EcoModder Forum Aerodynamics Seminar #3 - by Phil Knox

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Master EcoModder

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WonderWagon - '94 Ford Escort LX
Last 3: 51.52 mpg (US)

DaRedOne - '99 Ford Escort ZX2 Hot

DaBluOne - '99 Ford Escort SE
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The picture showing "formula for aerodynamic horsepower" was lost when this got moved to a new thread.
Here's one from Wikipedia.

Horsepower required to overcome aerodynamic drag -

Quote:
 is the force of drag, is the density of the fluid (Note that for the Earth's atmosphere, the density can be found using the barometric formula. It is 1.293 kg/m3 at 0 °C and 1 atmosphere.), is the speed of the object relative to the fluid, is the reference area, is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car), and is the unit vector indicating the direction of the velocity (the negative sign indicating the drag is opposite to that of velocity).

 02-14-2009, 08:47 PM #3 (permalink) Master EcoModder   Join Date: Jan 2009 Location: Maui, Hawaii Posts: 813 Thanks: 5 Thanked 32 Times in 25 Posts Don't you end up using 4x as much fuel at 2x the speed, because one of those 2x is cancelled out by getting to your destination in 1/2x the time?
 02-15-2009, 01:55 AM #4 (permalink) Master EcoModder   Join Date: Aug 2008 Location: South Dakota Posts: 460 WonderWagon - '94 Ford Escort LX Last 3: 51.52 mpg (US) DaRedOne - '99 Ford Escort ZX2 Hot DaBluOne - '99 Ford Escort SE 90 day: 48.97 mpg (US) Thanks: 0 Thanked 15 Times in 4 Posts Remember he's talking about how much additional HP is required to overcome drag - not average fuel economy. Less time burning fuel applies but, drag is only one factor. Parasitic losses in the engine, drive train and suspension go up as well - just not as fast as drag losses. So 2x speed (in same gear) means MORE than 4x fuel consumed.
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in 1/2x the time

Quote:
 Originally Posted by winkosmosis Don't you end up using 4x as much fuel at 2x the speed, because one of those 2x is cancelled out by getting to your destination in 1/2x the time?
We have to consider each vehicle on a case-specific basis, along with it's BSFC map under all operating regimes and changes to rolling resistance and any effects of road conditions,grades,temps,wind,etc.-------------------- For the aero portion only,doubling the velocity increases the road load power requirement by a factor of eight.The rolling resistance will not vary the same way.So you've got to calculate the total road load for any given speed.------------------- Using my truck for an example,at say 40 MPH you'd see low 40s mpg,and then pushing it to a steady 80 mph she falls off to around 30mpg.----------------- At the low speed the truck is mainly battling rolling resistance with only marginal air drag.At 80 mph,about 80% of available engine power is being absorbed by aero drag.--------------------- Every individual has to way out the implications of the higher velocity.