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Old 01-14-2015, 06:11 PM   #25 (permalink)
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n/a - '05 Ford Focus ZX3
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Originally Posted by some_other_dave View Post
No. You're making more power with the alternator on, so the total consumption goes up. ... -soD
Dave, what you say is true. However, what Jeff proposes is that if the alternator has to be run some of the time but not all of the time, what it is the most advantageous circumstance for having the alternator on? One answer to that is at times when using the alternator can contribute to raising the engine's efficiency to a higher level, i.e. using the BSFC plot to its best advantage. Doing so has a double benefit -- not only does it make the electrical generation more fuel efficient, it also makes the driving power delivered by the engine more fuel efficient. So, yes, turning the alternator on for that instant does raise the fuel consumption at that moment, but it is the best time to perform this necessary function. It is not an additional fuel requirement in the context of the length of a trip; it is a minimization of that necessary fuel requirement.
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