Day By Day

Monday, June 11, 2012

Government math

I've always wondered about government claims of "miles per gallon" for electric vehicles.  Since you don't put in a liquid fuel, how do you figure out a MPG figure?  But, I assumed they were making some kind of reasonable calculation, even though the number seemed wildly optimistic.

From the blog Green Explored and blogger Lindsay Leveen, we now know the EPA uses those same mathematical geniuses who are working on the Euro bailout.
The EPA allows plug in vehicle makers to claim an equivalent miles per gallon (MPG) based on the electricity powering the cars motors being 100% efficient. This implies the electric power is generated at the power station with 100% efficiency, is transmitted and distributed through thousands of miles of lines without any loss, is converted from AC to DC without any loss, and the charge discharge efficiency of the batteries on the vehicle is also 100%. Of course the second law of thermodynamics tells us all of these claims are poppycock and that losses of real energy will occur in each step of the supply chain of getting power to the wheels of a vehicle powered with an electric motor.

I started thinking about all of this last night when my wife asked me how the Honda Fit that is now available as an electric vehicle could get 118 MPG as the equivalent rating from the US EPA? I told my wife that was because the US EPA believes in Political Science and not Real Science and that I would investigate this claim for her. Well it is simple the US EPA uses a conversion factor of 33.7 kilowatt hours per gallon of gasoline to calculate the equivalent MPG of an electric vehicle.

Dr. Chu Chu of the Department of Entropy is instructing the EPA on thermodynamics in coming up with the 33.7 kwh per gallon. On a heating value of the fuel 33.7 kwh equals 114,984 BTUS which is indeed the lower heating value of gasoline. The fit needs 286 watt hours to travel a mile and the Green Machine agrees with this for the 2 cycle US EPA test with no heating, cooling or fast acceleration. Using this amount of energy per mile and the 33.7 kwh “contained” in a gallon of gas, the EPA calculates the Fit gets 118 MPG equivalent.

All of these calculations are in fact flawed as the generation of electricity, the transmission and distribution of electricity, the conversion of the AC electricity into DC electricity, and the charging and discharging of the vehicle batteries all have energy losses associated with these activities. The average efficiency of power generation is perhaps 42.5%, the transmission and distribution efficiency is perhaps 90%, the AC to DC conversion and the battery charge discharge efficiency is about 90%. Multiplying all these efficiencies one can calculate that the overall efficiency is 34.4% to get electric power from fuels at the power station into stored electrons within the plug in vehicle’s batteries.

On this basis the 118 MPG equivalent is 40.6 MPG actual for the Honda Fit which is not much of an improvement to the gasoline version of this vehicle that has an EPA rating of 35 MPG combined for city and highway driving.
Green?  You tell me.  And don't forget, that also is for a Fit with no heat, no air conditioning and no fast acceleration.  And no rational basis for existence.

A government agency cooking data to fit political goals?  To quote Captain Renault, "I'm shocked!"

Hat tip to Breitbart for the pointer.

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