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Old 10-21-2008, 10:58 PM   #1 (permalink)
Renaissance Man
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Join Date: Jan 2008
Location: In the Northeast dreaming of the Southwest
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Aegean C - '17 Honda Civic LX
90 day: 39.42 mpg (US)
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Question about shutting down at red lights

So I was at the dragstrip the other day (how many threads on Ecomodder start like that?) and while waiting in line to make a run I would shut the engine off, wait until the cars ahead of me had moved up several spaces, start it back up and move forward, etc etc, to help keep the engine cool. A friend of mine suggested that this was ultimately counter productive because the alternator would have to do extra work to replace the charge lost by cranking the starter. This got me thinking about one hypermiling technique I use, which is to shut the car off when stopped at red lights. Is it possible that it might actually take more fuel to replace the battery power used to restart the car than is saved by shutting the engine off? I read a statistic once that the threshold for whether you save fuel by shutting the engine off is six seconds, but I have no idea how that was calculated or whether it took this variable into account. There would also clearly be a large difference between the energy needed to start the 2 litre four cylinder engine in my Escort vs the 5.7 litre V8 with 10.4/1 compression in my Firebird. Would an amperage gauge possibly be useful in determining this? Let hear some theories.


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