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Old 04-25-2009, 10:37 PM   #1 (permalink)
Ernie Rogers
Ernie Rogers
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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Let's be fair on the high-mileage EVs

I see that the four highest-mileage cars listed on the left margin of the home page are all electric cars, with the exception of the Mustang HP2g (see more on that elsewhere). The data provided in the EV mileage logs indicates the assumed energy content of gasoline. Here are the numbers I found:

Citicar................36.6 kWh per gallon
Electro Metro......36.5?
1992 Metro.........33.6
Fiero EV.............36.6

Except for the third car, one wonders if these people are intent on violating the first law of thermodynamics. I think the most widely-accepted energy content (low heat value) for gasoline for use in an internal combustion engine is 116,090 BTU per gallon. This was adopted by Wang at Argonne Nat'l Laboratory (e.g., the GREET model) and (I think) is the value used for calculations for the Auto X Prize. From my CRC handbook, the conversion factor from kWh to mean BTUs is 3409.5. Dividing these numbers gives a HEAT energy content of gasoline of--

34.05 kWh (heat) per gallon.

I acknowledge that the higher value of approx. 36.6 kWh (heat) per gallon is obtained if you use 125,000 BTU per gallon, but I believe that would be incorrect for referencing to real gasoline in an internal combustion engine that exhausts products as gases.

Let's be kind and assume that the calculations are based on a strict 100% conversion of gasoline heat of combustion to electricity. However, this represents an implicit violation of the second law of thermodynamics, which states that heat cannot be converted entirely to work.

If the EV car owners would like to be entirely fair in representing the efficiency of their cars, then I suggest they agree to abide by both the first law of thermodynamics and the second law. We could be generous and let them use say the theoretical (Carnot) efficiency of a gasoline engine, or possibly a high (power plant+transmission) efficiency for an advanced power system, at about 50%.

In such a case, I would suggest that they use a kWh-gallon equivalence of either 17 or 18 kWh per gallon.

Making this adjustment will allow a much more fair comparison between EVs and highly-efficient IC cars.

Ernie Rogers

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