1 TITLE OF THE INVENTION

An oil free Auxiliary Power Unit for aero engine application

2 FIELD OF THE INVENTION

The invention relates to an aeronautical gas turbine-powered auxiliary power unit (APU), and more particularly to an APU that comprises an oil-free bearing and starter generator.

3 BACKGROUND OF THE INVENTION

Secondary power system based on gas turbine engine is usually used for starting the aircraft main engine at ground, and finds similar application in air during the emergency. The secondary power system comprises of an Auxiliary Power Unit (APU) and an Air Starter. APU is used for supplying compressed air to Air Starter for starting the main engines and AC power generation for operation of various aircraft systems on ground and upto an altitude. APU generally uses oil lubrication. An oil free APU is designed with the capability to fulfill the requirements of air supply and power generation at high altitudes. The design of an oil free rotor APU involves challenges related to rotordynamics i.e issues associated with the rotor systems. These problems are addressed by modifying the design of foil bearings through detailed rotordynamic analysis.

Prior Art

Patent US20060042270A1 discusses an Auxiliary power unit with an oil free compressor. It describes an auxiliary power unit for aircraft with a load compressor that delivers cabin air free of oil contamination by supporting a drive shaft for load compressor with oil free bearings.

Patent US007494282B2 discusses a radial foil

bearing for supporting a high speed rotating shaft. This invention is intended to improve the performance and economical efficiency by changing the structure of the bump foils and the top foil.
4 SUMMARY OF THE INVENTION

An oil free APU is designed that comprises of the following features:

The technologies involved in making an oil free Auxiliary Power Unit for aircraft application is the development of an oil free bearing and the elimination of gear systems, oil pump, oil seals etc. The fuel pump also removed from the APU and external driven fuel system is introduced.

Air Foil Bearing (AFB) is designed for oil free operation of the bearing. Once a series of thin sheet metal foils is introduced into the gas bearing, it is known as gas foil bearing and if the gas is air, it is called air foil bearing (AFB). Direct driven starter generators are used for eliminating the gear requirement and this replaced the starter motor and electrical generator used earlier.

Early attempts to incorporate foil bearings into gas turbines often encountered rotordynamic problems stemming from unfavorable bearing performance characteristics. These problems are addressed by modifying the design of foil bearings through detailed rotordynamic analysis.

5 BRIEF DESCRIPTION OF THE DRAWINGS

The invention with its advantages is illustrated with reference to Figures 1 and 2.

Fig-1 represents the existing APU rotor system.

Fig-2 shows the complete layout of APU with the incorporated modification.

6 DETAILED DESCRIPTION

The preferred embodiments of the present invention will be hereafter described in detail with reference to the accompanying drawings.

The APU is a single shaft gas turbine engine consisting of an air intake casing (2) with bleed air arrangement, reverse flow annular combustion chamber (4), exhaust duct (6).
A turbine shaft (10) connecting a single stage compressor (3) and two stage turbine (5) forms the main rotor system of APU. The layout of the APU with this rotor system is represented in Fig-1 where two deep groove ball bearings (9) are supporting the rotor system. Oil inlet pipe (7), oil outlet pipe (8), oil pump (15), fuel pump (14), dynamic oil seals (11) and the gear box (1) are shown in the layout. The starter motor (13) is used for starting the APU and electrical generator (12) provides electrical power to aircraft.

The general APU layout require oil lubrication system due to the presence of ball bearings (9) and gear box (1).

The disadvantages of oil lubrication such as periodical maintenance, greasy machinery, work space consumption etc. demanded the development of an oil free APU. The impending payoffs of oil free gas turbine system are higher reliability, soft failure, optimization of the gear box lubricant for military application, oil free cabin air etc.

An oil free APU is proposed by eliminating the gear train and replacing the ball bearings with oil free bearings. Direct driven starter generators are proposed for eliminating the gear train.

The rotor system is subjected to extensive dynamic analysis to identify the bearing stiffness and damping characteristics as well the critical speeds of the rotor system.

The placement of forward synchronous critical speeds with respect to the operating speed of the machine is the primary consideration in the design of rotor systems. Rotordynamics analyses are carried out on various designs by modifying the fluid film and foil characteristics to arrive at a design which operates away from critical speed and is free from instability effects. The air foil bearings design is arrived based on these analyses.

The new layout developed with this rotor system is shown in Fig-2. The new configuration includes the Air Foil Bearings (16) and direct driven electrical generators (18). The new design eliminated oil inlet pipe (7), oil outlet pipe (8), oil pump (15) and dynamic oil seals (10). Oil pump is a Line Replacement Unit (LRU) which gets separately designed and certified due to its complex design. These parts are required for the oil

lubrication system and the introduction of air foil bearing eliminated these parts.
The design is carried out in such a way that the foil bearings are supported on the existing rotor arrangement by introducing an additional removable rotational member called foil support (17). This eliminated the complexities associated with interchanging the ball and foil bearings.

The fuel pump (14) is removed from APU and external driven fuel system is introduced. The starter motor (13) and electrical generator (12) are replaced by a direct driven starter generator (18) which serves the function of both (12) and (13). Above modifications led to the elimination of the gear box (1) in new configuration. The bearings and the gears of gear box are complicated parts which require continuous supply of oil for lubrication. The introduction of direct driven electrical generator simplified the design by reducing the complexities associated with the gear box.

The removal of oil pump, fuel pump, gear box and different lubrication pipe lines resulted in significant weight reduction, improved reliability and simplification of the design.