Patent Publication Number: US-2020292433-A1

Title: Jet Fuel Thermal Oxidation Test Equipment

Description:
FIELD OF INVENTION 
     The present disclosure is related to jet fuel thermal oxidation testing and, more particularly, to equipment that may be used with jet fuel thermal oxidation testing rigs to improve accuracy, efficiency, and reliability. 
     BACKGROUND 
     Modern jet engine systems comprise gas turbine engines that run on jet fuel. Under normal operating conditions, jet fuel is heated by the hot components or regions of the gas turbine engines, which include the fuel nozzles, fuel nozzle support assemblies, and heat exchangers. Modern jet engine systems use the jet fuel&#39;s heat sink capability for cooling various aircraft systems, including hydraulic, electronic, and lubrication systems. However, heat management and, ultimately, performance of the jet engine system and airframe is a delicate balance between (i) running fuel systems cooler and incurring performance, cost, and weight penalties by use of air cooling, or (ii) running systems as hot as possible and causing problems associated with unacceptable deposition rates. Accordingly, engineers often design jet engine systems to take maximum advantage of the thermal stability of currently available fuels. 
     Trends in higher whole engine system performance as well as airframe and engine heat loads, coupled with simultaneous reductions in fuel consumption, are forcing fuel system temperatures to increase even further. Therefore, many modern high performance jet engine systems utilize thermally stressed fuels. At high temperatures, however, less stable species in the thermally stressed jet fuel may undergo oxidation reactions that produce gums, lacquers, particulates, and coke deposits. These resultants may cause a number of problems, including blockage of filters, loss of heat exchanger efficiency, stiction or hysteresis of sliding components in control units, and fouling of injectors and distortion of spray patterns. For example, oxidation of thermally stressed jet fuel may result in deposits or particulate that blocks engine fuel nozzles, thereby causing damage to the engine hot sections due to distorted fuel spray patterns, especially the combustor region. Accordingly, a jet fuel&#39;s thermal stability is critical to achieving optimum performance of modern gas turbine engines. 
     The current standard for evaluating a jet fuel&#39;s thermal oxidation is the Standard Test Method for Thermal Stability of Aviation Turbine Fuels, designation D3241, IP323, as published by the American Society for Testing and Materials International (“ASTM International”). This test method mimics the thermal stress conditions encountered by jet fuel in operation and, despite being developed in the early 1970s, remains the best method to evaluate jet fuel thermal stability. More specifically, the D3241 test method sets forth a procedure for rating the tendency of jet fuels to deposit decomposition products within a fuel system. The D3241 test method is performed in two (2) phases. The first phase mimics the fuel conditions present during airplane engine operation, whereas the second phase quantifies the oxidation thermal deposits formed during the first phase. 
     Various laboratory devices, known as rigs, have been developed since that time to facilitate the D3241 test method. These rigs subject an aluminum heater tube to sample jet fuel under conditions mimicking those encountered during actual engine operation. However, these rigs are difficult to use and require substantial expertise when installing the heater tube within the test section and when preparing the jet fuel sample. Moreover, these known rigs include pump systems that move the fuel sample through the test section, but often have leaks, inconsistent flow rates, and micro-ruptures, and are expensive to operate and maintain. Furthermore, these known rigs have primitive temperature control systems that impact the test results and reproducibility of the same. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, without departing from the scope of this disclosure. 
         FIG. 1A  is a perspective view of an example rig that may incorporate the principles of the present disclosure. 
         FIG. 1B  is a detailed perspective view of the example rig of  FIG. 1A , showing an example test section that may incorporate the principles of the present disclosure. 
         FIG. 2  is a side view of a disassembled test section utilized in the rig of  FIG. 1B . 
         FIG. 3A  is a detailed side view of the sleeve and heater tube assembly utilized in the test section of  FIG. 1B  and illustrates the fluid outlet when the heater tube is arranged within the sleeve. 
         FIG. 3B  is a cross-sectional side view of the fluid outlet of  FIG. 3A . 
         FIGS. 4A-4B  are side views of the sleeve and heater tube assembly of  FIG. 3A  and illustrate utilization of a gauge to position the heater tube within the sleeve. 
         FIG. 4C  is a cross-sectional side view of the gauge of  FIGS. 4A-4B , which may be used to position the heater tube within the sleeve. 
         FIG. 5  is a schematic that illustrates various functions of the rig of  FIG. 1A  that are utilized to aerate the fuel sample. 
         FIG. 6A  is a diagram that illustrates an example operation of a manual fuel sample aeration procedure. 
         FIG. 6B  is a diagram that illustrates an example operation of an automatic fuel sample aeration procedure. 
         FIG. 7  is a schematic illustrating an example operation of a pump system having a dual syringe arrangement. 
         FIG. 8  is a diagram illustrating the operation of a heating system utilized in the rig of  FIG. 1A . 
         FIG. 9A  is a diagram illustrating the operation of a bus bar cooling system utilized in the rig of  FIG. 1A . 
         FIG. 9B  is a schematic of the bus bar cooling system of  FIG. 9A . 
         FIG. 10  is schematic illustrating an example operation of a bus bar cooling system that independently controls the separate bus bars. 
         FIG. 11A  is a schematic illustrating clamping systems that may be utilized to secure the sleeve and heater tube assembly to the bus bars, for example, at the lower bus bar of  FIG. 1B . 
         FIG. 11B  is a schematic illustrating an alternate clamping system that may be utilized to secure sleeve and heater tube assembly to the bus bars. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments described herein provide positioning gauges for arranging a heater tube within the sleeve of a rig test section. Other embodiments described herein provide air control systems that provide automated aeration of fuel samples with automatic airflow control. Further, embodiments described herein provide pump systems having double syringe arrangements. Moreover, embodiments described herein provide cooling systems that independently control the separate bus bars. 
     The ASTM International jet fuel thermal oxidation test (D3241, IP 323) standard test method (the “test method”) is performed in two (2) phases. The first phase mimics the fuel conditions present during airplane engine operation, whereas the second phase quantifies the oxidation thermal deposits formed during the first part. A technician performs the first phase via an apparatus that simulates conditions present in gas turbine engine fuel systems during operation. The apparatus, referred to herein as a rig, includes a test section that generally comprises a tube-in-shell heat exchanger that holds a test coupon and directs fuel flow over the test coupon. The second phase consists of inspection of the test coupon either via an instrument that automatically measures thermal oxidation deposit thickness or through visual inspection. The following disclosure focuses primarily on the first phase of the test method and the rigs utilized therein to form the thermal oxidation deposit. 
       FIG. 1A  is a partial perspective view of an exemplary rig  100  that may incorporate the principles of the present disclosure. The depicted rig  100  is just one example testing rig that can suitably incorporate the principles of the present disclosure. Indeed, many alternative designs and configurations of the rig  100  may be employed, without departing from the scope of this disclosure. 
     In the illustrated embodiment, the rig  100  is configured to automatically perform the test method; however, it may also be configured to automatically perform other petroleum product tests such as ISO 6249. As illustrated, the rig  100  includes a sample container  102 , a waste container  104 , and a test section  110  that fluidly interconnects the sample container  102  and waste container  104  as hereinafter described. In use, a technician will place a fuel sample S in the sample container  102  and, upon activating the rig  100  to perform the test method, the rig  100  pumps the fuel sample S from the sample container  102 , through the test section  110 , and into the waste container  102  upon completion of the test method. 
       FIG. 1B  is a detailed view of the test section  110  of  FIG. 1A  according to one or more embodiments. As illustrated, the test section  110  may include a sleeve  112  with a heater tube  114  (partially obscured from view in  FIG. 1B ) hermetically sealed therein. Here, the heater tube  114  is secured within the sleeve  112  via a pair of nut assemblies  136   a , 136   b , however, other assemblies may be utilized to secure heater tube  114  within the sleeve  112  without departing from the present disclosure. The sleeve  112  is hollow and is open at each of its ends  112   a,   112   b  (obscured from view in  FIG. 1B ). The test section  110  also includes a fuel inlet  116  and an outlet  118  arranged on the sleeve  112  between the open ends  112   a,   112   b.  The fluid inlet  116  is fluidly connected to the sample container  102  and the fluid outlet  118  is fluidly connected to the waste container  104 . In addition, the test section  110  includes a test filter  120  that is arranged proximate to the outlet  118  at a location between the outlet  118  and the waste container  104 . 
       FIG. 1B  also illustrates the rig  100  comprising a pair of jaws or bus bars  122   a,   122   b  that are arranged to secure the test section  110  in a desired orientation via a clamping system which is further described below with reference to  FIG. 11A . However, alternate clamping systems may be utilized, for example, as described with reference to  FIG. 11B . As described below, the bus bars  122   a,   122   b  supply a controlled high amperage, low voltage current to the heater tube  114 , thereby making it possible to maintain an accurate temperature during the duration of the test method. Accordingly, the bus bars  122   a,    122   b  are directly or indirectly connected to a transformer or other power supply (not illustrated). In some embodiments, the bus bars  122   a,   122   b  are made from brass or other material having a lower thermal conductivity than the heater tube  114  material as hereinafter described. In addition, a thermocouple  124  is arranged to provide temperature measurements of the test section  110  as described below. 
       FIG. 2  illustrates a side view of the test section  110  when disassembled and detached from the rig  100 . As illustrated, the sleeve  112  is hollow and the fuel inlet  116  and outlet  118  are disposed between the open ends  112   a,   112   b  thereof such that the fuel inlet  116 , the outlet  118 , and the open ends of  112   a,   112   b  are in fluid commination with each other.  FIG. 2  also illustrates the heater tube  114  when extracted from the sleeve  112 , as may occur before and after the test method. As illustrated, the heater tube  114  includes a thin portion  130  interposed between a pair of shoulders  132   a,   132   b  disposed at opposing ends  134   a,   134   b  of the heater tube  114 . In operation, the heater tube  114  is inserted into and through the sleeve  112 , and secured thereto via a pair of clamping nut assemblies  136   a,   136   b  that permit a technician to remove the heater tube  114  from the sleeve  112 , for example, before and after performing the test method. In the illustrated embodiment, the clamping nut assemblies  136   a,   136   b  each include gaskets, washers, seals and nuts to secure the shoulder  132   a  of the heater tube  114  at the open end  112   a  of the sleeve  112  and to secure the shoulder  132   b  at the open end  112   b.  It will be appreciated, however, that the nut assemblies  136   a,   136   b  may be differently arranged with the same and/or different components without departing from the present disclosure. 
     The heater tube  114  also includes a thermocouple (obscured from view) arranged inside an interior volume thereof, and the heater tube  114  is resistively heated by conductance via the pair of bus bars  122   a,   122   b  that each clamp a respective one of the pair of shoulders  132   a,   132   b  of the heater tube  114 . In some embodiments, the heater tube  114  is an aluminum (or other metal) coupon controlled at elevated temperature by the bus bars  122   a,   122   b,  over which a fuel sample S is pumped. 
     As mentioned above, at various points before, during, and after the test method, the technician may need to assemble or disassemble the sleeve  112  and the heater tube  114 . For example, the test method may require the technician to precisely assemble the test section  110  (i.e., install the heater tube  114  within the sleeve  112  without any leakage) before beginning the test method and/or to disassemble the test section  110  at the end of the test method. In addition, the test method may call for the technician to clean, rinse, and dry the certain components during the disassembly phase. Accurate analysis and test method results depend on proper assembly, dismantling, cleaning, rinsing, and drying of the test method components. Thus, significant technician expertise is needed to properly perform these phases of the test method, which may consume a significant amount of time and resources. 
       FIG. 3A-3B  illustrates a side view of the heater tube  114  assembled within the sleeve  112  and secured therein via the clamping nut assemblies  136   a,    136   b.  The test method specifies that the heater tube  114  is to be manually positioned within the test section  110  by a technician. More specifically, the test method specifies that the heater tube  114  should be positioned precisely relative to the sleeve  112 , and visually adjusted to center a lip  302  of the upper shoulder  132   a  (of the heater tube  114 ) within an aperture  304  of the fuel outlet  118  as illustrated in  FIG. 3A-3B . This arrangement permits the fuel sample S may flow through the fuel outlet  118  and to other downstream instrumentation, such as the differential pressure measurement instrumentation as hereinafter described. 
     Once the lip  302  of the upper shoulder  132   a  has been centered within the fuel outlet  118 , the technician will tighten and secure the heater tube  114  within the sleeve  112 , for example, via the nut assemblies  136   a,   136   b.  Tightening the heater tube  114  within the sleeve  112  will help seal the interior volume through which the fuel sample S flows, however, the resulting clamping forces oftentimes cause unintended repositioning of the heater tube  114  relative to the sleeve  112  such that the lip  302  is no longer properly positioned as mentioned above. Consequently, an extreme fine adjustment is required to pre-position the lip  302  of the heater tube  114  to account or anticipate such displacement during tightening. Accordingly, technicians need significant expertise to properly install the heater tube  114  within the sleeve  112 . 
       FIGS. 4A-4B  illustrate a positioning gauge or gauge  402  that may be utilized to reliably position the heater tube  114  relative to the sleeve  112 , according to one or more embodiments. The gauge  402  may be provided as an accessory to assist technicians that would otherwise need to rely on the visual location of the lip  302  within the outlet  118  in order to prepare the test section  110 . In the illustrated embodiment, the gauge  402  is open at a first end  404  thereof, and an inner bore  406  of the first end  404  is threaded so that the gauge  402  may be screwed onto an end of the sleeve  112 , for example, at a plurality of threads  408  arranged at the open end  112   a.  In some embodiments, the gauge  402  is open at a second end thereof, and may include a threaded bore at the foregoing second end that includes the same or differently arranged threads, and such arrangements may provide the gauge  402  with the ability to be used with various test sections  110 . The body of the gauge  402  includes a central bore that extends a length through the body, and the length that the bore extends may be equal to the body length or shorter. In some embodiments, the bore extends through the body for a length that is shorter than the body and, in such embodiments, a shoulder may be provided along the inner bore surface to act as an abutment that inhibits further axial movement of the shoulder  132   a.    
       FIG. 4C  illustrates an example of the gauge  402 , according to one or more embodiments. In the illustrated embodiment, the gauge  402  includes a body  410  that is open at the first end  404  thereof. As illustrated, the body  410  includes a bore  412  extending there-through, from the first end  404  towards a second end  414  that, in the illustrated embodiment, is not open. Accordingly, the bore  412  extends into the body  410  through the first end  404 , but stops at a location  416  interposing the first and second ends  404 , 414 . As illustrated, the bore  412  includes the threaded inner bore  406  that extends into the body  410  and terminates at an abutment  418 . The bore  412  is also illustrated as including an unthreaded inner bore  420  that extends into the body  410  from the abutment  418  such that the abutment  418  interposes the threaded inner bore  406  and the unthreaded inner bore  420 . In the illustrated embodiment, the abutment  418  is arranged as a shoulder that reduces the diameter of the unthreaded inner bore  420  as compared to the threaded inner bore  406 ; however, in other embodiment, the abutment  418  may be provided as an a protrusion, ring, or other structure that may or may not affect the diameter of the unthreaded inner bore  420 . Here, the threaded inner bore  406  is arranged proximate to the first end  404  of the body  410  and includes a plurality of threads  422  arranged to mesh with the threads  408  at the open end  112   a  of the sleeve  112 , whereas the unthreaded inner bore  416  is arranged to interpose the abutment  418  and the second end  414  of the body  410 . 
     In use, a technician positions the first end  404  of the gauge  402  towards the open end  112   a  of the sleeve  112  in a first direction D 1  and screws the threaded inner bore  406  thereof onto the threads  408  of the sleeve  112  at the open end  112   a.  Then, the technician inserts the heater tube  114  in a second direction D 2  into the open end  112   b  at the bottom of the sleeve  112 . After positioning the heater tube  114  within the sleeve  112 , the technician clamps the heater tube  114  into position at the bottom end of the sleeve  112 , for example, via the nut assembly  136   b.  Then, the technician removes the gauge  402  and clamps the heater tube  114  into position at the top end of the sleeve  112 , for example, via the nut assembly  136   a.  Thereafter, the technician may tighten the heater tube  114  into position. 
     As previously mentioned, the test method is performed in two (2) parts. First, the test rig  100  is used to create the thermal oxidation deposit. Second, a dedicated instrument is utilized to quantify thermal oxidation deposit formed during the first phase.  FIG. 5  illustrates a sequence of functions  502  performed by the rig  100  during the first part of the test method to create the thermal oxidation deposit, according to one or more embodiments. As illustrated, the sequence of functions  502  includes an aeration step or procedure  504 , a pre-filtration step or procedure  508 , a bus bar cooling step or procedure, a tube heating step or procedure  510 , and a differential pressure measurement step or procedure  512 . The bus bar cooling will be detailed below. 
     The fuel sample S is a fixed volume of fuel and stored in the sample container  102 . The rig  100  utilizes a pump system  506  to move or pump the fuel sample S at a steady rate from the sample container  102 , through the test section  110  and across the heater tube  114 , and finally into the waste container  104 . The fuel sample S may degrade on the heated heater tube  114  to form thermal oxidation deposits that may appear as a visible film thereon. In addition, degraded materials from the fuel sample S may flow downstream from the heater tube  114  and, for example, be caught in the test filter  120 . 
     Accordingly, the fuel sample S is first prepared by aerating or saturating it with dry air via the aeration procedure  504 . After the aeration procedure  504 , the rig  100  subjects the fuel sample S to the pre-filtration step  508 , for example, by pumping the fuel sample S through a paper membrane. In one embodiment, the paper membrane of the pre-filtration step  508  is a 0.45-μm membrane filter. The pump system  506  then moves fuel sample S at a fixed volumetric flow rate into the test section  110  through the fluid inlet  116  of the sleeve  112 . The fuel sample S flows through the test section  110 , between an inner wall of the sleeve  112  and an outer wall of the heater tube  114 , and exits the sleeve  112  through the outlet  118  thereof. After exiting the sleeve  112 , the fuel sample S passes through the test filter  120  and the rig  100  performs the differential pressure measurement step  512 . 
     In the illustrated embodiment, the differential pressure measurement step  512  includes estimating an obstruction rate of the test-filter  120  by conducting a differential pressure measurement between the pressure in the lines upstream of the test filter (ΔP+) and the pressure in the lines downstream of the test filter (ΔP−). The obstruction rate, hereinafter referred to as a differential pressure drop (ΔP), across the test filter  120  is measured by mercury manometer or by electronic transducer. The rig  100  may also include a differential by-pass line having a valve that may be selectively opened or closed to facilitate flow of the fuel sample S through the by-pass line. If, for example, the differential pressure drop ΔP across the test filter  120  begins to rise sharply (and the technician desires to run the full test method), the valve of the bypass line may be opened in order to finish the test method. 
     As briefly detailed above, the test method requires a technician to prepare the fuel sample S via the aeration procedure  504 . More specifically, the test method directs the technician to inject dry air in the fuel sample S that is contained in the sample container  102  at a rate of 1.5 liters (“L”) per minute (“min”) for 6 minutes prior to performing the test method. Existing instruments, however, utilize manual airflow adjustment that may affect or influence the accuracy and reproducibility of the test method results.  FIG. 6A  illustrates an exemplary aeration procedure  502  comprising a number of manual aeration sequence  602  that is utilized by existing instruments. As illustrated, the manual aeration sequence  602  (sometimes referred to as the aeration phase) begins with providing air A at atmospheric pressure, and then pumping that air A through a filter  604  at a rate of 1.5 L/min via a pump  606 . The pre-filtered air A is then subject to a drying process, for example, via an air desiccant  608  and humidity sensor  610 , that collectively dry and measure the amount of moisture present within the air A. The air A is then directed into a variable area flowmeter  612  that is manually adjusted to ensure that the air A is injected into the sample container  102  at the desired rate to ensure adequate aeration. In the illustrated embodiment, the air A is directed from the variable area flowmeter  610  into a diffuser  614  arranged within the sample container  102  and, as prescribed by the test method, the diffuser  614  may be a coarse 12-millimeter (“mm”) borosilicate glass dispersion tube. As will be appreciated, aeration of the fuel sample S results in fumes that are vented from the system via a ventilation system. However, the aeration sequence  602  is manual and, depending on the technician&#39;s skill and operation of the variable area flowmeter  612 , the test method results may or may not be accurate. 
       FIG. 6B  illustrates an alternate aeration sequence  622  for automatically controlling the airflow during the test method, according to one or more embodiments. As with the manual aeration sequence  602 , the aeration sequence  622  similarly includes utilization of the filter  604 , the pump  606 , the air desiccant  608 , the humidity sensor  610 , and the diffuser  614  arranged within the sample container  102 . However, the aeration sequence  622  is performed automatically so that no manual action or adjustment is required to maintain the desired flow rate, thereby ensuring that the flow rate prescribed by the test method is utilized/obtained throughout the aeration sequence  622 . In the illustrated embodiment, the aeration sequence  622  thus utilizes an electronic flowmeter  624  (in lieu of the variable area flowmeter  610  of the manual aeration sequence  602 ), and the pump  606  includes a control loop or controller  626  associated with the electronic flowmeter  624  to maintain the desired flow rate as the air A is pumped through the air desiccant  608  and the humidity sensor  610  during at least a portion of the automatically controlled aeration sequence  622 . In one embodiment, the controller  626  is a servo control utilizing pulse width modulation to coordinate the operation of the pump  606  and the electronic flowmeter  624  such that the fuel sample S is appropriately aerated as prescribed. In other embodiments, however, the automatic airflow control of the aeration sequence  622  may be differently arranged, for example, the pump  606  and the electronic flowmeter  624  may include a plurality of sensors and use logic to maintain the prescribed flow rate. 
     As detailed above, the pump system  506  moves the fuel sample S at a steady rate from the sample container  102 , through the test section  110  and across the heater tube  114 , and finally into the waste container  104 . Indeed, the test method prescribes that the fuel sample S should flow at a rate of 3 mL/min with a pressure of 500 pounds per square inch (“PSI”). This low flow rate, coupled with the variability of the mechanical properties of the fuel sample S (i.e., viscosity, density, etc.), may hinder the ability to use conventional pump systems (i.e., membrane pumps, piston pumps, etc.) in a reliable manner and thus adversely impact the accuracy of the test method results. Moreover, the flow rate may impact the quality of the thermal oxidation deposit formed on the heater tube  114 . For example, at a low flow rate period, followed by a sharp increase in flow rate along with a large temperature gradient may result in axisymmetric instabilities (i.e., Taylor type toroidal vortices) near the hot surface, and these “local vortices,” while not making the overall flow through the heater tube  114  turbulent, may operate to remove thin layers of the thermal oxidation deposit from the heater tube  114  (as it forms thereon). Thus, the pump system  506  utilized should provide a smooth and steady rate of flow so as to not damage the resulting thermal oxidation deposit. 
     In the past, conventional pump systems  506  have comprised a single syringe, meaning that the whole fuel volume (i.e., the fuel sample S) necessary for the test was contained in the single syringe. This generation of instrument, however, had numerous issues related to the size of the syringe, as well as its handling and leaking. For example, where the single syringe is utilized having a volume that is less than the total volume of sample fuel S needed for the test method, a pause or gap in flow is inevitable at the time of the intermediate aspirations. Other prior pump systems  506  have utilized high-performance liquid chromatography (“HPLC”) pumps with dual pistons. HPLC pumps, however, are not satisfactory because there are micro ruptures at the end of each piston cycle. In addition, HPLC pumps are expensive to purchase and maintain. 
     In one embodiment, the pump system  506  has a dual syringe arrangement that ensures steady flow of the fuel sample S, regardless of the mechanical properties of the fuel sample S.  FIG. 7  illustrates a pump system  702  utilizing a dual syringe/piston arrangement, according to one or more embodiments. As illustrated, the pump system  702  includes two (2) syringes or piston assemblies  704 , 706  that are respectively operated by a pair of motors  708 , 710 . Thus, the first motor  708  operates to drive the first syringe assembly  704 , whereas the second motor  710  operates to drive the second syringe assembly  706 . 
     In the illustrated embodiment, each syringe assembly  704 , 706  includes a barrel  712  that is hollow and defines an interior volume  714  into which the fuel sample S may be pumped. The barrel  712  includes a tip portion  716  at a first end of the barrel  712  and an open end  718  at a second end of the barrel  712  that is oriented opposite of the tip portion  716 . Each syringe assembly  704 , 706  also includes a plunger (or piston)  720  that extends into the interior volume  714  of the barrel through the open end  718  thereof, and may slide therewithin so as to increase or decrease the amount of the fuel sample S that may fill the interior volume  714 . The piston  720  includes a head portion  722  and a shaft  724  that is connected to a rear face of the head portion  722 . The head portion  722  is dimensioned to fit within the interior volume  714  such that its outer perimeter or periphery abuts an interior wall of the barrel  712 , thereby forming a seal between the periphery of the head portion  722  and the interior wall of the barrel  712  to inhibit the fuel sample S from leaking or flowing out of the open end  718  of the barrel  712 . The shaft  724  extends away from the rear face of the head portion  722 , through the interior volume  714  and exits the barrel  712  via the open end  718 . 
     In addition, the shaft  724  includes an end  726  that is arranged opposite the head  722  and operatively coupled to one of the motors  708 , 710 . In one embodiment, the motors  708 , 710  are step motors that each include a ball screw transmission  728 , that in turn drive the piston  720 . In that embodiment, the ball screw transmissions  728  are connected to the end  726  of the shaft  724  to drive the head  722  of the plunger relative to the barrel  712 , thereby varying the size of the interior volume  714 . The feed speed of the piston  720  is imposed by the motors  708 , 710  via the ball screw transmission  728 . 
     Each syringe assembly  704 , 706  also includes a pair of check valves  730 , 732  to control the flow of the fuel sample S entering and exiting the interior volume  714  of the barrel  712 . Here, the check valves  730 , 732  are arranged at each tip portion  716 . The first check valve  730  is arranged on an input line  734  that fluidly interconnects the sample container  102  to the interior volume  714  of the barrel  712 , and permits flow of the fuel sample S from the sample container  102  into the interior volume  714  of the barrel  712 , but not in the reverse direction. Similarly, the check valve  732  is arranged on a fluid output line  736  that fluidly interconnects the interior volume  714  to other downstream systems such as those utilized in the pre-filtration step  508 , and permits flow from the barrel  712  to such downstream equipment, but not in the reverse direction. 
     The syringe assemblies  704 , 706  operate with an alternate firing sequence. For example, when the first syringe assembly  704  is drawing the fuel sample S into its respective barrel  712  (i.e., the suction phase), the second syringe assembly  706  is expelling the fuel sample S from its respective barrel  712  (i.e., the expulsion phase). With this arrangement, one of the syringe assemblies  704 , 706  is always performing an expulsion phase, thereby ensuring that the fuel sample S is provided to the downstream equipment at a constant flow rate, as prescribed by the test method. 
     The fuel sample S is drawn into and expelled out of the barrels  712 , via axial movement of the piston  720 , in and out of the barrels  712 . When the piston  720  is pulled from the first syringe assembly  704  in a first direction X 1  at a constant speed, a volume of the fuel sample S is sucked from the sample container  102 . At the same moment, the piston  720  of the second syringe assembly  706  is pushed into the barrel  712  at a fixed speed. When pushing the piston  720  into the second syringe assembly  706 , the fuel sample S in the respective barrel  712  is expelled at a rate that is dependent on the diameter of the head portion  722  and the speed at which it is displaced within the interior volume  714 . The pair of check valves  730 , 732  ensure the alternating operation of the suck phase and the expulsion phase as detailed above and, in some embodiments, the pair of check valves  730 , 732  are active valves, whereas in other embodiments the pair of check valves  730 , 732  are passive valves. 
     The pump system  702  pumps the fuel sample S with an imperceptible flow fluctuation during the switch from one of the syringe assemblies  704 , 706  to the other. This is achieved by accelerating one of the pistons  720  at the beginning of its stroke in the bottom of the barrel  712  (i.e., proximate to the open end  718 ), as it travels in the first direction X 2  towards the tip  716  and simultaneously decelerating the second piston  720  when it nears the end of its stroke (i.e., proximate to the tip  716 ). Thus, the deceleration of one piston  720  (e.g., of the first syringe assembly  704 ) at the end of the cycle is compensated by the acceleration of the other piston  720  (e.g., of the second syringe assembly  706 ), and vice versa. This phasing is provided such that the sum of the piston  720  speeds of the first and second syringe assembly  704 , 706  is always equal to the nominal feed rate, thereby ensuring a constant rate of flow rate for the chosen diameter of the barrel  712 . In the illustrated embodiment, the interior volume  714  of each barrel  712  is 5 mL, and the fuel sample S flow rate is 3 mL/min. In the illustrated embodiment, the switch period from one of the syringe assemblies  704 , 706  to the other is about 20% of the total cycle time, which thereby eliminates any flow fluctuation. 
     As the fuel sample S is pumped through the test section  110 , a steady current is applied to the heater tube  114  via the bus bars  122   a,   122   b  and, depending upon the temperature and/or quality of the fuel sample utilized in a particular test, a thermal oxidation deposit may form on the heating tube  114  as a visible film. The heater tube  114  is maintained at a relatively high temperature, for example, at 260° C.; however, this temperature may be higher or lower in some applications. The current applied to the heater tube  114  is controlled to maintain a steady temperature at the point of measurement. 
       FIG. 8  is a diagram that illustrates a conventional heating system  802  for heating the heater tube  114  via the bus bars  122   a,   122   b.  As illustrated, the conventional heating system  802  includes a power supply  804 , a control system  806 , a thermocouple  124  that measures a hot spot  808  of the heater tube  114  at a point P thereon, and the pair of bus bars  122   a,   122   b  that secure the heater tube  114 . The heater tube  114  is resistively heated by the conductance of high amperage, low voltage current from the power supply  804  through the heater tube  114 , which results in the heater tube  114  having a thermal profile as illustrated. Here, the position of the point P of measurement of the thermocouple  124  is located inside the heater tube  114 , and is fixed by the length of the shoulder  132   a,b  of the heater tube  114 , which per the test method is 39 mm. Therefore, this 39 mm point is in the hottest region (i.e., the hot spot  808 ) of the heater tube  114  utilized in the test method. 
     In the illustrated embodiment, the bus bars  122   a,   122   b  are relatively heavy and water-cooled so that they incur a relatively minimal temperature increase when supplied with current. The control system  806  serves as an indicator and/or controller. For example, it may automatically control the temperature and vary the power supplied from the power supply  804  as needed so that a steady source of heat is provided to the bus bars  122   a,   122   b  and heater tube  114 . Accordingly, the heating system  802  may be utilized to maintain a target temperature, for example, 260° C., as prescribed by the test method. The control system  806  may alternatively provide for manual operation and thus provide a technician only a temperature readout so that he or she may manually adjust the temperature as needed. 
     The thermal profile of the heater tube  114  and, therefore, the position of the hot spot  808  thereon, may be influenced by numerous factors. These factors include the thermal properties of the fuel sample S, the temperature of the bus bars  122   a,   122   b,  and the temperature difference (ΔT) between the bus bars  122   a,   122   b.  In addition, the ability to control the thermal profile of the heater tube  114  may improve test method results and reproducibility of the same. Conventional instruments, however, do not include control systems that permit fine-tuning of the heater tube  114  thermal profile. For example, while existing instruments do include cooling systems that remove heat going into the bus bars  122   a,   122   b  by conduction from the hot heater tube  114 , technicians may not control these existing cooling systems to optimize the heat profile of the heater tube  114 . 
     The bus bars  122   a,   122   b  of existing rigs  100  are cooled via water cooling systems that circulate water along a single path that flows through each bus bar  122   a,   122   b.  The water may be provided from an external source, for example a laboratory sink, or existing instruments may include an internally circulated and radiator cooled water system to circulate water.  FIG. 9A  is a diagram illustrating how an existing bus bar water cooling system  902  operates, and  FIG. 9B  illustrates an exemplary internal cooling system  904  that may be integrated into the existing instruments. These existing systems, however, are not temperature controlled, as they simply include a liquid pump  906  that circulates a liquid through the bus bars  122   a,   122   b  and then into a heat exchanger  908  that is associated with a fan  910  that blows air at ambient temperature, thereby cooling the liquid. 
     During operation of existing instruments, the initially unheated fuel sample S is introduced into the sleeve  112  proximate the lower bus bar  122 b, is heated along the length of the heater tube  114  while flowing upward there-along, and exits the sleeve  112  proximate to the top bus bar  122   a  at a higher temperature. Fuel samples S comprising fuels with good heat transfer properties will, however, decrease the temperature of the lower bus bar  122   b,  but such fuel samples S will not impart the same effect to the upper bus bar  122   a.  This will in turn affect the heat profile of the heater tube  114 , for example, by skewing the size of the hot spot  808  and/or by moving the hottest point P even closer to the upper shoulder  132   a.  These effects may adversely impact the test method results, as the temperature control system  806  is designed to take temperature measurements from a single point that is supposed to be the hottest point P on the heater tube  114 ; however, when the temperature profile is skewed and the hottest point P is shifted upwards along the heater tube  114 , the temperature control system  806  will no longer be measuring the hottest point P, and will therefore provide inaccurate results. Moreover, when performing successive tests, for example, when several tests are performed in quick succession, the cooling fluid may become warmer and the thermal conditions of the heater tube  114  will not be identical for each of the subsequent tests. 
       FIG. 10  illustrates a temperature system  1002  for controlling temperature in the bus bars  122   a,   122   b,  according to one or more embodiments. The temperature system  1002  individually controls the temperature of each of the bus bars  122   a,   122   b  such that they are controlled independently of each other, thereby maintaining a constant thermal profile of the heater tube  114 . In this way, the temperature difference (ΔT) between the bus bars  122   a,   122   b  may be minimized and/or locked or set to a desired value. In addition, by locking the temperature difference (ΔT) between the top and bottom bus bars  122   a,   122   b,  the temperature system  1002  may also limit the effects of the variability of the thermal properties of the tested fuel samples S. 
     The temperature system  1002  maintains a constant thermal profile of the heater tube  114  as a function of the test method temperature (e.g., 260° C. according to the test method). To do this, the temperature of each bus bar  122   a,   122   b  is perfectly controlled, and their temperature profiles are based on a typical temperature profile extracted from existing instruments in order to guarantee perfectly correlated results. The reproduced profile is the image of tests performed under normal ambient temperature and non-successive testing conditions. Moreover, if the test method protocols change or evolve in the future to require, for example, that the upper and lower bus bars  122   a,   122   b  maintain the same temperature (e.g., 35° C.), the temperature system  1002  will be compatible with such a new requirement while the existing instruments utilizing liquid circulation will be unable to satisfy such new requirement. 
     As illustrated, the temperature system  1002  includes an upper bus bar sub-system  1004  and a lower bus bar sub-system  1006  that control the temperature in the upper and lower bus bars  122   a,   122   b,  respectively. Each of the bus bar sub-systems  1004 , 1006  includes a cooling module  1010 , a heat sink  1012 , a controller  1014 , a forced convection device  1016 , and a thermocouple  1018  that measures the temperature of its respective bus bar  122   a,   122   b.  In the illustrated embodiment, the cooling module  1010  is a Peltier element and the forced convection device  1016  is a fan, but other cooling modules  1010  and/or forced convection devices  1016  may be utilized without departing from the present disclosure. As will be appreciated, each of the bus bar sub-systems  1004 , 1006  include a separate controller  1014  and componentry so that they may individually adjust the heat extracted from the bus bars  122   a,   122   b  by a respective heat pipe  1008 . 
     Electric power is supplied to the cooling module  1010  and, therefore, the amount of thermal energy transferred from the bus bars  122   a,   122   b  to their respective heat sink  1012  is controlled by a temperature measurement carried out on each of the bus bars  122   a,   122   b.  The measuring point utilized for these temperature measurements is located on the bus bars  122   a,   122   b  at a point that is close to the interface with the heater tube  114  and may each, for example, be located at the same point of measurement as made on bus bars of existing instruments. 
     The bus bars  122   a,   122   b  may have geometries that optimize heat transfer. For example, an exterior profile or shape  1019  of the bus bars  122   a,   122   b  may be contoured as illustrated so as to be able to use the entire exchange surface of the cooling module  1010 . Also in the illustrated embodiment, each bus bar  122   a,   122   b  includes a base  1020  and a bore  1022  extending inward therefrom, towards a tapered end  1024  that holds or secures the heater tube  114 ; and the heat pipes  1008  are inserted into the bores  1022  of the bus bars  122   a,   122   b.  Since the thermal conductivity of the heat pipe  1008  is higher than that of the bus bars  122   a,   122   b  (e.g., which may be made from brass), calories are more efficiently transferred from one end of each bus bar  122   a,   122   b  to the other. The temperature difference (ΔT) between their measuring points (i.e., the measuring points of the thermocouples  1018 ) and the bearing surface of the cold face of the cooling module  1010  may be reduced, which improves the efficiency of the cooling system  1002  and the response time of the control loop. Thus, the temperature system  1002  provides independent thermal control of the separate bus bars  122   a,   122   b,  while eliminating the impact of ambient temperature compared to a cooling solution based solely on heat exchange with the ambient temperature. 
       FIG. 11A  illustrates a clamping system  1102  that is utilized to secure the lower shoulder  132   b  of the heater tube  114  (within the sleeve  112 ) to the lower bus bar  122   b.  As illustrated, the clamping system  1102  includes a plate  1104  that is moveably positioned proximate to an end face  1106  of the lower bus bar  122   b,  and arranged to compress or clamp the lower shoulder  132   b  of the heater tube  114  that is positioned within the lower bus bar  122   b.  The clamping system  1102  further includes a pair of screws  1108  that extend through an outer surface  1110  and interior surface (obscured from view) of the plate  1106  and into the end face  1106  of the lower bus bar  122   b.  As will be appreciated, a technician may tighten or loosen the screws  1108  to compress or depress the plate  1104  relative to the lower bus bar  122   b.  Thus, when the lower shoulder  132   b  of the heater tube  114  (that is secured within the sleeve  112 ) is positioned between the interior face (obscured from view) of the plate  1104  and the end face  1106  of the lower bus bar  122   b,  the technician may tighten or loosen the screws  1108  to secure or remove the test section  110 . In some embodiments, either or both of the interior face (obscured from view) of the plate  1104  and the end face  1106  of the lower bus bar  122   b  are contoured to receive the lower shoulder  132   b  of the heater tube  114 . In addition, the screws  1108  may include a lever  1112  extending therefrom to facilitate tightening and loosening of the same. It will be appreciated that, while note illustrated, the clamping system  1102  is similarly arranged at the upper bus bar  122   a  to secure/unsecure the upper shoulder  132   a  thereto. 
     To install or uninstall the sleeve  112  and heater tube  114  assembly (i.e., the test section  110 ) relative to the lower bus bar  122 b, the technician must move the plate  1104  so that the plate  1104  no longer obstructs the location on the end face  1106  that receives the lower shoulder  132   b  of the heater tube  114 . In one method, the technician must fully remove one (1) of the screws  1108  and then loosen the other one (1) of the screws  1108  such that the plate  1104  may pivot on the (remaining) screw  1108 , thereby un-obstructing and presenting the lower shoulder  132   b  within the end face  1106  of the lower bus bar  122   b.  Alternatively, the technician may remove both of the screws  1108  to fully remove the plate  1104  from the end face  1106  of the lower bus bar  122   b  to install or uninstall the test section  110 . While not described, it will be appreciated that the foregoing described operation of the clamping system  1102  may be similarly utilized at the upper bus bar  122   a  to secure/unsecure the upper shoulder  132   a  thereto. 
     Alternate clamping systems may be utilized, however, that do not necessitate two (2) screws and that provide improved electrical and/or thermal contact between the shoulders  132   a,   132   b  and the bus bars  122   a,   122   b.  For example,  FIG. 11B  illustrates a clamping system  1120 , according to one or more embodiments. As detailed below, the illustrated clamping system  1120  utilizes a single screw that may be removed to install or uninstall the heater tube  114 , and may provide enhanced thermal and electrical contact. While the clamping system  1120  of  FIG. 11B  may be utilized with either or both of the upper and lower bus bars  122   a,   122   b,  it is hereinafter described with use on a single unspecified bus bar  122  that could be utilized as either the upper or lower bus bar  122   a,   122   b.    
     As illustrated, the bus bar  122  utilized in the clamping system  1120  is forked at the tapered end  1024 . Thus, the tapered end  1024  of the bus bar  122  includes a pair of forks or prongs  1122   a,   1122   b  extending therefrom away from the base  1020  of the bus bar  122 . The pair of prongs  1122   a,   1122   b  define a recess or gap  1124  there-between. Here, gap  1124  is sized such that the shoulder  132   a,   132   b  of the heater tube  114  may be inserted or retracted there trough as hereinafter described. In addition, the tapered end  1024  may be hollow to define a threaded bore  1126  that extends into the bus bar  122  for at least the length of prongs  1122   a,   1122   b.    
     In the illustrated embodiment, the clamping system  1120  further includes a screw  1128  having a threaded portion  1130  that is received within and meshes with the threaded bore  1126  of the bus bar  122 . Also, the clamping system  1120  includes a plate  1132  that is positioned within the gap  1124  between the pair of prongs  1122   a,   1122   b,  and the plate  1132  is arranged to slide between the prongs  1122   a,   1122   b  towards and away from an interior face  1134  of the bus bar  122  that will abut one of the shoulders  132   a,   132   b  of the heater tube  114 . In operation, one of the shoulders  132   a,   132   b  will be disposed proximate to the interior face  1134  of the bus bar  122 , and the screw  1128  may then be rotated to drive the threaded portion  1130  thereof into or out of the threaded bore  1126 , which in turn drives the plate  1132  towards or away from the interior face  1134  and thus compresses or decompresses one of the shoulders  132   a,   132   b  that is positioned there-between. When the screw  1128  and the plate  1132  are withdrawn from the tapered end  1024  of the bus bar  122 , the gap will be unobstructed such that the shoulder  132   a,   132   b  of the heater tube  114  may be inserted or withdrawn. In the illustrated embodiment, the plate  1132  and the interior face  1134  each include a seat  1132 ′, 1134 ′ that is contoured to receive the shoulders  1132   a,   1132   b′.    
     Also in the illustrated embodiment, the screw  1128  is hollow and includes a bore  1136  having a narrow portion  1137 a and a wide portion  1137 b, and the plate  1132  includes a shaft  1138  that is hollow and defines a bore  1140  that is coaxial with the bore  1136  of the screw  1128 . As illustrated, the shaft  1138  and its bore  1140  extend from the plate  1132 , through the narrow portion  1137   a  and into the wide portion  1137   b  of the bore  1136  of the screw  1128  in a direction away from the base  1020  of the bus bar  122 . 
     A locking device  1142  maybe be utilized to limit or inhibit the amount of axial movement of the plate  1132  within the gap  1124  relative to the screw  1128  while permitting rotation of the screw  1128  relative to the plate  1132 . The locking device  1142  is secured within the bore  1140  of the plate  1132 . In addition, the locking device  1142  may include a flange  1144  that floats within the wide portion  1137   b  of the bore  1136  of the screw  1128 , and abuts a shoulder  1146  within the bore  1136  of the screw  1128  (i.e., that is disposed between the narrow and wide portions  1137   a,   1137   b ) when the screw  1128  is retracted from the bore  1126  of the bus bar  122 . Also, the plate  1132  may be attached to the screw  1128  to permit relative rotation between the plate  1132  and the screw  1128 , but to inhibit the shaft  1138  of the plate  1132  from being fully withdrawn from the bore  1136  of the screw  1128  via interaction between the flange  1144  and the shoulder  1146 . Thus, when the screw  1128  is withdrawn from the threaded bore  1126  of the bus bar  122 , the plate  1132  (that is attached to the locking device  1142 ) will be pulled by the (rotating) screw  1128  in the axial direction away from the base  1020  of the bus bar  122 . Stated differently, rotation of the screw  1128  translates to an axial displacement of the plate  1132  within the gap  1124 . Accordingly, the plate  1132  is carried by (or retracted with) the screw  1128 , which may be removed from the tapered end  1024  of the bus bar  124  to expose the gap  1124  so that the shoulder  132   a,   132   b  of the heater tube may be assembled or disassembled relative thereto, which facilitates removal of the heater tube  114  from the bus bar  122 . 
     In some embodiments, the bus bars  122  may one or both of a pair of recesses  1018   a,   1018  that are disposed at an upper or lower sides of the bus bar  122  and arranged to receive one of the thermocouple  1018  of the temperature system  1002 , as detailed above. 
     Therefore, the disclosed systems and methods are well-adapted to attain the ends and advantages mentioned, as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the teachings of the present disclosure may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope of the present disclosure. The systems and methods illustratively disclosed herein may suitably be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the elements that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted. 
     The use of directional terms such as above, below, upper, lower, upward, downward, left, right, and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward or upper direction being toward the top of the corresponding figure and the downward or lower direction being toward the bottom of the corresponding figure. 
     As used herein, the phrase “at least one of” preceding a series of items, with the terms “and” or “or” to separate any of the items, modifies the list as a whole, rather than each member of the list (i.e., each item). The phrase “at least one of” allows a meaning that includes at least one of any one of the items, and/or at least one of any combination of the items, and/or at least one of each of the items. By way of example, the phrases “at least one of A, B, and C” or “at least one of A, B, or C” each refer to only A, only B, or only C; any combination of A, B, and C; and/or at least one of each of A, B, and C.