Auxiliary power unit adaptive cooldown cycle system and method

1. An auxiliary power unit (APU) system, comprising: an APU configured to rotate at a rotational speed; one or more sensors disposed within the APU, each of the one or more sensors configured to sense an operational parameter of the APU and supply a sensor signal representative of the operational parameter; and an APU control unit in operable communication with the APU and coupled to receive the sensor signals from the one or more sensors, the APU control unit further coupled to receive a shutdown signal and configured, in response to the shutdown signal, to: (i) reduce the rotational speed of the APU to a predetermined cooldown speed magnitude that ensures combustor inlet temperature has reached a predetermined temperature value, (ii) determine, based on one or more of the sensor signals, when a lean blowout of the APU has occurred, and (iii) when a lean blowout has occurred, increasing the predetermined cooldown speed magnitude for use in a subsequent shutdown of the APU.

2. The APU system of claim 1 , wherein the APU control unit is configured, in response to the shutdown signal, to reduce the rotational speed of the APU at a predetermined rate.

3. The AU system of claim 2 , wherein the APU control unit is configured, in response to the shutdown signal, to reduce the rotational speed of the APU at the predetermined rate for a predetermined time period.

4. The APU system, of claim 1 , wherein the APU control unit is configured, in response to the shutdown signal, to: reduce the rotational speed of the APU to a predetermined post-shutdown speed value at a first speed reduction rate, and thereafter reduce the rotational speed of the APU to the predetermined cooldown speed magnitude at a second speed reduction rate.

5. The APU system of claim 4 , wherein the APU control unit is further configured to selectively adjust one or both of the first and second speed reduction rates.

6. The APU system of claim 1 , wherein:

the APU control unit includes at least one overspeed trip circuit, the at least one overspeed trip circuit is coupled to receive a speed signal representative of the rotational speed of the APU and is configured to supply an overspeed trip signal when the speed signal indicates the rotational speed of the APU has exceeded a threshold speed;

the APU control unit is further configured to supply a speed signal that indicates the rotational speed of the APU has exceeded the threshold speed to the at least one overspeed trip circuit when the rotational speed of the APU is reduced to the predetermined cooldown speed magnitude; and

the overspeed trip signal causes fuel flow to the APU to cease.

7. The APU system of claim 6 , wherein:

the APU control unit includes a plurality of overspeed trip circuits; and

the APU control unit is further configured to supply the speed signal that indicates the rotational speed of the APU has exceeded the threshold speed to a different one of the plurality of overspeed trip circuits each time it receives the shutdown signal.

8. The APU system of claim 1 , wherein the APU control unit is further configured to:

determine a rate of change of the rotational speed of the APU; and

determine that the lean blowout of the APU has occurred when the rate of change of the rotational speed of the APU is greater than a predetermined rate magnitude.

9. The APU system of claim 1 , wherein the APU control unit is further configured, upon occurrence of the lean blowout of the APU, to:

determine a lean blowout speed, the lean blowout speed being the rotational speed of the APU when the lean blowout occurred; and

increase the predetermined cooldown speed magnitude to greater than the lean blowout speed.