WO2014070244A1

WO2014070244A1 - Ammonia slip detection

Info

Publication number: WO2014070244A1
Application number: PCT/US2013/042778
Authority: WO
Inventors: Randall R. ROEPKE; Michael James Miller
Original assignee: International Engine Intellectual Property Company, Llc
Priority date: 2012-11-02
Filing date: 2013-05-25
Publication date: 2014-05-08

Abstract

A method of detecting ammonia in the exhaust system of an internal combustion engine includes detecting upstream and downstream NOx levels relative to the catalyst and producing respective upstream and downstream NOx signals. The method also detects a preselected engine operating condition of the engine, such as a no-combustion condition, and determines decay rates in the upstream and downstream NOx signals during the preselected engine operating condition. The method signals an ammonia slip condition in response to the decay rate of the upstream NOx signal exceeding the decay rate of the downstream NOx signal by more than a preselected amount.

Description

AMMONIA SLIP DETECTION

BACKGROUND

[0001] Selective catalytic reduction (SCR) is commonly used to remove NO_x

(i.e., oxides of nitrogen) from the exhaust gas produced by internal engines, such as diesel or other lean burn (gasoline) engines. In such systems, NO_x is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NO_x.

[0002] Ammonia is often used as the reductant in SCR systems. The ammonia is introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. The SCR catalyst, which is positioned in the exhaust gas stream, causes a reaction between NO_x present in the exhaust gas and a NO_x reducing agent (e.g., ammonia) to convert the NO_x into nitrogen and water.

[0003] Proper operation of the SCR system involves precise control of the amount (i.e., dosing level) of ammonia (or other reductant) that is injected into the exhaust gas stream. If too little reductant is used, an insufficient amount of NO_x will be converted by the catalyst. If too much reductant is used, a portion of the ammonia will pass unreacted through the catalyst in a condition known as "ammonia slip." Thus, it is desirable to be able to detect the occurrence of "ammonia slip" conditions in order to regulate dosing levels.

SUMMARY

[0004] Aspects and embodiments of the present technology described herein relate to one or more systems and methods for controlling detecting ammonia slip in an SCR system. According to at least one aspect of the present technology, a method of detecting ammonia in the exhaust system of an internal combustion engine includes detecting upstream and downstream NO_x levels relative to the catalyst and producing respective upstream and downstream NO_x signal. The method also includes determining the decay rates in the upstream and downstream NO_x signals during a predefined engine operating condition. An ammonia slip condition is signaled when the decay rate of the upstream NO_x signal exceeds the decay rate of the downstream Οχ signals by more than a preselected amount. In at least some embodiments, the predefined engine operating condition may be a "no-combustion" condition. In some embodiments, the no-combustion condition is detected by monitoring fuel flow to the engine. According to some embodiments, the no-combustion condition is detected when no fuel is being supplied to the engine. In some embodiments, the ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined amount.

[0005] According to at least one aspect of the present technology, a method of detecting ammonia in the exhaust system of an internal combustion engine includes detecting upstream and downstream NO_x levels relative to the catalyst and producing respective upstream and downstream NO_x signal. The method also includes detecting predetermined engine operating condition and a no-combustion condition of the engine and determining decay rates in the upstream and downstream NO_x signals during the no-combustion condition. The method signals an ammonia slip condition in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signals by more than a preselected amount.

[0006] At least some embodiments of the present technology relate to a system for detecting ammonia in an exhaust system of an internal combustion engine. The system includes an upstream NO_x sensor positioned to detect the level of NO_x in the exhaust stream at a location upstream of a catalyst and produce a responsive upstream NO_x signal. A downstream NO_x sensor is positioned to detect the level of NO_x in the exhaust stream at a location downstream of the catalyst and produce a responsive downstream NO_x signal. A controller is configured to receive the upstream and downstream NO_x signals. The control is also configured to detect a preselected engine operating condition in the engine, such as a no-combustion condition, and determine decay rates for the upstream and downstream NO_x signals during the preselected engine operating condition. The controller is configured to detect an ammonia slip condition in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signals by more than a preselected amount. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a schematic illustration of an internal combustion engine with an exhaust gas SCR system.

[0008] FIG. 2 is a flow diagram of an exemplary method for determining the

ΝΟχ level in an engine's exhaust system according to certain embodiments of the present technology.

[0009] FIGs. 3A and 3B are graphs that further illustrate the manner in which ammonia slip can be detected according to certain aspects of the present technology.

DETAILED DESCRIPTION

[0010] Various examples of embodiments of the present technology will be described more fully hereinafter with reference to the accompanying drawings, in which such examples of embodiments are shown. Like reference numbers refer to like elements throughout. Other embodiments of the presently described technology may, however, be in many different forms and are not limited solely to the embodiments set forth herein. Rather, these embodiments are examples representative of the present technology. Rights based on this disclosure have the full scope indicated by the claims.

[0011] Fig. 1 shows an exemplary schematic depiction of an internal combustion engine 10 and an SCR system 12 for reducing NO_x from the engine's exhaust. The engine 10 can be used, for example, to power a vehicle such as an over- the-road vehicle (not shown). The engine 10 can be a compression ignition engine, such as a diesel engine, for example. Generally speaking, the SCR system 12 includes a catalyst 20, a reductant supply 22, a reductant injector 24, an electronic control unit ("ECU") 26, an upstream NO_x detector 30 and a downstream NO_x detector 32.

[0012] The ECU 26 controls delivery of a reductant, such as ammonia, from the reductant supply 22 and into an exhaust system 28 through the reductant injector 24. The reductant supply 22 can include canisters (not shown) for storing ammonia in solid form. In most systems, a plurality of canisters will be used to provide greater travel distance between recharging. A heating jacket (not shown) is typically used around the canister to bring the solid ammonia to a sublimation temperature. Once converted to a gas, the ammonia is directed to the reductant injector 24. The reductant injector 24 is positioned in the exhaust system 28 upstream from the catalyst 20. As the ammonia is injected into the exhaust system 28, it mixes with the exhaust gas and this mixture flows through the catalyst 20. The catalyst 20 causes a reaction between NO_x present in the exhaust gas and a NO_x reducing agent (e.g., ammonia) to reduce/convert the NO_x into nitrogen and water, which then passes out of the tailpipe 34 and into the environment. While the SCR system 12 has been described in the context of solid ammonia, it will be appreciated that the SCR system could alternatively use a reductant such as pure anhydrous ammonia, aqueous ammonia or urea, for example.

[0013] The upstream NO_x sensor 30 is positioned to detect the level of NO_x in the exhaust stream at a location upstream of the catalyst 20 and produce a responsive upstream NO_x signal. As shown in FIG. 1, the upstream NO_x sensor 30 may be positioned in the exhaust system 28 between the engine and the injector 24. The downstream NO_x sensor 32 positioned to detect the level of NO_x in the exhaust stream at a location downstream of the catalyst 20 and produce a responsive downstream NO_x signal. The ECU is connected to receive the upstream and downstream NO_x signals from the sensors 30 and 32. The ECU 26 may be configured to control reductant dosing from the injector 24 in response to the upstream and/or downstream Οχ signals (and other sensed parameters). The present technology is not limited to any particular dosing strategy. Accordingly, the particulars of the dosing strategy are not detailed herein.

[0014] In addition to controlling the dosing or metering of ammonia, the ECU

26 can also store information such as the amount of ammonia being delivered, the canister providing the ammonia, the starting volume of deliverable ammonia in the canister, and other such data which may be relevant to determining the amount of deliverable ammonia in each canister. The information may be monitored on a periodic or continuous basis. When the ECU 26 determines that the amount of deliverable ammonia is below a predetermined level, a status indicator (not shown) electronically connected to the controller 26 can be activated.

[0015] FIG. 2 is a flow chart of an exemplary method 200 for detecting ammonia slip in an SCR system according to certain aspects of the present technology. The method 200 begins in step 205. Control is then passed to stop 210 where the method determines an upstream NO_x level, e.g., by reading the upstream Οχ signal from the upstream NO_x sensor 30. Control is then passed to step 215, where the method determines a downstream NO_x level, e.g., by reading the downstream NO_x signal from the downstream NO_x sensor 32.

[0016] Control is then passed to the step 215 where the method checks to see if the engine is in a preselected engine operating condition. In at least some embodiments, the preselected engine operating condition may be a "no-combustion" condition. In at least some embodiments, a no-combustion condition may correspond to a condition where no fuel is being injected into the engine. In some embodiments, the method can detect a "no-combustion" condition in instances where no fuel is being injected into the engine. In some embodiments, the method can detect the no fuel condition based on accelerator pedal position or fuel setting, for example. In at least some embodiments, the no fuel condition may occur when the engine is operated at idle or when the vehicle is "motoring (i.e. coasting downhill)," for example.

[0017] o_x sensors are typically cross-sensitive to ammonia. More specifically, most o_x sensors do not distinguish between o_x and ammonia. As will be appreciated, the engine does not produce o_x in the absence of combustion. Furthermore, since o_x is not produced during periods of no combustion, the SCR system 12 typically will not inject any, e.g., ammonia, into the exhaust system 28 during periods of no combustion. Accordingly, as shown in FIG. 3A, the signal from the upstream o_x sensor will typically drop to zero during periods of no combustion. Furthermore, if the correct amount of reductant is being injected into the system, the signal from the downstream o_x sensor will also drop to zero. However, if excess ammonia is present in the system, the signal from the downstream o_x sensor 30 will generally not drop to zero or will typically decay more slowly than the signal from the signal from the upstream o_x sensor. (See, e.g., FIG. 3B). The phenomena occurs because ammonia (N₀₃) is a relatively "sticky" molecule in comparison to o_x (NO or N₀₂), meaning N₀₃ takes longer to pass through the catalyst 20 and exhaust system 28 than does No_x. As a result, if too much reductant was being injected before the occurrence of the "no-combustion" condition, the presence of the ammonia can be detected [0018] Accordingly, the method continues to loop through steps 205-215 until a "no-combustion" condition is detected. Upon detecting a no-combustion condition, control is passed to step 220 where the method determines the decay rates upstream and downstream NO_x signals. Control is then passed to step 225, where the method compares the decay rate of the upstream NO_x signal to that of the downstream NO_x signal to determine if ammonia slip is occurring across the catalyst 20. In some embodiments, the method signals an ammonia slip condition in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signals by more than a preselected amount. In at least some embodiments, the ammonia slip condition may be signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined level. Accordingly, if the decay rates differ by more than the preselected threshold, the method passes control to step 230 to signal an ammonia slip condition. In some embodiments, the ammonia slip condition may be signaled by setting a software flag, for example. When an ammonia slip condition is identified, the ECU may be configured to adjust the dosing level of the reductant to reduce and/or eliminate the level of ammonia slip.

[0019] At least some embodiments of the present technology relate to a system for detecting ammonia in an exhaust system of an internal combustion engine. Referring again to Fig. 1, the system may generally include the upstream o_x sensor 30, the downstream o_x sensor 32 and a controller such as the ECU 26. The ECU 26 may be configured to receive the upstream and downstream o_x signals. The ECU 26 may be configured to detect a preselected engine operating condition in the engine, such as a no-combustion condition. The ECU 26 may further be configured to determine decay rates for the upstream and downstream N_0x signals during the preselected engine operating condition. The ECU 26 may also be configured to detect an ammonia slip condition in response to the decay rate of the upstream o_x signal exceeding the decay rate of the downstream o_x signals by more than a preselected amount.

[0020] While this disclosure has been described as having exemplary embodiments, this application is intended to cover any variations, uses, or adaptations using the general principles set forth herein. It is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the spirit and scope of the disclosure as recited in the following claims. Further, this application is intended to cover such departures from the present disclosure as come within the known or customary practice within the art to which it pertains.

Claims

1. A method of detecting ammonia in an exhaust system of an internal combustion engine comprising; detecting an upstream NO_x level relative to the catalyst and producing an upstream NO_x signal; detecting a downstream NO_x level relative to the catalyst and producing an downstream NO_x signal; detecting a preselected operating condition of the engine; determining a decay rate in the upstream NO_x signal during the preselected engine operating condition; determining a decay rate in the downstream NO_x signal during the preselected engine operating condition; signaling an ammonia slip condition in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a preselected amount.

2. The method of claim I, wherein the preselected engine operating comprises a no-combustion condition during which combustion is not occurring in the engine.

3. The method of claim 2, wherein the no-combustion condition is detected by monitoring fuel flow to the engine.

4. The method of claim 3, wherein the no-combustion condition is detected when no fuel is being supplied to the engine.

5. The method of claim I, wherein an ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined amount.

6. The method of claim 5, wherein an ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined amount.

7. The method of claim 6, wherein an ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined amount.

8. A system for detecting ammonia in an exhaust system of an internal combustion engine, the exhaust system including an SCR, the system comprising: an upstream NO_x sensor positioned to detect the level of NO_x in the exhaust stream at a location upstream of the catalyst and produce a responsive upstream NO_x signal; a downstream NO_x sensor positioned to detect the level of NO_x in the exhaust stream at a location downstream of the catalyst and produce a responsive downstream NO_x signal; a controller configured to receive the upstream and downstream NO_x signals; detect an engine operating condition; determine decay rates for the upstream and downstream NO_x signals during the preselected engine operating condition; and detect an ammonia slip condition in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a preselected amount.

9. The system of claim 8, wherein the preselected engine operating condition comprises a no-combustion condition during which combustion is not occurring in the engine.

10. The system of claim 9, wherein the controller is configured to detect the no-combustion condition in response to level of fuel being supplied to the engine.

1 1. The system of claim 10, wherein the controller is configured to detect the no-combustion condition and no fuel is being supplied to the engine.

12. The method of claim 8, wherein an ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than about a predetermined amount.

13. The method of claim 12, wherein an ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined amount.

14. The method of claim 13, wherein an ammonia slip condition is signaled in response to the decay rate of the upstream NO_x signal exceeding the decay rate of the downstream NO_x signal by more than a predetermined amount.