in exhaust upstream of a diesel oxidation catalyst which is upstream of where ammonia is introduced into the exhaust system.

8. The method set forth in Claim 5 comprising providing a look-up table containing transit times, each correlated with a corresponding exhaust flow rate, and using a determination of current exhaust flow rate to select a corresponding transit time from the look-up table as the calculated transit time.

9. A system for estimating a quantity of ammonia consumed by selective catalytic reduction (SCR) of NOx in exhaust flow through an exhaust system, the system comprising:

an SCR catalyst;

an ammonia supply;

an ammonia outlet through which ammonia from the ammonia supply is introduced into the exhaust flow; a downstream NOx sensor providing data indicative of a quantity of NOx present in exhaust flow downstream of the SCR catalyst;

a data source providing data indicative of a quantity of NOx present in exhaust flow at a location upstream of where ammonia is introduced into the exhaust system; and

a processor for processing, according to an algorithm, data including data indicative of exhaust flow rate, data provided by the data source at a first time, and data provided by the downstream NOx sensor at a second time which is later than the first time, to calculate an estimate of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂0,

10. The system set forth in Claim 9 in which the data source comprises an upstream NOx sensor.

11. The system set forth in Claim 10 including a diesel oxidation catalyst in the exhaust system upstream of the SCR catalyst, and in which the upstream NOx sensor is disposed upstream of the diesel oxidation catalyst.

12. The system set forth in Claim 9 comprising a look-up table containing transit times, each correlated with a corresponding exhaust flow rate, and the algorithm using a determination of current exhaust flow rate to select a