In high-altitude piston aircraft, what type of forced induction (boost) is used? Turbocharger or supercharger?
You can have either (turbo- or super-) or both (superturbo-), there is also the turbo-compound engine.
Most WWII aircraft engines had mechanically driven superchargers, and the turbosuperchargers (with intercoolers) were added to achieve desired altitude performance.
Modern aviation piston engines designed to run at high altitudes typically include a turbocharger rather than a supercharger. The change in thinking is largely due to economics. Aviation gasoline was once plentiful and cheap, favoring the simple, but fuel-hungry supercharger. As the cost of fuel has increased, the supercharger has fallen out of favor.
— Wikipedia [edited for context; citations missing from main article]
During WWII, two-speed two-stage intercooled superturbocharged engines were common.
(Frames from YouTube)
Compressed air via the exhaust-driven-turbine is channeled to a geared mechanically-driven supercharger. (The jet-engine must have had a laugh.)
You get the super part, which is a mechanically-driven compressor boost.
The turbo part, which is a turbine-driven compressor boost.
Intercooler for cooling the air.
The two speeds and stages are for low- and high-altitude operations. Otherwise the engine could blow at low-altitude.
- Above is an example of having both, however as noted in the comments, superturbocharger as a term is nowadays used to describe a turbocharger.
Here's a declassified training film—from which the above frames were taken: Turbosupercharger: Master of the Skies (1943)
Initially, all supercharging was done with, well, superchargers. Read NACA Technical Note 48 for a nice summary of German work on superchargers during WW I which reached the point of almost being ready for operational use when the war ended.
All of these projects failed in the sense that the war ended before the technology could yield military advantage. While we have nothing by way of the detailed reporting on German wartime research accorded to British and American efforts, it appears that Germany came closest to producing combat-ready supercharged engines. Certainly they had a denser supplier network than the Allies, more consistent research goals, and accumulated far more hours of test flying.
Turbochargers were investigated soon after but were used much less frequently for two reasons:
- Due to the higher inertia a turbocharger reacts only slowly to speed changes, which limits the pilot's options especially in fighters.
- To avoid overstressing the engine, turbochargers need a wastegate. Before automatic wastegates were developed, the risk of manual mishandling of the wastegate setting was all too real.
Turbochargers were first used in production aircraft engines such as the Napier Lioness in the 1920s, although they were less common than engine-driven centrifugal superchargers.
Only when bombers and airliners needed to maximise their range did the advantage of turbochargers, their higher efficiency, take effect. In many cases a supercharged engine had a turbocharger added because the high energy content of the exhaust gases offered easy gains in efficiency. Note that most early examples of turbocharged aircraft were types in which the engines are run at a constant power setting over many hours. What also helped was the development of jet engines which spurred both the research into high-temperature alloys and turbine aerodynamics. This was a mutually reinforcing development, because the first turbojet designs were based on then-existing turbochargers.
With automatic wastegates, little requirement to frequently adjust engine power, much experience in high-temperature materials and high fuel prices, today the turbocharger is the obvious choice for boosting engine pressure. The most recent piston-powered high altitude platforms, the Boeing Condor and the Grob Strato 2C, used turbochargers. In case of the Condor this was a two-stage turbocharger with intercooling, however, flight test revealed synchronisation problems between both stages. The Strato 2C had two turbochargers and one supercharger, all with intercooling.