Roots-type supercharger

Roots blower, rotary lobe blower without internal compression, similar to the gear and rotary vane pumps. Two usually three -bladed rotor transport in the gaps between the wings and the outer wall of a gas ( usually air) from the suction side to the pressure side, which is then compressed by the backflow of the compressed gas. They are (approx. 1 bar) used in small to medium power and mostly small pressure differences. They serve as a simple vacuum pumps in industrial drying and extraction, during loading and unloading of bulk commodities such as grain in ships, as suction pumps in sewage tankers or as charging or scavenging blower for combustion engines. There, they are usually driven by the crankshaft via a belt.

History

The name comes from the American brothers Philander and Francis Roots, which the fan in 1860 could be patented. At that time, Roots blower was used mostly as a wind generator for blast furnaces, but also in other industries.

Paul Daimler built in 1921 as first a Roots supercharger in a car one.

Method of construction

Roots blower consist of a housing, in which two shafts rotate, which are connected outside of the housing with a gear stage synchronously in opposite directions the same speed. Each shaft carries a runner whose profile initially an "8" was similar; other profiles are known today. In the space of one night each a head tip of the other eight runs; the two profiles " comb " with one another, they are together and seal to the housing, but is not possible friction arise.

Production

The early Roots'schen wind and weather fan were built of wood, with about two meters rotor diameter. Today's Roots housing and rotor are highly sophisticated components, which are manufactured with substantial demands on the dimensional tolerances, material qualities and on the processing methods and are therefore very expensive to manufacture. However, all other comparable compressors such as screw compressor or scroll compressors even more expensive, so that even for the simple Roots blower remains a market share.

Criticisms

With increasing temperature requiring the thermal expansion of the components in the cold running allowing a play between them, which the sealing of the Roots blower is deteriorated, as long as the machine has not yet reached its operating temperature. In case of temperature, the runner can jam or seize the same reason.

Roots blowers are very sensitive to contamination; such small deviations in the compressor can significantly reduce the charge pressure and thus the performance.

Roots blower must be checked against unbalances; they will also test driven at mechanical engineering use cases, usually with 15 to 20% overspeed on a test stand.

Use

Two-stroke diesel engines had almost always Roots blower (eg Commer TS3 engine, or the Krupp Titan). The Roots blower must - in spite of today's twisted, three - or four-wing rotors - not to be confused with a screw loader, which differs by internal compression from the Roots blower. For the most efficient heat engines ever, the two-stroke marine diesel engines to Roots blower often as auxiliaries and Anfahrgebläse. They can do this instead of being driven by the crank mechanism in the fixed speed ratio, also have an exclusive manufacturer, speed decoupled drive.

Roots of the charger of an engine is mechanically driven and therefore consumes part of the power, as opposed to the exhaust gas turbocharger, which draws its driving power from the exhaust stream. Therefore, some manufacturers provide the Gebläseantriebsrad with a magnetic coupling ( principle of air conditioning compressor), with the fan in the lower part load range can be switched off. Advantage of the piston supercharger is immediately available when needed boost pressure, while the turbocharger about the system of turbine and compressor can only build up pressure with a certain delay.

If the blower is used as a loader, it runs dry; as a Roots compressor for greater pressures from 8 bar there is oil lubricated, so that it achieves its sealing effect.

Another application point were and are ventilation systems in explosion-protected areas (eg mines and bunkers ). Due to their design, they are well able to suppress pressure pulses from one side to the other.

Racing Cars

The advent of the Roots supercharger in racing mid -1920s ( Blower Bentley v Mercedes SSK) was the trigger for the racing Formula discussions: as far as engine capacity racing classes criterion, the cubic capacity of a supercharged engine with a correction factor has since been often applied to him to gain a competitive advantage against unaufgeladene engines. This means a turbocharged engine must have a smaller relative to the naturally aspirated engine capacity. This is often independent of the principle of the loader, whether turbo or supercharger.

Particularly successful racing cars were in addition to the supercharged racing cars of the 1920s, the Auto Union Grand Prix car between 1934 and 1939. Too, the 500 - cc BMW racing motorcycles of the 1930s drove with Roots superchargers numerous racing victories. So won " Schorsch " Meier in 1939 as the first foreigner ever to the 500cc race at the famous Tourist Trophy on the Isle of Man.

Classification and function

Roots blowers are among the group of two -wave Rotationsverdrängermaschinen in rotary piston type. They work on the principle of external compression (also known as isochoric compression ) so that the working medium no densification by volume change in the closed working chamber experiences, but is pushed against the system backpressure. The resulting return flows from the pressure side in the opening working chamber at the beginning of Ausschiebevorgangs one hand are unfavorable in terms of the thermodynamic process control and partly from fluid mechanical point of view responsible for the noise emission, especially at high speeds.

Alternatives, like other principles

VW built some time in the 1990s, so-called G-Lader with a fixed and an oscillating, "G" - shaped component, which is just like a compressor or Roots supercharger driven mechanically. However, this " G" only a continuous compression molding, which produces little friction, but could be applied as a lubricating failure defect in question. However, this design principle seemed to have not been successful and disappeared after a few years.

Among the mechanically driven supercharger systems, there are also the so-called Comprex (or pressure cell ) charger. However, this is not strictly speaking a displacement, but the mechanical drive has only a control function.