WO2013039589A1

WO2013039589A1 - Selective use of lean nox trap

Info

Publication number: WO2013039589A1
Application number: PCT/US2012/044136
Authority: WO
Inventors: Nishant Singh; Michael UCHANSKI; Joao Silva; Brad J. Adelman
Original assignee: International Engine Intellectual Property Co LLC
Current assignee: International Engine Intellectual Property Co LLC
Priority date: 2011-09-16
Filing date: 2012-06-26
Publication date: 2013-03-21
Anticipated expiration: 2014-03-16

Abstract

One or more embodiments provide a selective lean NOx system and method for use. The LNT system includes a diverter valve positioned in the exhaust gas flow to divert at least a portion of the exhaust gas into a LNT trap loop that directs the exhaust gas to an LNT. The diverter valve is selectively operable, for example to turn on when a NOx spike is detected during a high-torque operation.

Description

SELECTIVE USE OF LEAN NOx TRAP

RELATED APPLICATIONS

This application makes reference to, claims priority to, and claims the benefit of United States Provisional Patent Application No. 61/535,436, which was filed on September 16, 2011 and is entitled "Selective Use of Lean NOx Trap." The disclosure of the above -identified Provisional Patent Application is hereby incorporated by reference in its entirety.

BACKGROUND

[0001] Exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique for internal combustion engines and used in petrol/gasoline and diesel engines. In EGR, a portion of the exhaust gas of the engine is routed back to the engine cylinders. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture.

[0002] It has been found that NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature. Conversely, EGR causes lower combustion chamber temperatures and thus reduces the amount of NOx generated by the combustion.

[0003] High EGR engines have a tradeoff between engine out soot and NOx emissions as well as torque response. To achieve the target engine out NOx and soot emissions, torque response may be compromised. On the other hand, achieving the desired torque response usually is at the expense of soot and or NOx emissions. It has been shown that torque and soot targets may be achieved if NOx emissions are allowed to rise during accelerations. However, these NOx spikes may make it difficult to meet the tailpipe emissions targets.

[0004] A well-known approach to attempt to contain these NOx spikes is to use a Lean

NOx Trap (LNT) but the current state of the art does not meet the needs for this application. In current designs, the LNT is exposed to the entire exhaust output where all the exhaust must pass through a LNT. Such a system is currently in production (for example, in light duty applications such as Mercedes or Volkswagen) or has been reported in the literature (for example, in SAE paper 2005-01-1760). These systems are designed so that the LNT is always exposed to NOx and when the LNT is capable of trapping NOx, it is be stored. However, once the LNT has reached its maximum NOx storage capability, it must be DeNOx-ed (have the NOx removed) before more NOx may be stored. Additionally, some prior systems use multi-leg LNT configurations which are used so that the LNT being regenerated (DENOX or DESOX) has signifimaytly lower exhaust flow through it so that it is easier to regenerate and consumes less fuel for said regeneration.

SUMMARY

[0005] A selective lean NOx trap (LNT) system is provided. The LNT system includes a diverter valve positioned in the exhaust gas flow to divert at least a portion of the exhaust gas into a LNT trap loop that directs the exhaust gas to an LNT. The diverter valve is selectively operable, for example to turn on when a NOx spike is detected during a high-torque operation.

BRIEF DESCRIPTION OF THE DRAWINGS [0006] Figure 1 illustrates a selective LNT system.

DETAILED DESCRIPTION

[0007] One or more embodiments provide a Lean NOx Trap (LNT) which allows selective, intermittent storage of NOx. For example, the LNT may be available on demand and may be able to counter the spikes in NOx emissions that occur at high torque response.

[0008] More specifically, one or more embodiments may be used to operate an exhaust aftertreatment system with a DOC (Diesel Oxidation Catalyst) and DPF (Diesel Particulate Filter) with the addition of a LNT which may be exposed to the exhaust gas only when a diverter valve is activated. When the diverter valve is deactivated the exhaust gas is exposed to the DOC and DPF but not exposed to the LNT. The LNT may be positioned on a bypass loop and the bypass loop may be positioned upstream, downstream or in between the DOC and DPF.

[0009] Actuation of the diverter valve may take place so that the LNT is exposed to the entire exhaust gas during the acceleration events when the NOx concentrations are elevated.

This is the first embodiment. An alternative method is to open the diverter valve so that the LNT is exposed to at least a portion of the exhaust gas during the acceleration events. Another alternative method is to open the diverter valve during steady state operation or idle where the flows are constant but NOx concentrations are lower. The duration of this event may be based on exposing the LNT for all or a portion of the acceleration event. For this method, communication between the diverter valve controller and the engine is needed (either by residing in the ECU or via MAY link). An alternative method is to monitor the NOx trapping efficiency across the LNT. This may be accomplished by monitoring the inlet and outlet NOx concentrations. Inlet concentrations may be determined directly with a NOx sensor placed within the exhaust flow and upstream of the LNT or may be determined using a NOx map/model based on the known engine operating parameters. NOx outlet concentrations require the use of a

NOx sensor downstream of the LNT system. The differences between the NOx concentrations entering and exiting the LNT is used to determine NOx trapping efficiency. Valve actuation may be triggered based on achieving a desired level of NOx trapping either during an acceleration event or during steady state operation or idle. [0010] DENOX (regeneration of the LNT) is based on known algorithms which monitor the fill state. When the fill state indicates that the LNT needs to be regenerated, the diverter valve is actuated so that the LNT is exposed to a net reducing environment. A ctivation of the diverter may be commanded either during an acceleration when AFR are low or during at steady state or idle when exhaust flows may be lower. DESOX (removal of the trapped sulfur from the LNT) is based on known algorithms which monitor the sulfur poisoning state. When this state indicates that the LNT needs to be desulfated, the diverter valve is actuated so that the LNT is exposed to a net reducing environment and elevated temperature. Activation of the diverter may be commanded during a DPF regeneration event when exhaust gas temperature are elevated and either during an acceleration when AFR are low or during at steady state or idle when exhaust flows may be lower.

[0011] Previous systems may use the concept of a diverter valve but that is for systems where the LNT is exposed to the entire exhaust flow and a diverter is used to minimize the flow into the LNT leg that is being regenerated. This diverts the flow so that the LNT which is not part of the main exhaust flow is now exposed to the regeneration event

[0012] Figure 1 illustrates a selective LNT system 100 according to one or more embodiments. The selective LNT system 100 includes an exhaust flow 110, a DOC 115, a DPF 120, a diverter valve 122, a LNT loop 125, a LNT 130, a downstream NOx sensor 135, an upstream NOx sensor 140, and a diverter valve control 145.

[0013] In Figure 1, the diverter valve 122 is located in the exhaust flow 110 upstream of the DOC 115 and DPF 120. When activated, the diverter valve 122 diverts at least a portion of the exhaust flow 110 to the LNT 130 in the LNT loop 125. Additionally the downstream NOx sensor 135 measures NOx downstream of the diverter valve 122 and the upstream NOx sensor measures NOx upstream of the diverter valve. Both the upstream NOX sensor 140 and the downstream NOx sensor 135 provide data to the diverter valve control 145 which controls the diverter valve 122. A NOx model or map may be substituted for the upstream NOx sensor 140. This model computes the engine out NOx levels based upon engine operating conditions and acts as a virtual NOx sensor.

[0014] In operation, when the diverter valve 122 is 'closed' the exhaust bypasses the LNT

130 and is exposed to the DOC 115 and DPF 120 only. Conversely, when the diverter valve 122 is 'opened' the entire exhaust flow 110 is sent to the LNT loop 125 where it passes through the LNT 130 brick then is reintroduced to the exhaust flow 110 upstream of the DOC 115 and DPF120.

[0015] The diverter valve control 145 determines whether to open or close the diverter valve based on engine operating conditions. In one method, the diverter valve 122 is Opened' during acceleration events where NOx concentrations are elevated. This event may be detected via various methods such as a signal from the ECU indicating an acceleration (change in pedal position, change in engine speed or other parameter). Likewise, the diverter valve 122 is 'closed' when steady state, idle or deceleration events occur. In an alternative method, the upstream NOx senor 140 may be used to 'open' the diverter valve 122 when a rapid increase in NOx is detected and then used to 'close' the diverter valve when a rapid decrease in NOx is detected. In another alternative method, if the diverter valve 122 is operated in a manner which allows the amount of exhaust which flows through the LNT system 100 to be part of but not all of the flow, the diverter valve control 145 determines whether to increase or decrease the opening of the diverter valve 122 based on the difference in the NOx readings.

[0016] The diverter valve 122 may be a single valve such as a 4-way valve.

Alternatively, the diverter valve 122 may have multiple valves to direct exhaust flow 110 either only to the DOC 115 and DPF 120 or to force exhaust through the LNT loop 125 and then the DOC 115 and DPF 120.

[0017] Alternative embodiments include installing the LNT system 100 upstream of the

DPF 120 but downstream of the DOC 115. Another alternative embodiment has the LNT system 100 downstream of both the DPF 120 and downstream of the DOC 115.

Claims

1. A selective lean NOx trap system, the system including:

a lean NOx trap, wherein the lean NOx trap traps NOx;

a lean NOx trap loop, wherein the lean NOx trap loop delivers exhaust gas containing NOx to the lean NOx trap; and

a diverter valve positioned in an exhaust gas flow, wherein the diverter valve may be selectively activated to :

divert at least a portion of the exhaust gas flow into the lean NOx trap, or

not divert the exhaust gas flow into the lean NOx trap.

2. The system of claim 1 further including a downstream NOx sensor downstream of the diverter valve.

3. The system of claim 2 further including an upstream NOx sensor upstream of the diverter valve.

4. The system of claim 3 further including a diverter valve control receiving NOx readings from the downstream NOx sensor and the upstream NOx sensor.

5. The system of claim 4 wherein the diverter valve control controls the diverter valve based on the increase or decrease in the upstream NOx sensor reading.

6. The system of claim 1 further including a diverter valve control, wherein an engine out map is used to calculate NOx concentration entering the LNT system when the diverter valve is opened and to control the diverter valve control based on the increase in the calculated NOx concentrations from the engine out map.

7. The system of claim 1 further including a diverter valve control receiving readings from the ECU.

8. The system of claim 7 wherein the diverter valve control controls the diverter valve based on engine operating conditions so that the LNT loop is only exposed to at least a portion of the exhaust gas flow during engine acceleration events where the NOx levels are elevated or during steady state operation or idle where flows are constant.

9. The system of claim 1 wherein the diverter valve is positioned upstream of a diesel oxidation catalyst.

10. The system of claim 1 wherein the diverter valve is positioned upstream of a diesel particulate filter but downstream of the diesel oxidation catalyst.

11. The system of claim 1 wherein the diverter valve is positioned upstream of a diesel particulate filter but downstream of the diesel oxidation catalyst."

12. A method for selectively trapping NOx, the method including:

positioning a diverter valve in the exhaust gas flow;

selectively activating the diverter valve to divert at least a portion of the exhaust gas flow into a lean NOx trap loop, wherein the lean NOx trap loop delivers the at least a portion of the exhaust gas flow to a lean NOx trap; and

trapping at least a portion of the NOx in the exhaust gas flow at the lean NOx trap.

13. The method of claim 12 further including controlling the diverter valve based on the increase or decrease in an upstream NOx sensor reading.

14. The method of claim 12 wherein an engine out map is used to calculate NOx concentration entering the LNT system when the diverter valve is opened and to control the diverter valve control based on the increase in the calculated NOx concentrations from the engine out map.

15. The method of claim 12 wherein a diverter valve control controls the diverter valve based on engine operating conditions so that the LNT loop is only exposed to at least a portion of the exhaust gas flow during engine acceleration events where the NOx levels are elevated or during steady state operation or idle where flows are constant.