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Turbocharging[edit]
Main article:
Turbocharger
A mechanically driven supercharger has to take its drive power from the engine. Taking a single-stage single-speed supercharged engine, such as an early
Rolls-Royce Merlin, for instance, the supercharger uses up about 150
hp (110
kW). Without a supercharger, the engine could produce about 750
horsepower (560
kilowatts), but with a supercharger, it produces about 1,000 hp (750 kW)—an increase of about 400 hp (750 - 150 + 400 = 1000 hp), or a net gain of 250 hp (190 kW). This is where the principal disadvantage of a supercharger becomes apparent. The engine has to burn extra fuel to provide power to drive the supercharger. The increased air density during the input cycle increases the
specific power of the engine and its
power-to-weight ratio, but at the cost of an increase in the
specific fuel consumption of the engine. In addition to increasing the cost of running the
aircraft a supercharger has the potential to reduce its overall range for a specific fuel load.
As opposed to a supercharger driven by the engine itself, a
turbocharger is driven using the otherwise wasted exhaust gas from the engine. The amount of power in the gas is proportional to the difference between the exhaust pressure and air pressure, and this difference increases with altitude, helping a turbocharged engine to compensate for changing altitude. This increases the height at which maximum power output of the engine is attained compared to supercharger boosting, and allows better fuel consumption at high altitude compared to an equivalent supercharged engine. This facilitates increased
true airspeed at high altitude and gives a greater operational range than an equivalently boosted engine using a supercharger.
The majority of aircraft engines used during
World War II used mechanically driven superchargers, because they had some significant manufacturing advantages over turbochargers. However, the benefit to operational range was given a much higher priority to American aircraft because of a less predictable requirement on operational range, and having to travel far from their home bases. Consequently, turbochargers were mainly employed in American aircraft engines such as the
Allison V-1710 and the
Pratt & Whitney R-2800, which were comparably heavier when turbocharged, and required additional ducting of expensive high-temperature
metal alloys in the
gas turbine and preturbine section of the exhaust system. The size of the ducting alone was a serious design consideration. For example, both the
F4U Corsair and the
P-47 Thunderbolt used the same
radial engine, but the large barrel-shaped fuselage of the turbocharged P-47 was needed because of the amount of ducting to and from the turbocharger in the rear of the aircraft. The F4U used a two-stage intercooled supercharger with more compact layout. Nonetheless, turbochargers were useful in high-altitude
bombers and some fighter aircraft due to the increased high altitude performance and range.
Turbocharged piston engines are also subject to many of the same operating restrictions as those of gas turbine engines. Turbocharged engines also require frequent inspections of their turbochargers and exhaust systems to search for possible damage caused by the extreme heat and pressure of the turbochargers. Such damage was a prominent problem in the early models of the American
Boeing B-29 Superfortresshigh-altitude
bombers used in the
Pacific Theater of Operations during 1944–45.
Turbocharged piston engines continued to be used in a large number of postwar airplanes, such as the
B-50 Superfortress, the
KC-97 Stratofreighter, the
Boeing Stratoliner, the
Lockheed Constellation, and the
C-124 Globemaster II.
In more recent times most aircraft engines for
general aviation (light airplanes) are
naturally aspirated, but the smaller number of modern aviation piston engines designed to run at high altitudes use turbocharger or turbo-normalizer systems, instead of a supercharger driven from the crank shafts. 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 ordinary supercharger has fallen out of favor. Also, depending on what
monetary inflation factor one uses, fuel costs have not decreased as fast as production and maintenance costs have.
Effects of fuel octane rating[edit]
Until the late 1920s all automobile and aviation fuel was generally rated at 87
octane or less. This is the rating that was achieved by the simple distillation of "light crude" oil. Engines from around the world were designed to work with this grade of fuel, which set a limit to the amount of boosting that could be provided by the supercharger, while maintaining a reasonable compression ratio.
Octane rating boosting through additives was a line of research being explored at the time. Using these techniques, less valuable crude could still supply large amounts of useful gasoline, which made it a valuable
economic process. However, the additives were not limited to making poor-quality oil into 87-octane gasoline; the same additives could also be used to boost the gasoline to much higher octane ratings.
Higher-octane fuel resists
auto ignition and
detonation better than does low-octane fuel. As a result, the amount of boost supplied by the superchargers could be increased, resulting in an increase in engine output. The development of 100-octane aviation fuel, pioneered in the USA before the war, enabled the use of higher boost pressures to be used on high-performance aviation engines, and was used to develop extremely high-power outputs – for short periods – in several of the pre-war speed record airplanes. Operational use of the new fuel during World War II began in early 1940 when 100-octane fuel was delivered to the British
Royal Air Force from refineries in America and the East Indies.
[19] The German
Luftwaffe also had supplies of a similar fuel.
[20][21]
Increasing the knocking limits of existing aviation fuels became a major focus of aero engine development during World War II. By the end of the war, fuel was being delivered at a nominal 150-octane rating, on which late-war aero engines like the
Rolls-Royce Merlin 66
[22][23] or the
Daimler-Benz DB 605DC developed as much as 2,000 hp (1,500 kW).
[24][25]
The last part about octane rating is why the manual recommends 93, and why you won't get peak power without it.