Posted by Brian in Oregon on Tuesday, June 19, 2001 at 4:01AM :
In Reply to: Too Good to Be True posted by Mark in Houston on Tuesday, June 19, 2001 at 0:03AM :
Well, I am an electronics technician. And yes, power, as you call it, or more accurately wattage, is amps x volts.
Your figures are correct. However, it is not so simple. You are comparing NET wattage consumption of the electric fan to the GROSS power loss of the engine driven fan.
We can skip resistive losses, as most wiring in a car is reasonably large enough to make those losses negligable.
The real loss is that the alternator consumes engine power. You have belt and pulley loss. And the natural electrical resistive loss to being turned (load). Plus bearing and brush losses. You also get some minor losses in the diode bridge.
The battery will also have some losses. Ever feel a charging battery? It gives off waste heat. Heat = power loss.
I would guestimate that the losses would result in your 132 watt figure in being more like 165 watts adding in a conservative 25% system losses just for electrical losses. I cannot see this figure being less, and in fact it could be 50% to 100% depending on a number of factors, including ambient heat, wire size, other loads, rpm (meaning if the alt is running at peak efficiency for the rpm), etc.
Ah, but then there is those friction losses. How much? Estimates vary, but it appears there is a 1/2 to three HP gain when the alternator belt is not hooked up (but the water pump is using an idler). That's a wide variation, probably due to various alternators and their load. This is borne out by increases in ETA of roughly .05 second. This is not a hard and fast rule of thumb, as this is a subjective figure based on a lot of factors. But it is close enough to confirm the HP figures.
So, my feeling is that there is no such thing as a free lunch. To move a certain volume of air, you MUST use a certain amount of power regardless of the system. And you have pulley & belt losses regardless of whether you use the engine or alternator to turn the fan.
However, the electrical fan can be turned off when not needed. Like at highway speeds. The engine driven fan can be reduced via fluid or mechanical clutches or flex blades, but it cannot be eliminated. The electric fan runs at a steady rate when it is on, while the engine driven fan varies and is designed for max cooling even at times it is not needed. During accelleration, the electric fan is more fuel efficient because it is not as much of a drag on the engine, since the battery will spread the charge rate out a bit. These are definite pluses for an electric fan.
One more thing... many electric fans on factory cars continue to run while the engine is off. Good for cooling. However, you have to replace those lost amps when you fire the engine up. Again, no such thing as a free lunch.
The bottom line is, which works best for ALL conditions including extremenly hot and cold, and which is more reliable. I say the engine driven fan is more reliable. And you can carry a spare belt easier than a spare fan. Further, if you blow the alternator, or the diodes, or the regulator, an electric fan will suck the battery down much faster than simply powering the distributor and coil. This is a major factor to consider if you go long distances from repair centers.
So, electric fans do save some HP over engine driven fans, but at the debateable expense of reliability, imperfect replacement replacement under all conditions (particularly with heavy hauling applications) and ease of repairs in the field.
Now, as for water pumps, the typical little electric motor used on race cars, primarily bracket cars, work soley because they do not have to deal with a lot of heat, such as that found in heavy trafic or hauling. You'd need a much bigger motor to move more water for those conditions, and a bigger motor will mean more power or wattage from the electrical system. I do not believe there is a suitable electrically driven water pump on the market for conditions found in most driving. Then like the electric fan there is the reliability issue.
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