1. Technical Field
This invention relates generally to an exhaust system having a controllably variable flowpath for exhaust gas circulation, and more particularly to the use of such a system to control exhaust gas flow through regenerable exhaust gas treatment devices.
2. Background Art
Worldwide emissions regulations slated for introduction in the near future impose very stringent emissions regulations. The Tier 2 regulations in the United States require that Diesel vehicles have the same ultra-low emissions levels as spark ignited vehicles. Moreover, Tier 3 requirements, which phase in for different engine levels over the next three years call for a 40% reduction in NOx (oxides of nitrogen) from the Tier 2 levels now in existence.
Various combustion modes, directed to addressing both in-cylinder (engine-out) and exhaust gas treatment device requirements, have been proposed. For example, U.S. Pat. No. 5,732,554, issued Mar. 31, 1998 to Shizuo Sasaki, et al. for an EXHAUST GAS PURIFICATION DEVICE FOR AN INTERNAL COMBUSTION ENGINE describes a method by which the normal fuel lean operating mode of an engine is switched to a rich premixed charge compression ignition, more accurately and preferably referred to as premixed controlled compression ignition (PCCI), combustion mode.
U.S. Pat. No. 5,937,639 granted Aug. 17, 1999 to Shizuo Sasaki, et al. for INTERNAL COMBUSTION ENGINE describes an alternative method for lowering the combustion temperature, i.e., low temperature combustion (LTC) to minimize smoke generation during rich, or near rich, combustion. LTC and PCCI combustion are alternative combustion modes which normal Diesel lean combustion can be transitioned to during engine operation.
Perhaps of most concern to the Diesel engine market are the proposed very tight future reductions in terms of oxides of nitrogen (NOx) and particulate matter (PM) emissions. One of the most promising technologies for NOx treatment is NOx adsorbers, also known as “lean NOx traps.” Diesel particulate filters, also known as Diesel particulate traps, and lean NOx traps are the most likely, at least in the foreseeable future, means by which emissions will be reduced. Lean NOx traps and Diesel particulate filters need to be regenerated periodically to restore their efficiencies. The regeneration of lean NOx traps is usually done by providing reductants, such as CO and HC under oxygen-free conditions. A regenerated lean NOx trap not only adsorbs NOx emissions, but also adsorbs sulfur carried in the exhaust gas stream. Sulfur removal (desulfation) must be undertaken at a temperature above 600° C. under oxygen-free conditions, i.e., combustion of a stoichiometric or richer air/fuel ratio. Under typical Diesel lean combustion operation, such very high temperatures cannot normally be obtained except under very high load conditions. Diesel particulate filter regeneration is carried out by oxidizing soot and other particles “trapped” in the Diesel particulate filter at a high temperature and a lean air/fuel ratio.
Frequent lean NOx trap (LNT) regeneration is necessary when the engine-out NOx is high, for example, when operating under high loads, but frequent generation at higher loads can cause the temperature of the LNT to increase rapidly. The rapid temperature increase results from the exothermic reaction associated with the rich combustion products carried in the exhaust that are used to regenerate the LNT. Diesel particulate filters (DPF) also require high temperature to be regenerated. When the LNT is located downstream of the DPF, the exothermic reaction taking place in the DPF during regeneration will result in an increase in the outlet exhaust gas temperature of the DPF. The LNT inlet exhaust gas temperature is therefore also increased. When the temperature of the LNT increases above a critical temperature, as a result of the frequent regeneration of the LNT or the regeneration of the DPF, the absorption efficiency of NOx by the LNT is very low and tailpipe NOx emissions accordingly are high.
The present invention is directed to overcoming the problems set forth above with respect to the critical temperature requirements associated with catalyzed and other exhaust gas aftertreatment device operation and regeneration. It is desirable to have an exhaust system in which LNTs, selective catalytic reduction (SCR) catalysts, and like regenerable exhaust gas treatment devices can be optimally positioned within the exhaust system to provide efficient operation over a wide range of engine operating conditions and regeneration requirements. It is also desirable to have a flexible exhaust system in which the exhaust gas flowpath can be selectively varied to control regeneration temperatures to meet differing operation and regeneration requirements. It is also desirable to have a method by which LNT temperature can be managed for the best efficiency during cold start, regeneration and DPF regeneration. It is also desirable to have an exhaust system and method of temperature control by which unregulated emissions can be reduced by management of the exhaust gas temperature passing through various components of the exhaust system.
In accordance with one aspect of the present invention, an exhaust system having a variable flowpath has a three-way engine-out exhaust control valve adapted to direct exhaust gas flow through selectable first and second outlet ports. A regenerable gas treatment device has an inlet in direct communication with the first outlet port of the three-way engine-out exhaust control valve, and a second exhaust gas treatment device has an inlet in direct communication with the second outlet port of the three-way engine-out exhaust control valve. A second three-way exhaust control valve has a first inlet port in communication with the outlet of the second exhaust gas treatment device, and a second inlet port in communication with the inlet of the regenerable exhaust gas treatment device. The second three-way exhaust control valve has an outlet port in direct communication with the ambient environment. A third three-way exhaust control valve has an inlet port in direct communication with the outlet of the regenerable exhaust gas treatment device.
Other features of the exhaust gas system embodying the present invention, include a third exhaust gas treatment device having an inlet in direct communication with the outlet of the second exhaust gas treatment device.
In accordance with another aspect of the present invention, a method for controlling exhaust gas flow through a regenerable exhaust gas treatment device includes passing exhaust gas through a relatively short passageway from the engine to an inlet of the regenerable exhaust gas treatment device by which the internal temperature of the regenerable exhaust gas treatment device, during engine operation, is maintained above a predetermined low temperature. The method further includes sending exhaust gas produced by the engine through a relatively longer second exhaust duct to the regenerable gas treatment device by which the internal temperature of the regenerable exhaust gas treatment device, during engine operation, is maintained below a predefined high temperature.
Other features of the method for controlling exhaust gas flow through a regenerable exhaust gas treatment device, in accordance with the present invention, includes passing exhaust gas produced by the engine through a relatively shorter first exhaust duct when the engine is operating in a low to light load condition, and through the relatively longer second duct when the engine is operating in a medium to high load condition.
Yet another feature of the method for controlling exhaust gas flow through a regenerable exhaust gas treatment device, embodying the present invention, includes passing exhaust gas produced by the engine through the relatively short first exhaust duct during cold start-up of the engine.
A more complete understanding of the exhaust system and method for controlling exhaust gas flow through a regenerable exhaust gas aftertreatment device, in accordance with the present invention, may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:
At cold start, the best way to increase LNT substrate temperature is to increase the exhaust gas temperature so that hot exhaust gas will quickly heat the LNT and raise its substrate temperature. In addition to using in-cylinder or external means to increase exhaust gas temperature as fast as possible, such as by increasing engine idle speed, increasing exhaust back pressure whereby engine load is increased at idle, retarding combustion so that the exhaust temperature will be higher, or by electrical heating, optimizing the location of each treatment device in the exhaust system is also very important.
Ideally, an LNT should be positioned as close as possible to the exhaust manifold or turbocharger outlet so that the exhaust gas will heat the LNT first. However, a close-coupled LNT will have lower efficiency at higher engine loads when the LNT has warmed up, increasingly high LNT substrate temperatures will result in a reduced capability to the LNT to absorb NOx emissions. When the LNT is mounted remotely, i.e., further away, from the turbocharger outlet, it is easier to maintain the LNT substrate temperature for best efficiency under most speed-load condition after the LNT warms up. However, the remote location also contributes to cool-down of the LNT during low or light load conditions and, accordingly, the LNT will lose its conversion efficiency.
In the following description of preferred embodiments of the present invention, a flexible configuration of exhaust gas treatment system components permits the functional location of the LNT, or other aftertreatment device, in the exhaust system to be readily changed by controlling the exhaust gas flowpath. A close-coupled LNT can be provided for fast warm-up during cold start and warming up operation and LNT substrate temperatures can be maintained within a desirable temperature range under light load engine operation. By lengthening the exhaust gas flowpath and behind other components of exhaust gas treatment system, the LNT can be advantageously positioned for optimum efficiency under higher engine speed and load operation.
The first embodiment of the present invention is illustrated in
As shown in
The exhaust system 10 has a three-way engine-out exhaust valve 38 positioned in close proximity to the turbine 30. A conduit 36, preferably having a very short length, extends between an outlet of the turbine 30 and an inlet port of the three-way engine-out exhaust valve 38. If desired, the three-way engine-out exhaust valve 38 could even be mounted directly on the outlet of the turbine 30. The three-way engine-out exhaust valve 38 has first and second outlet ports arranged so that the valve can controllably direct all, or portions, of the exhaust gas flow through either of the outlet ports.
A first exhaust duct 40, also preferably having a short length, extends between the first outlet port of the three-way engine-out exhaust valve 38 and an inlet to a regenerable exhaust gas treatment device 42 which, for the purpose of illustrating the present invention, is a lean NOx trap. A second exhaust duct 44 extends between the second outlet port of the three-way engine-out exhaust valve 38 and a second exhaust gas treatment device 46, for example, a Diesel oxidation catalyst. In the illustrated embodiments, the exhaust gas treatment system 10 also has a third exhaust gas treatment device 48, such as a Diesel particulate filter. A third exhaust duct 50 provides fluid communication between an outlet of the Diesel particulate filter 48 and a first inlet port of a second three-way exhaust valve 52. An outlet port of the second three-way exhaust valve 52 provides direct communication with the ambient environment. A third three-way exhaust valve 54 has an inlet port in direct communication with an outlet of the lean NOx trap 42. A fourth exhaust duct 56 extends from a first outlet port of the third three-way exhaust valve 54 to the second exhaust duct 44. A fifth exhaust duct 58 extends from the inlet of the LNT 42 to the second inlet port of the second three-way exhaust valve 52. A sixth exhaust duct 59 extends from a second outlet port of the third three-way exhaust valve 54 to the third exhaust duct 50.
During cold start and warm-up if an engine control unit, not shown, detects temperatures in the LNT 42 that are less than a predefined lower value, the configuration of the flexible exhaust system 10, illustrated in the
Under normal operating conditions, the flexible exhaust system 10 embodying the present invention is reconfigured to the arrangement illustrated in
During low to light load operation, if the LNT temperature is less than a desired minimum LNT efficiency temperature, the variably configurable exhaust system 10 directs the flow of exhaust gas through the close-coupled LNT configuration shown in
During regeneration of the LNT 42, the close-coupled configuration illustrated in
During LNT regeneration, the regeneration frequency, duration and air/fuel ratio are determined by a specific regeneration strategy that is a function of engine-out NOx emissions and current engine operating conditions. At high load, due to the high engine-out NOx emissions, regeneration must be carried out at more frequent intervals. Accordingly, the temperature of the LNT 42 increases rapidly if the period between two adjacent regenerations is not long enough for the LNT substrate temperature to be cooled. The only way to reduce the internal temperature of the LNT is to cool down the exhaust gas entering the LNT between two consecutive LNT regenerations. Ways in which the exhaust gas can be cooled during LNT regeneration are discussed below in the embodiments illustrated in
During LNT desulfation at high load operation, in which PCCI is the preferred combustion mode for increasing LNT temperature while providing exhaust gas consisting of products of stoichiometric combustion for regeneration of the LNT 42, in the remotely-mounted LNT configuration illustrated in
The variably reconfigurable exhaust system 10 embodying the present invention, provides at least two methods by which exhaust gas temperature can be cooled down between consecutive LNT regenerations.
Moreover, as illustrated in the lower portion of the
Although the present invention is described in terms of preferred illustrative embodiments, those skilled in the art will recognize that variations on, or combinations of, the described embodiments can be made in carrying out the present invention. For example, LNT desulfation can be carried out in the close-coupled LNT configuration illustrated in
Other aspects, features and advantages of the present invention may be obtained from the study of this disclosure and the drawings, along with the appended claims.
This is a non-provisional application claiming priority to U.S. Provisional Application Ser. No. 60/625,847 filed Nov. 8, 2004.
Number | Date | Country | |
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60625847 | Nov 2004 | US |