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Old 09-05-2012, 09:47 AM   #1
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Optimizing Accel and Decel

Acceleration

In order to see the effect of rate of acceleration on fuel economy I plugged in my Scangauge and went out to a country road. Conventional wisdom is that the slowest acceleration will give you the greatest fuel economy. Popular Mechanics recently said that’s not necessarily true; a faster acceleration will get you into top gear soonest given you the greatest economy. I found that conventional wisdom won out.

At a stop I reset the Avg MPG function on the Scangauge and then accelerated to 45 mph at different rates (ranging from a brisk 8 sec to a geriatric 30 sec). I then recorded the Avg MPG on the Scangauge upon reaching a point ˝ mile from the start point while travelling at a constant 45 mph. Here are the results.



In general, I found you do get better economy with slower acceleration, though the benefit plateaus off. You don’t get significantly better economy with a geriatric 30 second time to 45 than you do with a moderate 22 second time.

Deceleration

There was a debate on here a while back about whether you get greater fuel economy by coasting in gear to a stop or coasting in neutral. I, like many others, assumed the greater economy was achieved by coasting in gear due to the fuel shutoff feature. Not so according to my Scangauge. This is assuming you have to continue travelling at speed closer to the stop in order to be at the same speed at the stop, since your speed will decrease more rapidly while coasting in gear than in neutral.

I got up to a steady 45mph on a country road. At a point I had determined was about ˝ mile from a stop sign, I reset the Scangauge’s Avg MPG calculation, let off the gas and shifted into neutral. At the stop sign I was doing roughly 30 mph and just before I hit the brakes I recorded the Avg MPG on the Scangauge. I did this in both directions. I then repeated by resetting the Scangauge about ˝ mile out but continuing at a steady 45 mph for another couple of tenths of a mile before letting off the gas and coasting in gear. I found that this also resulted in about a 30 mph speed at the stop sign. I similarly recorded the Avg MPG before hitting the brakes. Here are the results (average of 2 directions):

Coasting in Neutral: 124 mpg
Coasting in Gear: 72 mpg


Clearly coasting in neutral is more efficient, at least according to the Scangauge’s calculation.


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Old 09-05-2012, 09:56 AM   #2
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Interesting stuff, the acceleration is what I expected, though I do wonder about highway merging, going from 30-65, if going slowly still helps, I feel that when i go too slowly I am spending more time on the throttle and less time coasting, not that mashing the pedal is best, but probably like you found 20 seconds to desired speed is the best balance.
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Old 09-05-2012, 11:14 AM   #3
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Interesting stuff, the acceleration is what I expected, though I do wonder about highway merging, going from 30-65, if going slowly still helps, I feel that when i go too slowly I am spending more time on the throttle and less time coasting, not that mashing the pedal is best, but probably like you found 20 seconds to desired speed is the best balance.
Yeah there are probably dozens of test to be done to simulate real life driving. I'm bored with the Scanguage now though so I won't be doing such tests. But yeah a liesurely to moderate pace will probably always be the best balance.
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Old 09-05-2012, 11:28 AM   #4
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Hi kam327,

I also use my PLX Kiwi WiFi device paired with my iPod's Obie app and if you suggest that from a stand still start you see 25 mpg after 5 seconds of continuous acceleration, well, this is something I don't buy. From my readings in the same conditions and in a continuous acceleration I see either 5-7 mpg (if I push harder) or 8-11 mpg (if I drive like a granny). The moment the DCT is changing gears, yes, I see some mpg changes, but not even close to the 25 mpg you suggest after the 5 seconds of acceleration. If I stop accelerating, yes, the mpg goes up in no time, but not while accelerating. This is why I tend to give credit to Popular Mechanics for their findings.

As for the deceleration, as long as the car doesn't change gears (downshift) the mpg are at max (100 in the Obie app) from my readings. I didn't test it while in N because I think there is no important gain.
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Old 09-05-2012, 11:46 AM   #5
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Hi kam327,

I also use my PLX Kiwi WiFi device paired with my iPod's Obie app and if you suggest that from a stand still start you see 25 mpg after 5 seconds of continuous acceleration, well, this is something I don't buy. From my readings in the same conditions and in a continuous acceleration I see either 5-7 mpg (if I push harder) or 8-11 mpg (if I drive like a granny). The moment the DCT is changing gears, yes, I see some mpg changes, but not even close to the 25 mpg you suggest after the 5 seconds of acceleration. If I stop accelerating, yes, the mpg goes up in no time, but not while accelerating. This is why I tend to give credit to Popular Mechanics for their findings.

As for the deceleration, as long as the car doesn't change gears (downshift) the mpg are at max (100 in the Obie app) from my readings. I didn't test it while in N because I think there is no important gain.
Voldar, no, the 25 mpg is at the end of the 1/2 mile strip I marked out. The acceleration is only at the beginning of the 1/2 mile, then there are several tenths of a mile at steady 45 mph speed (more steady speed distance with quicker acceleration and vice versa). This seemed to me to be more realistic than just measuring it during accel only.

Sounds like your Obie app maxes out at 100 mpg? The Scangauge doesn't, so wouldn't it provide more accurate results in this test?

Not sure what you mean about not downshifting. You can't decel from 45-30 and remain in 6th gear the whole time.
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Old 09-05-2012, 11:59 AM   #6
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Voldar, no, the 25 mpg is at the end of the 1/2 mile strip I marked out. The acceleration is only at the beginning of the 1/2 mile, then there are several tenths of a mile at steady 45 mph speed (more steady speed distance with quicker acceleration and vice versa). This seemed to me to be more realistic than just measuring it during accel only.

Sounds like your Obie app maxes out at 100 mpg? The Scangauge doesn't, so wouldn't it provide more accurate results in this test?

Not sure what you mean about not downshifting. You can't decel from 45-30 and remain in 6th gear the whole time.
Okay, now I get it. I thought you had the 25 mpg after 5 seconds of driving from a stand still.

The Obie app maxes out at 100 mpg, yes. From my understanding, the 100 = no fuel consumption so whatever goes beyong 100 is not that important (for myself) in a real life driving experience.
I am not making statements about which is more accurate or not and I think each of them is great to have: Obie or Scangauge.
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Old 09-05-2012, 12:09 PM   #7
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Quote:
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The Obie app maxes out at 100 mpg, yes. From my understanding, the 100 = no fuel consumption so whatever goes beyong 100 is not that important (for myself) in a real life driving experience.
I am not making statements about which is more accurate or not and I think each of them is great to have: Obie or Scangauge.
Gotcha. I think what gets lost there though is you have to continue at cruising speed further before coasting in gear in order to reach a stop at the same speed as if you had started coasting in nuetral earlier. So you may burn 1 gal/hr for 20 seconds and then burn no fuel for 40 seconds. As opposed to burning 0.25 gal/hr for 60 seconds while coasting in nuetral. You can see in that very simple, generalized scenario coasting in neutral has a clear advantage - about half the fuel burn of coasting in gear. That's what my own simple test reinforced.
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Old 09-05-2012, 11:39 AM   #8
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This is assuming you have to continue travelling at speed closer to the stop in order to be at the same speed at the stop, since your speed will decrease more rapidly while coasting in gear than in neutral.

Yes, this appears to be the key. DFSO can't make up for the kinetic energy lost through drivetrain resistance.
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Old 09-05-2012, 12:47 PM   #9
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So it all depends on how far you can coast, longer coasts are better in neutral but potentially shorter ones are better in gear
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Old 09-05-2012, 12:50 PM   #10
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Cool stuff! One thing I noticed about this car is that even in gear on a flat surface it coasts a much longer distance than any other car I've owned, and I use that frequently now to boost my mileage.
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