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Supercharger : How does it work ?

By Unknown - February 05, 2018


Supercharger (also known as blower), is a gas compressor used to pump air into the cylinder of an internal combustion engine. The additional oxygen mass that is forced into the cylinder makes the engine burn more fuel, and increases the engine's volumetric efficiency and makes it more energetic. A supercharger is mechanically powered by a belt-pulley, chain-sprocket, and gear mechanism of the engine crankshaft.

The supercharger is similar to a turbocharger, but the turbocharger is powered by an engine output gas stream (exhaust) that drives the turbine. Superchargers can absorb as much as one-third of engine crankshaft power and in many applications are less efficient than turbochargers. In applications where large power is more important than other considerations, such as dragster top fuel and vehicles used in tractor pull competitions, superchargers are very common.

In 1848 or 1849, G. Jones of Birmingham, England brought out a Roots-style compressor. 

In 1860, brothers Philander and Francis Marion Roots, founders of Roots Blower Company of Connersville, Indiana, patented the design for an air mover for use in blast furnaces and other industrial applications. The world's first functional, actually tested engine supercharger was made by Dugald Clerk, who used it for the first two-stroke engine in 1878. Gottlieb Daimler received a German patent for supercharging an internal combustion engine in 1885. Louis Renault patented a centrifugal supercharger in France in 1902. An early supercharged race car was built by Lee Chadwick of Pottstown, Pennsylvania in 1908 which reportedly reached a speed of 100 mph (160 km / h).

The world's first series-produced cars with superchargers were Mercedes 6/25/40 hp and Mercedes 10/40/65 hp. Both models were introduced in 1921 and had Roots superchargers. They were distinguished as "Kompressor" models, the origin of the Mercedes-Benz badging which continues today.
On March 24, 1878 Heinrich Krigar of Germany obtained patent # 4121, patenting the first ever screw-type compressor. Later that same year on August 16 he obtained patent # 7116 after modifying and improving his original designs. His designs show a two-lobe rotor assembly with each rotor having the same shape as the other. Although the design resembled the Roots style compressor, the "screws" were clearly shown with 180 degrees of twist along their length. Unfortunately, the technology of the time was not sufficient to produce such a unit, and Heinrich made no progress with the screw compressor. Nearly half a century later, in 1935, Alf Lysholm, who was working for Ljungstroms Angturbin AB (later known as Svenska Rotor Maskiner AB or SRM in 1951), patented a design with five female and four male rotors. He also patented the method for machining the compressor rotors.

There are two main types of superchargers defined according to the method of gas transfer: positive displacement and dynamic compressors.

Positive displacement 
Positive-displacement pumps deliver a fixed volume of air per revolution at all speeds (minus leakage, which is almost constant at all speeds for a given pressure, so its importance decreases at higher speeds)
Major types of positive-displacement pumps include:
  • Roots
  • Lysholm twin-screw
  • Sliding vane
  • Scroll-type supercharger, also known as the G-Lader

Positive-displacement pumps are further divided into internal and external compression types.


Roots superchargers are external compression only (although high-helix roots blowers attempt to emulate the internal compression of the Lysholm screw).
  • External compression refers to pumps that transfer air at ambient pressure into the engine. If the engine is running under boost conditions, the pressure in the intake manifold is higher than that coming from the supercharger. That causes a backflow from the engine into the supercharger until the two reach equilibrium. It is the backflow that actually compresses the incoming gas. This is an inefficient process and the main factor in the lack of efficiency of Roots superchargers when used at high boost levels. The lower the boost level the smaller is this loss, and Roots blowers are very efficient at moving air at low pressure differentials, which is what they were invented for (hence the original term "blower").
All the other types have some degree of internal compression.
  • Internal compression refers to the compression of air within the supercharger itself, which, already at or close to boost level, can be delivered smoothly to the engine with little or no back flow. This is more effective than back flow compression and allows higher efficiency to be achieved. Internal compression devices usually use a fixed internal compression ratio. When the boost pressure is equal to the compression pressure of the supercharger, the back flow is zero. If the boost pressure exceeds that compression pressure, back flow can still occur as in a roots blower. Internal compression blowers must be matched to the expected boost pressure in order to achieve the higher efficiency they are capable of, otherwise they will suffer the same problems and low efficiency of the roots blowers.

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 compressor are:
  • Centrifugal
  • Variable ratio centrifugal
  • Multi-stage axial-flow
  • Pressure wave supercharger

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