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Old 02-22-2011, 03:16 PM   #1 (permalink)
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Turtle How many KWH does the alternator produce from 1 gallon of gas?

I'm trying to figure out how much gas is saved by changing to LED head lights. Does any know how many KWH does the alternator produce from 1 gallon of gas?

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Old 02-22-2011, 05:28 PM   #2 (permalink)
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Four.

Start with the roughly 33KWh/gal LHV (chemical energy) of gasoline.

A gas engine is about 25% efficient (ymmv, drastically) at turning chemical energy into torque, so you've got about 8 KWh of shaft work.

An alternator is about 50% efficient (ymmv, but not too drastically) at turning torque into electricity, so you're left with about 4KWh per gallon. I figure for a typical 200W econobox with the lights off, that's about .05GPH. Turn on the headlights, and it's about .10 GPH.

A typical hybrid is on the order of twice as efficient at generating 12VDC.
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Old 02-22-2011, 06:06 PM   #3 (permalink)
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my Jeep has a 135 amp alternator, and that is maximum capacity. 135A x 12V = 1620 Watts (i'm leaving out losses due to wire size/length/material etc., to keep it simple). if you can eliminate 100 Watts from your total load that equates to ~6% of total alternator capacity. this won't help a whole lot when you're trying to figure fuel consumption, but you can get an idea of how much of an effect you are having.
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Old 02-22-2011, 07:07 PM   #4 (permalink)
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Quote:
Originally Posted by Joenavy85 View Post
my Jeep has a 135 amp alternator, and that is maximum capacity. 135A x 12V = 1620 Watts (i'm leaving out losses due to wire size/length/material etc., to keep it simple). if you can eliminate 100 Watts from your total load that equates to ~6% of total alternator capacity. this won't help a whole lot when you're trying to figure fuel consumption, but you can get an idea of how much of an effect you are having.
Most alternators have a fuse of either 50 or 60 amps, you never use any where near the full capacity, or even get close to the full rating of the fuses of your alternator, it's kind of like saying that your car can go 120mph so how long does it take you to get home from work?
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Old 02-23-2011, 10:53 AM   #5 (permalink)
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Quote:
Originally Posted by Ryland View Post
Most alternators have a fuse of either 50 or 60 amps, you never use any where near the full capacity, or even get close to the full rating of the fuses of your alternator, it's kind of like saying that your car can go 120mph so how long does it take you to get home from work?
right , head lights only pull like 4a@14v and even if you put all accessories on your probably way lower than 50a .My 08 Honda with 85a alternator with most accessories on pulls like 35-40a max .
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Old 02-23-2011, 11:08 AM   #6 (permalink)
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i would say wire in an Amp meter to-from the battery and then shut off the field windings of the alternator to see how much of an electrical load you have. remember that Amps x Volts = Watts

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