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Old 12-09-2007, 09:10 AM   #11 (permalink)
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Thanks for bringing this back on track. That's a key point.

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Old 12-09-2007, 07:49 PM   #12 (permalink)
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Quote:
Originally Posted by brucepick View Post
Many of my mods decrease rolling resistance or air drag; some reduce engine load (e-fan). Without reducing rr and drag, P&G was nearly worthless. It wouldn't coast. Now with hard tires and some aero mods it will coast nearly anywhere, though still loses some speed of course.
Absolutely.

Ultimately, what we are attempting to do is minimize Delta E/ Delta s, or the net loss of energy over distance traveled. Since Delta E is the same as work, or Force * Delta s (force through distance), the Delta s' cancel and we are left with Force. Assuming an engine of constant efficiency, if we minimize force (drag + rolling resistance) we are maximizing fuel economy.

Pulse and glide is a method of driving in which the assumption of constant efficiency is a better one, since the engine is operating near its peak efficiency or is consuming negligible amounts of fuel otherwise. What will throw this out:
-engine is large and the car forced to idle (in the glide section)
-gearing such that the rpm are far above 2300, e.g. in my car, I'm using up to 17% more fuel than my engine needs to in order to produce a given power at 100kph.

Air resistance force increases as the square of speed. This means that if we can halve* the CdA, it's the same in terms of load as if we were driving at 1/(2^.5) or 71% of the current speed in the unmodified car. e.g. Driving at 100kph becomes like driving at 70kph the way you used to, or if you are driving at 80kph, it's like you were driving at 57kph in your old car.

So yeah, the long and the short of it is that there is a synergistic effect going on, pulse and glide works best with a low CdA. Pulse and glide with a high CdA car is a lot of effort for little gain and wear and tear on the clutch. Aero mods alone don't do much when the engine is expending lots of fuel to turn itself at high rpm.

*Halving the drag coefficient is a big ask. I'll suspect that I'll get close (est. 0.20) with the front skirts, but the only way I'm going to get better than that is to go basjoos style and extend the boattail further, or experiment with turbulators and a more extremely angled boattail.
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Old 12-09-2007, 10:03 PM   #13 (permalink)
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Originally Posted by newtonsfirstlaw View Post
So yeah, the long and the short of it is that there is a synergistic effect going on, pulse and glide works best with a low CdA.
Agreed. It's why the Honda Insight was/is a FE monster in the hands of a determined hypermiler. Light weight + low drag (aero, rolling & mechanical).

Another point I've been thinking about lately is that light weight (lower momentum) tends to reveal the impact of poor aerodynamics much more dramatically in a P&G scenario than a heavier vehicle with similar CdA.

I read an account of the Daimler smart car recently where the driver was somewhat surprised/perplexed by its poor coasting ability. At ~1600 lbs (-ish), with that shape, I wasn't surprised at all.
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Old 12-09-2007, 11:02 PM   #14 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
I read an account of the Daimler smart car recently where the driver was somewhat surprised/perplexed by its poor coasting ability. At ~1600 lbs (-ish), with that shape, I wasn't surprised at all.
Exactly. It's very similar to terminal velocity down a particular gradient. I'd forgotten that until creating my P&G calculator forced me go through the calculations. Ignoring driver and other weight will also throw out your drag coefficient calculations from coast down time.
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Old 12-09-2007, 11:22 PM   #15 (permalink)
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Winter beater Metro - '00 Chevrolet Metro
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Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

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I have to confess I didn't get to the GS thread about the P&G calculator.
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Old 12-10-2007, 12:33 AM   #16 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
I have to confess I didn't get to the GS thread about the P&G calculator.
I'm amending it for 4 valve per cylinder engines (such as mine), and improving it so that it's easier and less intimidating to use (i.e. automating engine map lookup). Will post here after it's finished.
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Old 12-10-2007, 08:46 AM   #17 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
I have to confess I didn't get to the GS thread about the P&G calculator.
I've put a new version on ecomodder. Now you have no excuse not to try it out, and it even looks up an approximate BSFC number for you and everything.

Being able to flick back and forth different speeds without checking up on a map really rams home both the difference going slower makes, and also the differences aero mods can make.

e.g. On my car (everything is P&G):
100kph, initial drag: 3.97l/100km
80kph, initial drag: 2.6337l/100km
100kph, modded to current state: 3.03l/100km
80kph, modded to current state: 2.1048 l/100km

That's a good 25% increase.

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