Abstract:
An apparatus and system are disclosed for enhancing aftertreatment regeneration. The system includes an internal combustion engine and an exhaust manifold directing the engine exhaust to an aftertreatment system. The system may further include an exhaust gas recycle system and a turbocharger. The system further includes a fuel injector mounted on the exhaust manifold that provides fuel to assist in regenerating an aftertreatment component. The fuel injector is mounted in an apparatus also including a flow dampener, an extender, and a residence chamber. The apparatus allows the fuel to be injected in a high temperature location where it will experience residence time at temperature, and experience shear forces passing through the turbocharger. The extender allows the fuel to be injected at a place in the exhaust manifold where recycling of injected fuel into the engine is minimized.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to exhaust gas aftertreatment systems and more particularly to an apparatus and system for enhancing aftertreatment regeneration. 
         [0003]    2. Description of the Related Art 
         [0004]    Environmental concerns motivate emissions requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NO x ), and unburned hydrocarbons (UHC). 
         [0005]    The need to comply with emissions requirements encourages the development of exhaust gas aftertreatment systems. Aftertreatment systems frequently include one or more of a diesel oxidation catalyst (DOC), a NO x adsorption catalyst (NAC), and a diesel particulate filter (DPF). The DOC oxidizes unburned hydrocarbons in the exhaust stream for cleanup and/or temperature generation. The NAC adsorbs NO x  from the exhaust gas and regenerates with periodic temperature events within the NAC. The DPF removes particulates from the exhaust gas stream. Furthermore, an exhaust gas recirculation (EGR) system may be implemented to reduce the formation of NO x  during combustion. 
         [0006]    Many aftertreatment components require temperature and/or UHC in the exhaust stream to facilitate regeneration, and many aftertreatment systems place a fuel injector (or “doser”) in the exhaust stream to provide the temperature and/or UHC. The placement of the fuel injector is a challenge in aftertreatment system design. In one embodiment of the present technology, the fuel injector is placed downstream of an exhaust manifold and turbocharger. Placement of the fuel injector, a precise mechanical device with sensitive electronic components, downstream of the exhaust manifold helps to ensure that commercially reasonable fuel injectors requiring relatively low operating temperature environments may be utilized. 
         [0007]    A common alternative method for dosing the exhaust gas is “in-cylinder dosing.” The dosing fuel is injected directly into the combustion chamber ensuring that the fuel is thoroughly mixed with the exhaust before reaching the aftertreatment system. However, some of the challenges of in-cylinder dosing include diluting the engine oil with fuel, fuel recycling through the EGR, and the necessity of including a post-injection capable fuel system that may be more expensive than desired (e.g. a common rail fuel system). 
         [0008]    Even if the fuel injector temperature limitations are overcome—perhaps through exotic materials and expensive cooling packages—placing the fuel injector into the exhaust manifold, or injecting in-cylinder, is difficult on engines with EGR. Fuel injected can be recirculated through the EGR path, potentially fouling an EGR cooler and EGR valve, and disrupting the designed torque and operation of the engine. Some engines may include grid heaters or other components in the air intake that are exposed to EGR flow and should not be exposed to unburned fuel. In the current technology, placing of a fuel injector in the exhaust manifold or dosing in-cylinder typically involves shutting off EGR and/or bypassing the EGR cooler. This results in increased emissions and/or lower power density of the engine. 
         [0009]    Placement of the aftertreatment fuel injector downstream of the turbocharger presently causes performance limitations on the aftertreatment system. The placement downstream of the turbocharger means the fuel is injected into a cooler, low shear and low turbulence environment, closer to the component of interest—usually the DOC—and therefore the fuel may not be completely evaporated and distributed in the exhaust stream. Also, in the environment downstream of the turbocharger, the fuel does not experience enough time at temperature to begin breaking down from large hydrocarbon chains to small hydrocarbon chains, further reducing the oxidizing effectiveness of the DOC or other aftertreatment component. 
         [0010]    An alternate placement of the aftertreatment fuel injector upstream of the turbocharger may allow for more flexibility of engine and aftertreatment design and permit fuel in the exhaust stream to experience higher temperatures, more turbulence, more shear forces, and longer residence time leading to superior oxidation and superior performance of the aftertreatment system. 
       SUMMARY OF THE INVENTION 
       [0011]    From the foregoing discussion, applicant asserts that a need exists for a system and apparatus to enhance aftertreatment regeneration. Beneficially, such a system and apparatus would allow placement of a fuel injector within an exhaust manifold providing a higher temperature environment, with greater turbulence and shear causing better mixing of injected fuel and exhaust gas. In a further beneficial improvement, the system and apparatus would allow for the continued normal use of EGR, while injecting fuel, compared to in-cylinder dosing. Additionally, the system and apparatus would provide a longer residence time for injected fuel compared to present methods of downstream dosing. 
         [0012]    The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available aftertreatment fuel injection systems and apparatus. Accordingly, the present invention has been developed to provide a system and apparatus for placing a fuel injector within a region of an exhaust manifold that overcome many or all of the above-discussed shortcomings in the art. 
         [0013]    An apparatus is disclosed to enhance aftertreatment regeneration. The apparatus includes a flow dampener comprising an orifice. The flow dampener may further include a wall segment comprising a frustum of a defining cone. The apparatus includes an extender coupled to the flow dampener configured to dispose the orifice within a normal flow region of an exhaust manifold. The normal flow region comprises a region of the exhaust manifold where an exhaust flow from an engine experiences minimal flow reversal. The extender may comprise a portion of the wall segment. The apparatus further includes a residence chamber disposed within the extender and the flow dampener, and a fuel injector configured to inject fuel into the residence chamber. In one embodiment, the apparatus includes an insulator ring placed between the fuel injector and the residence chamber. 
         [0014]    The apparatus may include the extender configured such that the injected fuel enters an exhaust stream in a location where minimal exhaust gas recycles to the engine intake. In one embodiment of the apparatus, the residence chamber has a volume such that the injected fuel fully vaporizes before diffusing through the orifice. The apparatus may include a flow dampener configured to dampen an exhaust flow convection through the orifice into the residence chamber such that the fuel injector maintains a temperature below a threshold temperature. 
         [0015]    A system is disclosed to enhance aftertreatment regeneration. The system comprises an internal combustion engine producing an exhaust stream and an exhaust manifold coupled to the engine to receive the exhaust stream. The system further comprises the apparatus coupled to the exhaust manifold and configured to inject fuel into the exhaust stream. The system may further comprise a turbocharger including a turbine inlet port receiving the exhaust stream from the exhaust manifold. 
         [0016]    Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
         [0017]    Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
         [0018]    These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]    In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
           [0020]      FIG. 1  is an illustration depicting one embodiment of a system to enhance aftertreatment regeneration in accordance with the present invention; 
           [0021]      FIG. 2A  is an illustration depicting one embodiment of an apparatus to enhance aftertreatment regeneration in accordance with the present invention; 
           [0022]      FIG. 2B  is an illustration depicting a side view of one embodiment of an apparatus to enhance aftertreatment regeneration in accordance with the present invention; 
           [0023]      FIG. 3  is an illustration depicting one embodiment of an apparatus to enhance aftertreatment regeneration including an insulator ring in accordance with the present invention; and 
           [0024]      FIG. 4  is an illustration of an insulator ring in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the apparatus and system of the present invention, as presented in  FIGS. 1 through 4 , is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. 
         [0026]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. 
         [0027]    Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of materials, fasteners, sizes, lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
         [0028]      FIG. 1  is an illustration depicting one embodiment of a system  100  to enhance aftertreatment regeneration in accordance with the present invention. The system  100  comprises an internal combustion engine  102  producing an exhaust stream  104 . The internal combustion engine  102  may be any type of internal combustion engine  102 . In one embodiment, the internal combustion engine  102  is a diesel engine  102 . The system  100  further comprises an exhaust manifold  106  coupled to the engine  102 . The exhaust manifold  106  is configured to receive the exhaust stream  104  coming from the engine  102 . The exhaust stream  104  may be from one exhaust bank, two exhaust banks, a plurality of exhaust banks, dual exhaust pipes with dual aftertreatment systems, and/or any other configuration of exhaust streams  104  coming from the combustion engine  102 . For example, a six-cylinder diesel engine  102  produces six exhaust streams  104  that collect in a pipe  106  configured as the exhaust manifold  106 . The exhaust manifold  106  is any apparatus configured to receive the exhaust stream  104  or exhaust streams  104  from the engine  102 . 
         [0029]    The system  100  further comprises a doser assembly  108 . The doser assembly  108  further comprises a flow dampener that is configured to reduce the heat transfer via convection from the exhaust stream  104  to the fuel injector. The flow dampener includes an orifice that restricts the flow of exhaust gas into the area around the fuel injector. In one example, the flow dampener is configured within a doser assembly  108  to support a fuel injector that is configured to function at 400 degrees F. in an exhaust manifold  106  experiencing standard diesel exhaust temperatures of about 1400 degrees F. 
         [0030]    The doser assembly  108  of the system  100 , in one embodiment, further comprises an extender coupled to the flow dampener. The extender disposes the orifice of the flow dampener into a normal flow region of the exhaust manifold  106 . The normal flow region may be a region of the exhaust manifold  106  where the exhaust flow  104  recirculating through to the exhaust gas recirculation (EGR) path  110  is minimal under normal operating conditions. For example, the normal flow region may be a region close to a turbine inlet port. In one embodiment, the normal flow region is within about three inches from a turbine inlet port. In an alternate embodiment, the normal flow region may be beyond an outlet of the exhaust manifold  106 . The extender may be configured such that the injected fuel enters the exhaust stream in a location where minimal exhaust gas recycles to the engine intake. 
         [0031]    The doser assembly  108  further comprises a residence chamber that is a volume disposed within the extender and the flow dampener. The residence chamber may have a volume such that the injected fuel experiences a sufficient residence time within the residence chamber such that the injected fuel fully vaporizes before diffusing through the orifice. For example, if simple testing indicates that liquid hydrocarbon is diffusing from the residence chamber, the residence chamber volume may be increased and/or the orifice size may be decreased to make the residence chamber volume sufficient to provide the residence time to vaporize the injected hydrocarbons. The doser assembly  108  may include an insulating ring interposed between the fuel injector and the residence chamber. 
         [0032]    The doser assembly  108  further comprises a fuel injector configured to inject fuel into the residence chamber. The fuel is injected to add energy to the exhaust flow and may be a hydrocarbon, hydrogen, alcohol, and/or other fuel, and may be the same fuel used by the combustion engine  102 . The fuel diffuses from the residence chamber through the flow dampener into the exhaust stream as exhaust gas pulses intermittently in and out of the flow dampener. 
         [0033]    The system  100  further comprises an EGR path  110  configured to recirculate a portion of the exhaust flow  104 . The EGR path  110  may include an EGR cooler  112  that cools the exhaust gas before the exhaust gas combines with an engine inlet air stream  114 . The EGR path  110  may further include an EGR valve  113  that restricts and allows EGR flow. The EGR valve  113  may be upstream or downstream of an EGR cooler  112 . The system  100  may further comprise a turbocharger  116  configured to receive an exhaust flow from the exhaust manifold  106 . The turbocharger  116  may be more than one turbocharger  118  configured in parallel or in series. The turbocharger  118  may be a standard turbocharger, a wastegate turbocharger, and/or a turbocharger with variable geometry (VGT). 
         [0034]    The system  100  further comprises an aftertreatment device  118  configured to treat an exhaust gas. The aftertreatment device  118  may be multiple devices configured to support each other, and/or be configured to treat multiple exhaust gas components. In a first example, the aftertreatment device  118  may burn a hydrocarbon to heat another aftertreatment device  118 . In a second example, a first aftertreatment device  118  may be a diesel oxidation catalyst (DOC), a second aftertreatment device  118  may be a NO x  adsorption catalyst (NAC), and a third aftertreatment device  118  may be a particulate filter. In the second example, at one operating point, the fuel injector injects diesel fuel into the exhaust gas, the DOC burns the diesel fuel upstream of the NAC, the heat generated by the DOC facilitates a regeneration event within the NAC, and a particulate filter removes particulates from the exhaust gas. 
         [0035]      FIG. 2A  is an illustration depicting one embodiment of an apparatus  200  to enhance aftertreatment regeneration in accordance with the present invention. The apparatus  200  comprises the doser assembly  108  coupled to the exhaust manifold  106  near a turbocharger interface  202 . The doser assembly  108  includes a flow dampener  204  comprising an orifice  206  that may, in one embodiment, comprise a diameter of about 10 mm. The flow dampener  204  may comprise only the orifice  206 . In alternate embodiments, the flow dampener  206  further includes a wall segment  208  comprising a frustum of a defining cone. The defining cone is illustrated as a right-angle cone in  FIG. 2A , and the orifice  206  is shown intersecting the cone at a right angle, but these can be any angle to meet the geometry of the system  100 . The orifice  106  angle, in one embodiment, is as close to a right angle with the cone as the system  100  geometry allows. 
         [0036]    The flow dampener  204  of the apparatus  200  is configured to provide a low heat transfer environment—especially a low convection environment—around a fuel injector  212  according to the expected temperatures and expected exhaust flow  104  conditions (e.g. peak rates, average rates, Reynolds number, etc.) within the exhaust manifold  106 . In one embodiment, the exhaust flow  104  through the exhaust manifold  106  may be turbulent and an angle θ of not more than 30 degrees is sufficient to maintain an operational temperature range of the fuel injector  212 . In an alternate embodiment, where the exhaust manifold  106  experiences a high steady-state exhaust flow  104 , an angle θ of not more than about 45 degrees is sufficient to maintain the operational temperature range of the fuel injector  212 . 
         [0037]    In one embodiment, the flow dampener is configured to dampen an exhaust flow convection through the orifice into the residence chamber  220 , such that the fuel injector  212  maintains a temperature below a threshold temperature. It is a mechanical step for one of skill in the art to determine a flow dampener  204  configuration, defined by an orifice  206  size and angle θ, to achieve a required heat transfer environment for a fuel injector  212  in a given embodiment of the system  100  based on the exhaust flow  104  temperature and conditions, the temperature requirements for the fuel injector  212 , and the disclosures herein. 
         [0038]    The doser assembly  108  further includes an extender  214  coupled to the flow dampener  204  configured to dispose the orifice  206  within a normal flow region  216  (refer to  FIG. 2B ) of the exhaust manifold  106 . The normal flow region  216  may be a region of the exhaust manifold  106  where the exhaust flow  104  from the engine  102  experiences minimal flow reversal. During ordinary engine  102  operation, different cylinders fire intermittently, causing pressure pulses within the exhaust manifold  106 . Some regions of the exhaust manifold  106  thereby experience significant reversals in the flow direction, and the regions experiencing such reversals for a given system  100  are ordinarily understood by one of skill in the art familiar with the particular system  100 . 
         [0039]    In one embodiment, the normal flow region  216  is the region  216  downstream of a plurality of cylinder exhausts. For example, a point in the exhaust manifold that is downstream of every cylinder exhaust will ordinarily experience minimal flow reversal, even though pulses in the flow magnitude will occur. In one embodiment, the normal flow region  216  is a region within about 3 inches of a turbine inlet port  218  (refer to  FIG. 2B ). The normal flow region  216  should be selected such that fuel injected into the normal flow region  216  does not significantly recirculate through the EGR path  110 . A simple check of whether unburned hydrocarbons are recirculating through the EGR path  110  will confirm whether the normal flow region  216  is selected such that minimum flow reversal is occurring. 
         [0040]    In one embodiment, the wall segment  208  of the doser assembly  108  includes a portion of the wall segment  208  comprising a part of the flow dampener  204  and a portion of the wall segment  208  comprising a part of the extender  214 . The length and diameter of the extender  214  are functions of the exhaust manifold  106  geometry, fuel injector  212  size, a required residence chamber  220  volume, location of the normal flow area  216 , mounting position of the doser assembly  108 , and other application specific parameters. It is a mechanical step by one of skill in the art to determine the length and diameter of the extender  214  based on the physical layout of a given system  100  and the disclosures herein. The extender  214  length and diameter should be selected such that the orifice  206  is within the normal flow region  216 , and that sufficient residence chamber  220  volume (discussed below) is available. In one embodiment, the extender  214  length is at least about 1.6 inches. In an alternate embodiment, the extender  214  length is about 40 mm, the extender diameter is about 35 mm, a flow dampener height  210  is about 20 mm, and the orifice  206  diameter is about 10 mm. In an embodiment where the normal flow region is accessible to a doser assembly  108  mounting location, the extender  214  length may be zero. 
         [0041]    The doser assembly  108  of the apparatus  200  further comprises the fuel injector  212  configured to inject fuel into the residence chamber  220 . The fuel injector  212  shown in  FIG. 2A  extends slightly into the residence chamber  220  to clearly illustrate the approximate placement of the injector  212 . The fuel injector  212  may also not extend into the residence chamber  220 , and may be recessed from the residence chamber  220  in some embodiments. 
         [0042]    The maximum fuel injection rate of the fuel injector  212  depends on the requirements of the aftertreatment system, the selected regeneration strategies for the aftertreatment system, and the thermal delivery capabilities and fuel system of the engine  102 . The maximum fuel injection rate for a given system  100  is ordinarily understood by one of skill in the art familiar with the particular system  100 . In one embodiment, for an approximately 6-Liter displacement engine  102  with a DOC, NAC, and particulate filter, the maximum fuel injection rate is about 60 cm 3 /minute. The maximum fuel injection rate may represent the maximum fuel injection rate the fuel injector is capable of injecting, and/or the maximum fuel injection rate expected by the design requirements of the aftertreatment device(s)  118 . For example, a fuel injector  212  may be capable of injecting 150 cm 3 /minute, but the aftertreatment device  118  required temperature and engine capabilities  102  may indicate a maximum fuel injection rate of 100 cm 3 /minute. 
         [0043]    The doser assembly  108 , in one embodiment, further includes the residence chamber  220  disposed within both the extender  214  and the flow dampener  204 . The fuel injector  212  injects fuel into the residence chamber  220 , where the fuel mixes into the gas of the residence chamber  220  and diffuses through the orifice  206  into the exhaust flow  104 . In one embodiment, the residence chamber  220  volume is sized to provide sufficient time for injected fuel to evaporate and break down before diffusion into the exhaust flow  104 . The required residence time depends on the fuel composition, the temperature in the residence chamber  220  at operating conditions, the catalyst composition of an aftertreatment device  118  oxidizing the fuel, and other parameters specific to a given embodiment of the system  100 . The available residence time depends on the maximum fuel injection rate, the volume of the residence chamber  220 , the size of the orifice  206 , and the exhaust flow  104  conditions in the normal flow area  216 . In one embodiment, the injected fuel is not completely vaporized within the residence chamber, but is entrained and well-mixed in the gas phase, and by passing through the mixing in the turbocharger  116  the injected fuel completes the vaporization process. 
         [0044]    One of ordinary skill in the art may determine the appropriate volume of the residence chamber  220  through simple experimentation. Specifically, if the system  100  exhibits unburned hydrocarbons at the outlet (e.g. the turbocharger outlet  116 , and/or the exhaust system outlet) at operating conditions and required fuel injection rates with a properly sized catalyst element in the aftertreatment device  118 , the residence chamber  220  size should be increased. In one embodiment, the volume of the residence chamber  220  comprises a volume of at least 0.5*V 1 , where V 1  is an expected fuel injection volume per minute. For example, the expected fuel injection volume per minute (V 1 ) for a system  100  is 60 cm 3 /minute and the volume of the residence chamber is at least 30 cm 3  (1.8 in 3 ). 
         [0045]    In one embodiment, a displacement volume V eng  of the engine  102  and a volume V rc  of the residence chamber  220  comprise a ratio V eng /V rc  of less than about 200. For example, the displacement volume V eng  for a system is 6,700 cm 3  (409 in 3 ) and the residence chamber volume V rc  is greater than about 33.5 cm 3  (2.0 in 3 ). In an alternate embodiment, the residence chamber  220  comprises a volume of about 35,000 mm 3 . 
         [0046]      FIG. 2B  is an illustration depicting a side view of one embodiment of an apparatus  200  to enhance aftertreatment regeneration in accordance with the present invention. The side view of the apparatus  200  is shown to enhance understanding of the positioning of the doser assembly  108  in relation to the exhaust manifold  106  and the turbocharger  116  for the embodiment of  FIG. 2A . The doser assembly  108  is coupled to the exhaust manifold  106  with the orifice  206  (not marked in  FIG. 2B  to avoid cluttering the Figure) within the normal flow region  216  of the exhaust manifold  106 . The normal flow region  216  is near the turbine inlet port  218  and the turbocharger interface  202  is fixed to the turbocharger  116 . 
         [0047]      FIG. 3  is an illustration depicting one embodiment of an apparatus to enhance aftertreatment regeneration including an insulator ring  302  in accordance with the present invention. In the embodiment of  FIG. 3 , the fuel injector  212  is recessed from the residence chamber  220 . The use of the flow dampener  206  can reduce the steady-state temperature of the fuel injector  212  by several hundred degrees F. The use of the insulator ring  302  can further reduce the steady-state temperature of the fuel injector  212  by tens of degrees F (e.g. 30 degrees F. for one embodiment). 
         [0048]      FIG. 4  is an illustration of an insulator ring  302  in accordance with the present invention. The thickness of the insulator ring  302  and the size of the center hole in the insulator ring  302  are limited by the geometry of the fuel injector  212 . Specifically, the amount of recession of the fuel injector  212  and the spray angle of the fuel injector  212  will define the maximum thickness and/or minimum hole size of the insulator ring  302 . It is a mechanical step for one of skill in the art to calculate the thickness and hole size of an insulator ring  302  based on a fuel injector  212  location relative to the residence chamber  220  and the spray angle of the fuel injector  212 . The insulator ring  302  may be any material suitable for the environment of the particular system  100 —preferably a material with low thermal conductivity, high temperature resistance, and easy manufacturability. In one embodiment, a ceramic fiber donut is suitable for an insulator ring  302 . 
         [0049]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.