Control logic for fuel controls on APUs

• US 7,168,254 B2
• Filed: 02/17/2004
• Issued: 01/30/2007
• Est. Priority Date: 02/17/2004
• Status: Expired due to Fees

• Current Assignee: Honeywell International Inc.
• Original Assignee: Honeywell International Inc.
• Inventors: Riley, Harold J.
• Primary Examiner(s): CASAREGOLA, LOUIS J
• Application Number:
  US10/781,154
• Publication Number:
  US 20060254282A1
• Time in Patent Office: 1,078 Days
• Field of Search: 60/392.81, 607/73, 607/78, 607/90
• US Class Current: 60/773
• CPC Class Codes: F02C 7/26 Starting; Ignition
• CPC Class Codes: F02C 9/28 Regulating systems responsive to plant or ambient parameters, e.g. temperature, pressure, rotor speed F02C9/30 - F02C9/38, F02C9/44 take precedence
• CPC Class Codes: F05D 2220/50 for auxiliary power units (APU's)
• CPC Class Codes: F05D 2270/021 to prevent overspeed
• CPC Class Codes: F05D 2270/095 by temporary overriding set control limits
• CPC Class Codes: F05D 2270/112 by limiting temperatures
• CPC Class Codes: F05D 2270/3032 excessive temperatures, e.g. caused by overheating
• CPC Class Codes: F05D 2270/705 proportional-integral

Abstract

Conventional auxiliary power units (APUs) may experience over-temperature shutdowns when attempting to start them at high altitudes. Further, such conventional APUs may also experience overspeed conditions when a generator load is removed during on-speed operations. A fuel control logic that controls the fuel flow cutback below the minimum blowout fuel schedule is provided. A temperature trim loop measures engine temperature to determine the onset of a possible over-temperature condition. The fuel flow may then be trimmed accordingly to correct this over-temperature onset. Further, when the onset of an overspeed condition is detected, such as when a generator load is removed, the fuel flow may be trimmed accordingly to correct this overspeed onset. The fuel control logic allows the control to find the individual minimum fuel flow for each fuel control without risking blowout of the APU itself.

Claims

1. A method for correcting and/or preventing an over-temperature condition in an auxiliary power unit comprising:

measuring an exhaust gas temperature of the auxiliary power unit;

comparing the exhaust gas temperature to a predetermined temperature trim limit to determine a temperature trim error;

selecting a fuel flow trim value based on the determined temperature trim error, the fuel flow trim value selected to be either (i) the determined temperature trim error, if the temperature trim error is greater than zero, or (ii) zero, if the temperature trim error is not greater than zero;

subtracting the selected fuel flow trim value from a starting fuel flow value to get a trimmed commanded fuel flow value; and

delivering fuel to the auxiliary power unit at said trimmed commanded fuel flow value.

2. The method according to claim 1, wherein the method is used during starting of the auxiliary power unit.

3. The method according to claim 1, further comprising determining the starting fuel flow value based upon speed, air pressure and temperature of the auxiliary power unit.

4. The method according to claim 3, further comprising providing an upper limit at a predetermined maximum fuel schedule and a lower limit at a predetermined minimum fuel schedule for the starting fuel flow value.

5. A method for correcting and/or preventing an over-temperature condition during starting of an auxiliary power unit of an aircraft comprising:

measuring an exhaust gas temperature of the auxiliary power unit;

comparing the exhaust gas temperature to a predetermined temperate trim limit to determine a temperature trim error;

selecting a fuel flow trim value based on the determined temperature trim error, the fuel flow trim value selected to be either (i) the determined temperature trim error, if the temperature trim error is greater than zero, or (ii) zero, if the temperature trim error is not greater than zero;

determining a starting fuel flow value based upon speed, air pressure and temperature of the auxiliary power unit;

providing an upper limit at a predetermined maximum fuel schedule and a lower limit at a predetermined minimum fuel schedule for the sag fuel flow value;

subtracting the selected fuel flow trim value from the starting fuel flow value to get a trimmed commanded fuel flow value; and

delivering fuel to the auxiliary power unit at said trimmed commanded fuel flow value.

6. A method for preventing and/or correcting undesired operating conditions of an auxiliary power unit comprising:

calculating an engine starting trimmed commanded fuel flow value by;

measuring an exhaust gas temperature of the auxiliary power unit;

comparing the exhaust gas temperature to a predetermined temperature trim limit to determine a temperature trim error;

selecting a fuel flow trim value based on the determined temperature trim error, the fuel flow trim value selected to be either (i) the determined temperature trim error, if the temperature trim error is greater than zero, or (ii) zero, if the temperature trim error is not greater than zero;

subtracting the selected fuel flow trim value from a staring fuel flow value to get the engine starting trimmed commanded fuel flow value;

starting the auxiliary power unit with a fuel flow rate at the engine starting trimmed commanded fuel flow value;

calculating an on-speed trimmed commanded fuel flow value by;

measuring the speed of the auxiliary power unit,

comparing the measured speed to a predetermined speed reference point to determine a speed error,

generating a commanded fuel flow from the determined speed error,

comparing the commanded fuel flow to a predetermined maximum fuel schedule to determine which is lower, and selecting the lower as a commanded fuel flow upper limit,

selectively limiting the commanded fuel flow upper limit to an on-speed trimmed command fuel flow value; and

continuing the running of the auxiliary power unit with a fuel flow rate at either the command fuel flow upper limit or the on-speed trimmed commanded fuel flow value.

7. The method according to claim 6, further comprising determining the starting fuel flow value based upon speed, air pressure and temperature of the auxiliary power unit.

8. The method according to claim 7, further comprising providing an upper limit at the predetermined maximum fuel schedule and a lower limit at a predetermined minimum fuel schedule for the starting fuel flow value.

9. The method according to claim 6, further comprising:

determining an on-speed blowout prevention rate;

comparing the commanded fuel flow upper limit to a predetermined minimum fuel schedule to determine a possibility of blowout at the commanded fuel flow upper limit; and

providing fuel to the auxiliary power unit at the on-speed blowout prevention rate when the possibility of blowout is present, thereby preventing blowout of the auxiliary power unit.

10. The method according to claim 9, further comprising providing an upper limit on the on-speed fuel flow trim value at a predetermined maximum filet schedule.