I was (partially) wrongAugust 20, 2009
I recieved comment form the GM spokesman, Rob Peterson, about my last two posts lambasting the supposed 230 mile per gallon Chevy Volt. Here is Rob’s comment in its entirety.
This is Rob Peterson from GM.
Although the Volt has a 16 kWh battery, only 8 kWh is used. This will significantly impact the rest of your calculations and your synopsis. Please post a correction based on this fact.
As for the Volt’s city fuel efficiency rating of 230mpg – this is based on the EPA’s draft methodology. The same methodology which will be used for all other vehicles of this type.
I responded to Rob with two comments, which you can read here. One of those comments questions his sincerity about “blaming” the 230 mile per gallon claim on the EPA. However, he is essentially right about the the charging cycle of the 16 kWh battery only using about half of that. He has asked me to “Please post a correction based on this fact.” I have done so, but the final numbers for the vaunted Volt are still underwhelming.
Here is a table comparing miles per gallon, kWh per mile, and pounds of CO2 per mile for the Chevy Volt, the Toyota Prius, and the a couple of ancient Honda Civics. You can read the details of how I derived the numbers for the Volt, using Rob’s partial capacity charge cycle scheme in the text below. Note that the prices for the Honda Civics have been adjusted for inflation to 2009 dollars for a fair comparison.
Now that I have posted a correction, can I expect Rob Peterson to post a retraction of GM’s preposterous 230 mile per gallon claim? Not Likely.
I have not yet been able to find an official specification for the number of kilowatt-hours per mile for the Volt. I am hoping Rob will provide one. I have found Rob’s description of the charging scheme for lithium-ion batteries to be essentially correct. That is, the battery is typically charged by the electrical grid to around 90% of total capacity. Then the car is propelled entirely off of battery power until it reaches about 30% capacity. This is known as the “charge depletion” mode. When the battery gets to about 30% of capacity the gasoline powered generator kicks in and maintains the charge at about 30% capacity. This is known as the “charge sustaining” mode.
Then, when the battery is plugged into the electrical grid it is recharged with grid energy from about 30% capacity back up to about 90% capacity That is a range of about 60% of the total capacity. So, for a 16 kilowatt-hour battery, a complete charge off the electric grid puts about 9.6 kWh (0.6 x 16 kWh) into the battery. But an extra 10% or so is lost due to transmission line and battery conversion losses. So the amount of power taken from the electrical grid will be about 10.6 kilowatt-hours. If that charge will propel the car for 40 miles, then that works out to 3.8 miles per kWh (or about 0.27 kilowatts per mile)
I cannot find the value of about 0.27 kWh per mile anywhere in the specifications for the Volt, but I did find this somewhat cryptic statement at Chevy.com:
“Under the new procedure, the EPA weights plug-in electric vehicles as traveling more city miles than highway miles on only electricity. The EPA procedure would also note 25 kilowatt-hours/100 miles electrical efficiency in the city cycle.
So, lets accept the value of 25 kilowatt-hours/100 miles (0.25 kWh per mile) for the moment. What is the affect that this will have on the numbers I reported for CO2 emissions?
The number of pounds of CO2 emitted per mile while powering the car with gasoline (known as the “charge sustaining” mode) will remain unchanged. There are 19.4 pounds of CO2 produced per gallon of gasoline burned, and GM claims 50 miles per gallon in “charge sustaining” mode. So:
( 19.4 lbs of CO2 / gallon) / (50 miles / gallon) =
0.39 lbs of CO2 per mile
This 0.39 lbs of CO2 per mile for the Volt running on gasoline (charge sustaining mode) is the same as for the Toyota Prius, because it also gets 50 miles per gallon.
Here is the same calculation for my ancient 1988 Honda Civic hatchback that got 47 miles to the gallon:
( 19.4 lbs of CO2 / Gallon) / (47 miles / gallon) =
0.41 lbs of CO2 per mile
And for the 19
7887 Honda Civic Coupe HF, which got 57 miles per gallon:
( 19.4 lbs of CO2 / Gallon) / (57 miles / gallon) =
0.34 lbs of CO2 per mile
Lets assume now that the Volt uses 0.25 kilowatt-hours per mile (“25 kilowatt-hours/100 miles’) when running off of power provided to the battery by the electric grid (known as the “charge depleting” mode). On the average the grid yields 1.34 pounds of CO2 per kilowatt-hour. The grid transmission losses and grid to battery conversion losses add up to about 10%. So the amount of CO2 yielded per mile will be:
(1.34 lbs of CO2 per grid kWh) x (0.25 kWh per mile) x 1.1 =
0.37 lbs of CO2 per mile
Almost identical to the CO2 emitted when it is running off of gasoline (0.39 lbs of CO2 per mile). And it is also nearly identical to the amount of CO2 per mile as the much cheaperPrius generates while running off of gasoline.
But here it the rub. If the Volt is driven in an area where the electricity is predominantly generated with coal (by far the most common source or electricity generation in the US), then the CO2 emissions go way up. That is because Coal emits about 2.1 pounds of CO2 per kilowatt-hour generated for the electric grid. So again we can asume 10% for the sum of the grid transmission losses and grid to battery conversion losses. Then the amount of CO2 that the Volt yields per mile driven in a region where coal is the primary source of electricity will be:
(2.1 lbs of CO2 per grid kWh) x (0.25 kWh per mile) x 1.1 =
0.58 lbs of CO2 per mile
If we really concerned about reducing CO2 (I’m not), saving energy (I am), creating American jobs (I am), and saving money (I am), then we should support the production of an American car that is similar to a 1988 Honda Civic. Why argue the merits of a $40,000 car that few people will ever be able to afford? A $15000 dollar car that gets as good or better mileage and generates as little or less CO2 would be bought by millions and have a much greater impact.