Wax thermostatic element

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Car engine wax thermostatic element Carthermostat.jpg
Car engine wax thermostatic element

The wax thermostatic element was invented in 1934 by Sergius Vernet (1899–1968). [1] 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).

Plumbing Systems for conveying fluids

Plumbing is any system that conveys fluids for a wide range of applications. Plumbing uses pipes, valves, plumbing fixtures, tanks, and other apparatuses to convey fluids. Heating and cooling (HVAC), waste removal, and potable water delivery are among the most common uses for plumbing, but it is not limited to these applications. The word derives from the Latin for lead, plumbum, as the first effective pipes used in the Roman era were lead pipes.

Sweden constitutional monarchy in Northern Europe

Sweden, officially the Kingdom of Sweden, is a Scandinavian Nordic country in Northern Europe. It borders Norway to the west and north and Finland to the east, and is connected to Denmark in the southwest by a bridge-tunnel across the Öresund Strait. At 450,295 square kilometres (173,860 sq mi), Sweden is the largest country in Northern Europe, the third-largest country in the European Union and the fifth largest country in Europe by area. The capital city is Stockholm. Sweden has a total population of 10.3 million of which 2.5 million have a foreign background. It has a low population density of 22 inhabitants per square kilometre (57/sq mi) and the highest urban concentration is in the central and southern half of the country.

Switzerland federal republic in Central Europe

Switzerland, officially the Swiss Confederation, is a sovereign state situated in the confluence of western, central, and southern Europe. It is a federal republic composed of 26 cantons, with federal authorities seated in Bern. Switzerland is a landlocked country bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. It is geographically divided between the Alps, the Swiss Plateau and the Jura, spanning a total area of 41,285 km2 (15,940 sq mi), and land area of 39,997 km2 (15,443 sq mi). While the Alps occupy the greater part of the territory, the Swiss population of approximately 8.5 million is concentrated mostly on the plateau, where the largest cities are located, among them the two global cities and economic centres of Zürich and Geneva.


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.

Thermal expansion The tendency of matter to change volume in response to a change in temperature

Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature.

Wax class of chemical compounds that are plastic (malleable) near ambient temperatures.

Waxes are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures. They include higher alkanes and lipids, typically with melting points above about 40 °C (104 °F), melting to give low viscosity liquids. Waxes are insoluble in water but soluble in organic, nonpolar solvents. Natural waxes of different types are produced by plants and animals and occur in petroleum.

A wax motor is a linear actuator device that converts thermal energy into mechanical energy by exploiting the phase-change behaviour of waxes. During melting, wax typically expands in volume by 5% to 20%.

Automotive thermostats

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. [2] 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" [3] and so such a fan is not the mechanism for primary control of the internal temperature.

Internal combustion engine Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine (ICE) is a heat engine where the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, rotor or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy.

An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the maximum operating temperature. Outside this range of safe operating temperatures the device may fail.

A coolant is a substance, typically liquid or gas, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

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. [4]

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. [5] [6] 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.

Cooling circuits have a small bypass path even when the thermostat is closed, usually by a small hole in the thermostat. This allows enough flow of cooling water to heat the thermostat when warming up. It also provided an escape route for trapped air when first filling the system. A larger bypass is often provided, through the cylinder block and water pump, so as to keep the rising temperature distribution even. [5]

Work on cooling high-performance aircraft engines in the 1930s led to the adoption of pressurised cooling systems, which became common on post-war cars. As the boiling point of water increases with increasing pressure, these pressurised systems could run at a higher temperature without boiling. This increased both the working temperature of the engine, thus its efficiency, and also the heat capacity of the coolant by volume, allowing smaller cooling systems that required less pump power. [6] A drawback to the bellows thermostat was that it was also sensitive to pressure changes, thus could sometimes be forced shut again by pressure, leading to overheating. [6] The later wax pellet type has a negligible change in its external volume, thus is insensitive to pressure changes. [6] It is otherwise identical in operation to the earlier type. Many cars of the 1950s, or earlier, that were originally built with bellows thermostats were later serviced with replacement wax capsule thermostats, without requiring any change or adaption.

This most common modern form of thermostat now uses a wax pellet inside a sealed chamber. [6] Rather than a liquid-vapour transition, these use a solid-liquid transition, which for waxes is accompanied by a large increase in volume. The wax is solid at low temperatures, and as the engine heats up, the wax melts and expands. The sealed chamber 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°C (160 to 200°F). [7] Modern engines run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce the emission of pollutants.

While the thermostat is closed, there is no flow of coolant in the radiator loop, and coolant water is instead redirected through the engine, allowing it to warm up rapidly while also avoiding hot spots. The thermostat stays closed until the coolant temperature reaches the nominal thermostat opening temperature. The thermostat then progressively opens as the coolant temperature increases to the optimum operating temperature, increasing the coolant flow to the radiator. Once the optimum operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the coolant recirculation flow and coolant flow to the radiator to maintain the engine temperature in the optimum range as engine heat output, vehicle speed, and outside ambient temperature change. Under normal operating conditions the thermostat is open to about half of its stroke travel, so that it can open further or reduce its opening to react to changes in operating conditions. A correctly designed thermostat will never be fully open or fully closed while the engine is operating normally, or overheating or overcooling would occur.

Double valve engine thermostat Double valve automotive thermostat.jpg
Double valve engine thermostat

Engines which require a tighter control of temperature, as they are sensitive to "Thermal shock" caused by surges of coolant, may use a "constant inlet temperature" system. In this arrangement the inlet cooling to the engine is controlled by double-valve thermostat which mixes a re-circulating sensing flow with the radiator cooling flow. These employ a single capsule, but have two valve discs. Thus a very compact, and simple but effective, control function is achieved.

The wax used within the thermostat is specially manufactured for the purpose. Unlike a standard paraffin wax, which has a relatively wide range of carbon chain lengths, a wax used in the thermostat application has a very narrow range of carbon molecule chains. The extent of the chains is usually determined by the melting characteristics demanded by the specific end application. To manufacture a product in this manner requires very precise levels of distillation.

Types of elements

Flat diaphragm element

The temperature sensing material contained in the cup transfers pressure to the piston by means of the diaphragm and the plug, held tightly in position by the guide. On cooling, the initial position of the piston is obtained by means of a return spring. Flat diaphragm elements are particularly noted for their high level of accuracy, and therefore mainly used in sanitary installations and heating.

Squeeze-push elements

Squeeze-Push elements contain a synthetic rubber sleeve-like component shaped like the 'finger of a glove' which surrounds the piston. As the temperature increases, pressure from the expansion of the thermostatic material moves the piston with a lateral squeeze and a vertical push. As with the flat diaphragm element, the piston returns to its initial position by means of a return spring. These elements are slightly less accurate but provide a longer stroke.


The stroke is the movement of the piston in relation to its starting point. The ideal stroke corresponds to the temperature range of the elements. According to the type of element, it can vary from 1.5 mm to 16 mm.

The temperature range lies between the minimum and maximum operating temperature of the element. Elements can cover temperatures ranging from -15 °C to +120 °C. Elements may move in proportion to the temperature change over some part of the range, or may open suddenly around a particular temperature depending on the composition of the waxes.

Hysteresis is the difference noted between the upstroke and down stroke curve on heating and cooling of the element. Hysteresis is caused by the thermal inertia of the element and by the friction between the parts in motion. [8]

See also

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Petrol engine internal combustion engine designed to run on gasoline

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Four-stroke engine engine

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.
Thermostat component which maintains a setpoint temperature

A thermostat is a component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.

Internal combustion engine cooling uses either air or liquid to remove the waste heat from an internal combustion engine. For small or special purpose engines, cooling using air from the atmosphere makes for a lightweight and relatively simple system. Watercraft can use water directly from the surrounding environment to cool their engines. For water-cooled engines on aircraft and surface vehicles, waste heat is transferred from a closed loop of water pumped through the engine to the surrounding atmosphere by a radiator.

Variable valve timing process of altering the timing of a valve lift event

In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.

Cylinder (engine) central working part of a reciprocating engine or pump, the space in which a piston travels, often equipped with a cylinder liner

A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work. Cylinders may be sleeved or sleeveless. A sleeveless engine may also be referred to as a "parent-bore engine".

Reed valve

Reed valves are a type of check valve which restrict the flow of fluids to a single direction, opening and closing under changing pressure on each face. Modern versions often consist of flexible metal or composite materials.

A hypereutectic piston is an internal combustion engine piston cast using a hypereutectic alloy–that is, a metallic alloy which has a composition beyond the eutectic point. Hypereutectic pistons are made of an aluminum alloy which has much more silicon present than is soluble in aluminum at the operating temperature. Hypereutectic aluminum has a lower coefficient of thermal expansion, which allows engine designers to specify much tighter tolerances.

Thermostatic radiator valve

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A pneumatic control valve actuator converts energy into mechanical motion. The motion can be rotary or linear, depending on the type of actuator.

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The cam-in-block valvetrain layout of piston engines is one where the camshaft is placed within the cylinder block, usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine. This contrasts with an overhead camshaft (OHC) design which places the camshafts within the cylinder head and drives the valves directly or through short rocker arms.

Heater core

A heater core is a radiator-like device used in heating the cabin of a vehicle. Hot coolant from the vehicle's engine is passed through a winding tube of the core, a heat exchanger between coolant and cabin air. Fins attached to the core tubes serve to increase surface for heat transfer to air that is forced past them, by a fan, thereby heating the passenger compartment.

Uniflow steam engine

The uniflow type of steam engine uses steam that flows in one direction only in each half of the cylinder. Thermal efficiency is increased in the compound and multiple expansion types of steam engine by separating expansion into steps in separate cylinders; in the uniflow design, thermal efficiency is achieved by having a temperature gradient along the cylinder. Steam always enters at the hot ends of the cylinder and exhausts through ports at the cooler centre. By this means, the relative heating and cooling of the cylinder walls is reduced.

A shutdown valve is an actuated valve designed to stop the flow of a hazardous fluid upon the detection of a dangerous event. This provides protection against possible harm to people, equipment or the environment. Shutdown valves form part of a safety instrumented system. The process of providing automated safety protection upon the detection of a hazardous event is called functional safety

Mahle GmbH company

MAHLE GmbH is an automotive parts manufacturer based in Stuttgart, Germany. It is one of the largest automotive suppliers worldwide. As a manufacturer of components and systems for the combustion engine and its periphery, the company is one of the three largest systems suppliers worldwide for engine systems, filtration, electrics, mechatronics, and thermal management. In 2018, Mahle GmbH sales amounted to over €12.5 billion

Radiator (engine cooling) heat-exchanging component of liquid cooled engines

Radiators are heat exchangers used for cooling internal combustion engines, mainly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plant or any similar use of such an engine.


  1. Vernet, Sergius "Thermostat," U.S. Patent no. 2,115,501 (filed: 1934 October 1 ; issued: 1938 April 26).
  2. "Engine cooling". Automotive Handbook (3rd ed.). Bosch. 1993. p. 413. ISBN   0-8376-0330-7. It is recommended that a temperature-sensitive thermostat incorporating an expansion element be installed to regulate temperature
  3. Bosch & Automotive Handbook , p. 414
  4. Sir Harry Ricardo - Memories and machines: the pattern of my life Constable, London, 1968. P218
  5. 1 2 Know Your Car (5th ed.). Autocar. 1957. pp. 57–58.
  6. 1 2 3 4 5 Setright, L. J. K. (1976). "Cooling". In Ian Ward (ed.). Anatomy of the Motor Car. Orbis. pp. 61–62. ISBN   0-85613-230-6.
  7. Don Knowles, Jack Erjavec Basic automotive service and maintenance Cengage Learning, 2004 ISBN   1-4018-5208-4 page 140
  8. "Hysteresis Curve Diagram". Rostra Vernatherm. 2011.