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{{main|Wax thermostatic element}}
[[Image:carthermostat.jpg|thumb|right|Car engine thermostat]]
Perhaps the best example of purely mechanical technology in widespread use today is the [[internal combustion engine]] cooling thermostat. These are used to maintain the core temperature of the engine at its optimum [[operating temperature]] by regulating the flow of [[coolant]] to an external [[heat sink]], usually an air cooled [[radiator (engine cooling)|radiator]]. Also, research in the 1920s showed that cylinder wear was
This type of thermostat operates mechanically. It makes use of a [[wax]] [[Pelletizing|pellet]] inside a sealed chamber. The wax is solid at low temperatures but as the engine heats up the wax melts and expands. The sealed chamber has an expansion provision that operates a rod which opens a [[valve]] when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90°[[Celsius|C]] (160 to 200°[[Fahrenheit|F]]). Modern engines run hot, that is, over 80°C (180°F), in order to run more efficiently and to reduce the emission of pollutants. Most thermostats have a small bypass hole to vent any gas that might get into the system, ''e.g.'', air introduced during coolant replacement, which also allows a small flow of coolant past the thermostat when it is closed. This bypass flow ensures that the thermostat experiences the temperature change in the coolant as the engine heats up; without it a stagnant region of coolant around the thermostat could shield it from temperature changes in the coolant adjacent to the [[combustion chamber]]s and [[cylinder bore]]s.
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