Wax thermostatic element

The wax thermostatic element was invented in 1936 by Sergius Vernet (1899-1968). Its principal application is in automotive thermostats used in the engine cooling system. The first applications in the plumbing and heating industries were in Sweden (1970) and in Switzerland (1971).

Wax thermostatic elements transform heat energy into mechanical energy using the thermal expansion of waxes when they melt. This wax motor principle also finds applications besides engine cooling systems, including heating system thermostatic radiator valves, plumbing, industrial, and agriculture.

The internal combustion engine cooling thermostat maintains the temperature of the engine near its optimum operating temperature by regulating the flow of coolant to an air cooled radiator. This regulation is now carried out by an internal thermostat. Conveniently, both the sensing element of the thermostat and its control valve may be placed at the same location, allowing the use of a simple self-contained non-powered thermostat as the primary device for the precise control of engine temperature. Although most vehicles now have a temperature-controlled electric cooling fan, "the unassisted air stream can provide sufficient cooling up to 95% of the time" and so such a fan is not the mechanism for primary control of the internal temperature.

Research in the 1920s showed that cylinder wear was aggravated by condensation of fuel when it contacted a cool cylinder wall which removed the oil film. The development of the automatic thermostat in the 1930s solved this problem by ensuring fast engine warm-up.

The first thermostats used a sealed capsule of an organic liquid with a boiling point just below the desired opening temperature. These capsules were made in the form of a cylindrical bellows. As the liquid boiled inside the capsule, the capsule bellows expanded, opening a sheet brass plug valve within the thermostat. As these thermostats could fail in service, they were designed for easy replacement during servicing, usually by being mounted under the water outlet fitting at the top of the cylinder block. Conveniently this was also the hottest accessible part of the cooling circuit, giving a fast response when warming up.

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