How does Roots blower work?

In this paper, the design and operation of Roots blower and its application as supercharger are introduced.

1 Operating principle

2 Roots supercharger

3 Rotor design

3.1 Double vane vane

3.2 Three blade rotary vane

working principle

The working principle of Roots blower (Roots supercharger) with a pair of double blade rotating blades (rotors) below is named after its inventor Francis Roots. Fluid is pumped from the suction side (inlet) to the pressure side (outlet) between each engaging vane and the housing. The gaseous fluid is compressed due to the back pressure on the pressure side.

The rotor is usually driven by a belt and a pair of gears mounted at the rear. Therefore, the rotor will not be driven by the contact of its engaged blades! On the contrary, even a small gap must be kept between the rotating blades, otherwise the high speed will lead to unacceptable heat generation and rapid wear of the rotor.

However, three or even four blade rotors are usually used instead of two blade rotors. In order to achieve more continuous compression and thus avoid pressure fluctuations, the rotor is also twisted along its axis of rotation. The cross section profile of the rotor consists of cycloid. However, due to non-contact meshing, it is not really a cycloidal gear!

Roots blower is not only used as compressor of supercharged internal combustion engine. They are also used to pump liquids or food, such as rice or grain.

Roots supercharger

Air is drawn in through the front slot. Air enters the housing through the air inlet equipped with heat sink.

Roots supercharger is equipped with three blade rotating blades (rotors), which pump the air between the rotor and the inner wall of the housing from the suction side (top) to the discharge side (bottom). Due to the "accumulation" of air on the discharge side, the pressure there increases and the air is compressed accordingly.

The rotating blade is driven by the belt, and the driving pulley directly drives only one of the two rotors. The second rotor is driven by two helical gears.

Once the air is pumped to the pressure side, the compressed air can be used for the combustion process. The unwanted temperature rise during compression is offset by the heat sink on the housing.

The pressure ratio between the pressure side and the suction side of Roots supercharger is usually maximum 2. According to the speed, the efficiency in these cases is about 50%. The greater the pressure ratio, the faster the efficiency will decline! Basically, the smaller the pressure ratio, the higher the efficiency of Roots supercharger.

Double vane vane

For the construction of double blade rotor, the rolling circle of epicycloid and hypocycloid is selected as one fourth of the diameter of the base circle (see the article "Design and construction of cycloid gear").

Trifoil

For the construction of a three blade rotor, the rolling circle used to generate the hypocycloid and epicycloid will be selected as one sixth of the diameter of the base circle.

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