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Old 04-02-2008, 02:00 PM   #15 (permalink)
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Location: Texas
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PaleCivic (retired) - '96 Honda Civic DX Sedan
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Originally Posted by trebuchet03 View Post
While cruising, keeping around BSFC is secondary (unless your engine is small enough to do so)...
And none of our engines are that small, so... P&G is all about keeping the engine at its peak BSFC, or not running at all. The average is higher than running at steady-state and lower BSFC efficiency.

Originally Posted by trebuchet03 View Post
Don't look at your average mpg while trying to figure out acceleration (effectively instantaneous) optimal. Miles/Gallon does not include a time variable - so trying to incorporate time (by watching average) will only bring false confidence. My average will stay really high if I've been driving for a long time compared to when I accelerate just after start up - but in both of those cases, it doesn't change my instantaneous fuel efficiency in the acceleration event.
Of course, the same acceleration will affect your average less if you're averaging over a longer distance. That doesn't change the actual fuel consumption during that accel. I'm comparing the same points on my daily drive from one day to the next using different methods - apples to apples. At this one point, about 5 miles into my drive:

Light acceleration will bring the average down by maybe 2mpg vs 4mpg at heavier accel. The faster accel will get you up to speed 100-200 yards sooner. After 5 miles, you just can't make up that 2mpg difference in 200 yards, even if you EOC.

Here's my theory: If you could have the ideal situation and coast all the way to zero at the end of the drive, or the next light, or whatever you stop, then the higher accel rate would work. But if you use your brakes at the other end, at all, then you can't make up the deficit.

Originally Posted by trebuchet03 View Post
Shoot for peak thermal efficiency while accelerating - then peak specific consumption while cruising (which will likely not be peak thermal). Then later, when you consider the time variable - your figure will be higher.
I think we're trying to say the same thing here.

11-mile commute: 100 mpg - - - Tank: 90.2 mpg / 1191 miles
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