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.
History
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'. 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. 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. 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.
Cars
Today, BMW, Audi, Volkswagen and Mercedes-Benz limit their production cars to 250 km/h (155 mph). Certain Audi Sport GmbH and AMG cars, and the Mercedes/McLaren SLR are exceptions. The BMW Rolls-Royces are limited to 240 km/h (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 km/h (155 mph), 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 km/h (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 km/h (155 mph). The Chrysler 300C SRT8 is limited to 270 km/h (168 mph). Most Japanese domestic market vehicles are limited to only 180 km/h (112 mph) or 190 km/h (118 mph). The top speed is a strong sales argument, though speeds above about 300 km/h (190 mph) are not likely reachable on public roads. Some performance cars can be limited to a speed of 250 km/h (155 mph) for reasons such as limiting insurance costs of the vehicle, reducing the risk of tires failing, and abiding by the laws of certain jurisdictions if they wish to sell their models in those areas.
Mopeds Mopeds in the United Kingdom have required a 30 mph (48 km/h) speed limiter since 1977. 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 (105 km/h). Urban public buses often have speed governors which are typically set to between 65 km/h (40 mph) and 100 km/h (62 mph).
Trucks All heavy vehicles in Europe and New Zealand have law/by-law governors that limits their speeds to 90 km/h (56 mph) or 100 km/h (62 mph). Fire engines and other emergency vehicles are exempt from this requirement.
Example uses
Aircraft 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.
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