Supercharger
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For other uses, see Supercharger (disambiguation).
Roots type supercharger on AMC V8 enginefor dragstrip racing
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A supercharger is an air compressor used for increasing the pressure, temperature, and density of
air supplied to an internal combustion engine. This compressed air supplies a greater mass of oxygen per
cycle of the engine to support combustion than available to a naturally aspirated engine, enabling for
more fuel to be burned and more work to be done per cycle, thus allowing to increase the power produced
by the engine.
Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or chain
connected to the engine's crankshaft. When power is provided by a turbine powered by exhaust gas, a
supercharger is known as a turbosupercharger[1] – typically referred to simply as aturbocharger or
just turbo. Common usage restricts the term supercharger to mechanically driven units.
Contents
[hide]
1 History
2 Types of supercharger
o 2.1 Positive displacement
2.1.1 Compression type
2.1.2 Capacity rating
o 2.2 Dynamic
o 2.3 Supercharger drive types
2.3.1 Mechanical
2.3.2 Exhaust gas turbines
2.3.3 Other
o 2.4 Temperature effects and intercoolers
3 Two-stroke engines
4 Automobiles
o 4.1 Supercharging versus turbocharging
4.1.1 Twincharging
5 Aircraft
o 5.1 Altitude effects
o 5.2 Effects of temperature
o 5.3 Two-stage and two-speed superchargers
o 5.4 Turbocharging
o 5.5 Effects of fuel octane rating
6 See also
7 Notes
8 References
9 External links
History[edit source | editbeta]
In 1860, brothers Philander and Francis Marion Roots, founders of Roots Blower Company of Connersville,
Indiana, patented the design for an air mover, for use in blast furnaces and other industrial applications.
The world's first functional, actually tested[2] engine supercharger was made by Dugald Clerk, who used it
for the first[3] two-stroke engine in 1878. Gottlieb Daimler received a German patent for supercharging an
internal combustion engine in 1885. Louis Renault patented a centrifugal supercharger in France in 1902.
An early supercharged race car was built by Lee Chadwick of Pottstown, Pennsylvania in 1908 which
reportedly reached a speed of 100 mph (160 km/h).
The world's first series-produced cars[4] with superchargers were Mercedes 6/25/40 hp and Mercedes
10/40/65 hp. Both models were introduced in 1921 and had Roots superchargers. They were distinguished
as "Kompressor" models, the origin of the Mercedes-Benz badging which continues today.
On March 24, 1878 Heinrich Krigar of Germany obtained patent #4121, patenting the first ever screw-type
compressor. Later that same year on August 16 he obtained patent #7116 after modifying and improving
his original designs. His designs show a two-lobe rotor assembly with each rotor having the same shape as
the other. Although the design resembled the roots style compressor, the "screws" were clearly shown with
180 degrees of twist along their length. Unfortunately, the technology of the time was not sufficient to
produce such a unit, and Heinrich made no further progress with the screw compressor. Nearly half a
century later, in 1935, Alf Lysholm, who was working for Ljungstroms Angturbin AB (later known as
Svenska Rotor Maskiner AB or SRM in 1951), patented a design with five female and four male rotors. He
also patented the method for machining the compressor rotors.
Types of supercharger[edit source | editbeta]
There are two main types of superchargers defined according to the method of gas transfer: positive
displacement and dynamic compressors. Positive displacement blowers and compressors deliver an
almost constant level of pressure increase at all engine speeds (RPM). Dynamic compressors do not
deliver any pressure at all at low speeds, past the threshold speed then increasing pressure with increasing
speed.[5]
Positive displacement[edit source | editbeta]
An Eaton MP62 Roots-type supercharger is visible at the front of this Ecotec LSJ engine in a 2006 Saturn Ion Red Line.
Lysholm screw rotors with complex shape of each rotor, which must run at high speed and with close tolerances. This
makes this type of supercharger expensive. (This unit has been blued to show close contact areas.)
Positive-displacement pumps deliver a nearly fixed volume of air per revolution at all speeds (minus
leakage, which is almost constant at all speeds for a given pressure, thus its importance decreases at
higher speeds).
Major types of positive-displacement pumps include:
Roots
Lysholm twin-screw
Sliding vane
Scroll-type supercharger, also known as the G-Lader
Compression type[edit source | editbeta]
This section does not cite any references or sources. Please help improve this
section by adding citations to reliable sources. Unsourced material may be
challenged and removed. (May 2010)
Positive-displacement pumps are further divided into internal and external compression types.
Roots superchargers are external compression only (although high-helix roots blowers attempt to emulate
the internal compression of the Lysholm screw).
External compression refers to pumps that transfer air at ambient pressure into the engine. If the
engine is running under boost conditions, the pressure in the intake manifold is higher than that
coming from the supercharger. That causes a backflow from the engine into the supercharger until the
two reach equilibrium. It is the backflow that actually compresses the incoming gas. This is an
inefficient process and the main factor in the lack of efficiency of Roots superchargers when used at
high boost levels. The lower the boost level the smaller is this loss, and Roots blowers are very
efficient at moving air at low pressure differentials, which is what they were invented for (hence the
original term "blower").
All the other types have some degree of internal compression.
Internal compression refers to the compression of air within the supercharger itself, which, already at
or close to boost level, can be delivered smoothly to the engine with little or no back flow. This is more
effective than back flow compression and allows higher efficiency to be achieved. Internal compression
devices usually use a fixed internal compression ratio. When the boost pressure is equal to the
compression pressure of the supercharger, the back flow is zero. If the boost pressure exceeds that
compression pressure, back flow can still occur as in a roots blower. Internal compression blowers
must be matched to the expected boost pressure in order to achieve the higher efficiency they are
capable of, otherwise they will suffer the same problems and low efficiency of the roots blowers.
Capacity rating[edit source | editbeta]
Positive-displacement superchargers are usually rated by their capacity per revolution. In the case of the
Roots blower, the GMC rating pattern is typical. The GMC types are rated according to how many two-
stroke cylinders, and the size of those cylinders, it is designed to scavenge. GMC has made 2–71, 3–71,
4–71, and the famed 6–71 blowers. For example, a 6–71 blower is designed to scavenge six cylinders of
71 cubic inches (1,163 cc) each and would be used on a two-stroke diesel of 426 cubic inches (6,981 cc),
which is designated a 6–71; the blower takes this same designation. However, because 6–71 is actually
the engine's designation, the actual displacement is less than the simple multiplication would suggest. A 6–
71 actually pumps 339 cubic inches (5,555 cc) per revolution (but as it spins faster than the engine, it can
easily put out the same displacement as the engine per engine rev).
Aftermarket derivatives continue the trend with 8–71 to current 16–71 blowers used in different motor
sports. From this, one can see that a 6–71 is roughly twice the size of a 3–71. GMC also made 53 cubic
inches (869 cc) series in 2-, 3-, 4-, 6-, and 8–53 sizes, as well as a “V71” series for use on engines using a
V configuration.
Dynamic[edit source | editbeta]
Dynamic compressors rely on accelerating the air to high speed and then exchanging that velocity for
pressure by diffusing or slowing it down.
Major types of dynamic compressor are:
Centrifugal
Multi-stage axial-flow
Pressure wave supercharger
Supercharger drive types[edit source | editbeta]
Superchargers are further defined according to their method of drive (mechanical—or turbine).
Mechanical[edit source | editbeta]
Belt (V-belt, Synchronous belt, Flat belt)
Direct drive
Gear drive
Chain drive
Exhaust gas turbines[edit source | editbeta]
Axial turbine
Radial turbine
Other[edit source | editbeta]
Electric motor
Auxiliary Power Unit in some large industrial applications.
All types of compressor may be mated to and driven by either gas turbine or mechanical linkage. Dynamic
compressors are most often matched with gas turbine drives due to their similar high-speed characteristics,
whereas positive displacement pumps usually use one of the mechanical drives. However, all of the
possible combinations have been tried with various levels of success. In principle, a positive displacement
engine could be used in place of an exhaust turbine to improve low speed performance. Electric
superchargers are all essentially fans (axial pumps). A form ofregenerative braking has been tried where
the car is slowed by compressing air for future acceleration.
Temperature effects and intercoolers[edit source | editbeta]
Supercharger CDT vs. Ambient Temperature. Graph shows how a supercharger's CDT varies with air temperature
and altitude (absolute pressure).
One disadvantage of supercharging is that compressing the air increases its temperature. When a
supercharger is used on an internal combustion engine, the temperature of the fuel/air charge becomes a
major limiting factor in engine performance. Extreme temperatures will cause detonation of the fuel-air
mixture (spark ignition engines) and damage to the engine. In cars, this can cause a problem when it is a
hot day outside, or when an excessive level of boost is reached.
It is possible to estimate the temperature rise across a supercharger by modeling it as an isentropic
process.
Where:
= ambient air temperature
= temperature after the compressor
= ambient atmospheric pressure (absolute)
= pressure after the compressor (absolute)
= Ratio of specific heat capacities = = 1.4 for air
= Specific heat at constant pressure
= Specific heat at constant volume
For example, if a supercharged engine is pushing 10 psi (0.69 bar) of boost at sea level (ambient
pressure of 14.7 psi (1.01 bar), ambient temperature of 75 °F (24 °C)), the temperature of the air
after the supercharger will be 160.5 °F (71.4 °C). This temperature is known as the compressor
discharge temperature (CDT) and highlights why a method for cooling the air after the compressor
is so important.
In addition to causing possible detonation and damage, hot intake air decreases power in at least
one way. At a given pressure, the hotter the air the lower its density, so the mass of intake air is
decreased, reducing the efficiency and boost level of the supercharger.
Two-stroke engines[edit source | editbeta]
Two-stroke engines do not have an induction stroke drawing air into the cylinder and so called
scavenging pumping is required to fill the cylinder with combustion air and purge exhaust gasses.
In small engines this is commonly achieved by using the crankcase as a blower, the descending
piston during the power stroke compresses air in the crankcase used to purge the cylinder.
Larger engines usually use a blower for scavenging and it was for this type of operation that the
Roots blower was developed. Scavenging blowing should not be confused with supercharging, no
charge compression takes place.
Simple two-stroke engines with ported inlet and exhaust cannot be supercharged since the inlet
port always closes first. For this reason, two-stroke Diesel engines usually have mechanical
exhaust valves with separate timing to allow supercharging. Regardless of this, two-stroke
engines require scavenging at all engine speeds and so turbocharged two-stroke engines must
still always also have a blower, usually Roots type. This blower may be mechanically or
electrically driven, in either case the blower may be disengaged once the turbocharger starts to
deliver air.
Automobiles[edit source | editbeta]
1929 "Blower" Bentley. The large "blower" (supercharger), located in front of the radiator, gave the car its
name.
In 1900, Gottlieb Daimler, of Daimler-Benz (Daimler AG), was the first to patent a forced-induction
system for internal combustion engines, superchargers based on the twin-rotor air-pump design,
first patented by the American Francis Roots in 1860, the basic design for the modernRoots type
supercharger.
The first supercharged cars were introduced at the 1921 Berlin Motor Show: the 6/20 hp and
10/35 hp Mercedes. These cars went into production in 1923 as the 6/25/40 hp (regarded as the
first supercharged road car[6]) and 10/40/65 hp.[7] These were normal road cars as other
supercharged cars at same time were almost all racing cars, including the 1923 Fiat 805-405,
1923 Miller 122[8] 1924 Alfa Romeo P2, 1924 Sunbeam,[9] 1925Delage,[10] and the 1926 Bugatti
Type 35C. At the end of the 1920s, Bentley made a supercharged version of the Bentley 4½
Litre road car. Since then, superchargers (and turbochargers) have been widely applied to racing
and production cars, although the supercharger's technological complexity and cost have largely
limited it to expensive, high-performance cars.
Supercharging versus turbocharging[edit source | editbeta]
A G-Lader scroll-type supercharger on aVolkswagen Golf Mk1.
Keeping the air that enters the engine cool is an important part of the design of both
superchargers and turbochargers. Compressing air increases its temperature, so it is common to
use a small radiator called an intercooler between the pump and the engine to reduce the
temperature of the air.
There are three main categories of superchargers for automotive use:
Centrifugal turbochargers – driven from exhaust gases.
Centrifugal superchargers – driven directly by the engine via a belt-drive.
Positive displacement pumps – such as the Roots, Twin Screw (Lysholm), and TVS (Eaton)
blowers.
Roots blowers tend to be only 40–50% efficient at high boost levels, by contrast centrifugal
(dynamic) superchargers are 70–85% efficient at high boost. Lysholm-style blowers can be nearly
as efficient as their centrifugal counterparts over a narrow range of load/speed/boost, for which
the system must be specifically designed.
Mechanically driven superchargers may absorb as much as a third of the total crankshaft power of
the engine and are less efficient than turbochargers. However, in applications for which engine
response and power are more important than other considerations, such as top-fuel dragsters and
vehicles used in tractor pulling competitions, mechanically driven superchargers are very
common.
The thermal efficiency, or fraction of the fuel/air energy that is converted to output power, is less
with a mechanically driven supercharger than with a turbocharger, because turbochargers use
energy from the exhaust gas that would normally be wasted. For this reason, both economy and
the power of a turbocharged engine are usually better than with superchargers.
Turbochargers suffer (to a greater or lesser extent) from so-called turbo-spool (turbo lag; more
correctly, boost lag), in which initial acceleration from low RPM is limited by the lack of sufficient
exhaust gas mass flow (pressure). Once engine RPM is sufficient to start the turbine spinning,
there is a rapid increase in power, as higher turbo boost causes more exhaust gas production,
which spins the turbo yet faster, leading to a belated "surge" of acceleration. This makes the
maintenance of smoothly increasing RPM far harder with turbochargers than with engine-driven
superchargers, which apply boost in direct proportion to the engine RPM. The main advantage of
an engine with a mechanically driven supercharger is better throttle response, as well as the
ability to reach full-boost pressure instantaneously. With the latest turbocharging technology and
direct gasoline injection, throttle response on turbocharged cars is nearly as good as with
mechanically powered superchargers, but the existing lag time is still considered a major
drawback, especially considering that the vast majority of mechanically driven superchargers are
now driven off clutched pulleys, much like an air compressor.
Turbocharging has been more popular than superchargers among auto manufacturers owing to
better power and efficiency. For instance Mercedes-Benz and Mercedes-AMG previously had
supercharged "Kompressor" offerings in the early 2000s such as the C230K, C32 AMG, and S55
AMG, but they have abandoned that technology in favor of turbocharged engines released around
2010 such as the C250 and S65 AMG biturbo. However, Audi did introduce its 3.0 TFSI
supercharged V6 in 2009 for its A6, S4, and Q7, while Jaguar has its supercharged V8 engine
available as a performance option in the XJ, XF, XKR, and F-Type.
Twincharging[edit source | editbeta]
In the 1985 and 1986 World Rally Championships, Lancia ran the Delta S4, which incorporated
both a belt-driven supercharger and exhaust-driven turbocharger. The design used a complex
series of bypass valves in the induction and exhaust systems as well as an electromagnetic clutch
so that, at low engine speeds, boost was derived from the supercharger. In the middle of the rev
range, boost was derived from both systems, while at the highest revs the system disconnected
drive from the supercharger and isolated the associated ducting. [11] This was done in an attempt to
exploit the advantages of each of the charging systems while removing the disadvantages. In turn,
this approach brought greater complexity and impacted on the cars reliability in WRC events, as
well as increasing the weight of engine ancillaries in the finished design.
The Volkswagen TSI engine (or Twincharger) is a 1.4-litre direct-injection motor that also uses
both a supercharger and turbocharger.
Aircraft[edit source | editbeta]
Altitude effects[edit source | editbeta]
The Rolls-Royce Merlin, a supercharged aircraft engine from World War II
A Centrifugal supercharger of a Bristol Centaurus radial aircraft engine.
Superchargers are a natural addition to aircraft piston engines that are intended for operation at
high altitudes. As an aircraft climbs to higher altitude, air pressure and air density decreases. The
output of a piston engine drops because of the reduction in the mass of air that can be drawn into
the engine. For example, the air density at 30,000 ft (9,100 m) is 1⁄3 of that at sea level, thus
only 1⁄3 of the amount of air can be drawn into the cylinder, with enough oxygen to provide efficient
combustion for only a third as much fuel. So, at 30,000 ft (9,100 m), only 1⁄3 of the fuel burnt at sea
level can be burnt.[12] (An advantage of the decreased air density is that the airframe experiences
only about 1/3 of the aerodynamic drag. Plus, there is decreased back pressure on the exhaust
gases.[13] On the other hand, more energy is consumed holding an airplane up with less air in
which to generate lift.)
A supercharger can be thought of either as artificially increasing the density of the air by
compressing it or as forcing more air than normal into the cylinder every time the piston
moves down.[12]
A supercharger compresses the air back to sea-level-equivalent pressures, or even much higher,
in order to make the engine produce just as much power at cruise altitude as it does at sea level.
With the reduced aerodynamic drag at high altitude and the engine still producing rated power, a
supercharged airplane can fly much faster at altitude than a naturally aspirated one. The pilot
controls the output of the supercharger with the throttle and indirectly via the propeller governor
control. Since the size of the supercharger is chosen to produce a given amount of pressure at
high altitude, the supercharger is over-sized for low altitude. The pilot must be careful with the
throttle and watch the manifold pressure gauge to avoid overboosting at low altitude. As the
aircraft climbs and the air density drops, the pilot must continuously open the throttle in small
increments to maintain full power. The altitude at which the throttle reaches full open and the
engine is still producing full rated power is known as the critical altitude. Above the critical altitude,
engine power output will start to drop as the aircraft continues to climb.
Effects of temperature[edit source | editbeta]
Supercharger CDT vs. Altitude. Graph shows the CDT differences between a constant-boost
supercharger and a variable-boost supercharger when utilized on an aircraft.
As discussed above, supercharging can cause a spike in temperature, and extreme temperatures
will cause detonation of the fuel-air mixture and damage to the engine. In the case of aircraft, this
causes a problem at low altitudes, where the air is both denser and warmer than at high altitudes.
With high ambient air temperatures, detonation could start to occur with the manifold pressure
gauge reading far below the red line.
A supercharger optimized for high altitudes causes the opposite problem on the intake side of the
system. With the throttle retarded to avoid overboosting, air temperature in the carburetor can
drop low enough to cause ice to form at the throttle plate. In this manner, enough ice could
accumulate to cause engine failure, even with the engine operating at full rated power. For this
reason, many supercharged aircraft featured a carburetor air temperature gauge or warning light
to alert the pilot of possible icing conditions.
Several solutions to these problems were developed: intercoolers and aftercoolers, anti-detonant
injection, two-speed superchargers, and two-stage superchargers.
Two-stage and two-speed superchargers[edit source | editbeta]
In the 1930s, two-speed-drives were developed for superchargers. These provided more flexibility
for the operation of the aircraft, although they also entailed more complexity of manufacturing and
maintenance. The gears connected the supercharger to the engine using a system of hydraulic
clutches, which were manually engaged or disengaged by the pilot with a control in the cockpit. At
low altitudes, the low-speed gear would be used in order to keep the manifold temperatures low.
At around 12,000 feet (3,700 m), when the throttle was full forward and the manifold pressure
started to drop off, the pilot would retard the throttle and switch to the higher gear, then readjust
the throttle to the desired manifold pressure.
Another way to accomplish the same level of control was the use of two compressors (also known
as stages) in series. After the air was compressed in the low-pressure stage, the air flowed
through an intercooler radiator where it was cooled before being compressed again by thehigh-
pressure stage and then aftercooled in another heat exchanger. In these systems, damper doors
could be opened or closed by the pilot in order to bypass one stage as needed. Some systems
had a cockpit control for opening or closing a damper to the intercooler/aftercooler, providing
another way to control temperature. The most complex systems used a two-speed, two-stage
system with both an intercooler and an aftercooler, but these were found to be prohibitive in cost
and complicated. In the end, it was found that, for most engines, a single-stage two-speed setup
was most suitable.
Turbocharging[edit source | editbeta]
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 the 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 increases in the specific fuel consumption of the engine. These factors
could increases the cost of running the airplane and reducing its overall range -- except that with
more engine power, the airplanes could carry more fuel, especially in external drop tanks,
especially in the American P-38 Lightning, P-47 Thunderbolt, P-51 Mustang, and F6F
Hellcat fighter planes.
For example, with their external fuel tanks and supercharged engines, the P-38 and the P-51
could fly from England to Berlin and back, the P-47 could fly from England to the Ruhr and back,
and the F6F had the longest range of any fighter based on aircraft carriers of the war. Also, the P-
51 could fly even further - from Iwo Jima to Tokyo and back. These ranges were much longer than
those of any Nazi German, British, Japanese, Canadian, or Soviet fighter planes of World War II.
These American fighters also had excellent fighting performance at high altitudes.
As opposed to a supercharger driven by the engine itself, a turbocharger is driven using the
exhaust gases from the engines. 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.
The majority of high-altitude aircraft engines used during World War II used mechanically-driven
superchargers, because these had three significant manufacturing advantages over
turbochargers. Turbochargers - used by large American aircraft engines such as the Allison V-
1710 (used in the P-38) and the Pratt & Whitney R-2800, used extra piping, and required
expensive high-temperature metal alloys in the gas turbine and preturbine section of the exhaust
system, but they were very useful in high-altitude bombers and some fighter planes. The size of
the piping alone was a serious problem. For example, both the F4U Corsair and the P-47
Thunderbolt used the same multicylinder radial engine, but the huge barrel-shaped fuselage of the
P-47 was needed because of the amount of piping to and from the turbocharger in the back of the
engine. The F - 4U used a two stage supercharger with compact intercooler layout.
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 B-29 Superfortress high-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 F8F Bearcat, the F7F Tigercat, the B-50 Superfortress, the KC-97 Stratotanker, 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 source | editbeta]
Until World War II 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.[14] The German Luftwaffe also had supplies of a similar
fuel.[15][16]
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[17][18] or
the Daimler-Benz DB 605DC developed as much as 2,000 hp (1,500 kW).[19][20]
