The Roots type supercharger or Roots blower is a positive displacement type device 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 or, in a two-stroke diesel engine, by spur gears. It is named for the brothers Philander Higley and Francis Marion Roots, 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.
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Applications
Out of the three basic supercharger types the Roots has historically been considered the least fuel efficient. However, recent engineering developments by Eaton Corporation has resulted in a new Roots-type supercharger, the TVS, which yields a pump that is more efficient than all previous models. In addition, the Roots-type supercharger is simple and widely used and thus is invariably the most cost efficient. It is also more effective than alternative superchargers at developing positive intake manifold pressure (i.e., above atmospheric pressure) at low engine rpms, making it a popular choice for passenger automobile applications. Peak torque can be achieved by about 2000 rpm.
Although intercoolers are more commonly known for their use on turbochargers superchargers may also benefit from the use of an intercooler to remove heat produced during compression, but those producing manifold pressure above 5 psi derive the greatest improvement. 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 is commonly used on two-stroke diesel engines, which require some form of forced induction as there is no 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.. Roots blowers are typically used in applications where a large volume of air must be moved across a relatively small pressure differential. This includes low vacuum applications, with the roots blower acting alone, or use as part of a high vacuum system, in combination with other pumps. Some civil defense sirens used Roots blowers to pump air to the rotor(chopper). The most well known are the Federal Signal Thunderbolt Series, and ACA(now American Signal Corporation) Hurricane. These sirens are known as "supercharged sirens". Roots blowers are also used in reverse to measure the flow of gases or liquids, for example, in gas meters.
Technical considerations
Because rotary lobe pumps need to maintain a clearance between the lobes, a single stage roots blower can only pump gas across a limited pressure differential. If the pump is used outside its specification, the compression of the gas generates so much heat that the lobes expand to the point that they jam, damaging the pump. Roots pumps are capable of pumping large volumes but as they only achieve moderate compression, it is not uncommon to see multiple roots blower stages, frequently with heat exchangers (intercoolers) in between to cool the gas. The lack of oil on the pumping surfaces allows the pumps to work in environments where contamination control is important. The high pumping rate for hydrocarbons also allows the roots pump to provide an effective isolation between oiled pumps, such as rotary compression pumps, and the vacuum chamber. A variant uses claw-shaped rotors for higher compression.
Comparative advantages
The roots pump is never used as a stand alone pump in high vacuum applications but its high pumping speed can be used to reduce the end pressure and increase the pumping speed of other vacuum pump systems in the medium to low vacuum range.
References
- (April 5, 2002). Roots Type Superchargers Explained. SuperchargersOnline.com. Accessed September 11, 2004.
- Blower Briefing. Tom Henry Racing. Accessed September 11, 2004.
- Eaton TVS Supercharger The Eaton TVS Supercharger. Accessed November 11, 2006
The term blower is used to define a device placed on engines that can not breathe on their own and is needed for scavenging and a supercharger is used on engines to boost power even though they are the same device.
