The
Roots type superchargerA 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 provided and more work to be done per cycle,...
or
Roots blower is a
positive displacement pumpA pump is a device used to move fluids, such as gases, liquids or slurries. A pump displaces a volume by physical or mechanical action. One common misconception about pumps is the thought that they create pressure. Pumps alone do not create pressure; they only displace fluid, causing a flow. ...
which operates by pulling air through a pair of meshing lobes not unlike a set of stretched
gearsA gear is a component within a transmission device that transmits rotational torque by applying a force to the teeth of another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages that mesh with other gear teeth, allowing force to be fully...
. Air is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The supercharger is driven directly from the
engineAn engine is a machine that produces mechanical force and motion from another form of energy . It is also referred to as a prime mover. An automobile makes use of several motors to start the car and drive the car's various pumps – but the power plant that propels the car is called an engine...
's
crankshaftThe crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
via a
beltA belt is a looped strip of flexible material, used to mechanically link two or more rotating shafts. They may be used as a source of motion, to efficiently transmit power, or to track relative movement. Belts are looped over pulleys...
or, in a two-stroke
diesel engineA 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 during the final stage of compression...
, by spur gears.
It is named for the brothers Philander and Francis Marion Roots of
Connersville, IndianaConnersville is a city in Fayette County, Indiana, United States. The population was 15,411 at the 2000 census. The city is the county seat of Fayette County...
, who first
patentA patent is a set of exclusive rights granted by a state to an inventor or their assignee for a limited period of time in exchange for a public disclosure of an invention....
ed the basic design in 1860 as an air pump for use in
blast furnaceA blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions take place...
s and other
industrialAn industry is the manufacturing of a good or service within a category. Although industry is a broad term for any kind of economic production, in economics and urban planning industry is a synonym for the secondary sector, which is a type of economic activity involved in the manufacturing of raw...
applications. In 1900,
Gottlieb DaimlerGottlieb Daimler was an engineer, industrial designer and industrialist, born in Schorndorf , in what is now the Federal Republic of 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.
Applications
Of the three basic supercharger types, the Roots has
historicallyHistory is the study of the human past, with special attention to the written record. Scholars who write about history are called historians. It is a field of research which uses a narrative to examine and analyse the sequence of events, and it often attempts to investigate objectively the patterns...
been considered the least
fuel efficientFuel efficiency, is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance...
. However, recent
engineeringEngineering is the discipline, art and profession of acquiring and applying technical, scientific and mathematical knowledge to design and implement materials, structures, machines, devices, systems, and processes that safely realize a desired objective or inventions.The American Engineers' Council...
developments by
Eaton CorporationEaton Corporation is a diversified industrial manufacturer with 2008 sales of $15.4 billion...
have resulted in an improved Roots-type supercharger, known as the TVS, which is more efficient than their previous models. Unlike the basic illustration, most modern Roots-type superchargers incorporate three-lobe or four-lobe rotors. The latest design, introduced for the
Ford Mustang GT500 Super SnakeThe Shelby Mustang is a high performance variant of the Ford Mustang, built by Ford from 1965 through 1970. The shelbys' were a series of Ford Mustangs which were specially modified by Carroll Shelby's company and sold under the name Shelby GT with identification badges Shelby Cobra. From 1968 the...
, has four lobes per rotor, enhancing its efficiency through a reduction of pulsations.
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
passengerA 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.There are two common contexts in which the term is used:...
automobileAn automobile, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
applications. Peak
torqueTorque, also called 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
heatIn physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body....
is an important consideration in the operation of a
compressorCompressor may refer to:*Gas compressor, a mechanical device that compresses a gas *A device used to apply Video compression to a video signal*A device used to apply Audio data compression to an audio signal...
in an
internal combustion engineThe 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 pressure gases, which are produced by the combustion, directly applies force to a movable...
. Per the
ideal gas lawThe Ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behaviour 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
intercoolerAn 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...
s are more commonly known for their use on
turbochargerA turbocharger, or turbo, is a gas compressor that is used for forced-induction of an internal combustion engine. A form of supercharger, the purpose of a turbocharger is to increase the density of air entering the engine to create more power...
s, superchargers may also benefit from the use of an intercooler. Internal combustion is based upon a
thermodynamic cycleA thermodynamic cycle is 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. State Properties depend only on the thermodynamic state and cumulative variation of such...
, 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 the intercooling improvement can become dramatic. With a Roots-type supercharger, one method successfully employed is the addition of a thin
heat exchangerA heat exchanger is a device 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. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants,...
placed between the blower and the engine.
WaterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
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 dieselA two stroke diesel is a diesel engine that works in two strokes. A diesel engine is an internal combustion engine which operates using the Diesel cycle...
engines (popularized by the
Detroit DieselDetroit Diesel Corporation , is a diesel engine producer headquartered in Detroit, Michigan, USA. There are today two individual divisions that share this name: the off-highway division which is owned by Tognum, which EQT IV formed along with MTU Friedrichshafen, and the on-highway division which...
division of General Motors), which require some form of
forced inductionForced induction is a term used to describe internal combustion engines that are not naturally aspirated. A gas compressor is added to the air intake instead, thereby increasing the quantity of oxygen available for combustion...
, as there is no separate intake stroke. In this application, the blower does not provide significant compression and these engines are considered
naturally aspiratedA naturally-aspirated engine is a reciprocating internal combustion engine that depends solely on atmospheric pressure to draw in combustion air...
;
turbochargerA turbocharger, or turbo, is a gas compressor that is used for forced-induction of an internal combustion engine. A form of supercharger, the purpose of a turbocharger is to increase the density of air entering the engine to create more power...
s are generally used when significant "boost" is needed. The
Rootes Co. two-stroke diesel engine, used in
CommerCommer was a British manufacturer of commercial vehicles which existed from 1905 until 1979. Commer vehicles included car derived vans, light vans, medium to heavy commercial trucks, military vehicles and buses. The Commer brand was exported widely, with examples, albeit collectors' vehicles,...
and
KarrierKarrier 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
nameA name is a label for a noun, normally used to distinguish one from another. 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...
s are not connected.
The superchargers used on top fuel engines,
funny carFunny Car is a drag racing car class. In the USA, 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' showroom models...
s, and other
dragsterDrag racing is a competition in which vehicles compete to be the first to cross a set finish line, usually from a standing start, and in a straight line. First gaining popularity in the USA after World War II , the sport steadily grew in popularity and spread across the globe...
s, as well as
hot rodHot 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 explanation is that the mufflers were...
s, are in fact
derivativeIn calculus 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 a quantity is changing at a given point; for example, the derivative of the position of a vehicle with respect to time is the instantaneous velocity...
s of
General Motors Coach Division blowers for their industrial
diesel engineA 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 during the final stage of compression...
s, which were adapted for automotive use in the early days of the
sportSport is an activity that is governed by a set of rules or customs and often engaged in competitively. Sports commonly refer to activities where the physical capabilities of the competitor are the sole or primary determinant of the outcome , but the term is also used to include activities such as...
of
drag racingDrag racing is a competition in which vehicles compete to be the first to cross a set finish line, usually from a standing start, and in a straight line. First gaining popularity in the USA after World War II , the sport steadily grew in popularity and spread across the globe...
. 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
cylinderA cylinder is the central working part of a reciprocating engine, 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 precision features are machined into it...
s of 71
cubic inchA cubic inch is a non-SI unit of volume, equal to the volume of a cube with sides of one inch.Cubic inches are still sometimes used as a unit of measurement in the United States and Canada, although SI is continuing to gradually displace non-SI usage.-Notation conventions:The following symbols...
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 sirenA civil defense siren is a mechanical or electronic device for generating sound to provide warning of approaching danger and sometimes to indicate when the danger...
s used Roots blowers to pump air to the rotor (chopper). The most well known are the
Federal Signal Thunderbolt SeriesThe 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 motors...
, 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 meterA 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
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
pumpA pump is a device used to move fluids, such as gases, liquids or slurries. A pump displaces a volume by physical or mechanical action. One common misconception about pumps is the thought that they create pressure. Pumps alone do not create pressure; they only displace fluid, causing a flow. ...
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 exchangerA heat exchanger is a device 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. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants,...
s (
intercoolerAn 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...
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
pumpA pump is a device used to move fluids, such as gases, liquids or slurries. A pump displaces a volume by physical or mechanical action. One common misconception about pumps is the thought that they create pressure. Pumps alone do not create pressure; they only displace fluid, causing a flow. ...
s, such as
rotary compressionA 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), 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.
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 of other vacuum pump systems in the medium to low vacuum range.
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".