Supercharger
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.
Types
To supercharge means to fill beyond
its actual physical capacity. Any
device that does this to an engine
is a supercharger.
There are two main types of
supercharger defined according to
the method of compression. Positive
displacement and dynamic
compressors.
Positive displacement
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:
Roots
Lysholm screw
Sliding Vane
Piston
Wankle
G-lader
(Positive displacement pumps are
further divided into internal
compression and external compression
types.)
Dynamic
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:
Centrifugal
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
charge.)
Superchargers are further defined
according to their method of drive.
Mechanical drive and exhaust gas
driven
· Types of Mechanical
drive:
Belt(V belt, Toothed belt, Flat
belt)
Direct drive
Gear drive
Chain drive
· Exhaust gas driven:
Axial turbine
Radial turbine
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
of success.
Automobiles
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
performance cars.
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.
Aircraft
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.
Altitude effects
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.
Altitude efficiency
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 engine.
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.
Multiple stages
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 engine performance.
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).
Article From
Wikipedia, the free encyclopedia
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