Abstract:
A compressor active surge control system that includes an air recirculation line that has a first end connected downstream of a compressor outlet and a second end connected upstream of a compressor inlet. The gas recirculation line recirculates compressed air from the compressor outlet to the compressor inlet. Also included in the system is a mixer positioned to receive both ambient air and recirculated air. Mixing of the air homogenizes the air prior to its introduction into the compressor. An air cooler may be included that cools the recirculated air prior to its introduction into the compressor. Cooling and mixing of the recirculated air expands the operating range of the compressor by reducing the incidence of surge. Various existing components, such as intercoolers and filters, may serve the additional duty of cooling or mixing the recirculated air.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to the use of compressor systems (e.g., turbochargers) for boosting the power output of combustion engines, and in particular to the use of recirculated compressor discharge air to facilitate the operation of such compressors. 
   2. Description of Related Art 
   Vehicle engine turbochargers provide an advantageous boost to engine power, especially at higher engine speeds. A turbocharger uses the exhaust gasses from an engine to drive a turbine that drives a compressor, which, in turn, increases the pressure of the engine intake air. The compressed engine intake air results in the boost of engine power. In some turbocharger engine systems, a portion of the exhaust gases from the engine is recirculated back to the intake of the engine to control emissions. In other systems, the compressor drive may be electrically assisted or the turbine may include a variable geometry nozzle to further increase performance. 
   In certain circumstances, a turbocharger can experience what is commonly referred to as a “surge” condition. Generally, surge occurs when the compressor is driven into low-flow, high pressure-ratio conditions, the result of which is that the compressor blades are forced to operate at such high incidence angles that significant flow separation occurs on the blades. Surge can result in severe aerodynamic fluctuations within the compressor of the turbocharger and can even cause damage to the engine or its intake pipe system. 
   In one instance, surge can occur when compressor exhaust gas recirculation flow is relatively high at low engine speed. The remaining engine cylinder volume for fresh air at this point becomes smaller. In order to maintain engine torque and load performance, the boosting pressure needs to increase (thereby increasing air density) to keep the same mass flow rate of fresh air into the engine. As a result, the compressor has to work at a relatively higher load and pressure ratio, while the airflow remains relatively low. 
   In another instance, surge can occur when a relatively high specific power output (such as 70 to 80 kilowatts per liter) is required of the engine and electrically assisted boosting of the compressor is employed. In still another instance, surge can occur when a quick compressor response is required using electrical boosting and/or the use of variable turbine nozzle vanes. In another instance, sudden deceleration of the engine due to closing of the throttle valve can result in surge. 
   It would be advantageous, therefore, to reduce the occurrence of surge in a turbocharger. Reducing such occurrence would allow an expanded range of conditions in which the turbocharger would be able to boost engine power. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
       FIG. 1  is a schematic of a surge control system of one embodiment of the present invention; 
       FIG. 2  is a schematic of open loop control logic of another embodiment of the surge control system of the present invention; 
       FIG. 3  is a schematic of a closed loop control logic of still another embodiment of the surge control system of the present invention; 
       FIG. 4  is a schematic of a continuation of the closed loop control illustrated in  FIG. 3 ; 
       FIG. 5  is a schematic of a surge control system of another embodiment of the present invention including an air cleaner upstream of a junction between the inlet line and the recirculation line; 
       FIG. 6  is a schematic of a surge control system of still another embodiment of the present invention including an air cleaner serving as a mixer; 
       FIG. 7  is a schematic of a surge control system of yet another embodiment of the present invention including an air cleaner as mixer and a dedicated gas recirculation cooler; 
       FIG. 8  is a schematic of a surge control system of another embodiment of the present invention including an air cleaner serving also as a mixer and as a cooler; 
       FIG. 9  is a schematic of a surge control system of another embodiment of the present invention including a tip turbine fan cooler; 
       FIG. 10  is a schematic of a surge control system of still another embodiment of the present invention including the use of an exhaust gas recirculation line cooler; and 
       FIG. 11  is a graphical depiction of the improved compressor operating range resulting from compressed air recirculation by the surge control system of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
   A compressor active surge control system  10  of one embodiment of the present invention, as shown in  FIG. 1 , includes a compressor  11 , a compressor inlet line  12 , a compressor discharge line  13 , a recirculation line  18  connecting the inlet and discharge lines, an air cooler  50  and an air mixer  20 . Generally, the compressor  11  supplies compressed air through the line  13  to a combustion engine  14  so as to increase power output by the engine. An exhaust gas line  15  conducts exhaust from the engine  14  to a turbine  16  which drives the compressor  11 . An exhaust gas recirculation line  17  may optionally be used to recirculate exhaust gas back to the engine  14 &#39;s intake for control of engine emissions. 
   The compressor inlet pipe or line  12  is connected to an ambient air source, such as by being connected to an intake scoop (not shown) beneath the hood of an automobile. The term “ambient” as used herein broadly defines air from a general supply of air, and not necessarily air at a temperature, pressure or cleanliness of the surrounding environment. Generally, then, the ambient air is any air (or other gas) that is useable for combustion, but is at a pressure lower than the air discharged from the compressor  11 . 
   The term “line” as used herein may include pipes, tubes, hoses or any other conduit, device or method for conducting air or other fluids from one place to another. In addition, although the term “line” may be used in singular form, it may indicate use of several lines, tubes, conduits, portions or combinations thereof, that are effective in conducting a fluid (such as air) from one place to another. For instance, the present invention need not be limited to a single compressor inlet line  12  as multiple inlet hoses could be used depending upon the amount of ambient air required for compression and combustion. Also, as used herein “line” does not necessarily indicate that the pipe or hose travels directly from one place to another, but could also include intervening stops for supplying other devices, branches or twists and turns. 
   The inlet line  12 , which is downstream of an air filter (not shown) in the embodiment depicted in  FIG. 1 , is connected at a downstream end to the recirculation mixer  20 . As illustrated, the mixer  20  includes a length of pipe  26  containing a cylindrical filter or baffle  25  housed within the pipe. Defined by the cylindrical baffle is a central opening that extends co-axially with the opening of the pipe  26 . The upstream end of the mixer pipe is fitted to the downstream end of the inlet line  12  while the opposite, downstream end of the mixer pipe is fitted to an inlet duct  23 . The inlet duct is part of a shroud or housing  24  of the compressor  11  and can have a diameter the same as the mixer pipe  26 . 
   Defined within the sidewall of the mixer pipe  26  is an opening to which a second end  28  of the recirculation line  18  is attached. Connection with the recirculation line allows entry of the recirculated air into the mixer  20  so that it can be mixed with the main, ambient air supply from the inlet line  12 , as will be described in more detail below. An advantage of the cylindrical baffle or filter of the mixer  20  is that its central opening is oriented with the direction of the ambient air so that it mixes with the recirculated airflow with minimal impedance of the flow. 
   Although the mixer  20  is illustrated in  FIG. 1  as having the pipe  26  and cylindrical filter  25 , other mixer devices may also be employed. For instance, a baffle could be used that does not filter, but redirects airflow to cause turbulence and mixing. Also, conventional, pre-existing air filters could be used such as off-the-shelf paper or oil-impregnated foam filters. In such a case, an additional opening may be formed in the filter and a fitting used to attach the second end  28  of the recirculation line  18 . In another alternative (as described below) flow from the inlet line  12  and the recirculation line  18  is combined upstream of the mixer  20  by a direct connection between the lines. 
   Referring again to the embodiment illustrated in  FIG. 1 , the compressor  11  is a rotationally driven compressor having a plurality of compressor blades  30  mounted to a rotatable shaft  31  driven by the turbine  16 . A volute  24  surrounds the compressor for receiving pressurized air discharged from the compressor. A discharge or outlet duct  32  is connected from the volute  24  to an upstream end of the discharge line  13 . 
   As noted above, the compressor  11  is driven by the turbine  16 . The turbine  16  includes an inlet volute  33  that receives exhaust gases from the engine  14  via the line  15  and via an inlet  35  to the volute. The turbine includes turbine blades  34  mounted on the shaft  31 . The turbine housing defines a turbine outlet  36  through which the exhaust gases are discharged after passing through the turbine. The turbine outlet  36  is attached to the downstream exhaust system (not shown) of the power plant. 
   Although the compressor in the illustrated embodiment is shown as being driven by a turbine powered by engine exhaust gas (e.g., a turbocharger), other alternative drives could be used to power the compressor  11 . For instance, the compressor could be mechanically driven by the engine drive shaft (e.g., a supercharger) or the compressor could be driven by an electrical motor. In another instance, the turbine drive of the illustrated embodiment could be assisted by another drive, such as an electrical drive (electrically assisted) or the engine drive shaft. 
   Returning to the intake air compression portion of the system  10 , the outlet duct  32  of the compressor is connected to discharge line  13  which extends to the air cooler  19 . The illustrated air cooler  19  is an intercooler which typically houses a plurality of tubes through which the relatively hot, compressed air flows from the compressor  11 . Generally, ambient air (or water) passes over the tubes and between fins that are attached to the tubes. Heat is then transferred from the hot tubes and fins to the cool ambient air. The air cooler  19  includes an upstream, inlet opening  37  which is attached to the discharge line  13  and a downstream, outlet opening  38  that is attached to a downstream portion of the discharge line  13  (which may also be referred to as an engine intake line). It should be noted that other types of air coolers or heat exchangers could also be used in lieu of the illustrated intercooler. 
   The discharge line  13  includes a connection to a first end  27  of the recirculation line  18 , a non-return valve  39 , and a connection to the exhaust gas recirculation line  17 . The non-return valve  39  is a one-way valve that prevents backflow of the recirculated exhaust gases and the engine intake air. The exhaust gas recirculation line  17  includes a first end  40  connected to the exhaust gas line  15  and a second end  41  connected to the downstream portion of the discharge line  13 . The exhaust gas recirculation system includes its own cooler  42  and control valve  43 . The exhaust gas recirculation valve  43  controls the flow of recirculated exhaust gases and may be positioned upstream, or downstream, of the exhaust gas cooler  42 . Operation of the exhaust gas recirculation valve  43  is controlled by an engine control unit  21 , as will be described in more detail below. 
   As mentioned above, the compressed air recirculation line  18  has its first end  27  connected to the discharge line  13  upstream of the air cooler  19 . The opposite, second end  28  of the compressed air recirculation line  18  is connected to the mixer  20 , as also described above. A recirculation valve  22  is positioned between the ends  27 ,  28  of the line for controlling the flow in the compressed air recirculation line  18 . The recirculation valve is connected in communication with, and controlled by, the engine control unit  21 , as indicated by the dashed line. The recirculation valve  22  in one embodiment is open-loop controlled by a pneumatic actuator; in another embodiment the valve  22  can be closed-loop controlled by a rotary electric actuator. Also within the recirculation line  18 , is a dedicated air cooler  50  for cooling recirculated air. 
   In addition to being connected to the valves  22 ,  43 , the engine control unit is also connected to an engine speed sensor  44 , a compressor inlet pressure sensor  45  and a compressor outlet pressure sensor  46 . For open loop control, opening of the recirculation valve  22  is a function of the engine speed as measured by the sensor  44  and the boost pressure as measured by sensor  46 . For closed loop control, opening of the valve is a function of engine speed and the compressor inlet pressure as measured by sensor  45 , which is capable of sensing surge. As another alternative, the recirculation valve  22  may be controlled in response to the exhaust gas recirculation rate (intake throttling) or the difference in gas pressure between the engine inlet (as measured by engine inlet pressure sensor  46 ) and turbine inlet (as measured by a turbine inlet pressure sensor  47 ). 
   Control of the compressed air recirculation valve  22  opening may also be coordinated with opening of the exhaust-gas recirculation valve  43  and control of the variable turbine nozzle to optimize air flow and exhaust gas recirculation rates. For instance, the compressed air recirculation valve  22  can be used to absorb the extra turbine power when the variable turbine nozzle is suddenly closed during engine braking. 
   Open loop control of the compressed air recirculation valve  22  of another embodiment of the present invention is illustrated by the flowchart of  FIG. 2 . Input data, including the degree of opening of the exhaust gas recirculation valve  43  and the degree of opening of the variable nozzle turbine, are collected in step  100 . The compressed gas recirculation valve opening is set to zero in step  101 . Then in step  102 , the engine speed (Ne) as measured by the engine speed sensor  44  is compared to a predetermined Ne threshold (Ne(setting)). If Ne is less than the threshold Ne(setting) then the recirculation valve  22  is opened in step  103 . The amount of opening is defined by the following equation:
 
 RVO=A/Ne+B/P 2 C  
 
wherein RVO is the amount of compressed air recirculation valve opening, A is a predetermined constant, B is a predetermined constant, Ne is the engine speed and P2C is the outlet pressure measured by sensor  46 . If Ne is determined to be more than the threshold Ne(setting) in step  102 , the subroutine ends and the recirculation valve  22  is left closed.
 
   Closed loop control of the compressed air recirculation valve  22  of another embodiment of the present invention is illustrated by the flowchart of  FIG. 3 . The first four steps  100 ,  101 ,  102  and  103  are the same as described above for open loop control. However, after opening the compressed air recirculation valve  22 , the compressor inlet pressure P 1 C as measured by sensor  45  is compared to a threshold pressure P 1 C(setting) in step  104 . If above the threshold, the logic returns to step  103 . If below the threshold (in a more severe surge condition), the logic proceeds to step  105  wherein data is collected on the exhaust gas recirculation rate (EGR) by determining the difference between engine inlet pressure (P 1 E) as measured by sensor  47  and turbine inlet pressure (P 1 T) as measured by sensor  48 , as shown in  FIG. 4 . 
   In a step  106  the EGR is compared to an exhaust gas recirculation threshold EGR(setting). If the EGR is greater than EGR(setting) then the subroutine ends. If the EGR is less than EGR(setting) then the logic proceeds to step  107  wherein the exhaust gas recirculation valve  43  is opened or, alternatively, the variable nozzle turbine is closed and the compressed air recirculation valve  22  is opened. EGR is compared again to EGR(setting) in a step  108  and if still less than EGR(setting), the logic returns to step  107 . The exhaust gas recirculation valve  43  is opened further, or the compressed air recirculation valve  22  is opened further. If EGR is greater than EGR(setting) then the subroutine ends. 
   One measure of EGR rate is based on the “mass ratio” or “mole ratio.” The mass ratio is the mass flow of EGR versus the total mass flow within the engine cylinder. Generally, the desired EGR rate is about 10 to 15% at full load with medium engine speed, or high engine speed. At full load and low engine speed for European and North American commercial vehicles the EGR rate is dropped to zero. For vehicles having higher emission requirements, the EGR rate would preferably be increased to about 20% at full load with low engine speed and more than 30% at full load with medium engine speed, or at high engine speed. Typically, to maintain high mass flow of fresh air, a higher pressure ratio is necessary and the compressor more easily runs into surge. 
   For the EGR(setting) at a given engine condition (speed and load), the EGR(setting) is a target rate that is related to the NOx reduction rate in the cylinders. If the EGR rate is lower than the target value, NOx contents at the engine exhaust manifold will be too high and cannot clear the emission target values even with use of a de-NOx catalyst. 
   Other embodiments of the compressor active surge control system of the present invention are illustrated schematically in  FIGS. 5-10 . In one embodiment shown in  FIG. 5 , an air cleaner  49  is positioned upstream of the mixer  20  in the inlet line  12  and the dedicated air cooler  50  is positioned in the compressed air recirculation line  18 . The junction between the first end  27  of the recirculation line  18  and the discharge line  13  is upstream of the intercooler  19 . 
   In another embodiment shown in  FIG. 6 , the mixer  20  is also an air cleaner which eliminates the need for an independent mixer. The junction of the first end  27  of the recirculation line  18  and the discharge line  12  is downstream of the intercooler  19 , allowing the intercooler to also cool the compressor discharge air prior to its recirculation. In still another embodiment shown in  FIG. 7 , the mixer  20  is an air cleaner that is downstream of the junction between the inlet line  12  and the second end  28  of the recirculation line  18 . In an embodiment shown in  FIG. 8 , the mixer  20  is an air cleaner and also serves as an air cooler, allowing the junction between the recirculation line  18  and discharge line  13  to be upstream of the cooler  19 . 
   In yet another embodiment shown in  FIG. 9 , the cooler  50  is in the compressed air recirculation line  18  and is a tip turbine fan. The tip turbine fan absorbs some of the thermal energy of the compressed air allowing it to be cooled more easily by the air cleaner type mixer  20 . Another embodiment is shown in  FIG. 10 , which is similar to the embodiment of  FIG. 9  except for the use of the exhaust gas recirculation cooler  42 . In this embodiment, the exhaust gas cooler  42  can aid the intercooler  19  if the cooling effectiveness of the intercooler is not enough. 
   Surge control by the present invention has many advantages. For instance,  FIG. 11  illustrates the expansion of the working area of the compressor  11  when the surge control system  10  of the present invention is used. The working envelope without compressed air recirculation is defined by a solid line  200 . The solid line  202  defines the compressor operating line without exhaust gas recirculation. The dashed line  203  represents how the operating line is shifted with exhaust gas recirculation. It can be seen that the operating line at low air flow conditions is into the surge area of the map. The dotted line  201  shows how the operating envelope is expanded by compressed air recirculation in accordance with the invention; as a result, even with exhaust gas recirculation, the operating line  203  falls within the non-surge region of the map. Expansion of the operating range of the compressor system is enhanced through cooling of the recirculated air and/or mixing of the recirculated air with the intake air such that substantially uniform flow conditions exist at the entrance to the compressor. Without adequate mixing, non-uniform flow conditions would tend to hurt compressor efficiency. 
   Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.