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

1. A method for controlling a compressor inlet-adjustment mechanism disposed in an air inlet of a centrifugal compressor, the inlet-adjustment mechanism being switchable between two binary positions P 1 and P 2 for altering a flow area of the inlet, the method comprising:

identifying a threshold line on a compressor map of pressure ratio PR versus corrected flow rate W c for the compressor, the threshold line being a line on which the pressure ratio and the corrected flow rate of the compressor are the same for the two binary positions of the inlet-adjustment mechanism at equal rotational speeds;

continually detecting PR and W c of the compressor during operation as an operating point of the compressor moves around on the compressor map; and

placing the inlet-adjustment mechanism 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 on the compressor map.

2. The method of claim 1 , further comprising:

providing a first compressor map describing performance of the compressor with the inlet-adjustment mechanism in the P 1 position, the first compressor map being in the form of compressor pressure ratio PR versus corrected flow rate W c the first compressor map including at least two constant-speed lines N 1 P1 and N 2 P2 each representing PR versus W c with rotational speed of the compressor held constant;

providing a second compressor map describing performance of the compressor with the inlet-adjustment mechanism in the P 2 position, the second compressor map being in the form of compressor pressure ratio PR versus corrected flow rate W c , the second compressor map including at least two constant-speed lines N 1 P2 and N 2 P2 each representing PR versus W c with rotational speed of the compressor held constant, where the value of N 1 P1 is equal to the value of N 1 P2 and the value of N 2 P1 is equal to the value of N 2 P2 ;

wherein the identifying step further comprises identifying a threshold line comprising a curve fit of at least two points each represented by coordinates of PR versus W c at which the N 1 P1 speed line crosses the N 1 P2 speed line and the N 2 P1 speed line crosses the N 2 P2 speed 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 P 1 and P 2 positions, wherein the threshold line divides the first compressor map into two regions R 1 and R 2 respectively 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 W c to the threshold line and determining which of the two regions R 1 and R 2 the current operating point falls into; and

wherein the placing step further comprises placing the inlet-adjustment mechanism in the P 1 position when the operating point falls into the region R 1 and placing the inlet-adjustment mechanism in the P 2 position when the operating point falls into the region R 2 .

3. The method of claim 2 , wherein the first compressor map includes at least a third speed line N 3 P1 and the second compressor map includes at least a third speed line N 3 P2 providing at least a third point represented by coordinates of PR versus W c at which the N 3 P1 speed line crosses the N 3 P2 speed line, the threshold line being a curve fit of at least three points.

4. The method of claim 2 , further comprising storing the first compressor map in an engine control unit (ECU), and inputting the detected PR and W c into 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, and further comprising outputting a command from the ECU to an actuator for the inlet-adjustment mechanism for placing the inlet-adjustment mechanism in one of the P 1 and P 2 positions or the other of the P 1 and P 2 positions based upon results of the comparing step.

5. The method of claim 2 , wherein the first and second compressor maps are based on test data for the compressor operated with the inlet-adjustment mechanism respectively in the P 1 position and in the P 2 position.

6. The method of claim 1 , wherein the first and second compressor maps are based on model-predicted data for the compressor with the inlet-adjustment mechanism respectively in the P 1 position and in the P 2 position.