[Part one is here.]
Yesterday, I stated repeatedly that miles per gallon is a stupid way of rating a plug-in electric hybrid like the Chevy Volt, which is now claiming an EPA estimated 230 MPG for city driving. Let's take this apart one piece at a time.
1) In a gasoline car that gets 30 miles per gallon, you can drive 30 miles and use one gallon of gas. In the Chevy Volt, based on our estimates of 40 miles of electric range and 50 MPG on gasoline backup, you will actually consume 3.8 gallons of gasoline if you drive 230 miles
2) The EPA rates gasoline engines for city and highway driving for two reasons. The first is that city driving involves idling, which consumes fuel while not actually moving. The second is that the efficiency of gasoline engines varies with speed. However, electric motors have much more forgiving efficiency-speed curves than gasoline engines, and do not need to consume energy while idling. If you drove 100 miles on the freeway or 100 miles in the city, there will be little difference in how much fuel you've consumed.
3) Rating the engine based on gasoline consumption alone ignores the cost and environmental impacts of the electricity consumed during the first 40 miles of travel. Just because you've used no gas after 40 miles doesn't mean you haven't used energy, spent money, or emitted carbon. Its electricity, not pixie dust.
And its this last point that is most important. What is the fuel efficiency of the electric batteries?
And how does this compare to the fuel efficiency of a gasoline engine?
According to the same CNN article
from yesterday, 8
kilowatt-hours of energy are needed to go 40 miles. Aha! So you get 5
miles per kilowatt-hour.
Uh, does that mean anything to you?
Me neither. Fortunately, we can convert kilowatt-hours to gallons of gasoline. Gallons of gasoline, like
kilowatt-hours, barrel of oil, calories, or tons of cooling, is just one
of the many difficult to decipher units of energy that we use on a
regular basis. But if you have the right unit of conversion, comparisons can be made. In this case, the right unit of conversion is:1 gallon of gas = 36.6 kWh
(5 miles) / (1 kWh) * (36.6 kWh) / (1 gallon of gas ) = 185 MPG equivalent
Here is where Chevy's claim of 230 MPG falls apart faster than an Ikea bookcase. When you are running just on the electric batteries, you are only getting 185 miles per gallon of gas equivalent used
(let's call this a gallon of electrons, or Ge for short). Then, when you switch to gasoline, the efficiency drops to 50 MPG.
it gets even more complicated from here. Let's say that I
bought a Chevy Volt tomorrow. I pay roughly 12 cents per kWh for
electricity, and $2.50 per gallon of gas. Based on my electric rate, my
gallon of electrons costs $4.40. So even though the car gets 185 MPGe,
that fuel costs me 75% more. If we calibrate MPGe for cost, we are down
to 105 MPGe-dc (dollar calibrated*).
We can also do the same thing for carbon emissions. Using myself as an example again, every kWh of electricity results in just under one pound of carbon emissions
That means my gallon of electrons produces 36.2 pounds of carbon.
Compare this to a gallon of gasoline, which produces 19 pounds of
carbon when burned in an internal combustion engine. The gallon of electrons produces 91% more carbon than a gallon
of gasoline! If we calibrate MPGe for carbon, we are down to only 97
MPGe-cc (carbon calibrated*).
Chevy-EPA smoke blowing bullshit rating: 230 MPG
Efficiency of electric drive: 185 MPGe
Calibrated for dollars: 105 MPGe-dc
Calibrated for carbon: 97 MPGe-cc
Once you are out of power: 50 MPG
Here's yesterday's graph, showing how MPG will vary with distance driven, now with lines for MPGe, MPGe-dc, and MPGe-cc added.
The vehicle is supposedly going to be marketed to those drivers who drive only short distance each day. But we can see from this graph that its these people that are being most drastically lied to about the fuel efficiency of the car.
230 MPG rating is for marketing, but once the cars get out there, the
backlash will begin. People will not get the fuel efficiency they
thought they would. Their electric bills will increase. The cost of
driving the car in terms of environmental impact will be much, much less
than a regular gasoline vehicle, but much, much more
than Chevy is advertising.
So couldn't GM have just come out and said this? The car gets 185 MPGe
for the first 40 miles, and 50 MPG after that. Why did they need to
come up with this bogus 230 MPG number that is just dying to be picked
apart by every nerd with a calculator and a blog? Why not rely on your
consumer to be smart enough to figure out what they are buying? Because
anybody who can't understand this is also too stupid to earn enough
money to buy a $40,000 vehicle.
Yes. But that is a topic for tomorrow.
*Exact dollar-calibrated and carbon-calibrated values will differ depending on where you live, how you get your energy, and how energy prices vary with time. Obviously.