Sergius Vernet's thermostat design helped to improve engine efficiency and reliability

Managing the thermal characteristics of engines helps to boost performance and in the early development of cars decreased cylinder wear

Keeping a combustion engine close to its ideal operating temperature of between 70°C and 90°C helps to reduce tailpipe emissions, improve efficiency and performance, while also increasing durability. And it is the wax thermostat that has been the key technology in meeting that goal for the past 70 years.

The instrument was developed in the 1930s by the gregarious US inventor Sergius Vernet. He told his local newspaper the Springfield News how he came to plug what he perceived as a technology gap: “On many occasions I have been faced with the necessity of using a thermostat which would be accurate, simple and powerful,” he said. “No such thing existed although there were many which were satisfactory on cases where power was not necessary. I determined to make a thermostat for which there would unquestionably be a good market.”

Vernet was born in Philadelphia, and moved to work at Antioch College in Ohio. He joined the industrial research institute which had been set up to attract inventors to the college so it could share in the profits from new technologies. His laboratory was in a basement but gave him the facilities to patent more than 100 inventions.

Vernet’s thermostat technology helped to calibrate tiny changes in the engine’s temperature, addressing one of the biggest challenges at the time: cylinder wear. Research conducted in the 1920s had shown a link between wear and condensation of fuel when it contacted a cool cylinder wall. Vernet’s development of the wax thermostat during the following decade solved this problem by ensuring fast engine warm-up.

Vernet said in his 1938 patent: “One object is to provide a device having positive and accurate response over a predetermined temperature range. Another object is to provide a device which will have in its desired temperature operating range a high coefficient of expansion but which will have a minimum of expansion outside said range.”

The key to achieving his goal was the use of a wax pellet at the centre of the device. This remained solid at low temperatures but as heat built up the wax melted and expanded. 

Operation of Vernet’s device was simple. The thermostat started in the closed position, but as the temperature rose the crystalline material went from solid to liquid and increased in volume. The change in volume was transmitted to a diaphragm which pushed towards a piston. As the crystals continued to fuse, the change in volume became greater until the piston was moved vertically, altering the direction of a vane and opening the system. When the temperature fell, the crystalline material began to solidify and the volume shrank. Tension on the spring acted on the piston to move the vane to the closed position.

“The invention relates to that class of device wherein crystalline material is made to undergo modification within preassigned volume-temperature limits and preferably in the presence of a non-modifying non-solvent acting as the integrating medium for the contributory pressures of the elementary mass particles whether in distinctly crystalline or substantially fused state,” said Vernet.

Vernet’s thermostat technology, although first adopted for the automotive sector, was also used in military applications including planes and armoured vehicles. Vernet’s device has changed little since he first developed it. It controls the thermal management of combustion engines to this day, be it in city cars, SUVs or sportscars.

tags: Oct 2011 Emissions Powertrain