Wax thermostats

Original thermostats worked on the phase change of a substance between the liquid and vapour states

When people first made automobiles, they needed to cool the motor. This was done by circulating water through a radiator.

Later, since most automobiles were used in temperate or cold climates, somebody thought it would be a good idea to make a thermostat which would prevent circulation through the radiator until the motor had properly warmed up. These original thermostats worked on the phase change of a substance between the liquid and vapour states: the hot vapour was sealed into an expanded bellows which contracted as the vapour cooled and changed to liquid, pulling a cap in to close the circulation passage. They were fail-safe, because loss of the substance would allow the bellows to expand and water to circulate through the radiator.

Contrast this with the modern wax thermostat which relies on the phase change from solid to liquid. Here, the increase in volume which takes place is used to force a piston upward against a spring in order to open the circulation passage. This makes the device fail-unsafe, because loss of the substance leaves the passage closed, preventing circulation of water through the radiator and immediately causing the motor to overheat.

Having just had to have yet another of these devilish devices replaced - the fourth on three different vehicles - I am becoming exasperated with the auto industry, which in nearly 50 years has failed to come up with a fail-safe alternative with the same small form factor which made the wax unit so attractive.

Tony Marshallsay, Riyadh, Saudi Arabia

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