A fan clutch discussion

Question:

I read the January issue when it came, and this morning was going through it again. Something bothered me, and I wanted to let you know about it. Maybe it is significant, maybe not. In answer to a guy named Mel, you explained how a fan clutch worked, and you are a bit confused. I checked my thinking, and I was right. You stated that “as the temperature increases, the coupling force within the clutch mechanism increases, causing the fan to turn faster...” Later you say, “When the temperature decreases, the coupling force diminishes to the point where it releases and disengages the fan from the engine.”

A fan clutch is on the engine because at highway speed, the fan is not needed to get air through the radiator, so it disengages the fan and thus reduces engine drag (helping gas mileage a bit). So the clutch ideally works with the engine at low speed, but drops out at high speed. This attachment talks about how they work: http://www.aa1car.com/library/ coolingfanclutch.htm.

There are two kinds of fan clutches, and he describes (incorrectly) the fluid type. The fluid type reacts only to engine speed, not temperature. The second type uses bimetallic springs (no fluid) which do react to the temperature of the air flowing through the radiator, engaging the fan at higher temperatures, but dropping out as the engine cools itself (and it certainly could do that at highway speed). You talk about closing the gap between the parts (before media blasting), and fluid leakage, etc., which indicate the fluid type clutch, but you describe the bimetal spring type as far as how it works.

Maybe the difference is not important, but you decide. I just think that it should be the purpose of the magazine to put out accurate info, so you handle it as you wish.

Answer:

With all due respect, I stand by my answer in the January 2011 issue.

I read the information at the link you cite and believe that it does support my answer. That answer states that a thermostatic fan clutch is designed to increase or decrease fan speed in accord with engine temperature. This is why it’s called a “thermostatic” fan clutch.

As the temperature increases, the coupling force within the clutch mechanism increases, causing the fan to turn faster and thereby move more air through the radiator and across the engine. When the temperature decreases, the coupling force diminishes to the point where it releases and thereby disengages the fan from the engine. The coupling mechanism is usually hydraulic fluid, and a valve normally controls the fluid. The valve in turn is most often controlled by a bi-metallic spring assembly. The bi-metallic spring assembly expands or contracts in response to temperature change and the movement of the spring one way or the other opens or closes the valve. The valve controls the flow of hydraulic oil in the clutch assembly and determines whether and to what extent the two primary sections of the clutch assembly—the one coupled to the engine and the one coupled to the fan— are coupled.

So, I believe that your division of fan clutches into two different types, one with fluid and the other with bi-metallic springs, is incorrect. Though designs vary, in most instances the function of the bi-metallic spring is to open or close the valve that controls hydraulic fluid flow.