Quote:
Originally Posted by drmiller100
http://ecomodder.com/forum/tool-aero-rolling-resistance.php?Weight=1000&WeightUnits=lbs&CRR=.00 8&Cd=.25&FrontalArea=12&FrontalAreaUnits=ft^2&Fuel Wh=33557&IceEfficiency=.22&DrivetrainEfficiency=.9 5&ParasiticOverhead=0&rho=1.22&FromToStep=5-200-5
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Hi Doug,
The calculator you linked to is useful for certain very limited purposes. As you probably know, the urban cycle has an average speed of 20 mph. I plugged in numbers for a Prius (plus driver) using its peak fuel efficiency of 38%. At 20 mph the Prius gets 196 mpg, according to the calculator. We know the Prius really gets 50 mpg on the urban cycle, so the calculator would appear to be off by a factor of 4.
the calculator
The "problem" is that the calculator is applying one efficiency number to all speeds. If you look at 85 mph (on the perfectly flat road represented by the calculator) then the number is close to 50 MPG, meaning that at that speed the engine might be operating at a point near peak efficiency. But we would have no way of knowing that, because we didn't look up the efficiency at the power required for 85 mph. (Because we are working backwards, we can conclude that the Prius engine must be close to 38% efficient at 85 mph, where it is producing 27 hp. So at some speed, the calculator produces a number that is close. At all other speeds, it produces a number that is incorrect -- any where from wrong to incredibly wrong. And we have no way of knowing where it is producing a correct number be cause we can't feed in the right BSFC for each load condition.
To use the calculator, you'd have to plug in actual efficiency figures (from a BSFC chart) that correspond to the engine efficiency at each speed. (This would be much easier to do with another spread sheet made for the purpose.
That other spread sheet, or series of spread sheets, could assess the power required at each five seconds on the UDDS, lookup the relevant efficiency at that power point, and use that in doing the math.
For many cars, you can come close to a correct combined figure if you calculate the fuel efficiency while going up a 2.5% grade at 45 mph. You calculate the hp required (more or less as as the linked calculator does, but incorporating grade) then find the efficiency at that hp level and then do the math to find mpg.
Optimizing for running near peak efficiency is why a Prius gets twice the mileage of an Accord. You can't just plug in a single efficiency value for either and get a good mpg number.
Of course, you can skip the math, and just go ahead and build it. It will probably get pretty good mileage.
Regards, Ken