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An auxiliary power unit for an aircraft includes a rotary intermittent internal combustion engine drivingly engaged to an engine shaft, a turbine section having an inlet in fluid communication with an outlet of the engine(s), the turbine section including at least one turbine compounded with the eng

An auxiliary power unit for an aircraft includes a rotary intermittent internal combustion engine drivingly engaged to an engine shaft, a turbine section having an inlet in fluid communication with an outlet of the engine(s), the turbine section including at least one turbine compounded with the engine shaft, and a compressor having an inlet in fluid communication with an environment of the aircraft and an outlet in fluid communication with a bleed duct for providing bleed air to the aircraft, the compressor having a compressor rotor connected to a compressor shaft, the compressor shaft drivingly engaged to the engine shaft. The driving engagement between the compressor shaft and the engine shaft is configurable to provide at least two alternate speed ratios between the compressor shaft and the engine shaft.

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1. An auxiliary power unit for an aircraft, comprising: a rotary intermittent internal combustion engine drivingly engaged to an engine shaft;a turbine section having an inlet in fluid communication with an outlet of the rotary intermittent internal combustion engine, the turbine section including a

1. An auxiliary power unit for an aircraft, comprising: a rotary intermittent internal combustion engine drivingly engaged to an engine shaft;a turbine section having an inlet in fluid communication with an outlet of the rotary intermittent internal combustion engine, the turbine section including at least one turbine compounded with the engine shaft; anda compressor having an inlet in fluid communication with an environment of the aircraft and an outlet in fluid communication with a bleed duct for providing bleed air to the aircraft, the compressor having a compressor rotor connected to a compressor shaft, the compressor shaft drivingly engaged to the engine shaft, the driving engagement between the compressor shaft and the engine shaft configurable to provide at least two alternate speed ratios between the compressor shaft and the engine shaft, wherein the compressor rotor connected to the compressor shaft defines the compressor on one side and an additional compressor on an opposed side, the additional compressor having an inlet in fluid communication with the environment of the aircraft and an outlet in fluid communication with an inlet of the rotary intermittent internal combustion engine. 2. An auxiliary power unit for an aircraft, comprising: a rotary intermittent internal combustion engine drivingly engaged to an engine shaft;a turbine section having an inlet in fluid communication with an outlet of the rotary intermittent internal combustion engine, the turbine section including at least one turbine compounded with the engine shaft; anda compressor having an inlet in fluid communication with an environment of the aircraft and an outlet in fluid communication with a bleed duct for providing bleed air to the aircraft, the compressor having a compressor rotor connected to a compressor shaft, the compressor shaft drivingly engaged to the engine shaft, the driving engagement between the compressor shaft and the engine shaft configurable to provide at least two alternate speed ratios between the compressor shaft and the engine shaft, wherein the compressor shaft is drivingly engaged to the engine shaft through a transmission defining the at least two different alternate speed ratios between the compressor shaft and the engine shaft, the at least two different alternate speed ratios being discrete speed ratios. 3. An auxiliary power unit for an aircraft, comprising: a plenum in fluid communication with an environment of the aircraft through a main inlet;a rotary intermittent internal combustion engine drivingly engaged to an engine shaft;a turbine having an inlet in fluid communication with an outlet of the rotary intermittent internal combustion engine, the turbine compounded with the rotary intermittent internal combustion engine;a first compressor having an inlet in fluid communication with the plenum and an outlet in fluid communication with an inlet of the rotary intermittent internal combustion engine;a second compressor having an inlet in fluid communication with the plenum and an outlet in fluid communication with a bleed duct for providing bleed air to the aircraft; anda compressor shaft connected to a rotor of at least one of the first and second compressors, the compressor shaft drivingly engaged to the engine shaft, the driving engagement between the compressor shaft and the engine shaft configurable to provide at least two alternate speed ratios between the compressor shaft and the engine shaft, wherein the compressor shaft is connected to a first rotor of the first compressor and to a second rotor of the second compressor. 4. The auxiliary power unit as defined in claim 3, wherein the rotor connected to the compressor shaft defines the first compressor on one side and the second compressor on an opposed side. 5. The auxiliary power unit as defined in claim 3, wherein the compressor shaft is drivingly engaged to the turbine section. 6. The auxiliary power unit as defined in claim 3, wherein the compressor shaft is drivingly engaged to the engine shaft through a transmission defining the at least two different alternate speed ratios between the compressor shaft and the engine shaft, the at least two different alternate speed ratios being discrete speed ratios. 7. The auxiliary power unit as defined in claim 3, wherein the compressor shaft is drivingly engaged to the engine shaft through a continuously variable transmission. 8. The auxiliary power unit as defined in claim 3, wherein the compressor shaft is drivingly engaged to the engine shaft through an electrical link between an electric motor drivingly engaged to the compressor shaft and an electric generator drivingly engaged to the engine shaft. 9. The auxiliary power unit as defined in claim 3, wherein the turbine is a first stage turbine, the auxiliary power unit further comprising a second stage turbine having an inlet in fluid communication with an outlet of the first stage turbine, the first and second stage turbines having different reaction ratios from one another. 10. The auxiliary power unit as defined in claim 3, wherein the rotary intermittent internal combustion engine is a Wankel rotary engine including a rotor having three apex portions mounted for eccentric revolutions within an internal cavity defined in a housing, the internal cavity having an epitrochoid shape with two lobes. 11. The auxiliary power unit as defined in claim 3, wherein the inlet of at least one of the first and second compressors includes variable inlet guide vanes. 12. An auxiliary power unit for an aircraft, comprising: a rotary intermittent internal combustion engine drivingly engaged to an engine shaft;a turbine section having an inlet in fluid communication with an outlet of the rotary intermittent internal combustion engine, the turbine section including at least one turbine compounded with the engine shaft; anda compressor having an inlet in fluid communication with an environment of the aircraft and an outlet in fluid communication with a bleed duct for providing bleed air to the aircraft, the compressor having a compressor rotor connected to a compressor shaft, the compressor shaft drivingly engaged to the engine shaft, the driving engagement between the compressor shaft and the engine shaft configurable to provide at least two alternate speed ratios between the compressor shaft and the engine shaft, wherein the outlet of the compressor is also in fluid communication with an inlet of the rotary intermittent internal combustion engine, and wherein the compressor is a first compressor and the compressor shaft is a first compressor shaft, the auxiliary power unit further comprising:a second compressor having an inlet in fluid communication with the environment of the aircraft, an outlet in fluid communication with an inlet of the rotary intermittent internal combustion engine and with the environment of the aircraft, and an additional compressor rotor connected to a second compressor shaft, anda turbine shaft connected to a rotor of the turbine, the first and second compressor shafts being alternately engageable to the turbine shaft through a clutch. 13. The auxiliary power unit as defined in claim 12, wherein the first compressor shaft is drivingly engaged to the turbine section. 14. The auxiliary power unit as defined in claim 12, wherein the first compressor shaft and/or the second compressor shaft is/are drivingly engaged to the engine shaft through a transmission defining the at least two different alternate speed ratios, the at least two different alternate speed ratios being discrete speed ratios. 15. The auxiliary power unit as defined in claim 12, wherein at least one of the first compressor shaft and the second compressor shaft is drivingly engaged to the engine shaft through a continuously variable transmission. 16. The auxiliary power unit as defined in claim 12, wherein at least one of the first compressor shaft and the second compressor shaft is drivingly engaged to the engine shaft through an electrical link between an electric motor drivingly engaged to the at least one of the first compressor shaft and the second compressor shaft and an electric generator drivingly engaged to the engine shaft. 17. The auxiliary power unit as defined in claim 12, wherein the turbine is a first stage turbine, the auxiliary power unit further comprising a second stage turbine having an inlet in fluid communication with an outlet of the first stage turbine, the first and second stage turbines having different reaction ratios from one another. 18. The auxiliary power unit as defined in claim 12, wherein the rotary intermittent internal combustion engine is a Wankel rotary engine including a rotor having three apex portions mounted for eccentric revolutions within an internal cavity defined in a housing, the internal cavity having an epitrochoid shape with two lobes. 19. The auxiliary power unit as defined in claim 12, wherein the inlet of the compressor includes variable inlet guide vanes. 20. An auxiliary power unit for an aircraft, comprising: a plenum in fluid communication with an environment of the aircraft through a main inlet;a rotary intermittent internal combustion engine drivingly engaged to an engine shaft;a turbine having an inlet in fluid communication with an outlet of the rotary intermittent internal combustion engine, the turbine compounded with the rotary intermittent internal combustion engine;a first compressor having an inlet in fluid communication with the plenum and an outlet in fluid communication with an inlet of the rotary intermittent internal combustion engine;a second compressor having an inlet in fluid communication with the plenum and an outlet in fluid communication with a bleed duct for providing bleed air to the aircraft; anda compressor shaft connected to a rotor of at least one of the first and second compressors, the compressor shaft drivingly engaged to the engine shaft, the driving engagement between the compressor shaft and the engine shaft configurable to provide at least two alternate speed ratios between the compressor shaft and the engine shaft, wherein the outlet of the first compressor is also in fluid communication with the bleed duct and the outlet of the second compressor is also in fluid communication with the inlet of the rotary intermittent internal combustion engine, the compressor shaft is a first compressor shaft and is connected to the rotor of the first compressor, the auxiliary power unit further comprising:a second compressor shaft connected to a rotor of the second compressor, anda turbine shaft connected to a rotor of the turbine, the first and second compressor shafts being alternately engageable to the turbine shaft through a clutch.