Diesel Engine Nox Reduction

Abstract

An engine system for treating nitrogen oxides present in an exhaust gas generated by the combustion of fuel. The engine system includes one or more long breathing lean nitrogen oxide traps that is/are configured to store at least a portion of the nitrogen oxide in the exhaust gas when the lean nitrogen oxide trap operates in an absorption mode. The lean nitrogen oxide trap is also configured for the conversion of the nitrates stored by the lean nitrogen oxide trap during a regeneration event. The engine-out nitrogen oxide levels may be reduced to extend the duration of the absorption process, thereby reducing both the frequency of regeneration events and the associated fuel penalty. The system may include primary and secondary exhaust gas recirculation systems, with exhaust gas from the secondary system being returned to the engine cylinder to reduce the level of nitrogen oxides in that exhaust gas.

Description

BACKGROUND

Combustion engines may employ emission controls or systems that are configured to reduce the amount of nitrogen oxides (NO_x) present in the engine's exhaust gas. One approach to controlling such emissions may include the use of a Selective Catalytic Reduction system (SCR). The SCR typically uses a catalyst and a reductant to convert NO_x in the exhaust gas into at least nitrogen gas and water. The reductant may be a liquid or gas, such as, for example, urea, anhydrous ammonia, or aqueous ammonia, among others, that is injected into the stream of exhaust gas generated by the engine.

For example, in instances in which urea is injected into an exhaust gas stream having normal diesel exhaust gas temperatures, the urea undergoes a thermal decomposition and hydrolysis to produce ammonia. The ammonia may then react with NO_x on an SCR catalyst substrate to produce nitrogen and water. As the NO_x conversion efficiency of SCR catalysts may exceed 80% over a relatively wide range of exhaust gas temperatures, high engine-out NO_x levels can be tolerated. Thus, in at least some applications in which an SCR is used with diesel engines, the diesel engines may be calibrated to run near optimum performance in terms of fuel consumption, as the SCR may be able to handle engine-out NO_x levels of the order of several hundred parts-per-million (ppm), and may even tolerate NO_x levels exceeding 1000 ppm.

However, the reductants employed by an SCR, such as urea fluid, are typically not naturally found on-board a vehicle. Thus, an SCR often requires that a vehicle include an on-board storage and delivery system for the reductant or for the solution used to provide the reductant. However, with such systems, it is typically the responsibility of the end-user to maintain a supply of reductant for the supply system. For example, the end-user is typically responsible for re-filling a depleted storage system with reductant, such as, for example, re-filling or replacing reductant depleted storage cartridges. However, placing such responsibility on the end-user to monitor and maintain the SCR in functioning condition may be problematic.

Another approach to controlling NO_x emissions for a lean burn engine may include the use of a lean NO_x trap (LNT). The LNT is typically a high surface area ceramic substrate coated with a washcoat which usually contains at least a NO_x storage substance, such as barium oxide (BaO), and a precious metal. Under typical diesel oxygen rich exhaust conditions, NO_x in the exhaust gas may be stored as solid nitrates on the LNT catalyst surface. The LNT catalyst however may become at least partially saturated with NO_x over a relatively short time period, which may severely reduce the NO_x storage capacity of the LNT catalyst due to a finite amount of NOx storage sites. Moreover, with traditional diesel combustion, typical engine-out NO_x levels are relatively high, such as being on the order of hundreds of ppm, and consequently the LNT saturation process is relatively quick.

Accordingly, an LNT catalyst requires a NO_x regeneration or desorption processes for the release and conversion of the stored nitrates to nitrogen. Regeneration may be achieved via a temporary fuel rich spike in the exhaust gas, which may be provided by the injection of fuel into the exhaust gas stream, or by calibration of the engine's fuel injection map to account for the need for LNT regeneration. Under fuel rich conditions, the solid nitrates are released from the catalyst surface of the LNT and reduced to nitrogen. At least some catalysts used with LNTs may convert up to over approximately 80% of the stored nitrates under optimal conditions. To prevent excessive NO_x slip through the LNT, regeneration typically takes place before the LNT is fully saturated with nitrates, implying that a fuel rich spike is required every couple of minutes under traditional operation of a diesel engine, as illustrated by FIG. 1. However, as illustrated, frequent regeneration and the associated dosing of exhaust gas with fuel may lead to a relatively high fuel consumption penalty, as well as relatively frequent NO_x slip.

BRIEF SUMMARY

According to certain embodiments, an engine system is provided for treating nitrogen oxides present in an exhaust gas generated by the combustion of fuel. The engine system includes a diesel engine configured to generate the exhaust gas by the combustion of a fuel. The diesel engine may be calibrated, through the use of in-cylinder NO_x reduction strategies which may incur a minor fuel penalty, for the generated exhaust gas to have an engine-out nitrogen oxide level of less than around 100 parts-per-million. The engine system also includes a lean nitrogen oxide trap that is configured to store at least a portion of the nitrogen oxide in the exhaust gas when the lean nitrogen oxide trap operates in an absorption mode. The lean nitrogen oxide trap is also configured for the conversion of a plurality of nitrates stored by the lean nitrogen oxide trap during a regeneration event. According to certain embodiments, the ratio of a duration that the lean nitrogen oxide trap is in the absorption mode to a duration of the regeneration event may be approximately 6:1, however this ratio may vary for other embodiments.

Additionally, certain embodiments provide an engine system for treating nitrogen oxides present in an exhaust gas generated by the combustion of fuel by an engine. The engine system includes a primary exhaust gas recirculation system configured for at least a portion of the exhaust gas generated by the diesel engine to be circulated to an intake manifold of the engine. The engine system also includes at least one injector configured to inject a fuel into the exhaust gas as well as a plurality of lean nitrogen oxide traps. Additionally, the engine system includes a secondary exhaust gas recirculation system that is configured to recirculate at least a portion of the exhaust gas that exits one or more of the lean nitrogen oxide traps to a location upstream of the engine.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates the duration and frequency of absorption processes, regeneration events, and fuel injection, as well as NO_x slippage, for an LNT that is operating under conventional diesel engine operating parameters.

FIG. 2 illustrates a diesel engine system that includes an exhaust gas after-treatment system having a long-breathing LNT.

FIG. 3 illustrates the duration and frequency of absorption processes, regeneration events, and fuel injection, as well as NO_x slippage, for an LNT that is operating under engine operating parameters that allow for a long breathing LNT.

FIG. 4 provides after-treatment flow bench test data under optimized conditions illustrating an absorption process for a long breathing LNT in which the simulated exhaust NO_x level has been reduced to approximately 50 ppm.

FIG. 5 illustrates a diesel engine system that includes an exhaust gas after-treatment system having a long-breathing LNT and a secondary EGR system.

FIG. 6 presents experimental results of the removal of NO_x from intake air that was dosed with NO_x from a bottled cylinder by a diesel engine that was operating in the low temperature combustion mode.

FIG. 7 illustrates a diesel engine system that includes an exhaust gas after-treatment system having a plurality of long-breathing LNTs and associated switches and injectors as well as a secondary EGR system.

DETAILED DESCRIPTION

FIG. 2 illustrates a diesel engine system 10 that includes an exhaust gas after-treatment system 14 having a long-breathing LNT 46. As shown, air for use in the operation of the engine system 10, such as, for example, for use during an internal combustion process, may flow along an intake line 20 that includes various hoses and/or tubes. For example, air passes along a first portion of the intake line 20 and into a low pressure compressor 22 before flowing along a second portion of the intake line 20 to the interstage cooler 24. The air then flows through a high pressure compressor 26 and high pressure charged air cooler 28 before flowing through another portion of the intake line 20 to an intake manifold 30.

The air may flow through the intake manifold 30 and to cylinders 32 of the engine 34, where the air may be used in a combustion event(s) that is used to displace the pistons of the engine 34, thereby transmitting the force of the combustion event(s) into mechanical power that is used to drive the drivetrain of the associate vehicle. The resulting hot exhaust gas and associated particulate matter, such as soot, produced by or during the combustion event(s) may then flow out of the cylinders 32 and engine 34 through an exhaust port(s) and along an exhaust line 36.

According to certain embodiments, at least a portion of the hot exhaust gas from the engine 34 may be diverted from the exhaust line 36 and to a primary exhaust gas recirculation (EGR) system 38. The primary EGR system 38 is configured to recirculate the diverted exhaust gas back to the intake manifold 30. However, before the primary EGR system 38 recirculates exhaust gas, the exhaust gas is typically cooled by an EGR cooler 40 or heat exchanger. A coolant, such as antifreeze mixtures or non-aqueous solutions, among others, typically circulates through the EGR cooler 40. By recirculating cooled exhaust gas back into the intake manifold, cooled exhaust gas may occupy a portion of the cylinder(s) 32 that may otherwise be occupied by a gas with a relatively high concentration of oxygen, such as fresh air, which may result in a reduction in the temperatures attained in the cylinder 32 during a combustion event. Because NO_x forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, lowering the temperature of the combustion event in the cylinder 32 through the use of the cooled exhaust gas re-circulated by the primary EGR system 38 may reduce the quantity of NO_x generated as a result of the combustion event.

According to certain embodiments, exhaust gas that is not diverted to the primary EGR system 38 may continue to flow along the exhaust line 36 and be delivered to a high pressure turbine 42. The exhaust gas, and the heat entrained therein, may then at least assist in driving the high pressure turbine 42. Power generated by the high pressure turbine 42 may at least in part be used to power or drive the high pressure compressor 26.

Exhaust gas exiting the high pressure turbine 42 may then flow along the exhaust line 36 to a low pressure turbine 44. The low pressure turbine 44 may also be configured to be driven by the exhaust gas, and the heat entrained therein. Additionally, operation of the low pressure turbine 44 may be used to power or drive the low pressure air compressor 22. Although FIG. 2 illustrates high and low pressure turbines 42, 44, according to certain embodiments, a single turbine or more than two turbines may be employed.

According to the embodiment shown in FIG. 2, exhaust gas exiting the low pressure turbine 44 passes through the exhaust line 36 and into the after-treatment system 14. The after-treatment system 14 is illustrated as including an LNT 46 and a diesel particulate filter (DPF) 48. However, the after-treatment system 14 may include a variety of other components, including, for example, a fuel reformer and fuel and/or water injector(s), among other components any of which may be arranged in any of the numerous possible configurations.

The LNT 46 may operate as a long-breathing LNT, in which engine 34 operating parameters allow for the extension of the duration in which the LNT 46 may absorb and/or store NO_x between regeneration events. Moreover, the LNT 46 may be a long-breathing LNT through a reduction in the NO_x levels in the exhaust gas exiting the engine 34 (or engine-out NO_x levels). For example, the engine-out NO_x levels may be reduced to around or below 100 ppm by the presence of cooled exhaust gases in the cylinder(s) 32, such as exhaust gas that has been cooled and circulated back to the intake manifold by the primary EGR system 38, as discussed above.

With reduced engine-out NO_x levels, the NO_x absorption or saturation process of the LNT 46 is slowed down. Accordingly, as the absorption process is slowed down, regenerations of the long breathing LNT 46 may be required less frequently when compared to the conventional operation of an LNT. For example, FIG. 3 illustrates a chart similar to that shown in FIG. 1, but for when an LNT 46 is allowed to operate as a long breathing LNT 46, such as, again, when the engine-out NO_x levels have been reduced. According to certain embodiments, the adsorption duration of a long breathing LNT may be at least three times longer than that of the adsorption duration of a conventional LNT operating under, or in, similar conditions. As shown, compared to the conventional LNT of FIG. 1, the long breathing LNT 46 of FIG. 3 is able to undergo the absorption process for a relatively significantly longer period of time. Such an extension of time of the absorption process not only reduce the frequency regenerations are required, and thus result in a smaller fuel penalty, but may also translate into overall less NO_x slip from the LNT 46.

FIG. 4 provides after-treatment flow bench test data under optimized conditions illustrating an absorption process for a long breathing LNT 46 in which the simulated exhaust NO_x level has been reduced to approximately 50 ppm. As shown, unlike conventional operation of an LNT in which the engine-out NO_x levels result in the absorption process lasting just a few minutes, the absorption process for the long breathing LNT 46 may be extended to the order of tens of minutes, such as the approximately 33.8 minute absorption processes attained by the testing reflected in FIG. 4. Further, the ratio of the duration of the absorption process to a regeneration event is also relatively significantly increased. With slower saturation and a higher absorption to regeneration duration ratio, the frequency of the fuel rich spike used for regeneration events is reduced, thereby reducing the fuel consumption penalty associated with the LNT 46.

As shown in at least FIGS. 3 and 4, typically, during the regeneration of the LNT 46, a spike in NO_x slip levels may occur, which may average several hundred ppm of NO_x. To prevent a relatively high level of NO_x that is released during the regeneration events from escaping untreated from the after-treatment system 14 and out of a tailpipe 50, according to certain embodiments, the after-treatment system 14 may include a secondary EGR system 52 that is positioned to receive exhaust gas that has passed through the LNT 46.

With a secondary EGR system 52, at least a portion of the NO_x slipping from the LNT 46, such as, for example, half of the NO_x slip, during the regeneration event may be sent back upstream of the LNT 46. For example, referencing FIG. 5, the secondary EGR system 52 may include a valve 54 that is used to control whether at least a portion of the exhaust gas that has passed through the LNT 46 is allowed to enter into the secondary EGR system 52. The exhaust gas that does flow into the secondary EGR system 52 may then be delivered to a position of the engine system 10′ that is upstream of the cylinders 32, including, for example, an intake manifold of the engine 34. According to the embodiment illustrated in FIG. 5, the secondary EGR system 52 may deliver exhaust gas through a secondary EGR line 56 to a portion of the intake line 20 that is upstream of the low pressure compressor 22. The secondary EGR system 52 may also include an EGR valve 58 that controls the flow of exhaust gas in the secondary EGR line 56 into the intake line 20.

During a regeneration event for the LNT 46, the engine system 10, 10′ may be calibrated for the engine 34 to operate at a low temperature combustion (LTC) mode, which, for example, may aim to reduce in-cylinder flame temperatures below approximately 1800 Kelvin, so as to limit the further formation of NO_x during combustion events in the cylinders 32. More specifically, the secondary EGR line 56 may deliver a high temperature combustion (high NO_x, meaning several hundreds of ppm of NOx) EGR gas from the LNT 46 to engine 34 that is operating in the LTC mode. By running the engine 34 in the LTC mode, a part of the NO_x slipping through the LNT 46 during regeneration is re-routed to the LTC operating engine 34 and partially destroyed by the high total hydrocarbon and carbon monoxide levels typically present inside the cylinder(s) 32 of an engine 34 that is operating in the LTC mode.

For example, FIG. 6 presents experimental data where the intake air of a diesel engine running in LTC mode was dosed with NO_x from a bottled cylinder. The presented data indicates that up to 40% of the NO_x that is present in the intake gas can be destroyed inside the engine 34 as the engine 34 operates in the LTC mode, such as, for example, the intake NO_x level being reduced from approximately 420 ppm to 250 ppm. Thus, using relatively high EGR ratios from exhaust gas delivered by the secondary EGR system 52, such as, for example, in the range of 50% to 70%, during the regeneration of the LNT 46 may allow for a large fraction of the NO_x that is released from the LNT 46 during regeneration to be routed back to the engine 34 and reduced inside the cylinder(s) 32.

Referencing FIG. 7, according to certain embodiments, the after-treatment system 14″ of the engine system 10″ may include more than one LNT 46a, 46b, switching valves 60a, 60b used to control flow along the exhaust line 36 downstream of the corresponding LNT 46a, 46b, and switching valves 60c, 60d for controlling flow of the exhaust gas into the secondary EGR system 52′. Additionally, the engine system 10″ may also include one or more injectors 62, which may be positioned upstream of the LNT 46a, 46b. Moreover, according to certain embodiments, a separate injector 62 may be positioned directly upstream of each LNT 46a, 46b, or a common injector 62 may be operably positioned upstream of a plurality of the LNTs 46a, 46b. The injector 62 may be controlled by an engine control unit, and may be configured to inject a fuel into the exhaust gas to create the necessary fuel-rich conditions for regeneration of the LNTs 46a, 46b.

The use of multiple LNTs 46a, 46b, may allow for various operating scenarios during the course of engine 34 operation. For example, according to one scenario, the LNTs 46a, 46b may be operated in the same mode, such as, for example, all of the LNTs 46a, 46b operating in a absorption process where the LNT 46a, 46b is absorbing NO_x, or all of the LNTs 46a, 46b undergoing a regeneration event. Alternatively, at least some of the LNTs 46a, 46b may be operating in different modes, such as at least one LNT 46a, 46b operating in an absorption process, while another LNTs is undergoing a regeneration event.

For example, referencing FIG. 7, a first LNT 46a may be undergoing a regeneration event. During or before the regeneration event, the injector 62 may inject fuel into the exhaust gas that is entering the first LNT 46a to create the necessary fuel-rich conditions for only the first LNT 46a. Further, a switching valve 60a downstream of the first LNT 46a may be in an off, or closed, position so that exhaust gas flowing out of the first LNT 46a does not continue flowing along the exhaust line 36 and toward the DPF 48 and tailpipe 50. Instead, as discussed above, because the exhaust gas flowing through the first LNT 46a during a regeneration event may have a relatively high level of NO_x slippage, a switch valve 60c that allows the exhaust gas from the first LNT 46a to flow into the secondary EGR system 52′ may be in an on, or open, position. With the switch valve 60c in the open position, NO_x released from the first LNT 46a during regeneration may have a chance for secondary reduction inside the cylinder 32 of the LTC operating engine 34, or for secondary storage on the second LNT 46b, thereby further improving the overall NO_x conversion efficiency of the after-treatment system 14″.

Similarly, while the first LNT 46a is undergoing a regeneration event, the second LNT 46b may be operating under an absorption process in which NO_x is being absorbed by the second LNT 46b. Accordingly, during the absorption process, the injector 62 may be configured or operated to not inject fuel into exhaust gas that is entering into the second LNT 46b, thereby allowing the second LNT 46b to continue operating under lean conditions. As the second LNT 46b is in the process of removing NO_x from the exhaust gas, a switching valve 60b downstream of the second LNT 46b may be in an on position so that exhaust gas flowing out of the second LNT 46b is able to continue flowing along the exhaust line 36 and toward the DPF 48 and tailpipe 50. Additionally, as the exhaust gas exiting the second LNT 46b may have a relatively low level of NO_x, a switch valve 60d that controls the flow of exhaust gas from the second LNT 46b into the secondary EGR system 52′ may be in an off, or closed, position so as to prevent exhaust gas that has exited the second LNT 46b from entering into the secondary EGR system 52′.

While the foregoing has been described with respect to first LNT 46a undergoing regeneration while the second LNT 46b undergoes NO_x absorption, the general process is equally applicable when the second LNT 46b undergoes regeneration and the first LNT 46b undergoes the absorption process. Additionally, due to the relatively high ratio of the duration of absorption process to regeneration of the long breathing LNT 46, both the first and second LNTs 46a, 46b may typically both be simultaneously storing NO_x for a significant portion of the period of time during which the engine 34 is operating. For example, according to certain embodiments, the absorption process to regeneration ratio for the long breathing LNT 46 may result in the first and second LNTs 46a, 46b operating in the absorption process during over 75% of the time that the engine 34 is operating. Accordingly, during simultaneous storage, NO_x release out of the exhaust pipe 50 may be relatively very low, such as, for example, less than 50 ppm, as indicated in FIG. 4.

Claims

1. An engine system for treating nitrogen oxides present in an exhaust gas generated by the combustion of fuel, the engine system comprising:

a diesel engine configured to generate the exhaust gas by the combustion of a fuel, the diesel engine calibrated for the generated exhaust gas to have, at least periodically, an engine-out nitrogen oxide level of less than around 100 parts-per-million;

a lean nitrogen oxide trap configured to store at least a portion of the nitrogen oxide in the exhaust gas when the lean nitrogen oxide trap operates in an absorption mode, the lean nitrogen oxide trap configured for the conversion of a plurality of nitrates stored by the lean nitrogen oxide trap during a regeneration event, the adsorption process being, at least periodically, at least three times longer than that of a conventional LNT operating under similar conditions.

2. The engine system of claim 1, further including a primary exhaust gas recirculation system configured for at least a portion of the exhaust gas generated by the diesel engine to be circulated to an intake manifold of the diesel engine, the exhaust gas circulated to the intake manifold being used to lower a temperature attained during the combustion of fuel by the diesel engine to reduce the engine-out nitrogen oxide level.

3. The engine system of claim 2, further including a secondary exhaust gas recirculation system configured to recirculate at least a portion of the exhaust gas that has exited the lean nitrogen oxide trap to a location upstream of the diesel engine.

4. The engine system of claim 3, wherein the secondary exhaust gas recirculation system is configured to recirculate at least a portion of the exhaust gas exiting the lean nitrogen oxide trap while the lean nitrogen oxide trap is undergoing a regeneration event.

5. The engine system of claim 3, wherein the secondary exhaust gas recirculation system does not recirculate exhaust gas during the absorption process in the lean nitrogen oxide trap.

6. The engine system of claim 3, wherein exhaust gas delivered by the secondary exhaust gas recirculation system is supplied to at least one cylinder of the diesel engine that is operating in a low temperature combustion mode, the level of nitrogen oxide of the supplied exhaust gas being lowered by the combustion of the fuel in the at least one cylinder.

7. An engine system for treating nitrogen oxides present in an exhaust gas generated by the combustion of fuel by an engine, the engine system comprising:

a primary exhaust gas recirculation system configured for at least a portion of the exhaust gas generated by the diesel engine to be circulated to an intake manifold of the engine;

at least one injector configured to inject a fuel into the exhaust gas;

a plurality of lean nitrogen oxide traps; and

a secondary exhaust gas recirculation system configured to recirculate at least a portion of the exhaust gas that exits one or more of the lean nitrogen oxide traps to a location upstream of the engine.

8. The engine system of claim 7, wherein the plurality of lean nitrogen oxide traps includes a first lean nitrogen oxide trap and a second lean nitrogen oxide trap, and wherein at least one of the first and second lean nitrogen oxide traps is configured to operate in an absorption mode for at least a portion of the time while the other of the first and second lean nitrogen oxide traps undergoes a regeneration event.

9. The engine system of claim 8, wherein the plurality of lean nitrogen oxide traps are configured to store at least a portion of the nitrogen oxide in the exhaust gas when at least one of the plurality of lean nitrogen oxide trap operates in an absorption mode, each of the plurality of lean nitrogen oxide traps configured for the conversion of a plurality of nitrates stored by the lean nitrogen oxide trap during a regeneration event, the adsorption process being, at least periodically, at least three times longer than that of a conventional LNT operating under similar conditions.

10. The engine system of claim 8, wherein exhaust gas delivered by the secondary exhaust gas recirculation system is supplied to at least one cylinder of the engine that is operating in a low temperature combustion mode, the level of nitrogen oxide of the supplied exhaust gas being lowered by the combustion of the fuel in the at least one cylinder.

11. The engine system of claim 7, wherein the engine system is configured for exhaust gas exiting each of the plurality of lean nitrogen oxide traps that are undergoing an absorption process to flow along an exhaust line, and wherein exhaust gas exiting each of the plurality of lean nitrogen oxide traps that are undergoing a regeneration event are diverted into the secondary exhaust gas recirculation system.

12. The engine system of claim 11, wherein the plurality of lean nitrogen oxide traps are simultaneously undergoing an absorption process at least approximately 75% of the time that the engine is operating.

13. The engine system of claim 11, wherein the secondary exhaust gas recirculation system is configured to deliver exhaust gas to a portion of an intake line upstream of a compressor.