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Old 05-14-2021, 01:22 PM   #31 (permalink)
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Originally Posted by Isaac Zachary View Post
If an alternator drags down the fuel economy by 10%, half of which is inefficiency, if you made it 99.9% efficient then you'd only increase overall efficiency by about 2.5%.

I do question whether alternator technology really hasn't improved over several decades. Are modern alternators really only 50% efficient?

As far as the battery goes the lead acid battery is tried and tested. You can make a lithium battery work. But now it needs thermal management, both cooling and heating, plus electrical management because they need to be balanced and if you leave your lights on you don't want a lithium ion battery discharged below it's threshold. Not to mention lithium ion can't be used in a 12V system, so you'd have to use another voltage and you couldn't use common 12V parts.
It would seem to me that if a 50% efficient alternator drops MPG by 10% and you could improve that efficiency to 99.9%, the alternator would then use only half the power from the engine and it would only drop the MPG by 5%. Could be wrong though.

Obviously making an alternator 99.9% efficient wouldn't be possible, but if the efficiency could be improved from, say, 50% to 80-90%, that could still show a measurable fuel savings and be worth doing depending on what the drawbacks are. My theory is since at least to the best of my knowledge the EPA's MPG test cycle doesn't include accessory use, the gains on the test cycle wouldn't be significant enough to be worth the cost or potential downsides.

Of course in the real world the efficiency of an alternator would become more important the more load it's under. It's not too uncommon for an alternator to be under a 700W load on more modern vehicles (50A X 14V). If the alternator is only 50% efficient then 700W of power is being wasted as heat, which I would consider pretty significant as that would require almost 1 extra HP from the engine to overcome the inefficiency.

Mechman alternators use a 6 phase design that they claim is more efficient than a standard 3 phase alternator by somewhere around 10-20% if I remember correctly. I put one of those on my buddy's Bronco because he has electric fans and a big sound system and it does run noticeably cooler than the stock alternator, but I can't say for certain that it runs cooler because it's more efficient as it probably has a better cooling fan also.

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Old 05-14-2021, 01:50 PM   #32 (permalink)
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It would seem to me that if a 50% efficient alternator drops MPG by 10% and you could improve that efficiency to 99.9%, the alternator would then use only half the power from the engine and it would only drop the MPG by 5%. Could be wrong though.
Thanks for the correction! I think I meant to put 75% and then put 99.9%.
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Old 05-14-2021, 03:36 PM   #33 (permalink)
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alternator efficiencies

From online:
* typical efficiency........................................ ............ 55% - 60%
* high-efficiency........................................ ............... 68% - 75%
* excited rotor type.............................................. .... up to 96%
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Old 05-14-2021, 03:46 PM   #34 (permalink)
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From online:
* typical efficiency........................................ ............ 55% - 60%
* high-efficiency........................................ ............... 68% - 75%
* excited rotor type.............................................. .... up to 96%
So does that mean that simply switching the roles the stator and the rotor play would increase efficiency? In other words, put the field in the stator housing and excite the rotor instead?
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Old 05-15-2021, 05:20 AM   #35 (permalink)
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There was a thread a long time ago over here. A guy with an SUV type american car (greenish if i recall) was looking at alternative diodes for the rectification circuit.
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Old 05-15-2021, 10:14 AM   #36 (permalink)
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Pretty sure to switch the stator and field requires rewinding at least the stator with smaller diameter wire to get more amp turns. Could also need a different winding pattern, but my motor theory is a bit soft here.
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Old 05-19-2021, 06:22 PM   #37 (permalink)
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Well that didn't take long, I already forgot to turn the fans back on. I noticed my AC wasn't cooling very well as I was pulling out of a grocery store parking lot and realized I forgot to re enable the fans. As soon as I turned the fans back on the AC started to cool. Hopefully I didn't damage my AC system, it seems to still work well so I think it's alright.

My system has a high pressure cutoff switch on the receiver dryer and the compressor has an over temp cutoff switch on it. I'm not sure if either activated or not, but either way I can't imagine it would be good for the system to overheat to the point of tripping a safety switch. Hopefully forgetting to turn the fans back on is a mistake I don't make again.

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Old 05-20-2021, 10:24 AM   #38 (permalink)
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Worst thing you'll do is cause a leak and then the low pressure shutoff (which works REALLY well) shuts down the A/C to prevent system damage from lack of lubricant.

If this occurs often then I suggest a different operational mode: system default is on, manual override to turn off, can be done with a cheap $5 car relay set up to latch open
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Old 05-20-2021, 10:53 AM   #39 (permalink)
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Worst thing you'll do is cause a leak and then the low pressure shutoff (which works REALLY well) shuts down the A/C to prevent system damage from lack of lubricant.

If this occurs often then I suggest a different operational mode: system default is on, manual override to turn off, can be done with a cheap $5 car relay set up to latch open
Yeah creating a leak isn't something I want to do. I completely rebuilt my AC system a few years ago and I don't want to damage it. I'm not sure what pressure the high pressure cutoff activates at, but I would think it would be significantly below the maximum pressure the system can actually withstand.

My fan switch's middle position is auto with down being off and up being on. I set it up with a normally closed relay that the switch activates when off to disable the fans. The problem isn't that the system malfunctioned, I just forgot to flip the switch back to auto when I got off the highway. That's why my preference was for the system to be automatic, but it doesn't seem that there's a practical way to do that, so the switch will have to due for now until I figure something else out.

One idea I came up with is to install an indicator light that comes on when the fan switch is off somewhere I would see it like by my Scangauge to remind me that the fans are off so I hopefully don't forget to turn them back on again.
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Old 05-20-2021, 01:13 PM   #40 (permalink)
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Logically it's easy to do this automatically using common stuff. Attach an A/C thermostat, preferably old style manual, and use the ac side which operates backwards: when its hot the system switches on. Bit of experimentation to do the high point setting, but a pair of needle nose pliers works wonders and it will sink 20 amps through the mercury switch and it's adjustable through 50 degrees. The other direction is the hot water heater controller on the radiator I mentioned but it's a bit insensitive and somewhat non repeatable.

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