History two-stroke engine used as the pioneering example of an APU, to turn over the central shaft of both World War II-era German
BMW 003 and
Junkers Jumo 004 jet engines (pullcord starter variant shown) During
World War I, the British
Coastal class blimps, one of several types of airship operated by the
Royal Navy, carried a
ABC auxiliary engine. These powered a generator for the craft's
radio transmitter and, in an emergency, could power an auxiliary air blower. One of the first military fixed-wing aircraft to use an APU was the British, World War 1,
Supermarine Nighthawk, an anti-Zeppelin
night fighter. During
World War II, a number of large American military aircraft were fitted with APUs. These were typically known as
putt–putts, even in official training documents. The putt-putt on the
B-29 Superfortress bomber was fitted in the unpressurised section at the rear of the aircraft. Various models of four-stroke
flat-twin or
V-twin engines were used. The engine drove a
P2, DC generator, rated 28.5volts and 200amps (several of the same
P2 generators, driven by
the main engines, were the B-29's DC power source in flight). The putt-putt provided power for starting the main engines and was used after take-off to a height of . The putt-putt was restarted when the
B-29 was descending to land. Some models of the
B-24 Liberator had a putt–putt fitted at the front of the aircraft, inside the nose-wheel compartment. Some models of the
Douglas C-47 Skytrain transport aircraft carried a putt-putt under the cockpit floor.
As mechanical "startup" APUs for jet engines The first German
jet engines built during the
Second World War used a mechanical APU starting system designed by the German engineer
Norbert Riedel. It consisted of a
two-stroke flat engine, which for the
Junkers Jumo 004 design was hidden in the engine nose cone, essentially functioning as a pioneering example of an auxiliary power unit for starting a jet engine. A hole in the extreme nose of the cone contained a manual pull-handle which started the piston engine, which in turn rotated the compressor. Two spark plug access ports existed in the Jumo 004's nose cone to service the Riedel unit's cylinders in situ, for maintenance purposes. Two small "premix" tanks for the Riedel's petrol/
oil fuel were fitted in the annular intake. The engine was considered an extreme short stroke (bore / stroke: 70 mm / 35 mm = 2:1) design so it could fit within the in the nose cone of jet engines like the Jumo 004. For reduction it had an integrated
planetary gear. It was produced by
Victoria in
Nuremberg and served as a mechanical APU-style starter for all three German jet engine designs to have made it to at least the prototype stage before May 1945 – the
Junkers Jumo 004, the
BMW 003 (which uniquely appears to use an electric starter for the Riedel APU), and the prototypes (19 built) of the more advanced
Heinkel HeS 011 engine, which mounted it just above the intake passage in the Heinkel-crafted sheetmetal of the engine nacelle nose. The
Boeing 727 in 1963 was the first jetliner to feature a
gas turbine APU, allowing it to operate at smaller airports, independent from ground facilities. The APU can be identified on many modern airliners by an exhaust pipe at the aircraft's tail.
Sections A typical gas-turbine APU for commercial transport aircraft comprises three main sections:
Power section The power section is the gas-generator portion of the engine and produces all the shaft power for the APU. In this section of the engine, air and fuel are mixed, compressed and ignited to create hot and expanding gases. This gas is highly energetic and is used to spin the turbine, which in turn powers other sections of the engine, such as auxiliary gearboxes, pumps, electrical generators, and in the case of a turbo fan engine, the main fan.
Load compressor section The load compressor is generally a shaft-mounted compressor that provides pneumatic power for the aircraft, though some APUs extract
bleed air from the power section compressor. There are two actuated devices to help control the flow of air: the inlet guide vanes that regulate airflow to the load compressor and the surge control valve that maintains stable or surge-free operation of the turbo machine. Onboard solid oxide fuel cell (
SOFC) APUs are being researched.
Manufacturers The market of Auxiliary power units is dominated by
Honeywell, followed by
Pratt & Whitney (a subsidiary of
RTX Corporation),
Motorsich and other manufacturers such as
PBS Velká Bíteš,
Safran Power Units,
Aerosila and
Klimov. Local manufacturers include
Bet Shemesh Engines and
Hanwha Aerospace. The 2018 market share varied according to the application platforms: • Large commercial aircraft: Honeywell 70–80%, Pratt & Whitney 20–30%, others 0–5% • Regional aircraft: Pratt & Whitney 50–60%, Honeywell 40–50%, others 0–5% • Business jets: Honeywell 90–100%, others 0–5% • Helicopters: Pratt & Whitney 40–50%, Motorsich 40–50%, Honeywell 5–10%, Safran Power Units 5–10%, others 0–5% On June 4, 2018,
Boeing and
Safran announced their 50–50 partnership to design, build and service APUs after regulatory and
antitrust clearance in the second half of 2018. Boeing produced several hundred
T50/
T60 small
turboshafts and their derivatives in the early 1960s.