A supercharger (also known as a blower) is an air compressor
used to compress air into the cylinders of an internal
combustion engine. The additional mass of oxygen containing
air that is forced into the cylinders improves the
volumetric efficiency of the engine which allows the engine
to burn more fuel and makes it more powerful. A supercharger
can be powered mechanically by belt-, gear- or chain-drive
from the engine's crankshaft. It can also be powered by a
gas turbine . When a centrifugal type compressor section is
mated to a gas turbine drive it is then referred to as a
turbo-supercharger, commonly called a turbocharger .
Positive displacement Superchargers may absorb as much as a
third of the total crankshaft power of the engine, and in
many applications are less efficient than turbochargers. In
applications where engine response and power is more
important than any other consideration, such as top-fuel
dragsters and vehicles used in tractor pulling competitions,
positive displacement superchargers are extremely common.
To supercharge means to fill beyond its actual physical
capacity. Any device that does this to an engine is a
There are two main types of supercharger defined according
to the method of compression. Positive displacement and
Positive displacement pumps deliver a fixed volume of air
per revolution at all speeds. The device divides the air
mechanically into parcels for delivery to the engine.
Mechanically moving the air into the engine bit by bit.
· Major types of positive displacement pumps:
(Positive displacement pumps are further divided into
internal compression and external compression types.)
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 compressors:
Multi stage axial flow
(Note:Comprex superchargers do not fit neatly into either
category, being part fish and part fowl. The Comprex design
uses the exhaust gas to directly compress the incomming
Superchargers are further defined according to their method
Mechanical drive and exhaust gas driven
· Types of Mechanical drive:
Belt(V belt, Toothed belt, Flat belt)
· Exhaust gas driven:
All the types of compressor may be mated to and driven by
either gas turbine or mechanical linkage. However since
dynamic compressors are well suited for gas turbine drive
due to their matching high-speed characteristics and are
most often matched together. While positive displacement
pumps usually use one of the mechanical drives. All of the
combinations mentioned have been tried with various levels
1929 "Blower" Bentley from the Ralph Lauren collection. The
large "blower" (supercharger) is located at the front, in
front of the radiator, and gave the car its name.
In cars, the device is used to increase the "effective
displacement" and volumetric efficiency of an engine, and is
often referred to as a blower. By pushing the air into the
cylinders, it is as if the engine had larger valves and
cylinders, resulting in a "larger" engine that weighs less.
In 1900 Gottlieb Daimler (of Daimler-Benz / Daimler-Chrysler
fame) became the first person to patent a forced-induction
system for internal combustion engines. His first
superchargers were based on a twin-rotor air-pump design
first patented by American Francis Roots in 1860. This
design is the basis for the modern Roots type supercharger.
It wasn't long after its invention before the supercharger
was applied to custom racing cars, with the first
supercharged production vehicles being built by Mercedes and
Bentley in the 1920s. Since then superchargers (as well as
turbochargers) have been widely applied to racing and
production cars, although their complexity and cost has
largely relegated the supercharger to the world of pricey
Boosting has made something of a comeback in recent years
due largely to the increased quality of the alloys and
machining of modern engines. Boosting used to be an
effective way to dramatically shorten an engine's life but,
today, there is considerable overdesign possible with modern
materials and boosting is no longer a serious reliability
concern. For this reason boosting is commonly used in
smaller cars, where the added weight of the supercharger is
smaller than the weight of a larger engine delivering the
same amount of power. This also results in better gas
mileage, as mileage is often a function of the overall
weight of the car and that is based, to some degree, on the
weight of the engine. Nevertheless, adding boost to a car
will often void the drivetrain warranty. Also, improperly
installed or excessive boost will greatly reduce life
expectancy of the engine as well as the transmission (which
may not have been designed to cope with additional torque).
There are three types commonly used in today's automotive
world: Roots type supercharger, twin-screw type
supercharger, and Centrifugal type supercharger.
A more natural use of the supercharger is with aircraft
engines. As an aircraft climbs to higher altitudes the
pressure of the surrounding air quickly falls off—at 6000 m
(18,000 ft) the air is at half the pressure of sea level.
Since the charge in the cylinders is being pushed in by this
air pressure it means that the engine will normally produce
half-power at full throttle at this altitude.
A supercharger remedies this problem by compressing the air
back to sea-level pressures, or even much higher. This can
take some effort. On the single-stage single-speed
supercharged Rolls Royce Merlin engine for instance, the
supercharger uses up about 150 horsepower (110 kW). Yet the
benefits are huge, for that 150 horsepower (110 kW) lost,
the engine is delivering 1000 hp (750 kW) when it would
otherwise deliver 750 hp (560 kW). And while the engine
might be fooled into thinking it's at sea level, the
airframe is quite aware of the halved air density and the
plane thus has half the drag. For this reason supercharged
planes fly much faster at higher altitudes.
A supercharger is only able to supply so much pressure
because the compression increases the air temperature, and
the engine is limited in maximum charge-air temperature
before pre-ignition occurs. The boost is typically measured
as the altitude at which the supercharger can still supply
sea level pressure (100 kPa or 1000 mbar) and is referred to
as the critical altitude. Throughout WWII British
superchargers generally had higher critical altitudes than
their German counterparts and, when combined with higher
octane fuels that the Americans supplied, that allowed for
higher boost levels. British engines were generally able to
outperform German ones.
Below the critical altitude the supercharger is capable of
delivering too much boost and must therefore be restricted
lest the engine be damaged. Unless other measures are taken,
this means that at least some of the power driving the
supercharger is wasted. Also, due to the denser air at lower
altitudes, the supercharger is not operating at its best
efficiency, and this can cause an additional load on the
For the early years of the war this was simply how it was
and this led to the seemingly odd fact that many early-war
engines actually delivered less power at lower altitudes,
because the supercharger was still using up power to
compress air that was not delivering any power back. As the
war progressed two-speed superchargers were introduced using
better controllers and, notably, hydraulic clutches, that
allowed the boost to be managed over a wide range of
altitudes by operating at low rpm down low and at high rpm
at higher altitudes. This generally "flattened out" the
power below the critical altitude.
Improving octane rating
In 1940 a batch of 100 octane fuel was delivered from the
USA to the RAF. This allowed the boost on Merlin engines to
be increased to 48 inHg (160 kPa) and the power to rise by
more than 10% (from 1030 to 1160 hp, or 770 to 870 kW). By
mid-1940 another increased boost yielded 1310 hp (980 kW).
Supercharging by itself could not have achieved these
improvements; however, when married with fuel improvements,
the engine could respond to both.
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. Ultimately it
was found that for most engines (excepting those in
high-performance fighters) a single-stage two-speed setup
was most suitable.
A final improvement was the use of two compressors in
series, which were introduced to solve the pre-ignition
problem. Compressing a gas always causes its temperature to
rise, and an overcompressed fuel-air mixture may therefore
prematurely ignite. In order to avoid pre-ignition the "two
stage" design was used. After being compressed "half-way" in
the low pressure stage the air flowed through an intercooler
radiator where it was partially cooled down before being
compressed the rest of the way in the high pressure stage
and then aftercooled in another air/air or coolant/air
radiator (heat exchanger). At low altitudes one stage could
be turned off completely. The two-stage Merlin was losing
400 hp (300 kW) to turn the supercharger but developing
between 1500 and 1700 hp (1125 to 1275 kW) at the propeller
shaft, depending on model.
It is interesting to compare all of this complexity to the
same system implemented with a turbocharger. Since the turbo
is driven off the exhaust gases, simply dumping some of the
exhaust pressure is sufficient to drive the compressor at
almost any desired speed. In addition the power in the
exhaust would otherwise be wasted (except to the extent that
the exhaust itself provided thrust) whereas in the
supercharger that power is being taken directly from the
engine. Thus at low altitudes the turbo robs nothing and, as
the altitude increases, it can use just as much power as it
needs and no more. Better yet the amount of power in the gas
is the difference between the exhaust pressure and air
pressure, which increases with altitude, so turbochargers
generally have much better altitude performance.
Yet the vast majority of WWII engines used superchargers,
because they maintained three significant manufacturing
advantages over turbochargers, which were larger, involved
extra piping, and required exotic high-temperature materials
in the turbine. The size of the piping alone is a serious
issue; consider that the Vought F4U and Republic P-47 used
the same engine but the huge barrel-like fuselage of the
latter was, in part, needed to hold the piping to and from
the turbocharger in the rear of the plane.
Supercharging versus Turbocharging
The physical space occupied by a turbocharger is
significantly less than its direct-drive counterpart. This
gives the opportunity of fitting multiple turbochargers to a
single engine, such as in a "sequential turbo", where one
turbo is tuned to give increased performance at low engine
speed and another turbo is tuned to increase the high-speed
An alternative arrangement utilizes two turbochargers of the
same type, known as a "twin turbo". This gives a large power
increase for a given engine speed at the cost of increasing
the lag-time for the exhaust to heat up sufficiently to
drive the turbochargers. This lag can be addressed by
reducing the size of each individual unit such that the
combined output is still as great as a single large
turbocharger without having to suffer the lag-time required
to reach operating speed.
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 the energy of the exhaust pressure is lost. For this
reason, both the economy and the power of a turbocharged
engine are usually better. The main advantage of an engine
with a mechanically driven supercharger is better throttle
response. This is important in dragsters and small sports
cars. It also tends to run less hot.
It is also possible to drive the blower from the crank shaft
and use an exhaust turbine for output power.
Supercharging in jet engines
Supercharging is not confined to superchargers - jet engines
rely on supercharging as one of the main routes to thrust
growth and improved fuel efficiency.
For example, adding an additional (i.e. zero) stage to a
compressor will not only increase the overall pressure ratio
of the cycle, but induce more airflow into the unit, by
supercharging the entry plane of the original compressor.
Ideally, the corrected (i.e. non-dimensional) speed of
original compressor should be maintained, by raising the
mechanical shaft speed by a factor √(Tstage1new/Tstage1old).
If stress considerations prevent any shaft speed increase,
there is only a modest increase in airflow.
Converting a turbojet into a turbofan, by adding a fan
spool, also supercharges the compression system, thereby
raising core flow.
Many of the large turbofan engine series (e.g. Pratt &
Whitney PW4000) have gained core flow by adding one or more
stages to the front of the gas generator, usually in the LP
(or IP) compressor. If the fan flow is not increased, the
bypass ratio will decrease.
Supercharging can also be achieved by improving the
aerodynamics of the existing blading. Core flow will
increase if the original compressor outlet (corrected) flow
size is maintained
Either way, raising core flow increases core power and,
thereby, the net thrust or shaftpower of the engine. Raising
overall pressure ratio tends to improve specific fuel
consumption (i.e. fuel efficiency).
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