Describe the function of the car thermostat?

Answer 1

A car 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.

Discussion

An internal combustion engine operates most efficiently at high temperature, with the coolant in the cooling system 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).

To remove heat from the engine, coolant is circulated in passageways around the engine block and head. Some of the coolant is recirculated around the engine, and some is diverted through the radiator. The proportion of coolant recirculated round the engine versus the proportion sent to the radiator 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 whilst heavily laden on a hot day, the thermostat will be approaching fully open because the engine will be 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 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 moving throughout its range, increasing or decreasing flow of engine coolant to the radiator, in response to changes in power output due to vehicle operating load, vehicle speed, and external temperature, always keeping the engine at its optimum operating temperature.