US 7,168,254 B2 · Control logic for fuel controls on APUs

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.

Claims10

Dependent claims

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.

3 more elements

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.

11 more elements

Dependencies are inferred from the claim text.

Control logic for fuel controls on APUs

Life of this patent

Control logic for fuel controls on APUs

Life of this patent

Abstract

Claims10