Until the 1930s, keeping an engine at the right temperature was largely the driver’s job. How a small capsule of wax took over that task, and why it still does it in many engines today.
How a capsule of wax took engine temperature out of the driver’s hands
Picture a sedan in the late 1920s leaving home on a January morning. The owner has strapped a quilted cover over the radiator so the engine will warm up, and the only warning of trouble is a thermometer mounted on the radiator cap, a common fitting in those years.
By early afternoon the sun is out and the car is grinding up a long grade behind a slow truck. Whether that cover stays on, or comes off before the water boils, depends on someone noticing the needle and pulling over, because on most ordinary cars of the time engine temperature was the driver’s job.
Too cold in the morning, too hot by afternoon
Water-cooled engines of that era moved their coolant with a pump or, on simpler cars such as the Ford Model T, by letting hot water rise and cooler water sink on its own. Either way the radiator had to cope with the worst day of summer, so on a cold morning it pulled out far more heat than the engine could spare.
An engine that runs cold burns its fuel poorly and wears faster, while one that runs too hot boils its coolant away, and for years, on most cars, the main defense against both was a driver willing to fiddle with covers and shutters.
Sergius Vernet and a capsule of wax
Carmakers did try to automate the job, and some expensive cars carried radiator shutters worked by a thermostat, or thermostats built around a sealed metal bellows filled with a liquid that boiled at a low temperature. The bellows worked but was delicate, so the industry still lacked a sensor simple enough for every engine.
In the 1930s an inventor named Sergius Vernet took a different route. He sealed wax inside a small metal capsule and let its expansion open the way for the coolant, which gave the engine a sturdier part that reacted to heat on its own. An idea like that tends to outgrow the workshop where it began, and this one became a business when, in 1946, Vernet founded Vernay, a company now headquartered in Griffin, Georgia.
Why melting wax can open a valve
The trick depends on a property anyone who has watched a candle cool has seen in reverse. When this kind of wax warms past its melting range it swells noticeably, and inside a sealed cup that growth has nowhere to go except against a small pin.
The pin pushes the valve open against a spring, and coolant starts flowing to the radiator. When the coolant cools again, the wax shrinks, and the spring closes the valve. On a cold start the thermostat stays shut, so the water circulates only around the engine and warms up quickly, which is precisely the job that the radiator cover used to do.
Wax also suited what came next. Once cooling systems were sealed under a pressure cap, which raises the boiling point of the coolant, the pressure could squeeze a bellows and keep it from opening when it should, whereas wax barely compresses. As a result the wax element is largely unaffected by system pressure, and it went on to become the standard design.
Electronics have not pushed it aside. Some modern engines whose computers manage temperature still use a wax thermostat fitted with a small electric heater, so the software warms the wax a little sooner instead of replacing it.
What the water temperature tells a race fan
Anyone who follows stock car racing knows how often the conversation on the team radio turns to water temperature. At speed the radiator depends on air pouring through the grille, and a piece of litter no bigger than a hot dog wrapper stuck on the front of the car can choke that airflow and send the readings climbing within a few laps.
Passenger cars live a quieter life, yet they rely on the principle Vernet worked out in the 1930s. On a cold morning the gauge on an ordinary dashboard settles near the middle of the scale a few minutes after startup and stays there, with no quilt on the radiator and no one climbing out to check.
Frequently Asked Questions (FAQ)
How does a wax thermostat work in a car engine?
A wax thermostat holds a sealed capsule of wax between the engine and the radiator. As the coolant heats up, the wax melts and expands, pushing a pin that opens a valve against a spring. When the coolant cools, the wax contracts and the spring closes the valve.
What happens if a car thermostat gets stuck open or stuck closed?
A thermostat stuck closed stops coolant from reaching the radiator, so the temperature gauge keeps rising and the engine can overheat, with a risk of damage such as a blown head gasket. One stuck open lets coolant flow all the time, so the engine is slow to warm up, the heater blows lukewarm air and fuel economy suffers.
Why should you never open the radiator cap on a hot engine?
A modern cooling system runs under pressure, which keeps the coolant from boiling even when it is hotter than the normal boiling point of water. Removing the cap releases that pressure all at once, and the coolant can flash into steam and spray out, causing serious burns. The safe approach is to wait until the engine has cooled down.
