History of the Thermostat Bypass

Everything has a history, including thermostat bypass systems. But before we get into that, we need to understand why there are bypass systems. And that's easy: from time to time, the engine will be running while the thermostat is closed. In this circumstance, there still needs to be some coolant circulation to address several issues: some flow is needed to prevent hot spots in the engine, circulation is needed through the intake manifold to promote vaporization, and finally, some circulation is needed so that hot coolant eventually reaches the thermostat. Let's look at the historical development of bypass systems.

In the beginning...
The very first automobile engine to incorporate thermostatic temperature control was the Cadillac V8, introduced in September 1914 as a 1915 model. The thermostatic bellows element was supplied by Sylphon, and controlled the engine inlet:

The bypass in this early Cadillac comprised a small tube leading from the carburetor water jacket to the pump inlet. This was sufficient to provide some circulation when the thermostat was closed, the clear intent being to encourage fuel vaporization. However, the open bypass was not designed to limit flow when the thermostat was open. As a result, some hot coolant always recirculated through the bypass line. The only limitation on the volume of recirculated coolant was the small diameter of the tube. Although somewhat primitive, this thermostat and bypass system allowed the Cadillac engine to develop an enduring reputation for reliability and economy which proved to be the foundation of the brand. 

Not to be outdone, Packard's V12, introduced in the Spring of 1915, incorporated a Sylphon bellows unit which controlled both a primary valve and a bypass:

The Packard engine, like the Jaguar V12, was essentially two six cylinder engines built on a common crank. Each bank had it's own water pump and thermostat, so the Packard was prone to some of the same problems as the Jag. The Sylphon bellows was still not built into a self contained thermostat, but operated a bellcrank system on which two butterfly valves were mounted, one controlling the bypass and one controlling the radiator passage. This allowed full volume bypass circulation, a big improvement over the Cadillac system. Thermal control was comparable to a modern engine. 

During the late teens and twenties, developement of bypass thermostats was sidetracked, as many cars were still being built with either thermosiphon systems or air-side shutters, neither of which requires a bypass. Where thermostats were used, they were often bimetallic valves. Some bypass circulation was provided by perforating the valve, or by allowing some circulation through the heater if so equipped. This circulation came at the expense of longer warm ups and poor thermal control. Bimetal thermostats were very susceptible to corrosion. 

By 1930, two events came together to advance cooling system design. The first was the development of the self contained thermostat. As we know it today, a brass or stainless thermostat includes the motor, valve, and support structure, and is built as a replaceable assembly. We don't even think of these things as separate parts today, but it was a new idea at the time. The other event was the acquisition of a series of thermostat companies by Reynolds Metals (as in Reynolds aluminum wrap), accompanied by a big sales push. Among the companies absorbed were Fulton Sylphon, Robertshaw, Bridgeport Brass and American Thermostat. A veritable juggernaut of thermal control. These companies would variously operate as independent or combined entities, aggressively dominating the thermostat market for many years. It's a very confusing corporate history. 

Buick 1931
Building on these events, a milestone in cooling system design was the 1931 Buick 8, which included a bellows thermostat. This relatively modern single poppet thermostat was new to the market, but for this story, the real news was the bypass mechanism:

The diagram is a little unclear, but what's going on is that a single poppet thermostat (T) controls the water exit to the radiator (R). But a completely separate spring loaded bypass valve (B) independently controls the bypass. The way this works is that when the thermostat is closed, it arrests flow, which creates a local area of high static pressure. This pressure opens the bypass valve, allowing coolant to recirculate to the pump (P) and back to the engine (E). When the thermostat is open, static pressure throughout the system will be fairly uniform, so the bypass valve closes.  Here are a few photos of the bypass valve:

Eventually, this system evolved into the open bypass systems used in most American cars of the 1960's. Open bypass will be discussed in the main chapter on bypass systems.

Chrysler 1934
The final stop on our history tour is the 1934 Chrysler Airflow straight 8, which incorporated a the first true dual-action bellows thermostat. This thermostat incorporated a secondary sleeve which closed the bypass as the main poppet opened. This influential design was adopted by most US manufacturers, and was used into the 1950's. It is the direct precursor of the sleeve bypass thermostat used in British cars of the 50's and 60's, as well as the dual poppet reverse thermostat which is so common today. 

Meanwhile, what was going on in Europe? 

Thermostat, 1938 Steyer 220. Very pretty, but years behind.

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