Governor (device)

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A governor, or speed limiter or controller, is a device used to measure and regulate the speed of a machine, such as an engine.


A classic example is the centrifugal governor, also known as the Watt or fly-ball governor on a reciprocating steam engine, which uses the effect of inertial force on rotating weights driven by the machine output shaft to regulate its speed by altering the input flow of steam.


Cut-away drawing of steam engine speed governor. The valve starts fully open at zero speed, and is closed as the balls rotate and rise. The speed sensing drive shaft is top right Centrifugal governor and balanced steam valve (New Catechism of the Steam Engine, 1904).jpg
Cut-away drawing of steam engine speed governor. The valve starts fully open at zero speed, and is closed as the balls rotate and rise. The speed sensing drive shaft is top right
Porter governor on a Corliss steam engine Ashton Frost engine governor.jpg
Porter governor on a Corliss steam engine

Centrifugal governors were used to regulate the distance and pressure between millstones in windmills since the 17th century. Early steam engines employed a purely reciprocating motion, and were used for pumping water – an application that could tolerate variations in the working speed.

It was not until the Scottish engineer James Watt introduced the rotative steam engine, for driving factory machinery, that a constant operating speed became necessary. Between the years 1775 and 1800, Watt, in partnership with industrialist Matthew Boulton, produced some 500 rotative beam engines. At the heart of these engines was Watt's self-designed "conical pendulum" governor: a set of revolving steel balls attached to a vertical spindle by link arms, where the controlling force consists of the weight of the balls.

The theoretical basis for the operation of governors was described by James Clerk Maxwell in 1868 in his seminal paper 'On Governors'. [1]

Building on Watt's design was American engineer Willard Gibbs who in 1872 theoretically analyzed Watt's conical pendulum governor from a mathematical energy balance perspective. During his Graduate school years at Yale University, Gibbs observed that the operation of the device in practice was beset with the disadvantages of sluggishness and a tendency to over-correct for the changes in speed it was supposed to control. [2]

Gibbs theorized that, analogous to the equilibrium of the simple Watt governor (which depends on the balancing of two torques: one due to the weight of the "balls" and the other due to their rotation), thermodynamic equilibrium for any work producing thermodynamic system depends on the balance of two entities. The first is the heat energy supplied to the intermediate substance, and the second is the work energy performed by the intermediate substance. In this case, the intermediate substance is steam.

These sorts of theoretical investigations culminated in the 1876 publication of Gibbs' famous work On the Equilibrium of Heterogeneous Substances and in the construction of the Gibbs’ governor. These formulations are ubiquitous today in the natural sciences in the form of the Gibbs' free energy equation, which is used to determine the equilibrium of chemical reactions; also known as Gibbs equilibrium. [3]

Governors were also to be found on early motor vehicles (such as the 1900 Wilson-Pilcher), where they were an alternative to a hand throttle. They were used to set the required engine speed, and the vehicle's throttle and timing were adjusted by the governor to hold the speed constant, similar to a modern cruise control. Governors were also optional on utility vehicles with engine-driven accessories such as winches or hydraulic pumps (such as Land Rovers), again to keep the engine at the required speed regardless of variations of the load being driven.

Speed limiters

Governors can be used to limit the top speed for vehicles, and for some classes of vehicle such devices are a legal requirement. They can more generally be used to limit the rotational speed of the internal combustion engine or protect the engine from damage due to excessive rotational speed.


Today, BMW, Audi, Volkswagen and Mercedes-Benz limit their production cars to 250 kilometres per hour (155 mph). Certain Quattro GmbH and AMG cars, and the Mercedes/McLaren SLR is an exception. The BMW Rolls-Royces are limited to 240 kilometres per hour (149 mph). Jaguars, although British, also have a limiter, as do the Swedish Saab and Volvo on cars where it is necessary.

German manufacturers initially started the "gentlemen's agreement", electronically limiting their vehicles to a top speed of 250 kilometres per hour (155 mph), [4] [5] since such high speeds are more likely on the Autobahn. This was done to reduce the political desire to introduce a legal speed limit.

In European markets, General Motors Europe sometimes choose to discount the agreement, meaning that certain high-powered Opel or Vauxhall cars can exceed the 250 kilometres per hour (155 mph) mark, whereas their Cadillacs do not. Ferrari, Lamborghini, Maserati, Porsche, Aston Martin and Bentley also do not limit their cars, at least not to 250 kilometres per hour (155 mph). The Chrysler 300C SRT8 is limited to 270 km/h. Most Japanese domestic market vehicles are limited to only 180 kilometres per hour (112 mph) or 190 kilometres per hour (118 mph). [6] The top speed is a strong sales argument, though speeds above about 300 kilometres per hour (190 mph) are not likely reachable on public roads.[ citation needed ]

Many performance cars are limited to a speed of 250 kilometres per hour (155  mph ) [7] to limit insurance costs of the vehicle, and reduce the risk of tires failing.[ citation needed ]


Mopeds in the United Kingdom have had to have a 30 mph (48 km/h) speed limiter since 1977. [8] Most other European countries have similar rules (see the main article).

Public services vehicles

Public service vehicles often have a legislated top speed. Scheduled coach services in the United kingdom (and also bus services) are limited to 65 mph. [9]

Urban public buses often have speed governors which are typically set to between 65 kilometres per hour (40 mph) and 100 kilometres per hour (62 mph).[ citation needed ]

Trucks (HGVs)

All heavy vehicles in Europe and New Zealand have law/by-law governors that limits their speeds to 90 kilometres per hour (56 mph) or 100 kilometres per hour (62 mph).[ citation needed ] Fire engines and other emergency vehicles are exempt from this requirement.

Example uses


Aircraft propellers are another application. The governor senses shaft RPM, and adjusts or controls the angle of the blades to vary the torque load on the engine. Thus as the aircraft speeds up (as in a dive) or slows (in climb) the RPM is held constant.

Small engines

Small engines, used to power lawn mowers, portable generators, and lawn and garden tractors, are equipped with a governor to limit fuel to the engine to a maximum safe speed when unloaded and to maintain a relatively constant speed despite changes in loading. In the case of generator applications, the engine speed must be closely controlled so the output frequency of the generator will remain reasonably constant.

Small engine governors are typically one of three types: [10]

Turbine controls

Operation of a flyball governor to control speeds of a water turbine

In steam turbines, the steam turbine governing is the procedure of monitoring and controlling the flow rate of steam into the turbine with the objective of maintaining its speed of rotation as constant. The flow rate of steam is monitored and controlled by interposing valves between the boiler and the turbine. [11]

In water turbines, governors have been used since the mid-19th century to control their speed. A typical system would use a Flyball governor acting directly on the turbine input valve or the wicket gate to control the amount of water entering the turbine. By 1930, mechanical governors started to use PID controllers for more precise control. In the later part of the twentieth century, electronic governors and digital systems started to replace mechanical governors. [12]

Electrical generator

For electrical generation on synchronous electrical grids, prime movers drive electrical generators which are electrically coupled to any other generators on the grid. With droop speed control, the frequency of the entire grid determines the fuel delivered to each generator, so that if the grid runs faster, the fuel is reduced to each generator by its governor to limit the speed.


Governors are used in elevators. It acts as a stopping mechanism in case the elevator runs beyond its tripping speed (which is usually a factor of the maximum speed of the lift and is preset by the manufacturer as per the international lift safety guidelines). This device must be installed in traction elevators and roped hydraulic elevators.

See also

Related Research Articles

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Steam engine Heat engine that performs mechanical work using steam as its working fluid

A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be transformed, by a connecting rod and crank, into rotational force for work. The term "steam engine" is generally applied only to reciprocating engines as just described, not to the steam turbine. Steam engines are external combustion engines, where the working fluid is separated from the combustion products. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In general usage, the term steam engine can refer to either complete steam plants, such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine.

Steam turbine Machine that uses steam to rotate a shaft

A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft, possibly using a renewable energy heat source. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam turbine involves advanced metalwork to form high-grade steel alloys into precision parts using technologies that first became available in the 20th century; continued advances in durability and efficiency of steam turbines remains central to the energy economics of the 21st century.

Gas turbine Type of internal and continuous combustion engine

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Centrifugal governor Mechanism for automatically controlling the speed of an engine

A centrifugal governor is a specific type of governor with a feedback system that controls the speed of an engine by regulating the flow of fuel or working fluid, so as to maintain a near-constant speed. It uses the principle of proportional control.

Cruise control System that automatically controls the speed of a motor vehicle

Cruise control is a system that automatically controls the speed of a motor vehicle. The system is a servomechanism that takes over the throttle of the car to maintain a steady speed as set by the driver.

Ram air turbine

A ram air turbine (RAT) is a small wind turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed of the aircraft.

Diesel locomotive Locomotive powered by a diesel engine

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Propulsion transmission Drivetrain transmitting propulsion power

Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term transmission properly refers to the whole drivetrain, including clutch, gearbox, prop shaft, differential, and final drive shafts. In the United States the term is sometimes used in casual speech to refer more specifically to the gearbox alone, and detailed usage differs. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycles, fixed machines, and where different rotational speeds and torques are adapted.

Rocketdyne J-2 Rocket engine

The J-2 is a liquid-fuel cryogenic rocket engine used on NASA's Saturn IB and Saturn V launch vehicles. Built in the U.S. by Rocketdyne, the J-2 burned cryogenic liquid hydrogen (LH2) and liquid oxygen (LOX) propellants, with each engine producing 1,033.1 kN (232,250 lbf) of thrust in vacuum. The engine's preliminary design dates back to recommendations of the 1959 Silverstein Committee. Rocketdyne won approval to develop the J-2 in June 1960 and the first flight, AS-201, occurred on 26 February 1966. The J-2 underwent several minor upgrades over its operational history to improve the engine's performance, with two major upgrade programs, the de Laval nozzle-type J-2S and aerospike-type J-2T, which were cancelled after the conclusion of the Apollo program.

Steam power developed slowly over a period of several hundred years, progressing through expensive and fairly limited devices in the early 17th century, to useful pumps for mining in 1700, and then to Watt's improved steam engine designs in the late 18th century. It is these later designs, introduced just when the need for practical power was growing due to the Industrial Revolution, that truly made steam power commonplace.

Variable-pitch propeller (aeronautics) Propeller with blades that can be rotated to control their pitch while in use

In aeronautics, a variable-pitch propeller is a type of propeller (airscrew) with blades that can be rotated around their long axis to change the blade pitch. A controllable-pitch propeller is one where the pitch is controlled manually by the pilot. Alternatively, a constant-speed propeller is one where the pilot sets the desired engine speed (RPM), and the blade pitch is controlled automatically without the pilot's intervention so that the rotational speed remains constant. The device which controls the propeller pitch and thus speed is called a propeller governor or constant speed unit.

Overspeed is a condition in which an engine is allowed or forced to turn beyond its design limit. The consequences of running an engine too fast vary by engine type and model and depend upon several factors, the most important of which are the duration of the overspeed and the speed attained. With some engines, a momentary overspeed can result in greatly reduced engine life or catastrophic failure. The speed of an engine is typically measured in revolutions per minute (rpm).

Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines-

  1. Internal combustion and
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Droop speed control is a control mode used for AC electrical power generators, whereby the power output of a generator reduces as the line frequency increases. It is commonly used as the speed control mode of the governor of a prime mover driving a synchronous generator connected to an electrical grid. It works by controlling the rate of power produced by the prime mover according to the grid frequency. With droop speed control, when the grid is operating at maximum operating frequency, the prime mover's power is reduced to zero, and when the grid is at minimum operating frequency, the power is set to 100%, and intermediate values at other operating frequencies.

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The Heilmann locomotives were a series of three experimental steam-electric locomotives produced in the 1890s for the French Chemins de Fer de l'Ouest. A prototype was built in 1894 and two larger locomotives were built in 1897. These locomotives used electric transmission, much like later-popular diesel-electric locomotives and various other self powered locomotives.

Automatic generation control

In an electric power system, automatic generation control (AGC) is a system for adjusting the power output of multiple generators at different power plants, in response to changes in the load. Since a power grid requires that generation and load closely balance moment by moment, frequent adjustments to the output of generators are necessary. The balance can be judged by measuring the system frequency; if it is increasing, more power is being generated than used, which causes all the machines in the system to accelerate. If the system frequency is decreasing, more load is on the system than the instantaneous generation can provide, which causes all generators to slow down.

High-speed steam engine

High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.

Cataract (beam engine)

A cataract was a speed governing device used for early single-acting beam engines, particularly atmospheric engines and Cornish engines. It was a kind of water clock.


  1. Bennett, Stuart (1992). A history of control engineering, 1930-1955. IET. p.  p. 48. ISBN   978-0-86341-299-8.
  2. Wheeler, Lynder Phelps (1947), "The Gibbs Governor for Steam Engines", in Wheeler, Lynder Phelps; Waters, Everett Oyler; Dudley, Samuel William (eds.), The Early Work of Willard Gibbs in Applied Mechanics, New York: Henry Schuman, pp. 63–78
  3. Wheeler, L. (1951). Josiah Willard Gibbs - the History of a Great Mind. Woodbridge, CT: Ox Bow Press.
  4. Bogdan Popa (28 July 2012). "Gentlemen's Agreement: Not So Fast, Sir!". autoevolution.
  5. van Gorp, Anke. "Ethical Issues in Engineering Design; Safety and Sustainability" page 16. Published by 3TU Ethics, 2005. ISBN   9090199071, 9789090199078 . ISSN 1574-941X
  6. "Why Japan finally got its foot off the brake | The Japan Times Online". 2008-04-13. Retrieved 2012-11-08.
  7. Mike Spinelli (11 February 2006). "So Long Guv'nor: Mercedes Will Unlock Top Speed on AMG Models in the US, for a Price". Jalopnik.
  8. Department for Transport (2008). "Reported Road Casualties Great Britain: 2008 Annual Report" (PDF). Retrieved 2010-01-09. p.179 states: "Mopeds redefined to 30 mph maximum design speed"
  9. "History of British road safety". Archived from the original on 2010-06-17. Retrieved 2010-01-20.
  10. "How does a small engine governor work? | Briggs & Stratton". Retrieved 2018-03-22.
  11. Rathore, M.M. (2010). Thermal engineering. New Delhi: Tata McGraw-Hill Education. ISBN   978-0-07-068113-2 . Retrieved 29 January 2015.
  12. Fasol, Karl Heinz (August 2002). "A Short History of Hydropower Control" (PDF). IEEE Control Systems Magazine. 22 (4): 68–76. doi:10.1109/MCS.2002.1021646. Archived from the original (PDF) on 6 November 2015. Retrieved 29 January 2015.