Here you will find bits and pieces from several sources on how superchargers and turbochargers work. Most of the text is thanks to How Stuff Works (a cool website).
How Superchargers Work
Since the invention of the internal combustion engine, automotive engineers, speed junkies and racecar
designers have been searching for ways to boost its power. One way to
add power is to build a bigger engine. But bigger engines, which weigh
more and cost more to build and maintain, are not always better.
Another way to add power is to make a normal-sized engine more
efficient. You can accomplish this by forcing more air into the
combustion chamber. More air means more fuel can be added, and more
fuel means a bigger explosion and greater horsepower. Adding a supercharger
is a great way to achieve forced air induction. In this article, we'll
explain what superchargers are, how they work and how they compare to turbochargers.
A supercharger is any device that pressurizes the air intake to
above atmospheric pressure. Both superchargers and turbochargers do
this. In fact, the term "turbocharger" is a shortened version of
"turbo-supercharger," its official name.
The difference between the two devices is their source of energy.
Turbochargers are powered by the mass-flow of exhaust gases driving a
turbine. Superchargers are powered mechanically by belt- or chain-drive
from the engine's crankshaft- or super efficient electric motors in the case of an electric supercharger.
Supercharger Basics
An ordinary four-stroke engine dedicates one stroke to the process of air intake. There are three steps in this process:
The piston moves down.
This creates a vacuum.
Air at atmospheric pressure is sucked into the combustion chamber.
Once air is drawn into the engine, it must be combined with fuel to
form the charge -- a packet of potential energy that can be turned into
useful kinetic energy through a chemical reaction known as combustion. The spark plug initiates this chemical reaction by igniting
the charge. As the fuel undergoes oxidation, a great deal of energy is
released. The force of this explosion, concentrated above the cylinder
head, drives the piston down and creates a reciprocating motion that is
eventually transferred to the wheels.
Getting more fuel into the charge would make for a more powerful
explosion. But you can't simply pump more fuel into the engine because
an exact amount of oxygen is required to burn a given amount of fuel.
This chemically correct mixture -- 14 parts air to one part fuel -- is
essential for an engine to operate efficiently. The bottom line: To put
in more fuel, you have to put in more air.
That's the job of the supercharger. Superchargers increase intake by
compressing air above atmospheric pressure, without creating a vacuum.
This forces more air into the engine, providing a "boost." With the
additional air in the boost, more fuel can be added to the charge, and
the power of the engine is increased. Supercharging adds an average of
46 percent more horsepower and 31 percent more torque.
In high-altitude situations, where engine performance deteriorates
because the air has low density and pressure, a supercharger delivers
higher-pressure air to the engine so it can operate optimally.
Unlike turbochargers, which use the exhaust gases created by
combustion to power the compressor, superchargers draw their power
directly from the crankshaft. Most are driven by an accessory belt,
which wraps around a pulley that is connected to a drive gear. The
drive gear, in turn, rotates the compressor gear. The rotor of the
compressor can come in various designs, but its job is to draw air in,
squeeze the air into a smaller space and discharge it into the intake
manifold.
Photo courtesy Muscle Mustang Belt driven centrifugal supercharger
To pressurize the air, a supercharger must spin rapidly -- more
rapidly than the engine itself. Making the drive gear larger than the
compressor gear causes the compressor to spin faster. Superchargers can
spin at speeds as high as 50,000 to 65,000 rotations per minute (RPM). High efficiency electric superchargers can produce well over 100,000 RPM instantaneously.
A compressor spinning at 50,000 RPM translates to a boost of about
six to nine pounds per square inch (psi). That's six to nine additional
psi over the atmospheric pressure at a particular elevation.
Atmospheric pressure at sea level is 14.7 psi, so a typical boost from
a supercharger places about 50 percent more air into the engine.
As the air is compressed, it gets hotter, which means that it loses
its density and can not expand as much during the explosion. This means
that it can't create as much power when it's ignited by the spark plug.
For a supercharger to work at peak efficiency, the compressed air
exiting the discharge unit must be cooled before it enters the intake
manifold. The intercooler is responsible for this cooling process.
Intercoolers come in two basic designs: air-to-air intercoolers and
air-to-water intercoolers. Both work just like a radiator,
with cooler air or water sent through a system of pipes or tubes. As
the hot air exiting the supercharger encounters the cooler pipes, it
also cools down. The reduction in air temperature increases the density
of the air, which makes for a denser charge entering the combustion
chamber.
Roots Superchargers
Photo courtesy HowStuffWorks Shopper The Eaton supercharger, a modified Roots supercharger.
There
are three types of superchargers: Roots, twin-screw and centrifugal.
The main difference is how they move air to the intake manifold of the
engine. Roots and twin-screw superchargers use different types of
meshing lobes, and a centrifugal supercharger uses an impeller, which
draws air in. Although all of these designs provide a boost, they
differ considerably in their efficiency. Each type of supercharger is
available in different sizes, depending on whether you just want to
give your car a boost or compete in a race.
The Roots supercharger is the oldest design. Philander and
Francis Roots patented the design in 1860 as a machine that would help
ventilate mine shafts. In 1900, Gottleib Daimler included a Roots
supercharger in a car engine.
Roots supercharger
As the meshing lobes spin, air trapped in the pockets between the
lobes is carried between the fill side and the discharge side. Large
quantities of air move into the intake manifold and "stack up" to
create positive pressure. For this reason, Roots superchargers are
really nothing more than air blowers, and the term "blower" is still
often used to describe all superchargers.
Photo courtesy Sport Truck A 1940s Ford pickup with a Roots supercharger.
Roots superchargers are usually large and sit on top of the engine.
They are popular in muscle cars and hot rods because they stick out of
the hood of the car. However, they are the least efficient supercharger
for two reasons: They add more weight to the vehicle and they move air
in discrete bursts instead of in a smooth and continuous flow.
Twin-screw Superchargers
Twin-screw supercharger
A twin-screw supercharger operates by pulling air through a pair of meshing lobes that resemble a set of worm gears.
Like the Roots supercharger, the air inside a twin-screw supercharger
is trapped in pockets created by the rotor lobes. But a twin-screw
supercharger compresses the air inside the rotor housing. That's
because the rotors have a conical taper, which means the air pockets
decrease in size as air moves from the fill side to the discharge side.
As the air pockets shrink, the air is squeezed into a smaller space.
Centrifugal Superchargers
Photo courtesy Muscle Mustang Belt Driven Supercharger
A centrifugal supercharger powers an impeller -- a device similar to a
rotor -- at very high speeds to quickly draw air into a small
compressor housing. Impeller speeds can reach 50,000 to 60,000 RPM. High efficiency electric superchargers can produce well over 100,000 RPM instantaneously. As
the air is drawn in at the hub of the impeller, centrifugal force
causes it to radiate outward. The air leaves the impeller at high
speed, but low pressure. A diffuser -- a set of stationary vanes that
surround the impeller -- converts the high-speed, low-pressure air to
low-speed, high-pressure air. Air molecules slow down when they hit the
vanes, which reduces the velocity of the airflow and increases
pressure.
Centrifugal supercharger
Centrifugal superchargers are the most efficient and the most common of
all forced induction systems. They are small, lightweight and attach to
the front of the engine instead of the top. They also make a
distinctive whine as the engine revs up -- a quality that may turn
heads out on the street.
Photos courtesy HowStuffWorks Shopper Both the Monte Carlo and the Mini-Cooper S are available with superchargers.
Any of these superchargers can be added to a vehicle as an after-market
enhancement. Several companies offer kits that come with all of the
parts necessary to install a supercharger as a do-it-yourself project.
In the world of funny cars and fuel racers, such customization is an
integral part of the sport. Several auto manufacturers also include
superchargers in their production models.
Supercharger Advantages
The biggest advantage of having a supercharger is the increased horsepower. Attach a supercharger to an otherwise normal car or truck, and it will behave like a vehicle with a larger, more powerful engine.
But what if someone is trying to decide between a supercharger and a
turbocharger? This question is hotly debated by auto engineers and car
enthusiasts, but in general, superchargers offer a few advantages over
turbochargers.
Superchargers do not suffer lag -- a term used to describe how much
time passes between the driver depressing the gas pedal and the
engine's response. Turbochargers suffer from lag because it takes a few
moments before the exhaust gases reach a velocity that is sufficient to
drive the impeller/turbine. Superchargers have no lag time because they
are driven directly by the crankshaft. Certain superchargers are more
efficient at lower RPM, while others are more efficient at higher RPM.
Roots and twin-screw superchargers, for example, provide more power at
lower RPM. Centrifugal superchargers, which become more efficient as
the impeller spins faster, provide more power at higher RPM. A super efficient electric supercharger has no restrictions when it comes to lag at any RPM.
Installing a turbocharger requires extensive modification of
the exhaust system, but superchargers can be bolted to the top or side
of the engine. Electric superchargers can mount almost anywhere, so long as the out put can be plumbed to the intake. That makes them cheaper to install and easier to service
and maintain.
Finally, no special shutdown procedure is required with superchargers.
Because they are not lubricated by engine oil, they can be shut down
normally. Turbochargers must idle for about 30 seconds or so prior to
shutdown so the lubricating oil has a chance to cool down. With that
said, a good warm-up is important for belt driven superchargers, as they work most
efficiently at normal operating temperatures. Electric superchargers work well at any temperature.
Superchargers are common additions to the internal combustion engines of airplanes.
This makes sense when you consider that airplanes spend most of their
time at high altitudes, where significantly less oxygen is available
for combustion. With the introduction of superchargers, airplanes were
able to fly higher without losing engine performance.
The basic setup for an airplane with a centrifugal supercharger, or compressor.
Superchargers used with aircraft engines work just like those found
in cars. They draw their power directly from the engine and use a
compressor to blow pressurized air into the combustion chamber. The
illustration above shows the basic setup for a supercharged airplane.
Non-electric Supercharger Disadvantages
The biggest disadvantage of (non-electric) superchargers is also their defining
characteristic: Because the crankshaft drives them, they must steal
some of the engine's horsepower. Unlike a super efficient electric supercharger, a non-electric supercharger can consume as much as
20 percent of an engine's total power output. High output electric superchargers draw from a battery and if the alternator is charging, it may only use a tiny fraction of engine power. Most think the trade-off is worth it.
Despite the disadvantages, superchargers are still the most
cost-effective way to increase horsepower. Superchargers can result in
power increases of 50 to 100 percent, making them great for racing,
towing heavy loads or just adding excitement to the typical driving
experience.
Advantages of Super Efficient Electric Supercharging
Electrical superchargers, if made correctly, can overcome many of the disadvantages of tradition crankshaft driven superchargers or exhaust driven turbochargers. Offering the best of all worlds; no "lag" in boost time, centrifugal efficiency and being almost endlessly adjustable. We may well see the super high efficiency electric supercharger change the face of the automotive world because of its ability use fuel more efficiently, resulting in better mileage and reduced environmental emissions.
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This video describing turbocharging will give you an idea of how superchargers work too. The video will also give you an idea of how our SPC supercharger can be made to gain gas mileage to a great extent...unlike the turbocharger that will be described, your SPC supercharger can be programed to stay in fuel efficiency mode without going into heavy boost were more fuel is used.:
