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Old 09-08-2010, 11:34 AM   #48 (permalink)
LoveLearn
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This has been a fun discussion thread. I used to run a "fan-away" all-mechanical centrifugal clutch between the belt-driven water pump shaft and its in-line supported radiator fan. The "fan-away" had, as I recall, three adjustable springs which pushed brake shoes against the inner driven cylinder. As that shaft's rpm increased, at a user-adjustable threshold those springs no longer held the brake shoes against the driven cylinder, releasing the fan load from the driven shaft. So when driving at constant highway speeds, you had the release-threshold rpm set low enough that the fan was not driven by engine power, unloading several horsepower of then-unneeded drag. At highway speeds, air flow generated by vehicle movement induced sufficient air flow to enable rejecting radiator heat. When you slowed down, you'd hear a little "squeal" as those little clutches bit against the drive cylinder's outer surface. This was back in the 1960s when I was occasionally participating in MPG contests.

Typical V-belt driving efficiency when fully warmed (yes, they are more efficient when warm and more flexible than when cold), can approach 97%. In-line concentrically-driven alternators, fans and other accessories don't need to pull that continual 3%-minimum efficiency loss.

Clearly the lowest power consuming way that's typically available to reject waste heat is with passively-generated air flow from vehicle movement. Adding radiator fan blades imposes some trade-offs. More driven blades can produce more air flow. More un-driven blades produces more air drag. So with more blades, passive air flow is reduced. Most road vehicle passengers dislike high fan noise levels. Yet from a purely-efficiency-judged intermittent use perspective, a 2-bladed high-speed fan seems to produce the best trade-off. When not driven, it imposes the least parasitic drag impeding passive air flow. When driven, 2-bladed props driven by equal power, spinning at higher rpms, generally are more efficient air movers than slower-spinning fans equally powered at lower rpms. But sounds like a small airplane preceding your vehicle in traffic with your Air Conditioning turned on would be an unacceptably high price to pay for the slight efficiency improvement compared to a higher-blade-count slower spinning fan.

While its obvious that powering no fan uses less energy than powering any fan, it seems reasonable to consider total fan-driving efficiency when they are being driven. Here, any of the best mechanically-driving systems are MUCH more energy efficient than electrically-driven systems. Let's assume for the moment that you compare two fan-driving systems which are as similar as possible except for their drive configurations. Same fan blade sets, same surrounds, just mechanical drive vs. electric drive. Nobody of whom I'm aware has been able to mechanically drive a radiator-cooling fan attached to crankshaft extension, perhaps with an in-line clutch to control when that parasitic load is added. So we have belt drives which can transmit about 97% of input energy to the driven shaft while converting about 3% to waste heat. Add your control clutch and fan. Someone's earlier speculation that spinning mass inherently converts input power to waste heat is simply wrong. Drag, not mass is how these systems loose energy.

By comparison, the first step in usual automotive electrically-driven fan systems is a belt-drive going to an alternator. Instantly multiply by 0.97 to account for that belt's losses. Then we have the ABYSMALLY low-efficiency claw-poll alternator which converts fully 50% of its input energy into waste heat. Perhaps you dropped $350 - $700 on a Permanent Magnet high-efficiency alternator as are becoming popular on big over-the-road trucks, and you are now talking about roughly 70% efficiency and only 30% being converted to waste heat. Or are you one of a handful who has a Polar Power alternator like those which performed so well in US Army tests producing 85% - 90% through-put efficiency and only converting 15% - 10% into waste heat. Some of us drive vehicles which cost less than one of those wonderful exotic alternators. Well over 99% of the on-the-road auto fleet run the usual bad-joke-efficiency claw-poll alternator configuration. So far we have 0.97 x .50 for power throughput on the electric fantastic. Let's assume you run sufficiently robust wiring that you have zero wiring system losses. That's not going to be true, but for a trivial 1% - 2% typical loss rate, we'll ignore wiring. But typical recent generation electric motors used to drive electric fans are 85% - 90% efficient. So the electrically driven system's crankshaft to fan-blade throughput efficiency prediction is at best the multiplied product of 0.97 x 0.5 x 0.9 = 0.4365. So a clean all-mechanical system can produce about 97% efficiency and the electrically driven system can produce about 44% efficiency WHEN DRIVEN.

The trick is to avoid driving those systems and just use passively-generated air flow.

Sorry to rupture some balloons, but totally ignoring required links in the power delivery chain is just silly.
John
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