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The Roots Supercharger

One Of The Better Power Adders

Roots Blower

Internal workings of a Roots type supercharger.

Roots Supercharger Diagram:

1 Rotary vane 1

2. Pump body

3. Rotary vane 2

a. Intake

b. Pumping

c. Forced air or air-fuel mixture into intake manifold

Roots type supercharger on a nostalgia front engine dragster.

The Roots type supercharger or Roots blower is a positive displacement pump which operates by pulling air through a pair of meshing lobes not unlike a set of stretched gears. Air is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The supercharger is driven directly from the engine's crankshaft via a belt.

It is named for the brothers Philander Higley and Francis Marion Roots of Connersville, Indiana, who first patented the basic design in 1860 as an air pump for use in blast furnaces and other industrial applications. In 1900, Gottlieb Daimler included a Roots-style supercharger in a patented engine design, making the Roots-type supercharger the oldest of the various designs now available.

The term "blower" is commonly used to define a device placed on engines with a functional need for additional airflow using a direct mechanical link as its energy source. The term blower is used to describe different types of superchargers. A screw type supercharger, Roots type supercharger, and a centrifugal super charger are all types of blowers. Conversely, a turbocharger, using exhaust compression to spin its turbine, and not a direct mechanical link, is not generally regarded as a "blower" but simply a "turbo."


Out of the three basic supercharger types, the Roots has historically been considered the least fuel efficient. However, recent engineering developments by Eaton Corporation have resulted in a new Roots-type supercharger, the TVS, which yields a pump that is more efficient than all previous models. Unlike the illustration, modern Roots superchargers feature three-lobe rotors in most applications. The latest design, introduced for use in the Corvette ZR1, has four lobes per rotor, even further enhancing its efficiency. In addition, the Roots-type supercharger is simple and widely used. It can also be more effective than alternative superchargers at developing positive intake manifold pressure (i.e., above atmospheric pressure) at low engine speed, making it a popular choice for passenger automobile applications. Peak torque can be achieved by about 2000 rpm.

Accumulated heat is an important consideration in the operation of a compressor in an internal combustion engine. Per the ideal gas law, a compression operation will raise the temperature of the compressed output, all else equal. Additionally, the operation of the compressor itself requires energy input, which is converted to heat and can be transferred to the gas through the compressor housing, heating it more. Although intercoolers are more commonly known for their use on turbochargers, superchargers may also benefit from the use of an intercooler application. Internal combustion is based upon a thermodynamic cycle, and a cooler temperature of the intake charge results in a greater thermodynamic expansion and vice versa. A hot intake charge robs the engine of efficiency and produces diminishing returns from the compression process, while an intercooling stage adds complexity but can improve the efficiency by wasting out some of the unneeded heat. Above about 5 psi the intercooling improvement can become dramatic. With a Roots-type supercharger, one method successfully employed is the addition of a thin heat exchanger placed between the blower and the engine. Water is circulated through it to a second unit placed near the front of the vehicle where a fan and the ambient air-stream can dissipate the collected heat.

The Roots design was commonly used on two stroke diesel engines (popularized by General Motors), which require some form of forced induction, as there is no separate intake stroke. In this application, the blower does not provide significant compression and these engines are considered naturally aspirated; turbochargers are generally used when significant "boost" is needed. The Rootes Co. two-stroke diesel engine, used in Commer and Karrier vehicles, had a Roots-type blower but the two names are not connected.

The superchargers used on top fuel engines, funny cars, and other dragsters, as well as hot rods, are in fact derivatives of General Motors Coach Division blowers for their industrial diesel engines, which were adapted for automotive use in the early days of the sport of drag racing. The model name of these units delineates their size; i.e. the once commonly used "6-71" and "4-71" blowers were designed for General Motors diesels having six cylinders of 71 cubic inches each, and four cylinders of 71 cubic inches each, respectively. Current competition dragsters use aftermarket GMC variants similar in design to the -71 series, but with the rotor and case length increased for added pumping capacity, identified as the 8-71, 10-71, 14-71 etc.

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