A Supercharger is an air compressor that increases the pressure or density of air supplied to an internal combustion engine. This gives each intake cycle of the engine more oxygen, letting it burn more fuel and do more work, thus increasing the power output.
Power of the supercharger can be provided mechanically by means of a belt, shaft, or chain connected to the engine's crankshaft.
Types of Superchargers :
There are three main types of superchargers; Roots-type, centrifugal, and twin screws. All work in a slightly different way, but ultimately they all do the same thing – compress the intake air and force it into the engine at a higher pressure. And because the speed at which the supercharger operates is directly linked to the engine rpm (remember the supercharger is driven by the crankshaft) there is no need for complicated boost control. Instead engine rpm and simple gearing can control everything. Simply by using smaller or larger diameter pulleys we can cause the supercharger speed to increase or decrease accordingly.
Each of the different types of superchargers works slightly differently and therefore has different characteristics, and different pros and cons. Choosing the right one for your application will depend on what you plan to use the car for and finding the type which best suits your needs.
ROOTS-TYPE SUPERCHARGER :
The oldest style of supercharger available is a Roots-type. So-called after the Roots brothers who founded the design. This design is still going strong today and is the most common type of supercharger used in OE production.
The main difference between a Roots-type supercharger and centrifugal and twin-screw types is that a Roots-type unit doesn’t actually compress the air within the supercharger itself. Instead, it is more like a pump, pumping air into the engine.
Because of this external compression nature, Roots-type superchargers produce more heat in the inlet air charge than either of the other styles of supercharger and therefore the Roots-type design is considered to be the least efficient of the three main styles available. For example, 1psi boost from a Roots-type supercharger will be hotter, and therefore less dense, than 1psi of boost generated from a centrifugal or twin-screw supercharger, and will therefore make less power. But of course, this can be negated by the use of a heat exchanger or intercooler setup.
However, Roots-type superchargers do have their advantages. Reliability is probably the biggest of which and is certainly one of the main reasons Roots-type superchargers are used in OE production cars. Roots-type superchargers are incredibly durable and require very little maintenance.
Boost throughout the rev range
Boost from low engine rpm
Least efficient supercharger design
Violent throttle response
Lots of space for installation
CENTRIFUGAL SUPERCHARGER :
A centrifugal supercharger is very similar to a turbocharger, but rather than having an exhaust housing with a turbine wheel being driven by exhaust gases, the supercharger has a pulley being driven by a belt from the crankshaft. But the front end of a centrifugal supercharger is very similar to that of a turbo, from head-on they even look the same.
However, in order to do this, the inducer needs to spin incredibly fast, just as it does with a turbocharger. To allow this, the belt-driven pulley found at the back of the unit is attached to a large gear, which in turn is meshed with a much smaller gear attached to the inducer wheel. The exact ratio used depends on the specific application, but this is how you can get a centrifugal supercharger spinning at over 100,000rpm while being powered by an engine rotating at just 6,000rpm.
And therein is another major difference between centrifugal superchargers and Roots-type or twin-screw units; centrifugal superchargers are dynamic compressors rather than positive displacement pumps.
Centrifugal superchargers have many other advantages too, as a much more compact unit, centrifugal superchargers are easier to install within the cramped confines of an engine bay, and don’t require you to cut huge holes in your bonnet to have the supercharger poking out from! They are also incredibly thermally efficient.
Good thermal efficiency
Flexibility to change impeller sizes
Compact design requires less space to install.
Not powerful at low revs
Installation requires removal of features like air con
Designed to fill the void between the Roots-type and the centrifugal style units, the twin-screw supercharger is a positive displacement supercharger like the Roots-type but is also an internal compression supercharger like the centrifugal units.
Inside a twin-screw supercharger, you will find, as the name suggests, two screw-like rotors. Unlike Roots-type units, which feature two identical rotors to pump air into the engine, these ‘screws’ are cleverly designed so they actually compress the air as they counter-rotate.
It is this reduction in volume along the length of the rotors that causes the compression of the air before it exits the supercharger.
This allows the twin-screw supercharger to produce boost pressure the moment you touch the throttle, and generally allows full boost to be reached very quickly at relatively low engine revs. This full boost is then available all the way to the redline.
The only real downside is price, as it costs considerably more to engineer the complex-shaped rotors used in a twin-screw supercharger. Also, a twin-screw unit is generally big and bulky like a Roots-type, so space in the engine bay can become an issue for installation.
Good thermal efficiency
Full boost at low rpm and held throughout the rev range
Excellent low rpm power
Expensive to produce
Big and bulky to install