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Old 05-05-2009, 02:01 AM   #22 (permalink)
Ernie Rogers
Ernie Rogers
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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I am just guessing on the accessories

To Noob:

Keeping in mind that I am mostly guessing on the accessories, that 250 watt guess was mostly for defrost. I could imagine doubling that for AC on a hot day. In a VW, the AC load is less when you have the fan on low. People see very little if any drop in mileage in a VW with AC and fan in the lowest position--it doesn't hurt the system either. Personally, I usually just drive with the AC off. That always worked when I was young and didn't have AC.

My recollection is that head lights are 90 watt bulbs. Add tail lights and all the other stuff and it is probably close to 250 watts total.

Now, about correcting for inefficiency of the alternator--I did divide by net power, after losses (the 6 kW). That means I was assuming 85% efficiency at the alternator, as with the engine and mechanicals. Remember that the alternator is always spinning and when you add a load, it is a marginal increase, which should be counted at a higher efficiency. (Much of the alternator losses are fixed, I think.)

Unchosen could easily be right that you should add a multiplier on the accessory numbers I got.


Originally Posted by theunchosen View Post
I forget the OP topic but the engine is stealing some power for ingition and other functions.

These don't look like very major differences with them on but something Ernie accidently dropped is your alt is nowhere near 100% efficient. On a good day its 60-75%. So it takes anywhere from 30-50% more power from the engine to make those watts. The more you draw the bigger the problem. Like I said that 200w in your cigarette lighter is going to be more like 300-350 watts(.4-.5 hp).

Also adding a few of those functions together and its serious gas mileage(for quick and easy just add 1/3 more than what it cost originally to account for ALT inefficiencies).
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