Improving fuel economy for the 1991 200q 20v
cord4530 at uidaho.edu
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.
Mechanical Engineer - Engine Research Facility
University of Idaho
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