Abstract:
A method for controlling a variable-geometry turbine ( 24 ) that includes a variable-geometry mechanism that is movable between a fully closed position closing a relatively greater portion of the turbine nozzle and a fully open position closing a relatively smaller portion of the nozzle. The turbine ( 24 ) further comprises a waste gate ( 40 ) movable between a closed position and an open position in which some of the exhaust gas bypasses the turbine ( 24 ). At low engine speeds, turbocharger boost us regulated by regulating the position of the wastegate. At high engine speeds, the variable geometry is fully open and the wastegate is opened again. When at medium engine speeds, the variable geometry mechanism is to be moved from its closed position toward its fully open position, the waste gate ( 40 ) is in an open position to cause a proportion of the exhaust gas to bypass the turbine ( 24 ) and thereby reduce the pressure of the exhaust gas in the chamber. While this reduced pressure of the exhaust gas is in effect, the variable-geometry mechanism is moved from the fully closed position toward the fully open position while simultaneously the waste gate begins to close.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to turbochargers having a variable-geometry mechanism for regulating the flow area of the nozzle leading into the turbine, and having a waste gate for causing exhaust gas to bypass the turbine under certain conditions. 
     In turbochargers of the above-described type, the variable-geometry mechanism (which can comprise a sliding piston or slide vane, or variable vanes in the nozzle, or the like) is typically movable between a fully closed position and a fully open position. In the fully closed position, the nozzle is significantly closed (and in some cases completely closed) by the mechanism. In the fully open position of the piston, the nozzle is significantly open (and in some cases completely open). 
     The variable-geometry mechanism can be difficult to control at the very beginning of the opening process. When the mechanism is opened just slightly from its fully closed position, it can create a significant amount of disturbance in the exhaust gas flow into the turbine, thereby decreasing efficiency of the turbocharger. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the above needs and achieves other advantages by providing a method for controlling a variable-geometry turbocharger having a turbine wheel mounted for rotation in a turbine housing, the turbine housing defining a chamber surrounding the turbine wheel for receiving exhaust gas from an engine and further defining a nozzle leading from the chamber generally radially inwardly to the turbine wheel through which exhaust gas flows from the chamber to the turbine wheel. The turbine includes a variable-geometry mechanism movable to regulate flow through the nozzle. The turbine further comprises a waste gate movable between a closed position in which substantially all of the exhaust gas is directed from the engine through the turbine nozzle to the turbine wheel and a plurality of open positions for allowing a variable proportion of the exhaust gas to bypass the turbine. The turbine further comprises a variable-geometry actuator operable to move the variable-geometry mechanism to regulate flow through the nozzle and a waste gate actuator operable to move the waste gate between the closed and open positions. 
     In accordance with the invention, at low engine speeds, turbocharger boost is regulated by regulating the position of the waste gate, while the variable-geometry mechanism is in its most-closed position (in which flow is still able to pass through the turbine). At intermediate engine speeds, boost is regulated by regulating the position of the variable-geometry mechanism, while the waste gate is closed or substantially closed. At high engine speeds, once again the boost is regulated by regulating the position of the waste gate, while the variable-geometry mechanism is open or substantially open. 
     In preferred embodiments of the invention, there is a transition region between the low-speed regime and the intermediate-speed regime. As speed increases through the transition region such that the variable-geometry mechanism is to be opened, the waste gate is positioned in a relatively open position and the variable-geometry mechanism begins to open while simultaneously the waste gate begins to close. It is thought that the open position of the waste gate reduces the pressure of the exhaust gas in the chamber and thereby reduces the amount of flow disturbance that occurs as the variable-geometry mechanism begins to open. 
     In other preferred embodiments of the invention, there is a second transition region between the intermediate-speed regime and the high-speed regime. As speed increases through the second transition region, the variable-geometry mechanism is in a relatively open position and the waste gate begins to open. 
     In some embodiments, the variable-geometry mechanism comprises a sliding piston. The turbine can include vanes mounted in the nozzle. When a sliding piston is employed, the vanes can be of fixed geometry. Alternatively, the piston can be omitted and variable-geometry vanes can be employed as the variable-geometry mechanism. 
    
    
     
       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 diagram of an engine and turbocharger system in accordance with one embodiment of the invention; 
         FIG. 2  is a graph schematically illustrating various engine operating regimes and the corresponding control strategies used therein in accordance with one embodiment of the invention; 
         FIG. 3  is a graph schematically illustrating a transition from engine operating regime I to engine operating regime II in accordance with an embodiment of the invention; and 
         FIG. 4  is a graph schematically illustrating a transition from engine operating regime II to engine operating regime III in accordance with an embodiment of the 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 inventions are shown. Indeed, these inventions 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. 
     An engine and turbocharger system  10  in accordance with one embodiment of the invention is depicted in diagrammatic fashion in  FIG. 1 . The system comprises an internal combustion engine  12  that is supplied with a mixture of fuel and air via an intake manifold  14  and exhausts the products of combustion, referred to as “exhaust gas”, via an exhaust manifold  16 . In a popular alternate arrangement, fuel can be supplied directly to each cylinder, though this will not affect the applicability of the present invention. The exhaust manifold is connected to an exhaust conduit  18 . The system further comprises a turbocharger  20 . The turbocharger comprises a compressor  22  and a turbine  24  mounted on a common shaft  26 . The turbine is supplied with exhaust gas from the exhaust conduit  18 , and expands the exhaust gas (which is at relatively high pressure and temperature) so as to extract mechanical power from the exhaust gas, after which the expanded exhaust gas is discharged from the turbine via an exhaust gas discharge  28 . The turbine rotatably drives the shaft  26  and thus drives the compressor  22 . The compressor draws in ambient air through a compressor inlet  30 ; typically the air is first passed through an air filter  32  to remove particulate matter that could damage the compressor. The compressor compresses the air to a higher pressure and discharges the pressurized air (also called “charge air”) through a charge air conduit  34 . It is frequently advantageous to pass the charge air through a charge air cooler (CAC)  36  to reduce the temperature of the charge air before it is supplied to the engine intake manifold  14 . 
     The system  10  employs a turbine  24  that has variable geometry for regulating the flow of exhaust gas through the turbine. Such a turbine is referred to herein as a “variable-geometry turbine” or “VGT”, but this generic term is intended to encompass turbines with any type in which a variable-geometry mechanism (illustrated diagrammatically by arrows  38  in  FIG. 1 ) is used for regulating exhaust gas flow through the turbine. As non-limiting examples, the variable-geometry mechanism  38  can comprise an axially slidable piston or “slide vane” as known in the art, or an array of vanes in the turbine inlet, either of which mechanisms can be variably positioned under the control of a suitable actuator so as to control the sizes and configurations of exhaust gas flow passages leading into the turbine and thereby regulate flow. 
     The system  10  also comprises a “waste gate” or bypass valve  40  that is connected between the exhaust gas conduit  18  and the exhaust gas discharge  28  of the turbine. The waste gate comprises a controllable valve that is variably positionable for regulating flow rate through the valve. The waste gate is connected to a suitable actuator for positioning the valve. When the waste gate is fully closed, all of the exhaust gas in the exhaust gas conduit  18  flows through the turbine  24 . When the waste gate is fully open, a substantial proportion of the exhaust gas in conduit  18  bypasses the turbine. At intermediate positions between the fully open and fully closed positions of the waste gate, a proportion of the exhaust gas flow, the magnitude of which is dependent on the valve position, bypasses the turbine. 
     The system  10  also comprises an engine control unit (ECU)  42  that is coupled with various components of the engine  12 , with the actuator for the variable-geometry mechanism  38  of the turbine, and with the actuator for the waste gate  40 . The ECU  42  comprises a programmed microprocessor with a storage or memory for data, operable to receive signals from various sensors associated with the engine  12 , with the variable-geometry mechanism  38 , and with the waste gate  40 , and to execute programmed instructions, taking into account the signals from the sensors. In particular, the ECU is operable to control the positions of the variable-geometry mechanism and the waste gate based on the signals from the sensors, as further described below. 
     In accordance with the invention, the positions of the variable-geometry mechanism and the waste gate are controlled for regulating the “boost” provided by the compressor  22 —i.e., the amount of pressurization of the charge air supplied to the engine intake manifold  14 . The amount of boost determines the degree to which the power output from the engine is augmented relative to the power output that would be obtained with no boost. Generally it is desired that the engine power output vary in a smooth fashion throughout the engine operating envelope, and the attainment of this goal requires close control of the boost level to avoid sudden surges of power and the like. 
     In accordance with the invention, the positions of the variable-geometry mechanism and the waste gate are controlled in different ways depending on the operating regime in which the engine is operating. More particularly, the positions of the variable-geometry mechanism and the waste gate are dependent on engine load and engine speed. One embodiment of the invention is depicted in  FIG. 2 . The engine operating envelope is divided into three regimes: Regime I encompasses relatively low engine speeds; Regime II encompasses intermediate engine speeds; and Regime III encompasses relatively high engine speeds. There is a first transition region T 1  between Regimes I and II, and a second transition region T 2  between Regimes II and III. 
     In Regime I, where engine speed is relatively low, the flow rate of exhaust gas from the engine generally is fairly low. In this regime, the variable-geometry mechanism  38  is maintained in a “closed” position. In the closed position of the variable-geometry mechanism, exhaust gas is still able to flow through the turbine. Boost is regulated by controlling the position of the waste gate  40  so as to bypass a variable amount of exhaust gas around the turbine. Various control schemes can be used for regulating boost via control of the waste gate, and the invention is not limited in that respect. 
     In Regime II, at intermediate engine speeds, a larger amount of exhaust gas flow is produced by the engine in comparison with Regime I. In this regime, the waste gate  40  is closed so that little or no exhaust gas bypasses the turbine, and boost is regulated by controlling the position of the variable-geometry mechanism  38 . 
     In Regime III, at high engine speeds, the exhaust gas flow rate from the engine is relatively high. Accordingly, in this regime, the variable-geometry mechanism is fully open so that as much flow as possible can pass through the turbine, and boost is regulated by controlling the position of the waste gate. 
     In the transition region T 1  between Regimes I and II, the variable-geometry mechanism and the waste gate are controlled in particular ways in order to accomplish a smooth transition as the variable-geometry mechanism begins to open for operation in Regime II. More particularly, as illustrated in  FIG. 3 , as the variable-geometry mechanism is initially moved from its fully closed position toward a more-open position, the waste gate simultaneously is moved from a relatively open position toward its closed position for operation in Regime II. This transition occurs, as shown in  FIG. 2 , at a predetermined engine speed for a given load, and the predetermined engine speed depends on engine load. Thus, in general, the transition occurs at a higher engine speed as load increases. However, various transition schemes can be used, and the illustrated one is presented merely for explanatory purposes. Although the applicant does not wish to be bound by theory, it is thought that positioning the waste gate in a relatively open position (i.e., not fully closed) at the moment that the variable-geometry mechanism begins to open serves to reduce the pressure of the exhaust gas entering the turbine via the variable-geometry mechanism and may reduce the amount of flow disturbance caused when the variable-geometry mechanism just begins to open. As the variable-geometry mechanism continues to open, the waste gate simultaneously can be closed. 
     In the transition region T 2  between Regimes II and III, the variable-geometry mechanism is fully open and the waste gate is moved from its fully closed position toward a more-open position. Again, as shown in  FIG. 2 , this transition occurs at a predetermined engine speed for a given load, and the predetermined engine speed depends on engine load. In general, the transition occurs at a higher engine speed as load increases. However, various transition schemes can be used, and the illustrated one is presented merely for explanatory purposes. 
     In  FIG. 2 , the locus of engine load-engine speed points delimiting the upper end of Regime I is denoted “N 1 ”. For a given engine load, N 1  is a predetermined engine speed at which the transition region T 1  begins; thus, below N 1 , the variable-geometry mechanism is closed and the waste gate is used for regulating boost. The locus of engine load-engine speed points delimiting the upper end of Regime II is denoted “N 2 ”. For a given engine load, N 2  is a predetermined engine speed at which the transition region T 2  begins; thus, above N 2 , the variable-geometry mechanism is open and the waste gate is used for regulating boost. At speeds between N 1  and N 2 , the waste gate is maintained in a relatively closed position and boost is regulated by regulating the position of the variable-geometry mechanism. 
     As shown in  FIG. 2 , there is also a predetermined engine speed (dependent on engine load) N 1 ′ that delimits the lower end of Regime II at which the transition region T 1  begins when speed is decreasing. The speed N 1 ′ may or may not be equal to the speed N 1  for a given engine load. Likewise, there is a predetermined engine speed (dependent on engine load) N 2 ′ that delimits the lower end of Regime III at which the transition region T 2  begins when speed is decreasing. The speed N 2 ′ may or may not be equal to the speed N 2  for a given engine load. As shown, N 1  is somewhat higher than N 1 , and N 2 ′ is somewhat higher than N 2 . 
     The operation of the apparatus and method of the invention is now explained by reference to specific examples, which are presented for explanatory purposes only and are not intended to be limiting in any way. As a first example, and with reference to  FIGS. 1 and 2 , consider an acceleration of the engine from a relatively low-speed, low-load condition A in Regime I to a higher-speed, higher-load condition B in Regime II. Initially, at condition A, the variable-geometry mechanism  38  is closed and the waste gate  40  is regulating boost, under the control of the ECU  40 . As speed and load increase to N 1 , the transition region T 1  is encountered. The ECU  40  commands the variable-geometry mechanism to begin opening and simultaneously commands the waste gate to begin closing (see  FIG. 3 ). The movement of each of the variable-geometry mechanism and the waste gate can be based on a predetermined function dependent on variables such as time, load, or speed, as stored in the ECU&#39;s memory, or can be at a predetermined fixed time rate of change stored in memory. 
     As a second example, consider moving from condition B to condition A in  FIG. 2 . Initially, at condition B, the waste gate  40  is closed and the variable-geometry mechanism  38  is regulating boost, under the control of the ECU  40 . As speed and load decrease to N 1 ′, the transition region T 1  is encountered. The ECU  40  commands the variable-geometry mechanism to begin closing and simultaneously commands the waste gate to begin opening (i.e., essentially the reverse of what is shown in  FIG. 3 ). The movement of each of the variable-geometry mechanism and the waste gate can be based on a predetermined function dependent on variables such as time, load, or speed, as stored in the ECU&#39;s memory, or can be at a predetermined fixed time rate of change stored in memory. 
     As a third example, consider moving from condition B to a condition C in Regime III, at constant load with increasing speed. Initially, at condition B, the waste gate  40  is closed and the variable-geometry mechanism  38  is regulating boost, under the control of the ECU  40 . As speed and load increase to N 2 , the transition region T 2  is encountered. By the time N 2  is reached, the variable-geometry mechanism is fully open, under the command of the ECU  40 , and the ECU commands the waste gate to begin opening (see  FIG. 4 ). The movement of the waste gate can be based on a predetermined function dependent on variables such as time, load, or speed, as stored in the ECU&#39;s memory, or can be at a predetermined fixed time rate of change stored in memory. 
     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.