Patent Application
20200130858
The application relates generally to auxiliary power units (APUs) and, more particularly, to a method and system for cooling an APU using aircraft fuel.
Auxiliary power units of the type including liquid cooled engines, such as intermittent internal combustion engines (e.g. Wankel engines), typically comprises an air cooler to cool the liquid coolant of the engines, as well as a blower to drive a cooling airflow through the cooler. In some aircraft applications, space may be limited for the installation of the air cooler and the blower. Also, the use of blowers increases weight and complexity.
Alternative APU cooling arrangements are thus desirable.
In one aspect, there is provided a method of cooling an auxiliary power unit (APU) of an aircraft, the method comprising: circulating a liquid coolant through a liquid-cooled engine of the APU, and circulating aircraft fuel in heat exchange relationship with the liquid coolant.
In another aspect, there is provided a method of cooling an auxiliary power unit (APU) of an aircraft having prime mover engines fed with aircraft fuel, the method comprising: using the aircraft fuel of the prime mover engines as a heat sink to absorb heat from a main source of heat of the APU.
In a further aspect, there is provided an aircraft power plant comprising: an aircraft fuel circuit; aircraft prime mover engines fluidly connected to the aircraft fuel circuit; and an auxiliary power unit (APU) comprising: an internal combustion engine having a housing in heat exchange relationship with a liquid coolant circuit, and a liquid-to-liquid heat exchanger in fluid communication with the liquid coolant circuit of the internal combustion engine and the aircraft fuel circuit.
Reference is now made to the accompanying figures in which:
The exhaust from the engine core 12′ is fed to one or more turbines 26, 22 of a compounding turbine section having an outlet fluidly connected to an exhaust duct 15 for discharging the hot combustion gases to atmosphere. A muffler 17 may be provided in the exhaust duct 15 downstream of the turbine section. One or more of the turbines 26, 22 is/are configured to compound power with the engine core 12′; in the embodiment shown, the turbine and engine shafts are coupled through a transmission provided by a gearbox 28. The compressor(s) 20, 21 may be driven by the turbines 26, 22 and/or the engine core 12′. In the embodiment shown, the compressors 20, 21 are driven by the turbines 26, 22, for example by being coupled to the same shaft or being engaged to the turbine shaft through a transmission provided in the gearbox 28. In another particular embodiment, the shaft(s) of the compressor(s) 20, 21 is/are engaged to the output shaft of the engine core 12′, either directly or through a transmission. As shown in
In a particular embodiment, the internal combustion engine(s) 12 is/are rotary intermittent internal combustion engines, for example Wankel engines; it is however understood that other types of intermittent internal combustion engines or other types of internal combustion engines may alternately be used.
As shown in
The rotor 34 is engaged to an eccentric portion 42 of an output shaft 16 to perform orbital revolutions within the rotor cavity. The output shaft 16 performs three rotations for each orbital revolution of the rotor 34. The geometrical axis 44 of the rotor 34 is offset from and parallel to the axis 46 of the housing 32. During each rotation of the rotor 34, each chamber 40 varies in volume and moves around the rotor cavity to undergo the four phases of intake, compression, expansion and exhaust.
An intake port 48 is provided through the peripheral wall 38 for admitting compressed air into one of the working chambers 40. An exhaust port 50 is also provided through the peripheral wall 38 for discharge of the exhaust gases from the working chambers 40. Passages 52 for a spark plug, glow plug or other ignition mechanism, as well as for one or more fuel injectors of a fuel injection system (not shown in
For efficient operation the working chambers 40 are sealed by spring-loaded peripheral or apex seals 56 extending from the rotor 34 to engage the inner surface of the peripheral wall 38, and spring-loaded face or gas seals 58 and end or corner seals 60 extending from the rotor 34 to engage the inner surface of the end walls 54. The rotor 34 also includes at least one spring-loaded oil seal ring 62 biased against the inner surface of the end wall 54 around the bearing for the rotor 34 on the shaft eccentric portion 42.
The fuel injector(s) of the engine 12, which in a particular embodiment are common rail fuel injectors, communicate with a source of Heavy fuel (e.g. diesel, kerosene (jet fuel), equivalent biofuel), and deliver the heavy fuel into the engine 12 such that the combustion chamber is stratified with a rich fuel-air mixture near the ignition source and a leaner mixture elsewhere.
The exemplary engine 12 is liquid cooled. Referring concurrently to
The liquid-to-liquid heat exchanger 24 further has one or more fuel conduits 67a connected to a fuel circuit 67 of the aircraft, the one or more fuel conduits 67a in heat exchange relationship with the one or more coolant conduit 63a to cool down the hot liquid coolant using aircraft fuel. For instance, in a single aisle or twin aisle aircraft, the quantity of aircraft fuel stocked in the aircraft fuel tanks T for feeding the aircraft prime mover engines E and other fuel-powered devices is significant (e.g. more than 30,000 lbs at take-off). Even after landing, when the APU is typically turned on, the quantity of fuel on the aircraft is still significant and often cooler after the flight. It is herein proposed to use this large quantity of “cold aircraft fuel” as a heat sink to dissipate the heat (about 1 million BTU/hr) generated by the internal combustion engine(s) 12 (the APU main source of heat), which is about 50-60% of the power produce by the APU. The APU is mostly used before take-off where fuel volume is very large so the temperature gain of the aircraft fuel resulting from the absorption of waste heat generated by the APU while the aircraft is on the ground is not problematic from a safety point of view. Also, the aircraft fuel, which is typically stored in the aircraft wings, will be cooled in flight by conduction through the aircraft wings and, thus, be colder again when the APU is used on the ground after flight when less fuel is available. On very hot days and long utilization of the APU after the flight, there could be a maximum fuel temperature limit preventing the use of the ground APU, but that would represent a very small percentage of the various missions. Sensors and a control unit operatively connected to fuel pumps could be used to shut down fuel flow through the heat exchanger 24 whenever need be.
As shown in
In a particular embodiment, the internal combustion engine 12 further includes a lubricant circuit 70 communicating with engine element(s) requiring lubrication (e.g. bearings, seals, etc.), so as to circulate a suitable lubricant (e.g. oil) thereto. A second heat exchanger 24′ has one or more lubricant conduit(s) 70a fluidly connected to the lubricant circuit 70. A lubricant pump 72 is positioned in the lubricant circuit 70 to move the lubricant in a continuous closed-loop cycle through the lubricant circuit 70 between the engine 12 and the second heat exchanger 24′. The second heat exchanger 24′ further has one or more fuel conduit(s) 67b in heat exchange relationship with the lubricant conduit(s) 70a. The one or more fuel conduit(s) 67b is fluidly connected to the aircraft fuel circuit 67.
Accordingly, the aircraft fuel can be used to cool down both the liquid coolant and the lubricant of the internal combustion engine(s) 12 of the APU. It is noted that the fuel flow through the fuel conduits 67a, 67b of the heat exchangers 24′ 24′ can be in series or in parallel. Also, the heat exchangers 24, 24′ can be separate or integrated in a same heat exchanger body/housing.
Embodiments disclosed herein include:
A: A method of cooling an auxiliary power unit (APU) of an aircraft, the method comprising: circulating a liquid coolant through a liquid-cooled engine of the APU, and circulating aircraft fuel in heat exchange relationship with the liquid coolant.
B: A method of cooling an auxiliary power unit (APU) of an aircraft having prime mover engines fed with aircraft fuel, the method comprising: using the aircraft fuel of the prime mover engines as a heat sink to absorb heat from a main source of heat of the APU.
Each of the embodiments, A and B, may have one or more of the following additional elements in any combination. Element 1: circulating the aircraft fuel and the liquid coolant through a liquid-to-liquid heat exchanger. Element 2: further comprising circulating a lubricant of the liquid-cooled engine in heat exchange relationship with the aircraft fuel. Element 3: pumping the aircraft fuel from fuel tanks fluidly connected to prime mover engines of the aircraft. Element 4: circulating the liquid coolant and the aircraft fuel while the aircraft is on the ground. Element 5: wherein the liquid-cooled engine comprises an internal combustion engine having a rotor mounted for eccentric revolution within an internal cavity of a housing, and wherein circulating the liquid coolant comprises circulating the liquid coolant through coolant passages extending through the housing.
According to at least some embodiments, the advantages in weight of using the aircraft fuel as a heat sink for the APU would be noticeable as there is no need for a blower anymore to drive a flow of cooling air through an air cooler, and the inlet and exhaust airflow in the aircraft tail cone can potentially be reduced by about half, reducing the size of doors, ducting, etc. Also, liquid-to-liquid heat exchangers can be more compact than the traditionally used air-to-liquid coolers. Furthermore, with fuel cooling instead of air cooling, there would be no concern of cooling drag should the APU be used in flight. The APU SFC could also be improved as there would no longer be a need for blower power to drive a flow of cooling air through a radiator-type cooler. Maintenance would also be simplified.
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the liquid-cooled engine of the APU could comprise an electric motor/generator and a battery pack. The aircraft fuel could be used to absorb the heat generated by the electric motor and the batteries. Also, the aircraft fuel could also be used to dissipate the heat generated by the generator 19. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
