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A long time ago, the first ever race between two cars took place. That exact moment in time is lost to history, but every since then there has been a non stop drive to increase engine power. One way to add power to a car is to wedge in a bigger engine. But that adds additional weight and size issues, and in some cases is not the best choice.
Another way to win the race is to make the engine that you already have
more powerful. Power comes from fuel, but to add more fuel to a motor
you have to be able to add more air. More air and more fuel means a
bigger explosion which means more horsepower. This is why so much
development has gone towards multi-valved heads, for example... it's
easier to get the air into them. Another way is called forced
induction, and a supercharger is one way of achieving forced induction.
Another way is turbochargers, and we'll talk a little about them, too.
A supercharger is really any device that pressurizes air in the intake
to levels above that of the ambient atmospheric pressure. Superchargers
and turbochargers both do this. The exact amount of pressurization is
measured in pounds of pressure above the ambient pressure, which is
standardized at 14.7 psi. So a supercharger boosting at 6 psi is
delivering air to the engine at 20.7 psi.
Turbochargers also pressurize air in the intake. The difference is in
the mechanism: Turbochargers are driven by exhaust gases while
superchargers are powered mechanically by a belt or a chain directly
from the engine's crankshaft.
Getting more fuel into an engine makes for a more powerful explosion.
But you can't simply pump more fuel into an engine because there is an
exact ratio of air to fuel that has to be met in order to burn a given
amount of fuel. That mixture is in the range of 14:1 and is essential
for an engine to operate properly. It is a hard and fast rule: if you
put more fuel into the mix, you have to add more air.
And that's what a supercharger does. They increase the amount of air
available to the engine by compressing air to a level above atmospheric
pressure. This forces more air into the engine. With the additional air
more fuel can be added and the engine power is increased. Superchargers
can add significant horsepower and torque, and at high altitudes where
air has a lower density and pressure, a supercharger can be used to
deliver higher pressure air to the engine so that it can run properly.
A turbocharger uses exhaust gas to power the compression process but a
supercharger draws power directly from the engine crankshaft. Usually
they are driven through an accessory belt which is connected to a drive
gear which in turn spins the compressor gear. The rotor of the
compressor can come in a variety of different designs but the job of it
is to draw air into a smaller space and exit it into the intake
manifold, and thereby into the engine.
In order to compress the air a supercharger has to spin very rapidly,
in fact more rapidly than the engine itself. Superchargers can be made
to spin as fast as 50,000 to 65,000 rpm.
A supercharger spinning at 50,000 rpm will create a boost from between
6 to 9 psi. As stated above, that's 6 to 9 psi above the standard 14.7
psi that ambient air has, so that is adding about 50% more air into the
intake.
As air is compressed it gets hotter which means that it is less dense
and will not expand as much when ignited, as well as causing potential
detonation issues. To combat this a variety of different intercoolers
can be employed, as well as chemical intercooling such as water/meth
injection. See the article on detonation for more information about
that.
There are three different types of supercharger: Roots, centrifugal and
twin-screw. The difference between them is how they move air to the
intake manifold. Roots and twin-screw types use a meshing lobe system,
while a centrifugal type uses an impeller to draw air in. The three
different designs all provide boost but they differ in terms of
efficiency.
Roots supercharger
The oldest design is the Roots supercharger. It was invented in 1860 as
a way of delivering air to mine shafts. Sort of a sophisticated fan.
Gottleib Daimler was the first man to attach a Roots supercharger to a
car engine.
As meshing lobes inside a Roots supercharger spin, air trapped in the
pockets between the lobes is moved between the intake side and the
discharge side. A large quantity of air moves into the intake manifold
to create positive pressure. Because of this a Roots supercharger is
often called a "blower" and that term is still applied to all
superchargers.
Roots superchargers are large and usually sit right on top of the
engine. Visually they are very impressive and because of that they are
very popular in show cars but they are the least efficient type of
supercharger because they weigh quite a bit and they deliver compressed
air in bursts instead of a smooth flow.
Twin screw supercharger
A twin screw supercharger compresses air by drawing it through a pair
of lobes that mesh like a set of worm gears. The air is trapped in
pockets, like a Roots style, but the air is compressed inside a rotor
housing because the lobes have a conical taper, decreasing the size of
the pocket as the air moves towards the discharge side. As the air
pockets shrink, the air is compressed.
This design makes twin screw superchargers more efficient but also more
costly as they require considerable precision in the manufacturing
process. Twin screw types will often sit on top of an engine, like a
Roots style, but can be detected at a distance by the considerable
amount of noise that the air makes as it whistles out of the discharge.
Centrifugal supercharger
A centrifugal type supercharger powers an impeller rotating at very
high speeds to quickly draw air into a compressor housing. As the air
is drawn into the hub of the impeller centrifugal force (Ok,
centripetal force for you physics majors) causes it to radiate outward
at high velocity but low pressure. A set of vanes converts the high
speed, low pressure air to low speed, high pressure air which is then
delivered to the intake manifold.
Centrifugal superchargers are the most efficient and the most common
type. They are small and lightweight and can be attached anywhere a
pulley can be mounted instead of on top of the motor.
Any of these superchargers can be added to a vehicle after market,
assuming for example that enough space can be found. There are several
companies that offer kits that come with all the various parts needed
to install a supercharger as a do-it-yourself style project or they can
be installed by a speed shop which is the recommended route since
engine tuning will most likely be required post install.
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.
Installing a turbocharger requires extensive modification of the
exhaust system, but superchargers can be bolted to the top or side of
the engine. 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 superchargers, as they work most
efficiently at normal operating temperatures.
Supercharger disadvantages
The biggest disadvantage of superchargers is also their defining
characteristic: Because the crankshaft drives them, they must steal
some of the engine's horsepower. A supercharger can consume as much as
20 percent of an engine's total power output. But because a
supercharger can generate as much as 46 percent additional horsepower,
most think the trade-off is worth it.
Supercharging puts an added strain on the engine, which needs to be
strong to handle the extra boost and bigger explosions. Most
manufacturers account for this by specifying heavy-duty components when
they design an engine intended for supercharged use. This makes the
vehicle more expensive. Superchargers also cost more to maintain, and
most manufacturers suggest high-octane premium-grade gas.
Despite their disadvantages, superchargers are still one of the most
cost-effective ways 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.
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