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Old 12-06-2014, 04:08 PM   #1 (permalink)
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This is something adapted from "The Optimization of Body Details-A Method for Reducing the Aerodynamic Drag of Road Vehicles",SAE Paper 760185,1976,by Hucho,Janssen,and Emmelmann.
* It shows some relationships between drag reduction,BSFC,gear-matching,top speed,and fuel economy.
* In the original work,gross drag reductions were shown,from Cd 0.5,down to 0.4,and 0.3.
* I've drawn in estimated positions for Cd 0.2 and Cd 0.10,to get it down to the theoretical (empirical ) minimum just to look at potentialities.
*As the drag is reduced it unloads the engine and allows higher revs which is dangerous for the engine,and also moves it on the BSFC map to a lower efficiency.
*If the final drive gearing is adjusted to maintain the original 100 rpm over rated power rpm,then the engine is protected,and it is moved back to it's original BSFC.
*The top speed increases with drag reduction,in this case,from 97-mph,to 130 mph.
*You can look at what the fuel economy does at any given velocity.
*I've included an expanded-mesh Rolling Resistance Force Coefficient table from Hucho for Radial tires which reflects radial tire technology for the test vehicle.
*With the aero data and R-R info,one could calculate the road load at any point and compare to the brake horsepower required to cover the road load.
This is how engineers would build your car within a numerical environment,knowing the cars performance before it was ever constructed,a Rosetta Stone if you will,for the language of streamlining.

*Taking the original top speed and factoring in the overall gear ratio you can reverse-engineer the dynamic tire circumference.
*The tire circumference allows the axle rpm to be determined.
*From an expanded-scale image,the original top speed engine rpm is found to be 5,818.This is your design engine rpm for all calculations.
*Rated power of 55kW is at 5,718 rpm.
*I went ahead and calculated the final drive ratios for the two added drag coefficients,along with new top speeds.
*From the tire curve you can see the non-linear relationship between high velocity and R-R.
*To use these relationships,one must first determine their cars performance at 100 rpm over rated power rpm.

*Combining knowledge about Low Rolling Resistance tires,lower viscosity lubricants,weight reduction,and improved powerplants,modders will have useful tools with which to predict future performance from mods before spending a penny on materials.
*You can see that even in 1976,streamlining would have produced a 32-mpg @ 100-mph car.
*With today's 'Skyactive' engine technology,with BSFC 0.28 lbs/bhp-hr,compared to 0.51 for 1976,you might image what modern cars could do with really low drag.

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Old 12-06-2014, 05:04 PM   #2 (permalink)
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Thanks. The only thing I see that needs to be added is number of wheels on the ground.

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