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Centrifugal Blowers
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This shot of the ProCharger 350Z kit shows off the
packaging advantage of centrifugal superchargers.
A centrifugal blower can be mounted most
anywhere on the engine (within reach of a drive
belt). That helps in packaging a blower in today’s
small and crowded engine bays. Also, since
centrifugals are best for blow-through applications,
they are excellent for plumbing dry, pressurized air
directly into today’s self-contained, self-
programming electronic fuel injection systems.
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The only similarity between a Roots-type blower and a centrifugal supercharger is the result they
produce, and even in this respect, they are significantly different. In the first place, a Roots is
literally a pump, whereas the centrifugal is more like a fan, or a literal blower. The Roots traps and
moves a given volume of air through it with each revolution of the rotors (not accounting for
leakage); consequently, it’s known as a positive-displacement pump. Its pumping output (again, not
counting inefficiency) is directly proportional to the speed at which it is being turned.
A centrifugal blower, however, is an inertial compressor. Its one primary moving part is a fixed-blade
fan called an impeller, which actually looks like a disk with fan blades protruding on one side.
Whereas the Roots blower is similar to a typical engine oil pump, a centrifugal blower is similar to a
typical engine water pump.
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The impeller is enclosed in a housing shaped more or less like a snail’s shell, with an inlet hole
above the center of the impeller, and an outlet at the end of the chamber of the “shell.” Air enters
the blower axially (along the rotating axis of the impeller), being sucked in by the low pressure
created by the “fan,” which is blowing air out of the housing. As soon as the air enters the blades of
the impeller, however, it is rapidly accelerated radially toward the outer tips of the fan blades by
centrifugal force.
Here’s how one source describes the action of a centrifugal blower: “The air-moving element is a
form of bucket wheel that hurls air outward from a central inlet against a collector housing called a
volute. As the heavy air is hurled outward from the center of the impeller, the partial vacuum thus
created at the center pulls more air into the inlet. It’s a centrifugal pump.”
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To get a better idea of the principle on which a centrifugal blower works, think of a merry-go-round
found in a schoolyard or city park. One or more children push the merry-go-round, while the rest
hang on and spin around on it. If you are sitting near the center of the merry-go-round, you won’t
be spinning very fast. But the farther toward the outside you move, the faster you go, even though
the merry-go-round itself is still turning at the same speed (that is, the same RPM). Furthermore, as
you may remember from your childhood, the spinning motion of the merry-go-round tends to force
your body towards the outside edge (that is, radially), and, if you don’t hang on tightly enough as it
picks up speed, it’ll hurl you off the edge with considerable velocity. The children’s game of crack
the whip operates on the same principle.
The tendency of a rotating body to want to fling objects on it, or contained by it, toward the edges is
called centrifugal force. It is a demonstration of the laws of inertia, which can be used in the case of
the centrifugal blower like an energy multiplier. For instance, let’s say you and a friend are playing
on a merry-go-round. You are pushing, and your friend is riding. If your friend is sitting on the
outside edge, it will be hard for you to get the merry-go-round going (a body at rest tends to remain
at rest), because you will have to accelerate his weight at the speed of the outer perimeter of the
merry-go-round. However, if he moves in toward the middle, it will be much easier to get the thing
going because you are pushing rapidly at the outside edge, but his weight is moving slowly in the
center. This is the same basic principle as reduction gearing: using a low gear to get an automobile
moving, for instance, and then shifting to higher gears once it’s under way. So, since you are an
intelligent kid, you have your friend sit in the middle while you get the merry-go-round under way.
As your friend moves outward, it will be a little harder to keep it going at a constant speed, but
nowhere near as hard as it would have been to get it going with him out there. As Newton put it, a
body in motion tends to remain in motion.
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The several machined discs in the
foreground represent the tedious
trial-and-error process of reproducing
an impeller for a 1920s Miller
supercharger at the Briggs
Cunningham auto museum. In the
background is the Miller blower
housing, which has a definite nautilus
shape.
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An interior view of the Miller blower
shows the relationship of rotor to
housing. This blower uses a
straight-blade impeller but also has a
diffuser ring that extends beyond the
impeller tips. It can be plainly seen in
this photo.
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This Miller centrifugal blower impeller
used forward-curved-tip blades as
early as the 1930s. However, the tips,
which were bent in a jig after the
blades were machined, tended to
break off under high boost pressures,
so the design was not used extensively.
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Centrifugal blowers were not only used
on several production automobiles in
America before World War II, they
dominated Indy-type racing, usually on
Miller or Duesenberg engines. This
supercharger is actually a Miller copy (a
1927 Cooper), which uses a
straight-bladed rotor and a slightly
snail-shaped housing. The inlet is in the
center.
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This view of a Paxton supercharger
clearly shows the snail shape of its
housing. The fan blades pull air into
the inlet, and the hole at the right side
is the outlet port.
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The primary operating component in a
centrifugal supercharger is its simple,
small (only about four inches across),
cast-aluminum impeller. The straight-
blade design of this particular impeller
is not very sophisticated, but it’s
certainly adequate and reliable for
street applications.
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The reason a body tends to be flung off a rotating object is that inertia (the fact that the moving
mass wants to remain in motion) is linear – that is, the body wants to keep moving in a straight line.
But the rotating object moves it in a curving line. If the body is firmly attached to the rotating object,
its mass will tend to keep the object rotating, but part of its inertial energy (also called kinetic
energy, or the energy “contained” in a moving body) will be lost.
To illustrate this principle, again think of yourself on the merry-go-round. If you are sitting on the
outside edge and your friend is pushing at a good speed, the weight of your body will keep the
merry-go-round spinning for quite a while after he stops pushing. This is the effect of inertia, or
kinetic energy – work continues to be done although no apparent energy is being put into the
system. But, in this case, it is only part of the energy available. If you are sitting on the outside
perimeter of the merry-go-round, you’re going to have to hang on with lots of your own energy to
stay in place – an amount of energy equal, but opposite, to the amount of kinetic energy that would
propel your body in a straight line (tangentially) off the merry-go-round as soon as you let go.
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Perhaps we’re overdoing the explanation of how a centrifugal blower works, but it does illustrate
why a centrifugal blower is much more efficient than a Roots or other type of positive-displacement
pump, which simply pushes the air with direct force. By using the energy multiplying effect of
centrifugal force (the inertia of the mass of the air molecules) to help compress the air, the
mechanical efficiency of a centrifugal blower is, consequently, much higher than other types,
meaning that it requires less horsepower from the engine to make good boost. The spinning blades
of the impeller in a centrifugal blower rapidly accelerate the air molecules, greatly increasing their
velocity, which gives them a considerable amount of kinetic energy.
How this velocity is transformed into pressure is a bit more difficult to understand. If you look at a
cross-section of an impeller and its housing, you will notice that most (though not all) types
decrease in cross-sectional area from the center to the tip of the blades. That is, the cavity is
enclosed by a pair of blades and the housing is wedge-shaped, tapering toward the outer edge. It
would appear, therefore, that air would be compressed in this cavity as it is “squeezed” outward into
the smaller area near the impeller tip. But such is not the case, for two reasons.
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First, although the cross-section (or the width) of the blade decreases toward the tip, the distance
between two adjacent fan blades, since they are radial, increases from the center to the edge, thus
increasing the enclosed volume. The combined effect might be a constant-area cross-section, but
in fact, many centrifugal housing designs (as used in turbochargers) actually have parallel walls.
Second, and much more significant, as the air is accelerated along the impeller blade, two things
happen to it. It tends to stack up against the face of the blade, which would compress the air. But at
the same time, as the blade accelerates the air molecules radially, they will stretch out as their
velocity is increased. This latter condition is similar to the effect of a carburetor venturi, which, as
you know, decreases the density and the pressure of the air by increasing its volume (the air
molecules are stretched farther apart).
All of this may be a little confusing, but don’t worry too much about exactly what is going on inside
the centrifugal blower housing. The one point we want to make here is that it’s nearly impossible to
compute a volumetric efficiency figure for a centrifugal-type blower, since it’s not a positive-
displacement pump. We don’t speak of the internal displacement of a centrifugal blower, as we do
of a Roots or a vane-type blower. In fact, the shape of the impeller blades and the housing have
nearly as much effect on the output of a centrifugal blower as does its physical size.
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The adiabatic efficiency of the centrifugal supercharger is very significant, however. Remember, the
energy imparted to the air by the blower (ultimately by the engine) is minimal in a centrifugal unit
because of its good mechanical efficiency. That is, there is comparatively little work done on the air
– since we’re actually using the weight of the air molecules to provide much of the energy – and
therefore the centrifugal supercharger doesn’t heat the air nearly as much as a Roots blower. Also,
the centrifugal blower doesn’t beat the air, so the flow of air through the blower is much smoother
than in a Roots.
Second, the compression process itself is much more efficient in the centrifugal supercharger. After
the air molecules are accelerated along the impeller blades, they are flung off the ends at a velocity
approximately equal to that of the tips of the blades. As this high-velocity air is hurled into the scroll
housing, it literally piles up against the air already packed into the housing and the manifold, which
means that the air actually compresses itself with its own momentum. The kinetic energy imparted
to the air molecules by the impeller in the form of velocity is immediately, and efficiently, converted
into static energy in the form of pressure. Some heat is certainly generated by this process, since
the volume of the air is reduced as it “stacks up” (thus increasing the density of the air at the same
time that it increases the pressure, which is what we want from a supercharger), but the heat gain in
a good centrifugal blower can be so low as to approach 100 percent adiabatic efficiency. Even the
less sophisticated types used for automobile superchargers are capable of 70 percent to 80
percent adiabatic efficiency.
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Previous | Next
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This has been a sample page from
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Sport Compact Turbos & Blowers
by Joe Pettitt
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Lightweight and high-revving, sport compacts are today’s most
popular cars. They have developed a cult following among today’s
youth and are fueling a multi-million dollar industry in modification
parts and equipment.
While most owners of sport compacts can afford the simple bolt-
ons available, some owners want to take their modifications a step
further. There is intense competition to be the fastest, and quite
often the only way to win is to go to the next level – by installing a
supercharger/blower or turbocharger on your engine.
This book is an enthusiast’s guide to understanding and using
turbochargers and superchargers on sport compact cars. It
covers the basics of each system and compares their pros and
cons. Building and tuning small-displacement 4- and 6-cylinder
engines to maximize performance and reliability with forced
induction is also covered.
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Click below to view sample
pages from each chapter!
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Chap. 1 - Exotic or Practical
Chap. 2 - Supercharging
Chap. 3 - Roots Blowers
Chap. 4 - Centrifugal Blowers
Chap. 5 - Turbocharging
Chap. 6 - Turbos & Compacts
Chap. 7 - Tuning for Boost
Chap. 8 - Building Engines
Chap. 9 - History
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Softbound
8-1/2 x 11
128 pages
300 black & white photos
Item: SA89
Price: $18.95
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Click here to buy now!
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Other items you might be interested in
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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Engine Management: Advanced Tuning
Engine Management: Advanced Tuning explains how the EFI system
determines engine operation and how to change the controlling
parameters to optimize actual engine performance. This book takes
engine-tuning techniques to the next level. It is a must-have for tuners
and a valuable resource for anyone who wants to make horsepower
with a fuel-injected, electronically controlled engine.
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Price:
$22.95
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MRE
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