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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nah with in 3 seconds my voltmeter levels off at 13.8ish, i would be more worried about heat soak :)
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I do have a manual fan switch hooked up in the Formula. It is a pretty consistent car, if it hooks up good most runs will be within a tenth, if not less.
The guy who told me that I should skip shutting it down over and over said that it takes twenty miles of driving to replace the charge used to start the car. I have no idea where he got that statistic, it seems very high to me. |
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500 amps x 20 seconds = 10,000 amp seconds 10,000 amps seconds / 60 seconds per minute = 167 amp minutes. Least amp rating I found in a quick search for an OEM replacement for you '97 Firebird was 105 Amps. Let's assume wipers, headlights, rear defroster, heater motor and stereo consume 85 of those amps - leaving 30 amps to recharge the battery. 167 amp minutes / 30 amps = 5.6 minutes Now lets assume more normal weather, warm engine and just stereo and heater fan (so at least 50 charging amps) ((300 amps x 5 seconds) / 60 seconds) 50 amps = 0.5 minutes. |
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