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Roots Compressor


One of the earliest compressor designs. It essentially consists of a series of rotating lobes on a set of rotors within a housing. Early designs had fewer lobes that were cut straight making them noisy and relatively inefficient. Modern roots blowers have the lobes twisted axially and have tighter tolerances and better housing designs. Efficiency has been improved greatly. While the roots blower is simplistic, reliable, and can build boost off idle, it is still somewhat hampered by the inherent inefficiency of the compressor design and by the fact that the bulky nature of the unit precludes adaptation into cramped engine bays.

Lysholm (aka Screw) Compressor


The screw has all of the pros of a roots compressor with efficiency as good as or greater than that of a centrifugal design. Screw compressors are internally similar to a roots compressor except that each rotor has an extra lobe, and the lobes are not ground in the same way. The lobe design allows near interlocking of the lobes which increases thermal efficiency. They also have better high boost characteristics than a basic roots type compressor.

Centrifugal Compressors


These simplistically consist of a “fan” (vaned wheel) inside a scroll type housing. The compressor sucks air in and the vanes push the air to the outside edges of the scroll, causing pressurization. This design relies on “centrifugal force” to compress the air (the author is aware that technically centrifugal force is not a real force, but that’s how this compressor got its name). Because centrifugal compressors are not positive displacement, they do not have good compression characteristics at low speeds, and must reach high speeds for any significant compression to occur. At high RPMs, however, this compressor type is very efficient. Crank driven centrifugal superchargers generally are internally geared to operate the wheel in the 10K RPM range, whereas turbochargers may operate at over 100K RPMs.

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