Roots type supercharger
Encyclopedia
The Roots type supercharger
or Roots blower is a positive displacement
lobe pump
which operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gear
s. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. It is frequently used as a supercharger in engines, where it 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 and Francis Marion Roots of Connersville, Indiana
, who first patent
ed the basic design in 1860 as an air pump
for use in blast furnace
s 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. Roots blowers are commonly referred to as air blowers or pd (Positive displacement) blowers.
Of the three basic supercharger types, the Roots design historically
possessed the worst thermal efficiency, especially at high pressure ratios. However, recent engineering
developments by Eaton Corporation
have resulted in a more efficient Roots-type supercharger, known as the TVS. This new Eaton TVS design has been proven to provide isentropic efficiencies as high as 76% while providing a significantly larger efficiency island than turbochargers. . Unlike the basic illustration, most modern Roots-type superchargers incorporate three-lobe or four-lobe rotors.
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.
Accumulated heat
is an important consideration in the operation of a compressor
in an internal combustion engine
. 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 intercooler
s are more commonly known for their use on turbocharger
s, superchargers may also benefit from the use of an intercooler. 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 intercooling stage adds complexity but can improve the efficiency by releasing some of the unneeded heat. Above about 5 psi (0.34473786465 bar) the intercooling 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
division 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 name
s are not connected.
The superchargers used on top fuel engines, funny car
s, and other dragster
s, as well as hot rod
s, are in fact derivative
s of General Motors Coach Division blowers for their industrial diesel engine
s, 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 cylinder
s of 71 cubic inch
es 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 siren
s 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 meter
s.
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 pump
s 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 exchanger
s (intercooler
s) 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 pump
s, 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.
Boost is given in terms of pressure ratio, which is the ratio of absolute air pressure before the blower to the absolute air pressure after compression by the blower. If no boost is present, the pressure ratio will be 1.0 (meaning 1:1), as the outlet pressure equals the inlet pressure. Fifteen psi boost is marked for reference (slightly above a pressure ratio of 2.0 compared to atmospheric pressure). At 15 psi (1 bar) boost, Roots blowers hover between 50% and 58%. Replacing a smaller blower with a larger blower moves the point to the left. In most cases, as the map shows, this will move it into higher efficiency areas on the left as the smaller blower likely will have been running fast on the right of the chart. Usually, using a larger blower and running it slower to achieve the same boost will give an increase in compressor efficiency.
The volumetric efficiency of the Roots-type blower is very good, usually staying above 90% at all but the lowest blower speeds. Because of this, even a blower running at low efficiency will still mechanically deliver the intended volume of air to the engine, but that air will be hotter. In drag racing applications where large volumes of fuel are injected with that hot air, vaporizing the fuel absorbs the heat. This functions as a kind of liquid aftercooler system and goes a long way to negating the inefficiency of the Roots design in that application.
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
or Roots blower is a positive displacement
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
lobe pump
Lobe pump
right|thumb|250px|Lobe pumps are used in a variety of industries including pulp and paper, chemical, food, beverage, pharmaceutical, and biotechnology...
which operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. It is frequently used as a supercharger in engines, where it is driven directly from the engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
's crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
via a belt
Belt (mechanical)
A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
or, in a two-stroke diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
, by spur gears.
It is named for the brothers Philander and Francis Marion Roots of Connersville, Indiana
Connersville, Indiana
At the 2000 census, there were 15,411 people, 6,382 households and 4,135 families residing in the city. The population density was 1,894.5 per square mile . There were 6,974 housing units at an average density of 857.3 per square mile...
, who first patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
ed the basic design in 1860 as an air pump
Air pump
An air pump is a device for pushing air. Examples include a bicycle pump, pumps that are used to aerate an aquarium or a pond via an airstone; a gas compressor used to power a pneumatic tool, air horn or pipe organ; a bellows used to encourage a fire; and a vacuum pump.The first effective air pump...
for use in blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...
s and other industrial
Industry
Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
applications. In 1900, Gottlieb Daimler
Gottlieb Daimler
Gottlieb Daimler was an engineer, industrial designer and industrialist born in Schorndorf , in what is now Germany. He was a pioneer of internal-combustion engines and automobile development...
included a Roots-style supercharger in a patented engine design, making the Roots-type supercharger the oldest of the various designs now available. Roots blowers are commonly referred to as air blowers or pd (Positive displacement) blowers.
Applications
History
History is the discovery, collection, organization, and presentation of information about past events. History can also mean the period of time after writing was invented. Scholars who write about history are called historians...
possessed the worst thermal efficiency, especially at high pressure ratios. However, recent engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
developments by Eaton Corporation
Eaton Corporation
Eaton Corporation is a global diversified power management company with 2010 sales of $13.7 billion. The company is a leading provider of electrical components and systems for power quality, distribution and control; hydraulics components, systems and services for industrial and mobile equipment;...
have resulted in a more efficient Roots-type supercharger, known as the TVS. This new Eaton TVS design has been proven to provide isentropic efficiencies as high as 76% while providing a significantly larger efficiency island than turbochargers. . Unlike the basic illustration, most modern Roots-type superchargers incorporate three-lobe or four-lobe rotors.
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
Passenger
A passenger is a term broadly used to describe any person who travels in a vehicle, but bears little or no responsibility for the tasks required for that vehicle to arrive at its destination....
automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
applications. Peak torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
can be achieved by about 2000 rpm.
Accumulated heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
is an important consideration in the operation of a compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
in an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
. In accordance with the ideal gas law
Ideal gas law
The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's 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 intercooler
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...
s are more commonly known for their use on turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...
s, superchargers may also benefit from the use of an intercooler. Internal combustion is based upon a thermodynamic cycle
Thermodynamic cycle
A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while varying pressure, temperature, and other state variables, eventually returning a system to its initial state...
, 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 intercooling stage adds complexity but can improve the efficiency by releasing some of the unneeded heat. Above about 5 psi (0.34473786465 bar) the intercooling improvement can become dramatic. With a Roots-type supercharger, one method successfully employed is the addition of a thin heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
placed between the blower and the engine. Water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
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
Detroit Diesel
As a corporation, Daimler Trucks North America has decided to rename the company "DETROIT".Detroit Diesel Corporation is an American-based diesel engine producer headquartered in Detroit, Michigan, USA...
division of General Motors), which require some form of forced induction
Forced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...
, as there is no separate intake stroke. The Rootes Co. two-stroke diesel engine, used in Commer
Commer
Commer became known in later years as a maker of vans for the British Post Office—particularly the Commer FC which was introduced in 1960 with many body styles, including a 1500 cc van. After engine and interior upgrades it was renamed the PB in 1967 and the SpaceVan in 1974...
and Karrier
Karrier
Karrier is a marque of car and commercial vehicle, the origins of which can be traced back to Clayton and Company, a 1904 company from Huddersfield, West Yorkshire, UK. In 1908, they started making Karrier cars and in 1920 changed the company name to Karrier Motors Ltd.In 1929, Karrier started...
vehicles, had a Roots-type blower but the two name
Name
A name is a word or term used for identification. Names can identify a class or category of things, or a single thing, either uniquely, or within a given context. A personal name identifies a specific unique and identifiable individual person, and may or may not include a middle name...
s are not connected.
The superchargers used on top fuel engines, funny car
Funny Car
Funny Car is a drag racing car class. In the United States, other "professional" classes are Top Fuel, Pro Stock, and Pro Stock Motorcycle. Funny cars have forward-mounted engines and carbon fiber automotive bodies over the chassis, giving them an appearance vaguely approximating manufacturers'...
s, and other dragster
Drag racing
Drag racing is a competition in which specially prepared automobiles or motorcycles compete two at a time to be the first to cross a set finish line, from a standing start, in a straight line, over a measured distance, most commonly a ¼-mile straight track....
s, as well as hot rod
Hot rod
Hot rods are typically American cars with large engines modified for linear speed. The origin of the term "hot rod" is unclear. One explanation is that the term is a contraction of "hot roadster," meaning a roadster that was modified for speed. Another possible origin includes modifications to or...
s, are in fact derivative
Derivative
In calculus, a branch of mathematics, the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a...
s of General Motors Coach Division blowers for their industrial diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s, which were adapted for automotive use in the early days of the sport
Sport
A Sport is all forms of physical activity which, through casual or organised participation, aim to use, maintain or improve physical fitness and provide entertainment to participants. Sport may be competitive, where a winner or winners can be identified by objective means, and may require a degree...
of drag racing
Drag racing
Drag racing is a competition in which specially prepared automobiles or motorcycles compete two at a time to be the first to cross a set finish line, from a standing start, in a straight line, over a measured distance, most commonly a ¼-mile straight track....
. 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 cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
s of 71 cubic inch
Cubic inch
The cubic inch is a unit of measurement for volume in the Imperial units and United States customary units systems. It is the volume of a cube with each of its 3 sides being one inch long....
es 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 siren
Civil defense siren
A civil defense siren is a mechanical or electronic device for generating sound to...
s used Roots blowers to pump air to the rotor (chopper). The most well known are the Federal Signal Thunderbolt Series
Thunderbolt Siren
The Thunderbolt siren is a discontinued electro-mechanical warning siren that was once manufactured in the United States by Federal Signal Corporation. The Thunderbolt siren consists of three major components: the blower, rotator and siren chopper. The blower, which is made up of a motor-driven...
, 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 meter
Gas meter
A gas meter is used to measure the volume of fuel gases such as natural gas and propane. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, as measured volumes are highly...
s.
Technical considerations
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.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 pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
s 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 exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s (intercooler
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...
s) 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 pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
s, such as rotary compression
Rotary compression
A rotary compression pump works in cycles. Over each cycle a volume is created in contact with the cavity where pressure is to be lowered. By random motion, molecules from the vacuum pass into the volume created by the pump. This volume is then cut off from the cavity and compressed in contact with...
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
Screw compressor
A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. They are commonly used to replace piston compressors where large volumes of high pressure air are needed, either for large industrial applications or to operate high-power air tools such...
), 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.
Roots Efficiency map
For any given roots blower running under given conditions, a single point will fall on the map. This point will rise with increasing boost and will move to the right with increasing blower speed. It can be seen that, at moderate speed and low boost, the efficiency can be over 90%. This is the area in which Roots blowers were originally intended to operate, and they are very good at it.Boost is given in terms of pressure ratio, which is the ratio of absolute air pressure before the blower to the absolute air pressure after compression by the blower. If no boost is present, the pressure ratio will be 1.0 (meaning 1:1), as the outlet pressure equals the inlet pressure. Fifteen psi boost is marked for reference (slightly above a pressure ratio of 2.0 compared to atmospheric pressure). At 15 psi (1 bar) boost, Roots blowers hover between 50% and 58%. Replacing a smaller blower with a larger blower moves the point to the left. In most cases, as the map shows, this will move it into higher efficiency areas on the left as the smaller blower likely will have been running fast on the right of the chart. Usually, using a larger blower and running it slower to achieve the same boost will give an increase in compressor efficiency.
The volumetric efficiency of the Roots-type blower is very good, usually staying above 90% at all but the lowest blower speeds. Because of this, even a blower running at low efficiency will still mechanically deliver the intended volume of air to the engine, but that air will be hotter. In drag racing applications where large volumes of fuel are injected with that hot air, vaporizing the fuel absorbs the heat. This functions as a kind of liquid aftercooler system and goes a long way to negating the inefficiency of the Roots design in that application.
Comparative advantages
Rotary lobe blowers, commonly called boosters in high vacuum application, are not used as a stand alone pump. In high vacuum applications the boosters pumping speed can be used to reduce the end pressure and increase the pumping speed.Related terms
The term "blower" is commonly used to define a device placed on engines with a functional need for additional airflow using a direct mechanical link as its energy source. The term blower is used to describe different types of super chargers. A screw type super charger, Roots type super charger, and a centrifugal super charger are all types of blowers. Conversely, a turbocharger, using exhaust compression to spin its turbine, and not a direct mechanical link, is not generally regarded as a "blower" but simply a "turbo".External links
- Flash Animation of Roots Pump
- http://www.everestblowers.com/technical-articles/understandin-blowers.pdf
- http://www.pdblowers.com/t17-positive-displacement-blower-calculations.php
- http://roystongroup.com/history.htm