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 speeds, making it a popular choice for passenger automobile applications. Peak torque can be achieved by about 2000 rpm. Unlike the basic illustration, most modern Roots-type superchargers incorporate three-lobe or four-lobe rotors.
Accumulated heat is an important consideration in the operation of a compressor in an internal combustion engine. Of the three basic supercharger types, the Roots design historically possessed the worst thermal efficiency, especially at high pressure ratios. In accordance with the ideal gas law, a compression operation will raise the temperature of the compressed output. 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 inter coolers are more commonly known for their use on turbochargers, superchargers may also benefit from the use of an inter cooler. 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 inter cooling stage adds complexity but can improve the efficiency by releasing some of the unneeded heat. Above about 5 psi (0.3 bar) the inter cooling 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 the Detroit Diesel [truck and bus] and Electro-Motive [railroad] divisions of General Motors), which require some form of forced induction, as there is no separate intake stroke. 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 "4–71" and "6–71" blowers were designed for General Motors diesels having four or six cylinders of 71 cubic inches each. 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.
No comments:
Post a Comment