Reactivation control apparatus and method

Abstract

The present disclosure relates to a control unit (8) for controlling reactivation of a lean NOx trap (LNT) disposed in an exhaust system (3) connected to an internal combustion engine (2), the control unit (8). The control unit (8) has at least one processor (11) configured to generate a reactivate flag (RF) for the LNT (6). A memory device (12) having instructions stored therein is coupled to the at least one processor (11). The at least one processor (11) is configured to generate the reactivate flag (RF) when the exhaust gas supplied to the LNT (6) is lean and an LNT temperature (T.sub.LNT) is greater than or equal to a predefined LNT temperature threshold (T1). The present disclosure also relates to a method of controlling reactivation of the LNT (6). The present disclosure also relates to a vehicle comprising reactivation control apparatus.

Claims

1. A control system for controlling reactivation of a lean NOx trap (LNT) disposed in an exhaust system connected to an internal combustion engine, the control system comprising: at least one processor configured to generate a reactivate flag for the LNT; and a memory device having instructions stored therein and coupled to the at least one processor; wherein the at least one processor is configured to generate said reactivate flag (RF) upon identification of the following conditions: an exhaust gas supplied to the LNT is lean; and an LNT temperature is greater than or equal to a predefined temperature threshold, and in dependence on an integral of the LNT temperature with respect to time during an interval when the exhaust gas supplied to the LNT is lean and the LNT temperature is greater than or equal to the predefined temperature threshold, and further wherein, in dependence on said reactivate flag, the at least one processor is further configured to generate a reactivation request signal when at least one predefined reactivation condition is identified, said at least one predefined reactivation condition comprising determining that the LNT temperature has subsequently fallen to a value equal to or less than a first reactivation temperature threshold, said first reactivation temperature threshold being less than the predefined temperature threshold.

2. The control system as claimed in claim 1, wherein the at least one processor is configured to generate said reactivate flag when the integral is greater than or equal to a predefined reactivation threshold.

3. The control system claimed in claim 1, wherein the at least one processor is configured to generate said reactivation request signal to an engine control unit to initiate operation of the internal combustion engine in a rich burn mode to reactivate the LNT.

4. The control system as claimed in claim 1, wherein the at least one processor is further configured to receive an engine operating mode signal, and to determine a composition of the exhaust gas by determining when the internal combustion engine is operating in a lean burn mode.

5. The control system as claimed in claim 1, wherein the at least one processor is further configured to receive a signal from an oxygen sensor disposed in the exhaust system, and to determine a composition of the exhaust gas by monitoring an oxygen content of the exhaust gas.

6. The control system as claimed in claim 1, wherein the at least one processor is further configured to determine the LNT temperature in dependence on a measured temperature of the exhaust gas supplied to the LNT.

7. The control system as claimed in claim 1, wherein the at least one processor is configured to model the LNT temperature.

8. The control system as claimed in claim 1, wherein the processor is responsive to a determination that an amount of NOx stored in the LNT exceeds a predetermined threshold to generate said reactivate flag or a further reactivate flag.

9. A vehicle comprising the control system as claimed in claim 1.

10. The vehicle as claimed in claim 9, further comprising an engine control unit configured to control operation of the internal combustion engine in dependence on said reactivate flag.

11. A method of controlling reactivation of a lean NOx trap (LNT) disposed in an exhaust system connected to an internal combustion engine, the method comprising: determining an LNT temperature and a composition of an exhaust gas supplied to the LNT; and generating a reactivate flag in dependence on identification of the following conditions: the exhaust gas supplied to the LNT is lean; and the LNT temperature is greater than or equal to a predefined LNT temperature threshold, and in dependence on an integral of the LNT temperature with respect to time during an interval when the exhaust gas supplied to the LNT is lean and the LNT temperature is greater than or equal to the predefined LNT temperature threshold, wherein, in dependence on said reactivate flag, the method further comprises generating a reactivation request signal when at least one predefined reactivation condition is identified, said at least one predefined reactivation condition comprising determining that the LNT temperature has subsequently fallen to a value equal to or less than a first reactivation temperature threshold, said first reactivation temperature threshold being less than the predefined temperature threshold.

12. The method as claimed in claim 11, wherein the reactivate flag is generated when the integral is greater than or equal to a predefined reactivation threshold.

13. The method as claimed in claim 11, further comprising operating the internal combustion engine in a rich burn mode to reactivate the LNT in dependence on said reactivate flag.

14. The method as claimed in claim 11, wherein determining the composition of the exhaust gas comprises identifying when the internal combustion engine is operating in a lean burn mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

(2) FIG. 1 shows a schematic representation of a vehicle incorporating an engine control unit and a reactivation control unit in accordance with an embodiment of the present invention;

(3) FIG. 2 is a first graph showing the relationship between time and operating temperature before the LNT is deactivated;

(4) FIG. 3A is a second graph showing the cumulative carbon monoxide conversion during a drive cycle; and

(5) FIG. 3B is a third graph showing the cumulative hydrocarbon conversion during a drive cycle.

DETAILED DESCRIPTION

(6) A vehicle 1 in accordance with an embodiment of the present invention is illustrated in FIG. 1. The vehicle 1 comprises an internal combustion engine 2 having an exhaust system 3 for exhausting gases to atmosphere. The exhaust system 3 comprises a selective catalytic reduction (SCR) device 4, a diesel particulate filter (DPF) 5 and a lean NOx trap (LNT) 6. The vehicle 1 comprises an engine control unit 7 for controlling operation of the internal combustion engine 2 in dependence on driver inputs; and a reactivation control unit 8 for controlling reactivation of the LNT 6.

(7) The internal combustion engine 2 in the present embodiment is a diesel engine. In particular, the internal combustion engine 2 is a light-duty diesel engine. Lambda (λ) of the internal combustion engine 2 is the ratio of the actual air/fuel ratio (AFR) to the stoichiometric air/fuel ratio (AFR.sub.stoich). The internal combustion engine 2 is operable in a lean burn mode (λ>1). When operating in said lean burn mode, lambda (λ) may be 1.3, for example. The internal combustion engine 2 is also operable in a rich burn mode (λ<1).

(8) As outlined above, the exhaust system 3 comprises a DPF 5. The DPF 5 is a particulate filter device and is periodically regenerated by raising the temperature to oxidise soot. The regeneration of the DPF 5 comprises raising the temperature of the exhaust gas, for example to between 550° C. and 800° C. for twenty (20) minutes. The DPF 5 is monitored in conventional manner to determine the need for regeneration.

(9) The LNT 6 controls the emission of lean burn gases from the exhaust system 3. More particularly, the LNT 6 is configured to control emission of NOx gases, typically comprising Nitric Oxide (NO), from the internal combustion engine 2. The LNT 6 has particular application at relatively low temperatures, for example in the range 20° C. to 200° C., which may occur after a cold-start of the internal combustion engine 2. The LNT 6 includes a NOx adsorber which is effective to bind the NOx gases during lean exhaust conditions. The NOx adsorber may be applied as a catalyst washcoat and may, for example, comprise palladium (Pd) and platinum (Pt); or cerium (Ce) and barium (Ba). It will be appreciated that other chemical compositions of the NOx adsorber are useful. The NOx adsorber may become deactivated when exposed to high-temperature, lean exhaust gas. This drop in activity can compromise oxidation of carbon monoxide (CO) and hydrocarbon (HC) from ambient temperatures; and a reduction in the ability of the NOx adsorber to store and reduce NOx in the exhaust gas. A first graph 50 illustrating the relationship between an LNT temperature T.sub.LNT and a deactivation time (seconds) is shown in FIG. 2. The LNT temperature T.sub.LNT indicated in FIG. 2 corresponds to an inlet temperature of the LNT 6. A first plot 55 represents the LNT temperature T.sub.LNT that can be sustained for a given time period before reactivation of the LNT 6 is required. The LNT 6 is deactivated when the LNT temperature T.sub.LNT is greater than a predefined LNT temperature threshold T1 which is indicated in FIG. 2. The LNT temperature threshold T1 in the present embodiment is defined as 350° C. It will be understood that the LNT temperature threshold T1 can be calibrated and may be defined as a value which is higher or lower than 350° C.

(10) When deactivated, the available storage capacity or the storage efficiency of the LNT 6, and CO and HC activity, is reduced. In order to increase the storage capacity or the storage efficiency of the LNT 6, and to regain desired levels of CO and HC activity, the NOx adsorber is periodically reactivated to release and reduce stored NOx. Reactivation comprises passing rich exhaust gas over the NOx adsorber to allow local oxygen to be consumed, to release the stored NOx and to reduce the released NOx, for example to nitrogen (N.sub.2). The rich exhaust gas can be sustained over the NOx adsorber for a period of time until rich exhaust gas is detected downstream of the NOx adsorber indicating that reactivation of the LNT 6 is complete (i.e. rich breakthrough detected). A lower operating temperature of approximately 250° C. may apply when detecting rich breakthrough. Alternatively, an open loop control strategy may be implemented, for example comprising determining the available capacity of the LNT 6 (for example by modelling NOx storage) and scheduling the supply of rich exhaust gas in dependence on the determined available capacity. Following reactivation, the NOx adsorber is available to store NOx during the following duty cycle, for example during the next cold-start cycle.

(11) The lean exhaust gas typically occurs when the internal combustion engine 2 is operating in said lean burn mode. The rich exhaust gas typically occurs when the internal combustion engine 2 is operating in said rich burn mode. In certain embodiments, the exhaust system 3 may comprise control devices for controlling the temperature and/or composition of the exhaust gas. For example, the exhaust system 3 may comprise a fuel injector for injecting fuel upstream of the LNT 6 selectively to provide rich exhaust gas suitable for regenerating the LNT 6. Alternatively, a fuel reformer may be used to create more reactive reductants. At least in certain embodiments, the composition of the exhaust gas may be adjusted within the exhaust system 3 at least partially independently of the operation of the internal combustion engine 2. For example, the composition of the exhaust gas may be controlled at least partially independently of the operating speed and/or load of the internal combustion engine 2.

(12) The engine control unit 7 comprises at least a first processor 9 connected to a first memory device 10. The first processor 9 is configured to implement a set of non-transitory computational instructions stored on said first memory device 10. When executed, the computational instructions cause the first processor 9 to implement an engine control strategy for controlling operation of the internal combustion engine 2. The reactivation control unit 8 comprises at least a second processor 11 connected to a second memory device 12. The second processor 11 is configured to implement a set of non-transitory computational instructions stored on said second memory device 12. When executed, the computational instructions cause the second processor 11 to implement a reactivation control strategy for controlling reactivation of the LNT 6. The second processor 11 models an LNT temperature T.sub.LNT in dependence on the operating conditions of the internal combustion engine 2. The LNT temperature T.sub.LNT may, for example, be indicative of an inlet temperature of the LNT 6, or a bed temperature of the LNT 6. The second processor 11 receives an engine operating mode signal SE1 from the engine control unit 7 to indicate whether the internal combustion engine 2 is operating in a lean burn mode or a rich burn mode. The engine operating mode signal SE1 could comprise lambda (λ). Furthermore, the second processor 11 receives an oxygen signal SO1 from an oxygen sensor 14 disposed downstream of the LNT 6 to determine the air/fuel composition of the exhaust gas exiting the LNT 6 (i.e. to determine whether the exhaust gas is rich or lean). Rather than model the LNT temperature T.sub.LNT, a temperature sensor (not shown) may be provided for measuring the temperature of the LNT 6. The temperature sensor may be disposed at an inlet of the LNT 6 or in the bed of the LNT 6.

(13) The reactivation control unit 8 is configured to identify operating conditions which are likely to result in deactivation of the NOx adsorber. In particular, the reactivation control unit 8 is configured to monitor exposure of the NOx adsorber to lean exhaust gas above the LNT temperature threshold T1. The reactivation control unit 8 models the LNT temperature T.sub.LNT and the composition of the exhaust gas. In the present embodiment, the reactivation control unit 8 is configured to determine when the exhaust gas has a net lean composition in dependence on the engine operating mode signal SE1 received from the engine control unit 7. Alternatively, or in addition, the reactivation control unit 8 may be configured to receive the oxygen signal SO1 from the oxygen sensor 14 to determine when the LNT 6 is exposed to lean exhaust gas. In alternate embodiments, a second oxygen sensor (not shown) may be provided between the internal combustion engine 2 and the LNT 6 to monitor the air/fuel composition of the exhaust gas to determine whether the exhaust gas introduced into the LNT 6 is rich or lean. The reactivation control unit 8 determines the time at which the LNT 6 is exposed to lean exhaust gas without exposure to rich exhaust gas while the LNT temperature T.sub.LNT is above the LNT temperature threshold T1. The reactivation control unit 8 determines an integral of the LNT temperature T.sub.LNT with respect to time during an interval when the exhaust gas supplied to the LNT is lean and the LNT temperature T.sub.LNT is greater than or equal to the LNT temperature threshold T1. The reactivate flag (RF) is generated when the integral is determined greater than or equal to a predefined reactivation threshold. By integrating the temperature with respect to time, the reactivation control unit 8 makes allowances for the magnitude of the temperature as well as the time period during which the LNT 6 is exposed to lean exhaust gas. It will be appreciated that the time threshold and/or the temperature threshold may be calibrated for particular applications.

(14) The reactivate flag RF provides an entry condition for reactivation of the LNT 6. The engine control unit 7 initiates reactivation of the LNT 6 in dependence on the reactivate flag RF when suitable conditions are identified. In dependence on said reactivate flag RF, the engine control unit 7 identifies the next available opportunity to reactivate the LNT 6. The engine control unit 7 may, for example, determine that reactivation of the LNT 6 may be performed when the LNT temperature T.sub.LNT is below a first (maximum) reactivation temperature threshold and above a second (minimum) reactivation temperature threshold. The first and second reactivation temperature thresholds may define a temperature range in which reactivation of the NOx adsorber may be performed. Upon identification of an opportunity to perform reactivation, the engine control unit 7 controls the internal combustion engine 2 to initiate a rich burn mode. The rich burn mode results in the generation of rich exhaust gas which is introduced into the LNT 6 to reactivate the NOx adsorber, as described herein. The internal combustion engine 2 may continue to operate in said rich burn mode until the oxygen sensor 14 detects rich gases downstream of the LNT 6, indicating that reactivation of the LNT 6 is complete. As outlined above, open loop control strategies may be implemented by modelling the available capacity of the LNT 6. The engine control unit 7 may then control the internal combustion engine to revert to a lean burn mode.

(15) The operation of the engine control unit 7 and the reactivation control unit 8 in accordance with an embodiment of the present invention will now be described. A vehicle operating cycle which may result in the deactivation of the LNT 6 is highway driving which may comprise driving the vehicle 1 at a relatively constant speed, for example 70 mph, over an extended period of time. An LNT temperature T.sub.LNT in excess of 300° C. may be sustained during this type of operation. Furthermore, during highway driving, the internal combustion engine 2 typically operates in said lean burn mode and the LNT 6 is exposed to lean exhaust gas without exposure to rich exhaust gas. Thus, highway driving of the vehicle 1 may result in deactivation of the LNT 6 which would reduce its ability to adsorb NOx. When the available capacity of the LNT 6 is reduced, the control of NOx emissions may be performed by the SCT 5 downstream of the LNT 6. The reactivation control unit 8 determines the integral of the LNT temperature T.sub.LNT with respect to time during an interval when the exhaust gas supplied to the LNT is lean and the LNT temperature T.sub.LNT is greater than or equal to the LNT temperature threshold T1. The reactivation control unit 8 generates the reactivate flag (RF) when the integral is determined greater than or equal to a predefined reactivation threshold. In dependence on the reactivate flag RF, the engine control unit 7 controls the internal combustion engine 2 to initiate a rich burn mode when the LNT temperature T.sub.LNT drops to a temperature suitable for reactivation, for example in the range 200° C. to 300° C. When the oxygen sensor 14 detects rich exhaust gas downstream of the LNT 6, the engine control unit 7 controls the internal combustion engine 2 to revert to the lean burn mode.

(16) The efficacy of the LNT 6 for different operating scenarios will now be described with reference to second and third graphs 100, 150 shown in FIGS. 3A and 3B respectively.

(17) The second graph 100 shows the cumulative carbon monoxide (CO) conversion (%) during a new European drive cycle (NEDC). A first plot 105 represents the CO conversion (%) of a deactivated LNT 6; and a second plot 110 represents the CO conversion (%) of the LNT 6 following reactivation. By way of comparison, a third plot 115 represents the CO conversion (%) of a regular diesel oxidation catalyst (DOC).

(18) The third graph 150 shows the cumulative total hydrocarbon (THC) conversion (%) during a new European drive cycle (NEDC). A first plot 155 represents the THC conversion (%) of a deactivated LNT 6; and a second plot 160 represents the THC conversion (%) of the LNT 6 following reactivation. By way of comparison, a third plot 165 represents the THC conversion (%) of a regular diesel oxidation catalyst (DOC).

(19) As described herein, the reactivation control unit 8 determines when the NOx adsorber is deactivated and generates the reactivate flag RF. It will be understood that references herein to the NOx adsorber being deactivated may refer to a decrease in the adsorption capacity or the adsorption efficiency of the NOx adsorber. It is not necessary that the NOx adsorber is completely deactivated; rather, the deactivation of the NOx adsorber may be partial. For example, the reactivation control unit 8 may determine that the NOx adsorber is deactivated if the NOx adsorption efficiency of the LNT 6 decreases below a predefined threshold. Conversely, the reactivation control unit 8 may determine that reactivation is complete or that the NOx adsorber is reactivated, when the adsorption efficiency or the adsorption capacity of the NOx adsorber increases above a predefined threshold. The performance of the LNT 6 may be modelled, for example in dependence on operating conditions of the internal combustion engine 2. Alternatively, or in addition, the effectiveness of the NOx adsorber may be determined in dependence on signals received from one or more sensor disposed in the exhaust system 3. The one or more sensor may comprise an oxygen sensor and/or a NOx sensor.

(20) In the embodiments described above, reactivation is carried out at least partly in response to the detection that the exhaust gas supplied to the LNT is lean while an LNT temperature is greater than or equal to a predefined temperature threshold. This reactivation may be supplemental to a periodic reactivation carried out in response to a natural build-up over time of NOx within the LNT (it will be appreciated that, since the LNT has the function of storing NOx, and has a maximum NOx storage capacity, the available NOx storage capacity will be reduced over time, requiring reactivation to purge NOx from the LNT to recover storage capacity for subsequent operation of the LNT). The amount of NOx stored in the LNT and/or the amount of NOx storage capacity remaining in the LNT can be estimated using established NOx storage models with inputs comprising temperature, sulphur exposure, high temperature exposure (permanent deactivation of NOx storage sites), input NOx flux and LNT storage efficiency.

(21) In one example implementation, a reactivation process (such as that described above) may be triggered by a flag which is set in response to two different triggers, one of these being the detection of the lean exhaust gas and high temperature conditions described earlier, and the other being a determination that the amount of NOx built up in the LNT has reached a predetermined threshold (or put another way, that the amount of NOx storage capacity remaining has fallen below a predetermined threshold). The predetermined threshold may be set in terms of a mass of NOx, or in terms of a proportion of the NOx storage capacity of the LNT. If the flag is set in response to either of these events, reactivation will subsequently occur, at a suitable time, based on the flag, in the manner described above.

(22) It will be appreciated that various changes and modifications may be made to the apparatus and method described herein without departing from the scope of the present invention.