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Old 12-05-2019, 01:40 PM   #5 (permalink)
ptitviet
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Join Date: Dec 2019
Location: Switzerland
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Yeah ok the obsolete refers to the diesel technology. The main topic for diesel is for me the choice between CO2/global warming and particles/Nitrogen pollution.

So I have done a big research and worked my math, and have done a nice Excel to calculate my aerodynamics and rolling resistance, and afterwards calculate a theoretical stabilized speed fuel efficiency. This could help me in the future to estimate the gains in fuel efficiency related to some mods.

First was the aerodynamics. They are made of 2 parts :
- Cd (Cx in France) : can be found for my car on the web : 0.27
- S : front surface. Could not find it. I first used a rough formula :
Width * Height * 0.85 = 2.486m2
As I was not happy with this value, I digitized in Autocad the owners manual dimension page and I got a much accurate value : 2.196m2

So in the end, the estimated S.Cd value is : 0.593m2

Second : roll resistance. 2 variables, difficult to measure I had to guess :
- 0.007 for my 195 65 15 inflated to eco pressure (2.6bar front 2.7 back), please tell me if it is wrong (Conti Winter TS860)
- weight : 1280kg on the datasheet, should be pretty accurate since I have few equipment on this car

Then I made a table with the force created by this two elements :
- rolling resistance : 87.90N at whatever speed (I know it is not totally correct but I have no way to calculate the aerodynamic lift)
- aerodynamics : 1/2*rho (took sea level with temp 20C) * V^2*S*Cd
176N at 80km/h
275N at 100km/h
397N at 120km/h

then I converted it to the equivalent engine power. Problem is (and I'd be happy to get a clue on it), very difficult to get realistic values of transmission losses. I estimated to 10% considering it is a front wheel drive (no transmission tunnel) and a manual gearbox.

Until here I think my guesses are not too wrong, as the top speed of the car is given for 204km/h, the estimated necessary power for driving 200km/h is 99.77hp, 114.70 for 210 and my car has a 110hp engine.

Then using the gearbox ratios and wheel diameter, and finding a correct bsfc curve on the web, I translated the bsfc curve on a table and using some interpolation extension I went to the estimated fuel consumption :

speed (km/h) fuel consumption (l/100km)
80 2.41
100 3.02
120 3.73

What is interesting to know is just to change the air temperature from 20C to 0C :
speed (km/h) fuel consumption (l/100km)
80 2.50
100 3.14
120 3.92

This illustrates the big importance of temperature in fuel economy.


I'm happy to share the OpenOffice sheet, if whoever wants to chek it and give me an opinion I'd be happy. To use it you need the interpolation extension. Next step will be to determine SCd and Crr experimentally in order to accurately measure the future improvements.
Attached Files
File Type: zip Calculs_Consommation.zip (86.0 KB, 7 views)
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