Improving fuel economy for the 1991 200q 20v

Dan Cordon cord4530 at
Sat Mar 5 18:20:54 EST 2005

Good points Alan. Especially on the 'too slow' issue. I don't anticipate 
there being much difference in fuel economy of driving 35 mph vs. 45 
mph. But aerodynamic loads certainly increase in the 65+mph range. And 
there's a good chance you would have worse fuel economy if you were 
driving the whole trip at 25 mph.

And you're correct on the energy too. That's a perfectly logical way to 
look at it as well.

For everyone else's sake:
Energy = Fd * distance
Power = Fd * velocity

Alan has a great point...using energy might be more intuitive, since 
it's looking directly at distance traveled. For a given distance, if you 
drive it at higher power, you'll probably get there in less time - 
perhaps confusing the comparison some.

Thanks for the different viewpoint.

alan cordeiro wrote:

> Energy used to overcome drag is proportional to the square of the velocity.
> (power used is velocity cubed...but gas used depends mostly on energy)
> Drag force,   Fd = A + BV + CV^2   
> and energy is Fd * distance.
> Unfortunately at low throttle angles the engine is running at lower
> volumetric efficiency, overall power conversion efficiencies run
> way below Carnot cycle theoretical, probably in the in the 
> 10-15% range, so going VERY slow can also be counterproductive
>>You didn't mention anything about aerodynamics. At highway speeds, the 
>>drag force is quite significant. Hard to say what would improve the 
>>aerodynamics of our cars....but it could be improved upon. Since drag is 
>>proportional to the cube of velocity (the power to overcome drag 
>>anyway), cruising at 60 mph will likely get you better fuel economy than 
>>cruising at 80 mph.

Dan Cordon
Mechanical Engineer - Engine Research Facility
University of Idaho

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