Roots-Type Superchargers
The Roots type blower is a positive displacement lobe pump , which operates by pumping a fluid with a pair of meshing lobes not unlike a set of stretched gears.

Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The most common application of the Roots type blower has been as the induction device on two-stroke Diesel engines, such as those produced by Detroit Diesel and Electro-Motive Diesel.

Root Blower Operational Logic


Roots type blowers are also used to supercharge Otto cycle engines, with the blower being driven from the engine crankshaft via a toothed or V-belt, a roller chain or a gear train.

Philander and Francis Marion Roots, who are inventors of this system, patented this system on 1860.

Roots blowers are commonly referred to as air blowers or PD (positive displacement) blowers, and can be commonly called "huffers" when used with the gasoline-burning engines in hot rod customized cars.

Most modern Roots-type superchargers incorporate three-lobe or four-lobe rotors; this allows the lobes to have a slight twist along the rotor axes, which in turn reduces pulsing in the input and output.

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.

 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 provokes detonation in a petrol engine, and can melt the pistons in a diesel, while an intercooling stage adds complexity but can improve the power output by increasing the amount of the input charge, exactly as if the engine were of higher capacity. An intercooler reduces the thermodynamic efficiency by losing the heat (power) introduced by compression, but increases the power available because of the increased working mass for each cycle. Above about 5 psi (0.3 bar), the intercooling improvement can become dramatic.

The Roots design was commonly used on two-stroke diesel engines, which require some form of forced induction, as there is no separate intake stroke.

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. One of these applications, very common in the industry, is to use roots blowers as air movers for pneumatic conveying systems: the roots blowers is delivering a high volume of air in a pipe, which is swiping away solids products. Pneumatic conveying allows to larger amount of bulk solids from 1 point to another.


Technical Consideration:

The simplest form of a Roots blower has cycloidal rotors, constructed of alternating tangential sections of hypocycloidal and epicycloidal curves. For a two-lobed rotor, the smaller generating circles are one-quarter the diameter of the larger. Real Roots blowers may have more complex profiles for increased efficiency. The lobes on one rotor will not drive the other rotor with minimal free play in all positions, so that a separate pair of gears provide the phasing of the lobes.

Root Blower – 3 Lobes EMD Type Root Blower – 3 Lobes


Because rotary lobe pumps need to maintain a clearance between the lobes, a single stage Roots blower can pump gas across only 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.


The Roots-type blower may achieve an efficiency of around 70% while achieving a maximum pressure ratio of two. Because a Roots type blower pumps air in discrete pulses (unlike a screw compressor), pulsation noise and turbulence may be transmitted downstream. If not properly managed (through outlet piping geometry) or accounted for (by structural reinforcement of downstream components), the resulting pulsations can cause fluid cavitation and/or damage to components downstream of the blower.




According to shown graph, roots are intended to have efficiency of 90%. So, they are designed to work in scientifically drawn graph.