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Old 12-08-2008, 12:51 PM   #1 (permalink)
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Arrow Online tool: shows aerodynamic drag, rolling resistance, power required & est. MPG

Aerodynamic & rolling resistance calculator - EcoModder.com

Back in the day, krousdb made a spreadsheet to calculate aero & rolling resistance forces against his del Sol, their respective power requirements, and effect on fuel consumption at various speeds.

I've ported/expanded the spreadsheet into an online tool which asks you to enter or accept default values for...
- vehicle weight
- Crr (coefficient of rolling resistance - .008 represents a low rolling resistance tire on a smooth surface - see Wikipedia for other sample values)
- Cd
- Frontal Area
- Fuel energy density (in Watt hours / US gallon)
- Engine efficiency
- Air density (rho)
... and it spits out a table showing resistance force values, estimated power requirements and fuel consumption across a range of speeds.

It's online at EM here: Aerodynamic & rolling resistance calculator - EcoModder.com

It's not perfect (e.g. engine efficiency changes with load in real life, but remains fixed at the value you supply for all speeds in the calculations for fuel consumption).

Discussion, questions, suggestions welcome.

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Old 12-09-2008, 09:11 AM   #2 (permalink)
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Thought of another flaw in the calculator: CRR entered is supposed to be for the tires' resistance, but rolling resistance involves some drivetrain resistance as well (bearings, possible brake drag, CV/U joints, differential).

Determining your actual Crr experimentally and entering it would be best. See:

Calculating Rolling Resistance

and/or help with simplified cDA/cRR formula
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Old 12-09-2008, 10:13 AM   #3 (permalink)
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Oh boy... I'm gonna be playing with this quite a bit!
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Old 12-09-2008, 11:22 AM   #4 (permalink)
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Added km/h and L/100 km to the output.

I plan to make the inputs more flexible as well (SI/Metric & old units).
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Old 12-09-2008, 12:11 PM   #5 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Determining your actual Crr experimentally and entering it would be best.
Ah! Found one. Point of reference:

IWillTry did just that with his '92 Metro with 145/r12 (I think) tires inflated to 45 psi: Crr = .0106

See his instructable: Measure the drag coefficient of your car
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Old 12-09-2008, 04:32 PM   #6 (permalink)
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bearings and such

Quote:
Originally Posted by MetroMPG View Post
Thought of another flaw in the calculator: CRR entered is supposed to be for the tires' resistance, but rolling resistance involves some drivetrain resistance as well (bearings, possible brake drag, CV/U joints, differential).

Determining your actual Crr experimentally and entering it would be best. See:

Calculating Rolling Resistance

and/or help with simplified cDA/cRR formula
Darin,the SAE considers bearing losses to be so low,they are basically ignored with respect to RR.-------------------------------

Power-train efficiency is a consideration,once road-load power is determined,to calculate engine power required to overcome loads.-----------------------

I've been using 95 % efficiency for manual-transmission vehicles,as this is a value ascertained by the SAE in early research on pickups and big trucks.Basically,once you've arrived at aero and RR horsepower necessary at the drive wheels,you divide that value by 0.95,and it spits-out Bhp at the flywheel.Multiplying that value by BSFC yields pounds of fuel,and dividing that figure by 6.1626 pounds/gallon( for gasoline ),it spits out gallons,then divided by distance for mpg.Sorry for the US Standards!--------------

Sorry,I do not have any data for automatic transmission vehicles.-----------------------

Thanks for creating the calculation tool as I'm sure it will pay dividends to anyone who uses it. Way cool!!!!!!!!!!!
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Old 12-09-2008, 09:45 PM   #7 (permalink)
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A suggestion for version 2.0 ...

- Tie in all the data values needed for the calculator in our vehicle entries in the Garage - Provide a link from one's vehicles to their own power-at-speed charts.

I'd imagine that gas/diesel vehicles will be somewhat easy to integrate ... you may have to work a little to figure out the same for HPV's and Electrics and Electric-Petro Hybrids.

The only big issue I could see would be the metric / imperial units confusion ... Some of the values already stored for cars are imperial (e.g. Frontal area), and the chart-script asks for the same in metric units.

And of course, thank you both, krousdb and MetroMPG for doing the legwork to get this up on the site! Takes me back to my highschool days, reading Korff's book borrowed from the library, and making these same charts on quad-paper for cars I wanted to design and build.
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Old 12-09-2008, 10:04 PM   #8 (permalink)
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Quote:
Originally Posted by aerohead View Post
Darin,the SAE considers bearing losses to be so low,they are basically ignored with respect to RR.
Do you know if the same thinking applies to losses in the CV/universal joints, potential brake drag, and differential/transaxle resistance - strictly for the purposes of calculating RR?

Quote:
I've been using 95 % efficiency for manual-transmission vehicles,as this is a value ascertained by the SAE in early research on pickups and big trucks.Basically,once you've arrived at aero and RR horsepower necessary at the drive wheels,you divide that value by 0.95,and it spits-out Bhp at the flywheel.
Good idea. Currently you can lump that into the "engine efficiency" figure in the calculator, if you're working backward from a known MPG at a certain speed.

Still some refinement to be done, for sure. Thanks for the feedback Phil.
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Old 12-09-2008, 10:05 PM   #9 (permalink)
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ChrstphrR - we're on the same wavelength! Those garage fields were put there from the beginning specifically thinking of this tool and integrating the two.

The conversions shouldn't be too hard to do.

Good point abot the hybrids/EVs, etc. For now, we'll focus on ICE vehicles since they're in the majority, and most in need of this kind of info to help people improve their efficiency.
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Old 12-09-2008, 10:26 PM   #10 (permalink)
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Diesel energy density figures for the drop-down.

Some energy-density figures for diesel fuel (since I'm an oil-burner... and want a nice simple drop down menu option too!)

http://www.biodiesel.org/pdf_files/f...al_Oct2005.pdf
HTML-ized version of:
Energy Content

Cites that:
  • No. 2 Diesel has an average energy density of: 129,500 Btu/gallon
  • B100 (100% Biodiesel) has an energy density of 118 296 Btu/gallon
  • B20 (20% Bio blend) has an average energy density of 127 259 Btu/gallon
  • Diesel stocks can vary in energy density by 15% between different suppliers.
  • Winter blends of Diesel fuel from a single supplier tend to be 15% lower than summer diesel blends.
  • This latter figure would mean the average energy density of Winter No. 2 Diesel is 110 075 Btu/gallon

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Last edited by ChrstphrR; 12-10-2008 at 01:52 AM.. Reason: Edit for calculation - winter diesel is 110k Btu, not just plain diesel.
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