Quote:
Originally Posted by Fitz
The alternator load on the drive belt is not a constant. It varies directly with the current draw on the alternator. The output power of the alternator in watts is the product of the output voltage and load current. My fans draw 25 amps when both run. At a constant 14V output this gives 350 watts. 746 watts is one horsepower so it takes about 0.5 hp to run my electric fans. According to Delco Remy the power conversion efficiency of my alternator is 65% which means for every 1 hp input at the pulley I get 0.65* 746 watts (484 ) out or just divide the power needed to drive a load like my fans by the decimal equivalent of the power conversion effiency of the alternator. In my case, 0.5 HP/0.65 = 0.75 HP from the engine. As you can see, the changes are small, but cannot be ignored if you're going to do an apples to apples comparison. Most drag racers prefer the electric fan and pump because the battery acts like a flywheel by smoothing out the load currents and spreading the fan and pump loads over a time period LONGER than the time required to do a 1/4 mile pass and there is no chance of damaging the engine by throwing a belt and losing circulation in the cooling system.
This information came from two sources. AC Delco provided the specs on my 105 amp alternator and the conversion factors came from my Thermodynamics text. Without bragging, I need to introduce myself. I am the Technical Director of Special Systems at Raytheon Missile Systems Company in Tucson. I have a Masters degree in Electrical Engineering from USC and I did Graduate level work at Stanford in Aerospace Engineering. So, that makes me a rocket scientist that likes to play with old trucks!
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While all this is technically accurate I think its miss applied. For the 3 minutes in maybe 10 minute intervals at idle or in traffic that the electric fan runs to cool the engine the above applies. Anything above 35 mph the air coming into the radiator cools the engine and the fan is not needed and free wheels saving up to 15 hp in most cases. If you have a mechanical fan pulling 5500 cfm it spins at full force at every rpm that the engine runs providing the engine doesn’t have a clutch fan. Most clutch fans spin longer than actually needed so they don’t free wheel prematurely and cause an over heating condition.
DC electric motors generate their own voltage when spinning under load which is approximately 2/3 the input voltage. Start up or Locked rotor state requires full voltage and current because it doesn’t generate this voltage until the motor is up to speed. In layman’s terms the fan motor requires full voltage and say 100 amps to start up momentarily but once spinning it starts generating power and reduces its required input voltage and current by 2/3’s. That’s why you get that huge spike when a Mark VIII fan starts up.
So while Fitz is correct, it is a very short duration that the fan is parasitic to the engine where a mechanical is always parasitic to the tune of 15 hp in most cases. For most automotive application in real world use they save a bunch on HP and MPG. Electric fans run maybe 10 minutes per hour while mechanical fans run 60 without a fan clutch and 30 -45 with a fan clutch.
I’m all for electric fans in most applications but not water pumps. SBC engines nee the variable rate water flow that a belt drive offers. Electric pumps are constant rate that may not be enough in a traffic condition.
"I'm not a rocket scientist but I did stay at a Holliday Inn Express"
Sorry I couldn't resist