Method for controlling a trim-adjustment mechanism for a centrifugal compressor

A method for controlling an inlet-adjustment mechanism in an air inlet for a compressor so as to switch the mechanism in a binary fashion between two positions P1 and P2 for adjusting a flow area of the inlet. The method includes identifying a threshold line on a compressor map of pressure ratio versus corrected flow rate for the compressor. The threshold line is a line on which the pressure ratio and flow rate of the compressor are the same for the P1 and P2 positions of the inlet-adjustment mechanism at equal speeds. When the operating point of the compressor on the compressor map crosses the threshold line, the inlet-adjustment mechanism is switched from one of its binary positions to the other.

BACKGROUND OF THE INVENTION

The present disclosure relates to centrifugal compressors, such as used in turbochargers, and more particularly relates to centrifugal compressors in which the effective inlet area or diameter can be adjusted for different operating points.

An exhaust gas-driven turbocharger is a device used in conjunction with an internal combustion engine for increasing the power output of the engine by compressing the air that is delivered to the air intake of the engine to be mixed with fuel and burned in the engine. A turbocharger comprises a compressor wheel mounted on one end of a shaft in a compressor housing and a turbine wheel mounted on the other end of the shaft in a turbine housing. Typically the turbine housing is formed separately from the compressor housing, and there is yet another center housing connected between the turbine and compressor housings for containing bearings for the shaft. The turbine housing defines a generally annular chamber that surrounds the turbine wheel and that receives exhaust gas from an engine. The turbine assembly includes a nozzle that leads from the chamber into the turbine wheel. The exhaust gas flows from the chamber through the nozzle to the turbine wheel and the turbine wheel is driven by the exhaust gas. The turbine thus extracts power from the exhaust gas and drives the compressor. The compressor receives ambient air through an inlet of the compressor housing and the air is compressed by the compressor wheel and is then discharged from the housing to the engine air intake.

Turbochargers typically employ a compressor wheel of the centrifugal (also known as “radial”) type because centrifugal compressors can achieve relatively high pressure ratios in a compact arrangement. Intake air for the compressor is received in a generally axial direction at an inducer portion of the centrifugal compressor wheel and is discharged in a generally radial direction at an exducer portion of the wheel. The compressed air from the wheel is delivered to a volute, and from the volute the air is supplied to the intake of an internal combustion engine.

The operating range of the compressor is an important aspect of the overall performance of the turbocharger. The operating range is generally delimited by a surge line and a choke line on an operating map for the compressor. The compressor map is typically presented as pressure ratio (discharge pressure Pout divided by inlet pressure Pin) on the vertical axis, versus corrected mass flow rate on the horizontal axis. The choke line on the compressor map is located at high flow rates and represents the locus of maximum mass-flow-rate points over a range of pressure ratios; that is, for a given point on the choke line, it is not possible to increase the flow rate while maintaining the same pressure ratio because a choked-flow condition occurs in the compressor.

The surge line is located at low flow rates and represents the locus of minimum mass-flow-rate points without surge, over a range of pressure ratios; that is, for a given point on the surge line, reducing the flow rate without changing the pressure ratio, or increasing the pressure ratio without changing the flow rate, would lead to surge occurring. Surge is a flow instability that typically occurs when the compressor blade incidence angles become so large that substantial flow separation arises on the compressor blades. Pressure fluctuation and flow reversal can happen during surge.

In a turbocharger for an internal combustion engine, compressor surge may occur when the engine is operating at high load or torque and low engine speed, or when the engine is operating at a low speed and there is a high level of exhaust gas recirculation (EGR). Surge can also arise when an engine is suddenly decelerated from a high-speed condition. Expanding the surge-free operation range of a compressor to lower flow rates is a goal often sought in compressor design.

One scheme for shifting the surge line of a centrifugal compressor to the left (i.e., surge is delayed to a lower flow rate at a given pressure ratio) and for shifting the choke flow line to the right (i.e., choke flow increases to a higher flow rate at a given pressure ratio) is to employ a trim-adjustment mechanism in the compressor inlet. Applicant is the owner of co-pending applications disclosing various trim-adjustment mechanisms of this type, see, e.g., application Ser. Nos. 14/537,339; 14/532,278; 14/642,825; 14/573,603; and 14/551,218; the entire disclosures of said applications being hereby incorporated herein by reference.

The present disclosure relates to methods for controlling such a trim-adjustment mechanism.

BRIEF SUMMARY OF THE DISCLOSURE

The present disclosure describes a method for controlling an inlet-adjustment mechanism in an air inlet for a compressor so as to switch the mechanism in a binary fashion between two positions P1and P2. The method includes identifying a threshold line on a compressor map of pressure ratio versus corrected flow rate for the compressor. The threshold line is a line on which the pressure ratio and flow rate of the compressor are the same for the P1and P2positions of the inlet-adjustment mechanism at equal speeds. In other words, the threshold line is a curve fit of points on the compressor map where a series of constant-speed lines achieved with the inlet-adjustment mechanism in the P1position cross their respective constant-speed lines achieved with the inlet-adjustment mechanism in the P2position. In accordance with the method, the inlet-adjustment mechanism is placed in one of the binary positions or the other based on a comparison of a location of the operating point relative to the threshold line.

In one embodiment, the method comprises:providing a first compressor map describing performance of the compressor with the inlet-adjustment mechanism in the P1position, the first compressor map being in the form of compressor pressure ratio PR versus corrected flow rate Wc, the first compressor map including at least two constant-speed lines N1P1and N2P2each representing PR versus Wcwith rotational speed of the compressor held constant;providing a second compressor map describing performance of the compressor with the inlet-adjustment mechanism in the P2position, the second compressor map being in the form of compressor pressure ratio PR versus corrected flow rate Wc, the second compressor map including at least two constant-speed lines N1P2and N2P2each representing PR versus Wcwith rotational speed of the compressor held constant, where the value of N1P1is equal to the value of N1P2and the value of N2P1is equal to the value of N2P2;identifying a threshold line comprising a curve fit of at least two points each represented by coordinates of PR versus Wc, at which the N1P1speed line crosses the N1P2speed line and the N2P1speed line crosses the N2P2speed line, such that for operating points falling on the threshold line, pressure ratio and corrected flow at equal speeds are the same with the inlet-adjustment mechanism in the P1and P2positions, wherein the threshold line divides the first compressor map into two regions R1and R2respectively located on opposite sides of the threshold line;comparing, for a current operating point of the compressor during operation, pressure ratio PR and corrected flow Wcto the threshold line and determining which of the two regions R1and R2the current operating point falls into; andplacing the inlet-adjustment mechanism in the P1position when the operating point falls into the region R1and placing the inlet-adjustment mechanism in the P2position when the operating point falls into the region R2.

In one embodiment described herein, the first compressor map includes at least a third speed line N3P1and the second compressor map includes at least a third speed line N3P2providing at least a third point represented by coordinates of PR versus Wcat which the N3P1speed line crosses the N3P2speed line, the threshold line being a curve fit of at least three points. In some embodiments, there can be four, five, six, seven, or more such speed lines, such that the threshold line is a curve fit of four, five, six, seven, or more points.

The method can include storing the first compressor map in an engine control unit (ECU), detecting PR and Wcof the compressor during operation, and inputting the detected PR and Wcinto the ECU, wherein the ECU is programmed to perform the comparing step periodically as the current operating point moves around on the first compressor map. The method further can include outputting a command from the ECU to an actuator for the inlet-adjustment mechanism for switching from one of the P1and P2positions to the other of the P1and P2positions upon the ECU detecting that the threshold line has been crossed.

In one embodiment the first and second compressor maps are based on test data for the compressor operated with the inlet-adjustment mechanism respectively in the P1position and in the P2position.

In another embodiment the first and second compressor maps are based on model-predicted data for the compressor with the inlet-adjustment mechanism respectively in the P1position and in the P2position.

DETAILED DESCRIPTION OF THE DRAWINGS

A compressor10in accordance with one embodiment of the invention is illustrated in cross-sectional view inFIGS. 1 and 2. The compressor10comprises a compressor housing12that defines an air inlet14extending along an axial direction of the compressor. A compressor wheel15, comprising a hub16from which a plurality of blades20extend generally radially outwardly, is disposed in the compressor housing and is affixed to one end of a shaft18that is rotatably driven in a suitable manner (e.g., by an exhaust gas-driven turbine, not shown) for rotatably driving the compressor wheel15. The compressor housing defines a diffuser22for receiving and diffusing air that is compressed as it passes through the compressor wheel. The diffuser delivers the diffused compressed air into a volute24defined by the compressor housing.

In accordance with the invention, the compressor of the turbocharger includes an inlet-adjustment mechanism30disposed in the air inlet14of the compressor housing and movable between an open or “low-trim” position (FIG. 1) and a closed or “high-trim” position (FIG. 2). The low-trim position is also referred to herein as the P1position, and the high-trim position is referred to as the P2position. The movement of the mechanism30can entail axial sliding, pivotal movement, or helicoidal (screw-type) movement of the mechanism, to name a few non-limiting examples. The details of the structure and operation of the inlet-adjustment mechanism30are not pertinent to the present disclosure, and any mechanism that operates to effectively alter the flow area of the air inlet14leading into the compressor wheel15can be employed in the practice of the present invention.

The inlet-adjustment mechanism30enables adjustment of the effective size or diameter of the inlet into the compressor wheel15. As illustrated inFIG. 2, when the inlet-adjustment mechanism is in the high-trim or P2position, the effective diameter of the inlet into the compressor wheel is relatively increased compared to the effective diameter in the low-trim or P1position ofFIG. 1. In order for this effect to be achieved, the axial spacing distance from the mechanism30to the compressor wheel must be as small as practicable.

At intermediate and high flow rates, the inlet-adjustment mechanism30can be placed in the high-trim position as inFIG. 2. This can have the effect of increasing the effective inlet diameter and thus of reducing the flow velocity into the compressor wheel.FIG. 3illustrates an exemplary compressor map for the high-trim position ofFIG. 2. The map is in the form of compressor pressure ratio (discharge pressure divided by inlet pressure) versus corrected flow rate (e.g., mass flow rate corrected to standard temperature and pressure inlet conditions). The map includes a series of constant-speed lines N1P2, N2P2, N3P2, N4P2, and N5P2at each of which the compressor rotational speed is held constant while corrected flow and pressure ratio vary. The map includes a high-trim surge line, which is the locus of points at which surge of the compressor is imminent; i.e., if the flow were reduced at the same pressure ratio, or the pressure ratio were increased at the same flow, the compressor would experience surge.

At low flow rates, the inlet-adjustment mechanism30can be placed in the low-trim position ofFIG. 1. This can have the effect of reducing the effective inlet diameter and thus of increasing the flow velocity into the compressor wheel. The result will be a reduction in compressor blade incidence angles, effectively stabilizing the flow (i.e., making blade stall and compressor surge less likely). In other words, the surge line of the compressor will be moved to lower flow rates (to the left on a map of compressor pressure ratio versus flow rate). This is illustrated inFIG. 4, showing the compressor map for the low-trim position. The low-trim map has the same values of speed lines N1P1, N2P1, N3P1, N4P1, and N5P1. That is, the value of N1P1is equal to the value of N1P2, and the same is true for the other speeds.

FIG. 5is a map that superimposes the speed lines for the low-trim or P1position ofFIG. 1, and the speed lines for the high-trim or P2position ofFIG. 2. It will be seen that for a given compressor speed, the respective low- and high-trim speed lines cross each other at a point denoted by a solid black circles inFIG. 5. This means that at the pressure ratio and corrected flow condition where the speed lines cross, the inlet-adjustment mechanism can be switched from one of its two positions to the other without there being any significant effect on the flow rate and pressure ratio of the compressor. Thus, for example, at the point labeled “A” inFIG. 5, the inlet-adjustment mechanism can be moved from the P1position to the P2position, or from the P2position to the P1position, with no significant effect on compressor behavior.

This characteristic speed-line crossing feature can be taken advantage of in designing the control scheme for controlling the operation of the inlet-adjustment mechanism. By establishing a “threshold” line on the superimposed maps inFIG. 5, representing a “best fit” of the speed-line crossing points, the operation of the inlet-adjustment mechanism can be regulated with reference to where on the compressor map the compressor is operating, and in particular where the operating point is located with respect to the threshold line. When the compressor is operating in a region located to the left of the threshold line inFIG. 5(referred to as the R1region), the inlet-adjustment mechanism can be placed in the low-trim or P1position (FIG. 1). When the compressor is operating to the right of the threshold line, in the region denoted as R2, the inlet-adjustment mechanism can be placed in the high-trim or P2position (FIG. 2). By switching between the P1and P2positions when the operating point moves so as to cross the threshold line, the transition from one position to the other will not cause any sudden change in compressor flow rate and pressure ratio.

Implementation of the above-described control scheme can be accomplished in various ways.FIG. 6illustrates the general architecture of a system for controlling the inlet-adjustment mechanism30. An actuator26of suitable type is coupled to the inlet-adjustment mechanism. The actuator can operate on any of various principles; for example, the actuator can employ an electric motor, a pneumatic device, a hydraulic device, or the like, for imparting movement to the inlet-adjustment mechanism to move it between its P1and P2positions. The actuator26is in communication with a control unit40. The control unit includes a processor50(such as a microprocessor), a memory60(such as non-volatile ROM, PROM, EPROM, or EEPROM memory), and interfaces70for communicating with other devices in the system. The memory can be programmed (e.g., in hardware and/or firmware and/or software) with control instructions that are executed by the processor for carrying out the functions of the control unit. In the illustrated embodiment the control unit is an engine control unit or ECU such as present on vehicles such as automobiles and trucks.

FIG. 7illustrates in further detail an embodiment of the invention for controlling the actuator26, which in turn controls the inlet-adjustment mechanism. The memory60of the control unit stores a base compressor map CM such as the high-trim map ofFIG. 3. The map can be stored in any of various forms such as a look-up table that includes the variables of corrected flow Wcversus pressure ratio PR along a series of constant-speed lines N1, N2, N3, etc. The memory also stores the threshold line TL representing the curve-fit of speed-line crossing points derived in the manner previously described. The threshold line can be stored in any of various forms, such as a table of PR versus Wcor a polynomial formula expressing PR as a function of Wc. The processor50receives inputs of PR and Wcand optionally compressor speed RPM. The pressure ratio and corrected flow are continually sensed by suitable sensors and the sensed values are sent to the processor (e.g., at regular time-step intervals such as every 0.1 second or other suitably selected interval). The processor compares the sensed PR and Wcto the threshold line to determine where the current compressor operating point (OP) is located relative to the threshold line TL. Based on that determination, the processor outputs a SETPOINT to the actuator26. For example, if the processor determines based on the sensed PR and Wcthat the current operating point is in region R1(e.g., OP1inFIG. 7), the SETPOINT causes the actuator to place the inlet-adjustment mechanism in the P1or low-trim position. If the processor determines that the operating point is in the region R2(e.g., OP2inFIG. 7), the SETPOINT causes the actuator to place the mechanism in the P2or high-trim position.

FIG. 8is a flow diagram illustrating a method in accordance with one embodiment of the invention. A threshold line is identified on the compressor map in a step100, in the manner generally as described above. In a step102, PR and Wcfor the compressor are detected for the current operating point of the compressor, using suitable sensors as well-known in the art. In a step104, the current operating point is located on the compressor map, based on the sensed PR and Wc. In a step106the location of the operating point is compared to the threshold line. For example, at the detected value of PR, the corresponding value of Wcis determined from the table look-up data or the formula representing the threshold line. The actual detected value of Wcis compared to the threshold value. In a step108, it is determined whether the actual Wcis less than the threshold Wcvalue; in other words, it is determined whether the operating point is located in the region R1(seeFIG. 5). If the operating point is in region R1, then in a step110the inlet-adjustment mechanism is set to the low-trim or P1position (FIG. 1). If the operating point is not in the region R1, then in a step112the inlet-adjustment mechanism is set to the high-trim or P2position. The process then returns to step102and the steps102through108are repeated at regular time steps. At each time step, the locations of the operating point relative to the threshold line for the previous time step as well as for the current time step are remembered and compared. By suitably selecting the time step interval to be short enough relative to the anticipated maximum rate at which the flow rate through the compressor can change, it can be ensured that the inlet-adjustment mechanism is switched from one position to the other at or sufficiently near the threshold line such that the switch has no significant effect on the flow rate and pressure ratio. Thus, there is no sudden perceptible change in compressor performance.

In accordance with the invention, the first and second compressor maps ofFIGS. 3 and 4can be based on test data for the compressor operated with the inlet-adjustment mechanism respectively in the P1position and in the P2position. Alternatively, the first and second compressor maps can be based on model-predicted data for the compressor with the inlet-adjustment mechanism respectively in the P1position and in the P2position.

In the illustrated embodiments of the invention, the threshold line is a curve-fit of five points representing the speed-line crossing points at five different compressor speed values. Alternatively, however, the curve fit could be simplified by using as few as two different speeds (thus, a linear threshold line) or using three or four speeds. Of course, more than five speeds can be used as well. The invention is not limited to any particular technique for defining the threshold line.