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Old 12-09-2007, 07:49 PM   #12 (permalink)
EcoModding Apprentice
Join Date: Dec 2007
Location: Melbourne
Posts: 152

The Mighty Mira - '92 Daihatsu Mira
90 day: 61.32 mpg (US)
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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.

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.
"Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." - Isaac Newton

Last edited by newtonsfirstlaw; 12-09-2007 at 08:59 PM..
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