Definitions

twin-screw

Twin-screw type supercharger

The twin-screw type supercharger or twin-screw blower is a positive displacement type device that operates by pulling air through a pair of meshing high-tolerance screws not dissimilar to a set of worm gears. Twin-screw superchargers are also known as Lysholm superchargers (or compressors).

Technical considerations

The intake is located at one end of the two screws, which overlap, but not completely, leaving a small pocket open. As the screws turn the pocket moves towards the exhaust side, pulling air in from the intake. The screws are machined so the pocket gets smaller, compressing the air as it moves through them. The supercharger is typically driven directly from the engine's crankshaft via a belt or gear drive. Unlike the Roots type supercharger, the twin-screw exhibits internal compression which is the ability of the device to compress air "within" the housing as it is moved through the device.

High-precision CNC manufacturing techniques make the screw type supercharger a more expensive alternative to other forms of available forced induction. Also due to the internal compression the air exiting the supercharger exhaust "pops", this can lead to a whining, whistling or screaming noise. Auto manufacturers utlize noise abatement devices and materials to lower the noise output from the supercharger.

All supercharger types benefit from the use of an intercooler to reduce heat produced during compression.

History

Heinrich Krigar in Germany on March 24th, 1878, patent #4121, first patented the principle of the screw compressor. He modified and improved his designs later that year and lodged a second patent (number 7116) on August 16th, 1878. Both of these patents are amongst the earliest on record, as the German Patent Office had only been formed just one year earlier.

Heinrich Krigar resided in Hannover and his illustrations clearly show a two-lobe rotor assembly, each rotor having the same profile as each other. In fact the rotor configuration resembles the Roots design, exhibited in Europe during 1867, with the exception that the rotors in the Krigar screw compressor twist through an angle of 180 deg along their length.

At that time it was not possible to develop the idea any further because of the lack of manufacturing technology.

Half a century later, a Swedish steam turbine manufacturer called Ljungstroms Angturbin AB appointed a new Chief Engineer, his name was Alf Lysholm and he was to be the pivotal in the development of the modern screw compressor. At that time, Lysholm was looking into lightweight compressors for gas and steam turbine use.

By this time the original patent rights had expired. Lysholm developed the profile of the screw compressor and tested various configurations and rotor lobe combinations. Not only was the shape of the rotors important, he solved the problem and patented the method for accurately machining the rotors. The 1935 patent clearly shows his asymmetric 5 female - 4 male lobe rotor design, although the shapes have been 'fine tuned' over the years, the screw compressor had at last been born.

Ljungstroms Angturbin AB changed its name to Svenska Rotor Maskiner AB in 1951. This company is well known throughout the world as SRM.

This type of compressor was designed, patented and manufactured by Svenska Rotor Maskiner AB(SRM) in Sweden in the 1930s, whose chief engineer was Alf Lysholm; this company still holds the patents on screw compressors. For years, because of the exceptionally high cost of machining the two mismatching convoluted rotors, screw compressors were used primarily on large stationary engines. Examples include the Elliot-Lycholm and the Broom Wade.

Screw compressors were also used in special applications requiring a highly reliable source of oil-free air, such as airplane or submarine cabin pressurizing. Over the years, SRM created hundreds of designs and sizes of screw compressors and licensed then to dozens of companies in many countries for various industrial and other applications. Between 1986-1988 in the U.S. Norm Drazy painstakingly developed a large screw-type supercharger, using a four-lobe male rotor and a six-lobe female rotor (turning 30% slower), called the PSI, hoping to introduce it to Top Fuel drag racing. In dyno testing, the 385 cu. in. prototype PSI made the same horsepower (1260 on alcohol) at 28 pounds of boost as a 14-71 (552 cu. in.) at 36 psi; plus the outlet temperature of the PSI was 85 degrees compared to the 150 for the 14-71. Unfortunately, the PSI was banned from the Top Fuel category by the NHRA, but the PSI and the newer large Whipple screw compressor have become standards in Top Alcohol drag racing, not only because of their high efficiency, but because they don't need to be "re-stripped" with Teflon after every round.

Manufacturers

Companies currently manufacturing twin-screw superchargers include Kenne Bell, Lysholm, Autorotor, Whipple, PSI, Ingersoll-Rand, Kobelco, and Sprintex.

Applications

The Ford GT, Mercedes-Benz SLR McLaren, Mercedes-Benz C32 and SLK32 AMG, Mercedes-Benz E55 AMG, Mazda Millenia S, Koenigsegg CC8S and Mercury Marine engines (Verado series) have Lysholm type superchargers.

Comparative advantages

A clear example of the technology applied by Autorotor in companies like Active Autowerk and Kennebell Supercharger systems can also demonstrate the effectiveness of the twin screw. While some centrifugal superchargers are consistent and reliable, they typically don't produce full boost until near peak engine rpm, while positive displacement superchargers such as Roots type superchargers and twin-screw types offer more immediate boost.

Many cold-air kits, exhaust upgrades, and most often any engine upgrade can often be retained when installing an aftermarket twin screw.

Related terms

The term "blower" is commonly used to define a device placed on engines with a functional need for additional airflow. The term blower is used to describe different types of super chargers. A screw type super charger, roots type super charger, and a centrifugal super charger are all types of blowers.

See also

Sources

  • Jim Bell's books on Turbocharging and Superchargers
  • http://www.procharger.com/

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