Patent Description:
The auxiliary power unit (APU) is a turbine engine or reciprocating engine usually mounted in the tail cone of an aircraft to provide autonomous electrical and mechanical power for the following purposes:.

Most medium and large aircraft use turbine engine auxiliary power units (APU). Known auxiliary power units (APU) comprise the auxiliary power unit (APU) engine, an AC electrical generator and engine mount brackets, among other elements.

More specifically, known auxiliary power units (APU) comprise an air inlet that provides air to a compressor of the auxiliary power unit (APU) and cooling air. The air inlet comprises an air diffuser duct, compressors air inlet duct, accessory cooling air duct and an air inlet door.

The elements inside the auxiliary power unit (APU) may have friction between the moving parts and require effective lubrication and cooling, as well as particle sweeping. Heat at these moving parts is the result of:.

Cooling of these elements is achieved through a lubricating oil. The lubrication system reduces friction between moving parts, prevents destructive scoring in gears and removes heat generated to keep system temperatures within limits. The lubricating oil is cooled on an air to liquid cooler, i.e., a heat exchanger, usually by ventilation air on the auxiliary power unit (APU) compartment.

The air flow for the auxiliary power unit (APU) compartment ventilation is achieved by either:.

It is known document <CIT> disclosing an auxiliary power unit (APU) to be mounted in an APU compartment that defines an inlet opening.

It is also known document <CIT> disclosing an air inlet assembly for bringing air to an auxiliary power unit mounted in the compartment of an aircraft. The assembly includes a duct extending from an intake contoured to conform to the to the aircraft fuselage to an exit coupled to the inlet plenum of the auxiliary power unit.

It is also known document <CIT> disclosing a power generation system for integration into an aircraft system, including a secondary power supply device having a generator turbine arranged in a duct running between a forward opening and a rearward opening on the fuselage of the aircraft.

It is also known document <CIT> disclosing a method of cooling an auxiliary power unit (APU) of an aircraft having prime mover engines fed with aircraft fuel.

As stated in the background of the invention, engine coolers heat and air flow are wasted to the environment and do not contribute to increase the efficiency or energy of the auxiliary power unit (APU) of the aircraft. The present invention relates to an aircraft auxiliary power unit (APU), of the turbine or reciprocating engine type, that provides electrical power and additionally bleed air.

According to the above, the auxiliary power unit system of an aircraft object of the invention comprises:.

The auxiliary power unit system object of the invention further comprises:.

Therefore, the aim of the invention is to harness the cooling air flow and use it to:.

It is also an object of the invention a tail cone of an aircraft comprising an auxiliary power unit system according to the above.

The tail cone is the last fuselage section of the aircraft at its rear part. The invention is not limited to an auxiliary power unit (APU) located in the tail cone of the aircraft, other locations along the aircraft are also possible.

It is also an object of the invention an aircraft comprising tail cone according to the above.

To complete the description and to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate preferred embodiments of the invention. The drawings comprise the following figures.

Referring to the embodiments of the accompanying figures, the main components of the auxiliary power unit and, and a brief description of each, are as follows:.

In the shown embodiments the air inlet door unit (<NUM>) is movable between:.

According to the above, the shown system has at least three operating positions:.

In the embodiments shown in <FIG> and <FIG>, the auxiliary power unit (APU) (<NUM>) is a reciprocating engine. The shown embodiment has lubricant oil and a cooling liquid, like glycol. For this reason, a first heat exchanger (<NUM>) and a second heat exchanger (<NUM>) are needed. The first heat exchanger (<NUM>) is a liquid-air heat exchanger for cooling the cooling liquid and the second heat exchanger (<NUM>) is a liquid-air heat exchanger for intercooling functions.

In another embodiment, up to three different heat exchangers may be located within the second duct (<NUM>) or even a single heat exchanger.

The reciprocating engine may have an additional heat exchanger located on the auxiliary power unit (APU) between a turbo compressor outlet and an engine intake inlet.

In the case of a turbine engine, typically a single heat exchanger would be located within the second duct (<NUM>), although there could be more than one heat exchanger.

The heat exchangers (<NUM>, <NUM>) shown in the figures are of the helical tube and shell type, but other heat exchanger types could also be used, like fin and plate. Both heat exchangers (<NUM>, <NUM>) are placed in series along the second duct (<NUM>).

In the embodiments shown in <FIG> and <FIG>, the auxiliary power unit (APU) also comprises a fan (<NUM>) located in the second duct (<NUM>) downstream the air turbine (<NUM>). The required air flow cooling the oil, glycol, and intercooler fluid at a reciprocating engine of an auxiliary power unit (APU) is too high as to be sufficient by just drawing air suctioned by the auxiliary power unit (APU) compressor, therefore a fan (<NUM>) or air amplifier would be required to increase the cooling air flow. The fan (<NUM>) or air amplifier will be usually OFF during flight and ON on ground auxiliary power unit (APU) system operations. In other embodiments, the fan (<NUM>) may also be ON in flight.

If a fan (<NUM>) is used it will be driven by an electrical motor. In an embodiment, the fan (<NUM>) may have variable pitch blades so that when the fan (<NUM>) is OFF the fan (<NUM>) blades are located at a feathered position, i.e., streamlined with the air flow. The fan (<NUM>) may have a brake so as to prevent its rotation when it is OFF.

If an air amplifier is used, pressurized air could be supplied to the air amplifier by a compressor driven by the auxiliary power unit (APU) (<NUM>) gearbox.

As previously stated, in the shown embodiments, the air inlet door unit (<NUM>) is movable between:.

In the embodiment shown in <FIG>, the auxiliary power unit (APU) (<NUM>) comprises a main door (<NUM>) and a secondary door (<NUM>). The main door (<NUM>) is configured to open and close the air inlet (<NUM>) and the secondary door (<NUM>) is configured to open and close the entrance of the first duct (<NUM>).

The main door (<NUM>) and the secondary door (<NUM>) are both hinged to a rear end of the air inlet (<NUM>). The rear end is the end of the air inlet (<NUM>) closest to the rear part of the aircraft.

The main door (<NUM>) and the secondary door (<NUM>) open in a direction opposite to the first and the second ducts (<NUM>, <NUM>), i.e., they open towards the outside of the tail cone in the embodiment shown in <FIG>.

A second embodiment is shown in <FIG>. The air inlet door unit (<NUM>) comprises a door (<NUM>) hinged to a front end of the air inlet (<NUM>). The front end of the air inlet (<NUM>) is the end of the air inlet (<NUM>) closest to the front part of the aircraft.

The free end of the door (<NUM>) opens towards the first and the second ducts (<NUM>, <NUM>), i.e., towards the inside of the tail cone. The free end of the door (<NUM>) is the end of the door (<NUM>) opposite to its hinge. The free end is movable between:.

In the shown embodiments, the first duct (<NUM>) and the second duct (<NUM>) are separated by a partition wall (<NUM>) longitudinal to the first and second duct (<NUM>, <NUM>) that splits the air flow downstream of the air inlet (<NUM>).

In both embodiments, in the first position, there is no air flow through the air inlet (<NUM>), see for instance <FIG>. The auxiliary power unit (APU) (<NUM>) is not required to be ON, and the aircraft operation does not require additional drag or auxiliary/emergency power generation. This is typical for a cruise flight wherein the auxiliary power unit (APU) (<NUM>) could be OFF and thus there is no need to draw air through the heat exchangers (<NUM>, <NUM>).

The air turbine (<NUM>) could be OFF since an airflow through the second duct (<NUM>) would create a drag penalty for the aircraft and therefore a fuel consumption increase. The closed air inlet door unit (<NUM>) keeps an aerodynamic tail cone with no additional penalties on drag caused by air flow through air inlet (<NUM>).

In the second position, the air inlet (<NUM>) is partially opened in the shown embodiments. In the embodiment shown in <FIG>, the main door (<NUM>) leaves the air inlet (<NUM>) partially open while the secondary door (<NUM>) allows air flow into the second duct (<NUM>) and prevents air flow into the first duct (<NUM>).

More specifically, the shown embodiment of the secondary door (<NUM>) disclosed in <FIG> comprises a first portion (<NUM>. <NUM>), see <FIG>, adjacent to the hinge comprising an opening adapted to the cross shape of the second duct (<NUM>). The secondary door (<NUM>) comprises a second portion (<NUM>. <NUM>) adjacent to the first portion (<NUM>. <NUM>) adapted to be located against the entrance of the first duct (<NUM>) and having no openings. Therefore, when the main door (<NUM>) is partially opened, the shape of the secondary door (<NUM>) allows the air flow to go through the second duct (<NUM>) but not to the first duct (<NUM>).

In the embodiment shown in the figures, the auxiliary power unit system comprises an actuator (<NUM>) attached to the main door (<NUM>) configured to open and close it. It may be an electrical actuator or a hydraulic actuator. In an embodiment, it is joined to the main door (<NUM>) by a spherical bearing joint and to the aircraft tail cone structure at its other end. It may keep the main door (<NUM>) at any position between fully closed and fully opened to meet the auxiliary power unit (APU) (<NUM>) airflow demand while reducing the aerodynamic to the minimum.

The main door (<NUM>) and the secondary door (<NUM>) are rotatably joined and the inlet door unit (<NUM>) comprises a torsion spring (<NUM>) between the main door (<NUM>) and the secondary door (<NUM>) to keep the secondary door (<NUM>) closing the first duct (<NUM>) when the main door (<NUM>) is in the second position.

The torsion spring (<NUM>) pushes the secondary door (<NUM>) against the entrance of the first duct (<NUM>) so that the secondary door (<NUM>) is kept at its fully closed position while the main door (<NUM>) is located between the first position and the second position.

Additionally, the main door (<NUM>) comprises a cam (<NUM>) configured to push the secondary door (<NUM>) in the positions ranging between the second and the third position. The cam (<NUM>) starts to push the secondary door (<NUM>) at the second position and moves the secondary door (<NUM>) to its fully open position at the third position while at any position between second and third positions the torsion spring (<NUM>) holds the secondary door (<NUM>) against the cam (<NUM>). In an embodiment, the cam (<NUM>) is attached to the main door (<NUM>) at both lateral sides of the main door (<NUM>).

Therefore, the secondary door (<NUM>) keeps closed until the main door (<NUM>) rotates beyond the second position, namely while running from the second position to the third position, the secondary door (<NUM>) opens and gets at the fully open position, see <FIG>, when the main door (<NUM>) is at the fully open third position.

In <FIG> the door (<NUM>) is hinged to a front end of the air inlet (<NUM>) and its free end moves from the rear end of the air inlet (<NUM>) so as to close the air inlet (<NUM>) to a cross end of the second duct (<NUM>) opposite the air inlet (<NUM>) so as to open the entrance of the second duct (<NUM>) but still closing the entrance to the first duct (<NUM>).

In the second position, therefore there is no air flow through the first duct (<NUM>), i.e., to the auxiliary power unit (APU), but there is air flow in the second duct (<NUM>). Heat exchangers (<NUM>, <NUM>) are not working, but the air turbine (<NUM>) may be ON.

This is applicable for descent and initial approach as well as final approach and landing phases, where the auxiliary power unit (APU) (<NUM>) is not required to be ON, but the air turbine (<NUM>) helps to provide additional drag to slow down the aircraft and convert the otherwise wasted energy into useful electrical energy by means of the generator coupled to the air turbine (<NUM>). This position is also used for emergency power generation. The electrical energy could be stored for instance on supercapacitors or high charge/discharge rate batteries or used while the air turbine (<NUM>) is ON.

In an embodiment, the air turbine (<NUM>) is coupled to an alternator that provides electrical energy to the aircraft, it could be stored at supercapacitors, at high charge/discharge rate batteries or feed the aircraft electrical system for instant power use.

<FIG>, <FIG> and <FIG> disclose embodiments of the third position of the air inlet door unit (<NUM>).

In the embodiment shown in <FIG> when the main door (<NUM>) is in the third position, the secondary door (<NUM>) is open, the auxiliary power unit (APU) (<NUM>) is ON, air flows through the first duct (<NUM>) and the second duct (<NUM>), heat exchangers (<NUM>, <NUM>) are working.

In the shown embodiments, the second duct (<NUM>) comprises a convergent exhaust (<NUM>) to the atmosphere downstream the air turbine (<NUM>). Therefore, the walls of the second duct (<NUM>) tend to approach each other.

In an embodiment, the air turbine (<NUM>) comprises variable pitch blades. It allows to trim the pitch at any desired angle and also at least to put the blades at the feathered position, i.e., streamlined with the air flow.

Near the rotor blades and upstream, the guide vanes have an aerodynamic and structural function to support the turbine and generator assembly. The guide vanes serve two functions:.

Two different scenarios are distinguished in the third position:.

Claim 1:
Auxiliary power unit system of an aircraft comprising:
- an auxiliary power unit (APU) (<NUM>),
- a cooling unit for the auxiliary power unit (APU) (<NUM>) comprising at least a heat exchanger (<NUM>),
- an air inlet (<NUM>) in fluid communication with the auxiliary power unit (APU) (<NUM>) and with the cooling unit,
- an air inlet door unit (<NUM>) located at the air inlet (<NUM>) to allow an air flow from outside the aircraft to enter the air inlet (<NUM>),
- a first duct (<NUM>) configured for drawing air into the auxiliary power unit (APU) (<NUM>) and having an entrance in fluid communication with the air inlet (<NUM>),
- a second duct (<NUM>) having an entrance in fluid communication with the air inlet (<NUM>), the heat exchanger (<NUM>) being at least partially located within the second duct (<NUM>),
the auxiliary power unit system being characterised in that it comprises:
- an air turbine (<NUM>) located within the second duct (<NUM>) downstream the heat exchanger (<NUM>), and
- an electrical generator coupled to the air turbine (<NUM>) so that the air turbine (<NUM>) and the electrical generator are configured to convert energy of an air flow in the second duct (<NUM>) into electrical energy.