Reverse Poppet Wax Thermostats

By the 1960's, engines were producing more power and running more accessories. And emissions became an issue. This created the need for better cooling. One of the keys to better cooling was the development of the wax motor/reverse poppet thermostat. Let's see how these work.

The core of the wax thermostat is the motor. Unlike the bellows thermostat, which used an expandable metal bellows filled with alcohol, the wax thermostat uses a rigid brass or copper cup filled with solid wax.  Waxes can be blended with any desired temperature range and the motor sized to produce any needed level of force, opening point, and hysteresis. When heated, the wax melts and rapidly expands,  acting against a rubber diaphragm, which in turn tranmits force to the pin. The pin is pushed out of the cup and reacts against the pin retainer, forcing the entire wax motor backwards. Wax motors in automotive thermostats typically have a stroke of 8mm.

The main poppet is permanently attached to the motor, so as the motor moves backwards, the flow path to the radiator is opened. The thermostat in the illustration is a bypass thermostat, which has poppets on either end. The bypass poppet closes the bypass passage as the main passage is opened (more about bypass systems in the next chapter.)

Wax motors have several advantages over bellows. The cup is perfectly rigid, and cannot be compressed. The wax is incompressible as well, whether in solid or liquid form. And since the poppet moves backwards against the direction of flow, there is no longer a problem of the poppet lifting in response to inertial forces. So pressure limitations are largely eliminated, allowing more aggressive water pumps and radiator caps to be used. This results in better water circulation and higher boiling points.

Boiling Point Elevation vs Pressure
(Jaguar applications usually use 7 or 13lb caps)

Pressure 0 PSI 4 PSI 8 PSI 12 PSI 16 PSI 20 PSI
Pure Water 212F 225F 233F 242F 252F 260F
33% Glycol 220F 230F 240F 253F 260F 268F
45% Glycol 224F 234 F 245F 257F 265F 272F
50% Glycol 226F 236F 248F 259F 267F 275F
60% Glycol 231F 241F 253F 264F 273F 280F

A few words here about thermostat waxes. One of the key properties of thermal waxes is that they are amorphous solids, which for this discussion means that they don't melt at a single temperature. Rather, they gradually liquify in an engineered temperature range. 

As the wax gradually transforms to liquid, it expands at a high rate. The expansion graph traces out a sort of "S" curve. The thermostat's start of open will occur near the lower inflection point, and its max open will occur near the higher inflection point, spanning a range of 10-15 degrees.

The specific heat of a substance is the amount of heat required to raise its temperature one degree. This heat must be released as it cools. Thermostat wax requires extra heat to increase or decrease in temperature between the critical start of open and max open temperatures. This thermal inertia effect creates hysteresis, or a slower reaction to changes in temperature. It's a desirable characteristic, which allows the thermostat to gradually achieve a stable equilibrium condition in response to average conditions, rather than continuously snapping open and closed in response to thermal transients. Adulterants such as powdered metal can be blended with the wax to fine tune reaction time.

One benefit of hysteresis is that it enables cold-side applications. A cold-side application places the thermostat at the engine inlet instead of the engine outlet. It's used in engines which have problems with thermal shock (due to cold water from the radiator entering a hot block) or where tight regulation of engine temperature is required for emissions control. A high-hysteresis thermostat is required for these applications because the thermostat sits between the hottest and coolest points in the system: if it was "fast", it would slam shut almost as quickly as it opened. More about this in the discussion of bypass systems.

This Youtube video shows how wax thermostats are manufacured:

I would be remiss to end this story without a little history. The wax thermostat was invented by Sergius Vernet. Vernet was born in Brooklyn and trained as an engineer at Pratt. Around 1930, he set up what we would now call an incubator company under the asupices of Antioch College in  Yellow Springs, Ohio. Vernet was a prolific inventor who already had a long list of patents by 1930, including the reciprocating windshield wiper mechanism. As Vernay Patent Company, he filed his most important patent in 1934 (US Patent 2115501.) Most of the elements were there, although his selection of paradichlorobenzene (mothballs) as the powering substance seems a bit humorous in retrospect. While Vernet primarily operated as a patent licensor, he also spun off several operating companies in the area of thermal controls and rubber products. Several of these spinoffs survive to this day...Vernay Laboratories, which produces a broad range of flow control products, Rostra Vernatherm, which produces thermal controls for heavy duty applications, and Vernet (of France), which produces thermostats. "Vernatherm" has become a generic term for oil cooler thermal valves. While Vernatherm produced oil cooler thermostats during WWII, automotive uses for wax motors seem to have come much later. The earliest example I've found of a wax thermostat for automotive applications is this Fulton-Sylphon thermostat from a '55 Packard. The design still pretty close to Vernet's original 1934 patent. It's probable that the expiration of the patent unleashed a flood of development:

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