Governor (device)
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A governor 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, which uses weights mounted on spring-loaded arms to determine how fast a shaft is spinning, and then uses proportional control to regulate the shaft speed.
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[edit] 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.
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, Gibbs observed that the operation of the device in practice was beset with the disadvantages of sluggishness and a tendency to overcorrect 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 the Gibbs' famous work On the Equilibrium of Heterogeneous Substances and in the construction of the Gibbs’ governor, shown adjacent. 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.[1]
[edit] Automobiles
Automobiles are a common application, and modern automobiles are equipped with such a mechanism for various reasons. There are two types of automobile governors, one limiting the rotational speed of the engine, the other limiting the speed of the vehicle. In small, low power applications, governors are used to protect the engine from damage due to excessive rotational speed, or pushing the engine past its peak abilities. In larger, higher performance engines governors are used to limit the vehicle speed. Many performance cars are limited to a speed of 250 km/h (155 mph)[2] to limit insurance costs of the vehicle and reduce the risk of tires (tyres) failing. All heavy vehicles in Europe have by law speed governors that limits the speed to 90 km/h (56 mph) (100 km/h for certain buses). Urban public buses often have speed governors which are typically set to between 40 and 55 miles per hour.
The German manufacturers initially started the gentlemen's agreement, since high speeds are more likely on the autobahn. This was done to reduce the political willpower to introduce a speed limit on the Autobahn. Today, BMW, Audi, Volkswagen and Mercedes-Benz limit their production cars to 155 mph (249 km/h). Certain AMG cars and the Mercedes/McLaren SLR are exceptions. The Rolls-Royce Phantom (BMW) is limited to 149 mph (240 km/h). Jaguar (Ford), despite being British, also have a limiter. As do the Swedish Saab and Volvo on cars where it is necessary.
In European markets, GM sometimes choose to discount the agreement, meaning that certain high-powered Opel or Vauxhall cars can exceed the 250 km/h 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 (156 mph). Chryslers in Europe such as the 300C SRT8 have no limiter. High-powered Japanese cars such as the Mitsubishi Evo or Nissan Skyline GT-R have no limiter. The top speed is a strong sales argument, even if it is so high, above 300 km/h (187.5 mph), that it is likely not reachable on a public road.
[edit] 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.
[edit] See also
[edit] References
- ^ Wheeler, L. (1951). Josiah Willard Gibbs - the History of a Great Mind. Woodbridge, CT: Ox Bow Press.
- ^ The "gentlemen's agreement" top speed of 250 km/h (155.2 mph)
