Variable Speed APU Control for Temperature-Induced Vibration

Solution Overview

Problem

Auxiliary power units (APUs) in aircraft face efficiency issues due to varying ambient temperatures, leading to mechanical vibration and resonance, as they are typically run at a constant mechanical shaft speed, which is suboptimal in extreme temperature conditions.

Innovation Solution

An adjustable APU system that varies shaft speed based on inlet air temperature, allowing operation within a selected range to maintain constant corrected speed, optimizing efficiency and preventing vibration by using a compressor map to determine optimal speed and efficiency parameters.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If the APU shaft speed is increased on hot days to improve compressor and turbine efficiencies, then the APU power and efficiency are improved, but mechanical vibration and resonance occur, increasing the likelihood of failures

Engineering Contradiction:

ImproveAPU power and efficiencyVSAvoidlikelihood of failures

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The APU shaft speed is made variable rather than constant, allowing it to adjust dynamically based on ambient temperature conditions. The control system modifies the shaft speed within an operating range to optimize efficiency while avoiding resonance conditions that cause mechanical failures.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The shaft speed parameter is changed as a function of ambient temperature. The control system adjusts this critical parameter to maintain optimal compressor and turbine efficiencies across different temperature conditions while avoiding problematic speed ranges that induce vibration and resonance.

Inventive Principle:

Principle #35Parameter changes

2Productivity

If the APU shaft speed is decreased on cold days to improve compressor and turbine efficiencies, then the APU power and efficiency are improved, but mechanical vibration and resonance occur, increasing the likelihood of failures

Engineering Contradiction:

ImproveAPU power and efficiencyVSAvoidlikelihood of failures

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The APU shaft speed is made variable rather than constant, allowing it to adjust dynamically based on ambient temperature conditions. The control system modifies the shaft speed within an operating range to optimize efficiency while avoiding resonance conditions that cause mechanical failures.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The shaft speed parameter is changed as a function of ambient temperature. The control system adjusts this critical parameter to maintain optimal compressor and turbine efficiencies across different temperature conditions while avoiding problematic speed ranges that induce vibration and resonance.

Inventive Principle:

Principle #35Parameter changes

3Ease of operation

If the APU is run at a constant mechanical shaft speed, then speed control is simple, but the APU experiences lower efficiencies and narrower compressor flow ranges in extreme temperature conditions

Engineering Contradiction:

Improvespeed control simplicityVSAvoidcompressor flow range and efficiency

Core Design Contradiction:

Ease of operationVSProductivity

Solution Approach 1:

The APU shaft speed is made variable rather than constant, allowing it to adjust dynamically based on ambient temperature conditions. The control system modifies the shaft speed within an operating range to optimize efficiency while avoiding resonance conditions that cause mechanical failures.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The shaft speed parameter is changed as a function of ambient temperature. The control system adjusts this critical parameter to maintain optimal compressor and turbine efficiencies across different temperature conditions while avoiding problematic speed ranges that induce vibration and resonance.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS7367193B1Auxiliary power unit control method and system

Publication Date: 2008.05.06 HAMILTON SUNDSTRAND CORP

AI summary

An adjustment method and system for an auxiliary power unit (APU) allows the APU speed to vary based on the inlet air temperature to the APU compressor. The APU is controlled by a control law that allows the APU speed to float within a selected range based on speed and electric power generators phase matching criteria that provides smooth power transfer between the APU and a main engine generator. The specific APU mechanical speed for a given temperature may be determined from a compressor map that identifies the optimum combination of pressure ratio, flow rate and efficiency for a given inlet temperature, and avoids running the APU near mechanical resonant vibration conditions.