David Boettcher - freelance engineer

Eur Ing D B Boettcher BSc(Hons) CEng MIEE MIET

Providing innovative solutions to engineering and business problems opportunities.

What does an engine thermostat do, and how does it do it?

An engine thermostat, such as a typical car or lorry water cooled engine thermostat, has two separate but interrelated functions:

1. To bring the engine up to optimum operating temperature as quickly as possible; and

2. To maintain the engine at optimum operating temperature thereafter.


Internal combustion engines operate most efficiently at relatively high temperatures, typically above 80°C - 85°C (176°F - 185°F). Wear on the moving parts is reduced and thermal efficiency is increased by operating at this temperature.

Lower engine temperatures result in inefficient combustion which causes increased fuel consumption, and increased wear with consequent reduced engine life.

However, if the engine temperature gets too high, boiling of the coolant leads to local steam pockets forming which severely reduce heat transfer in the affected area, usually the cylinder head, resulting in premature combustion of the fuel air mixture, also know as detonation or knocking, and ultimately damage to engine components (the cylinder head, valves and pistons).

Heat Source and Sinks

When an engine burns fuel, heat is produced. This heat increases the pressure of the resulting gas mixture, the remains of the intake air and the burnt fuel vapours, which forces the piston down and turns the crankshaft. But not all the heat produced by burning the fuel is turned into useful work; some of it remains in the gas and goes down the exhaust pipe, and some passes into the walls of the combustion chamber and cylinder and is removed by the engine cooling system.

So how much heat is involved? Internal combustion engines are quite efficient at turning heat into useful work at the crankshaft; in the most efficient high compression diesel engines the "thermal efficiency" (the amount of energy in the fuel that is turned into useful work) can approach 50% under ideal conditions. Petrol engines are not as efficient as diesel engines because of their lower compression ratios, and most engines certainly do not work under ideal conditions most of the time. So lets talk in round numbers: in a typical car engine, very roughly about one third (33%) of the energy in the fuel is turned into useful work to make the car go, one third of the heat goes down the exhaust pipe in the hot exhaust gas, and the final one third of the heat goes into the cooling system.

Using these proportions, we can see that a car engine, let's say a reasonably powerful one of 200 bhp (brake horsepower) is actually making 600 hp of heat when it is producing its maximum power. One horsepower is approximately 750 watts so this is about 450 kw (kilowatts), or as much as 150 electric kettles, which are typically about three kilowatt. One third of this 450 kw heat has to be removed by the cooling system.

Of course an engine is not producing its maximum power all the time. When it is ticking over doing no work it is producing very little heat, and the demands on the cooling system are much lower. This is evident when you are sitting in a traffic jam, even on a hot day. In a typical modern car with an electric radiator fan, the temperature of the engine will gradually rise until the fan kicks in to cool it down. Once the coolant has been cooled a few degrees the fan shuts off and the coolant can then absorb the heat output of the engine for a few minutes until it has heated up and the fan needs to kick in again to cool it down.

So the engine cooling system needs to able to remove its part of the heat being produced by the engine, either a lot or a little, and anything in between, whilst keeping the engine temperature stable at its optimum operating temperature.

The Role of the Thermostat

To remove heat from the engine block and head, coolant is circulated in passageways cast into those components. Some of the coolant is recirculated around the engine, and some is diverted through the radiator to be cooled. The proportion of coolant recirculated round the engine versus the proportion sent to the radiator and cooled is determined by the degree of opening of the thermostat.

To ensure that an engine is always operating at its optimum temperature, the thermostat modulates its opening to control the flow of coolant, and consequently the flow of heat, from the engine to the radiator. Coolant is cooled in the radiator and returned to mix with the coolant circulating around the engine to maintain a constant mixed temperature.

If the engine is producing little heat, for instance if it is idling, then a trickle of coolant through the radiator is sufficient to remove this heat and keep the engine at a constant temperature. If the engine is working hard, then more heat is being produced and more coolant must be circulated through the radiator to prevent overheating.

External temperature and vehicle speed, which change the ability of the radiator to reject heat, also affect the rate at which coolant must be circulated through the radiator, because they affect the temperature of the coolant returned to the engine from the radiator to mix with the coolant circulating round the engine.

To ensure that the engine reaches optimum operating temperature as quickly as possible, the thermostat restricts the flow of water from the engine to the radiator to virtually zero (a small flow is required so that the thermostat experiences changes to the water temperature as the engine warms up) until the engine reaches optimum temperature. The thermostat then gradually opens up to allow sufficient coolant to flow through the radiator to remove the heat being produced by the engine and prevent the temperature rising higher. If the engine is warming up whilst idling, and consequently generating only a small amount of heat, the thermostat will only need to open a little to remove the heat being generated.

With the engine at optimum temperature, the thermostat controls the flow of coolant to the radiator so that the engine is maintained at optimum operating temperature, even as the power output, and therefore heat output, of the engine changes with varying load and ambient conditions.

Under peak load conditions, such as labouring slowly up a steep hill on full throttle whilst heavily laden on a hot day, the thermostat will be approaching fully open because the engine is producing maximum power, the velocity of air flow across the radiator is low, and the temperature differential between the radiator and the cooling air will be low. (The velocity of air flow across the radiator and the temperature difference between the radiator and the cooling air have a major effect on its ability to dissipate heat.) Note that even with the engine operating at full power, the thermostat should not be fully open: there should always be a reserve margin of cooling capacity on the precautionary principle.

Conversely, when cruising fast downhill on a motorway on a cold night on a light throttle, the thermostat will be nearly closed, because the engine is producing little power and the radiator is able to dissipate much more heat than the engine is producing. Allowing too much flow of coolant to the radiator would result in the engine being over cooled and operating at lower than optimum temperature. A side effect of this would be that the passenger compartment heater would not be able to put out enough heat to keep the passengers warm.

The thermostat is therefore constantly modulating, that is, it is moving throughout its range in response to the temperature of the coolant flowing past it, increasing or decreasing flow of engine coolant to the radiator in response to changes in the temperature of the coolant due to changes in power output to respond to vehicle operating load, vehicle speed, and external temperature, always keeping the engine at its optimum operating temperature.

How Does a Thermostat Work?

A typical thermostat has a cylinder containing a heat sensitive wax and a piston which passes through the cylinder wall, to which is attached the operating disc of a valve and a return spring. Expansion of the wax as it is heated pushes the piston out of the cylinder, moving the disc of the disc valve. Contraction of the wax on cooling allows the piston to be pushed back into the cylinder by the return spring. At temperatures below the engine operating temperature range the wax is solid and the thermostat does not respond to changes in temperature. After the engine has been started and the coolant heats up, the wax liquifies when the temperature reaches the bottom of the operating temperature range. When the wax liquifies, the thermostat is at the point at which the piston begins to move the disc valve and divert some coolant flow to the radiator. As the engine warms up further, a steady flow of some of the coolant to the radiator removes surplus heat from the engine.

Once the engine is in its normal operating temperature range, the temperature of the coolant flowing past the thermostat will increase or decrease with changes in power output. The wax expands or contracts in proportion to the temperature change, pushing the piston out of the cylinder or drawing it in with the aid of the return spring. The disc valve acts as a proportional control valve, controlling the proportions of the coolant that either recirculates directly to the engine, or is sent to the radiator to be cooled and then mixed with the recirculated water.

The thermostat is designed so that it will go from fully closed to fully open over a small temperature range. The temperature rating of the thermostat e.g. 82, 88, 92 etc. is the nominal temperature in degrees centigrade at which the thermostat valve will start to open once the engine has warmed up. If the temperature of the coolant increases further, the valve will open further until fully open. The fully open temperature is normally 12-15 degrees above the opening temperature.

To test a thermostat, it is common practice to put it into a pan or kettle of water and bring this boil, observing that the disc valve goes from open to closed. However, testing like this can lead to misunderstanding about how the thermostat operates. The thermostat is designed to keep the temperature of the engine within a narrow range, and it does this by going from fully closed to fully open in over a temperature range of a few degrees.

Until the initial opening temperature of somewhere over 80 degrees is reached nothing happens, but once the opening temperature is reached the temperature of the water can rise so quickly through the thermostat's operating range that the proportional opening of the disc valve is not observed. This is why people mistakenly think that thermostats snap from closed to fully open in one step. To actually observe the proportional operation of a thermostat, the temperature of the coolant in which it is being tested should be raised very slowly over the operating range.

When testing higher temperature thermostats it should be noted that an 88 degree thermostat will not be fully open until 100-103 degrees, likewise a 92 degree thermostat will not be fully open until 104-107 degrees. The valve will not open fully when immersed in plain boiling water alone because the boiling point of water is 100 degrees centigrade at sea level. To test higher temperature thermostats they need to be heated in a water/ anti-freeze mixture or cooking oil, which will allow the temperature of the coolant to be raised above 100 degrees.

NB: Testing a thermostat by heating it in a liquid is potentially dangerous and should only be done by a competent person who is fully aware of the potential dangers, and with suitable safety precautions in place.

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This page updated November 2015. W3CMVS.