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Old 03-30-2012, 01:21 PM   #5 (permalink)
Piwoslaw
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Join Date: Oct 2008
Location: Warsaw, Poland
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Svietlana II - '13 Peugeot 308SW e-HDI 6sp
90 day: 58.1 mpg (US)
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Quote:
Originally Posted by redorchestra View Post
I always assumed that a car driving at 2000rpm was burning the same amount of fuel whenever it was at 2000 rpm. now I see that rpm has nothing to do with fuel consumption.
I see you are ready to enter the secret BSFC chamber...
In short, the amount of fuel your engine requires depends not only on rpms, but also on load (how much of its power-producing potential is needed at given rpm). Each engine has its own BSFC chart, ie how much fuel is used per unit of energy, usually in g/kWh, depending on engine speed and load. Here is an example:
Notice the little '250 island' in the middle? That's where you want to keep that engine as much as possible, since it uses the least amount of fuel for what it's doing. In other words, that engine is most efficient around 2600-3400 rpm and 70% load. The "sweet spot" is different for each engine, but it's usually around the max torque rpm and 60%-90% of load. Load depends on more than just how much your gas pedal is pressed, so something like a ScanGauge is worth its weight in gold if you're hoping to keep your engine in its most efficient area. A BSFC chart, like the one above, for your specific engine is also very valuable, but often hard to find. See this thread for more info.

Quote:
Originally Posted by redorchestra View Post
I am wondering now if I should be accelerating down hills and coasting as much as possible uphill.
The method Old Mech mentioned works well, but a few months ago someone here posted his test results which suggest that pulse&glide works well going uphill, even though the pulses are longer and the glides shorter.

Quote:
Originally Posted by redorchestra View Post
my commute is constant ascent and descent one after another with very little flat ground.
Are the hills long? Steep? Coasting down is the best, engine off if possible, then accelerating back up at most efficient BSFC (yes, that again). Or slowing down as you climb, then coasting back down. You'll have to compare.
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e·co·mod·ding: the art of turning vehicles into what they should be

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Piwoslaw's Peugeot 307sw modding thread

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