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Old 02-11-2008, 01:32 AM   #5 (permalink)
EcoModding Lurker
Join Date: Jan 2008
Location: Sydney, Australia
Posts: 76

Piaggio Fly 150 - '10 Piaggio Fly 150
90 day: 64.8 mpg (US)
Thanks: 1
Thanked 2 Times in 2 Posts
Note that this value accounts for any additional fuel required to recharge the battery to it's pre-crank state. A well tuned, warm engine starts very easily and 1 minute of idling is plenty to recharge the battery. Further testing would be required to determine how much of the 7 second value above results from the higher idle RPM and how much results from recharging the battery. Given how closely the injector duty cycle curve below matches the RPM curve, it seems likely that most of the additional fuel is due to higher RPM and that battery recharging has a negligible effect. A good additional test would be to measure battery power draw (voltage * current) during regular idling and just after starting for comparison.
It seems I misread/misremembered the experiment, but the higher idling is partly to do with the additional alternator load so i think it's a bit of a stretch to suggest the 0.2 second rule rather than the 7 second one. Its not that I don't believe it, it just that my fuel log seems to be defying this rule, the few times I have gone from shutting off the engine all the time to just engine braking and my FE increased mysteriously. Maybe that when I use EOC I am less cautious with the rest of my driving. I just wanted to do an experiment in my own car to make sure this was true. It also makes it easier for me to explain this to other people. I'm not trying to rock the boat just get more data to confirm the results.

But that doesn't make sense. In both cases (fuel cut-off state in top gear vs. engine-off in neutral) the engine is using no fuel. Yet you'll travel a shorter distance when engine braking in top gear than you will in neutral with the engine off. Whichever method takes you farthest in either case will yield maximum fuel economy overall.
They are both not using fuel but with engine braking the battery and accessories are being charged/powered by the cars decreasing momentum. It also doesn't need energy to start up again and recharge after losing the amps to start it. In my case the engine then needs to re-vacuum the vacuum reserve and re pressurise the power-steering. Most of my driving is on gently hills/ between traffic lights so I'm usually either accelerating up to speed, slowing up to a traffic light or coasting down a hill (requiring braking to not speed). I'm a pretty big fan of engine off at lights I just don't understand how changing nothing else gave me a FE boost.
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