jesse
@ August 12, 2009


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[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.

And 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*).

To summarize:

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.

CHEVY VOLT MPG-2.JPGThe 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.

The 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.

$40,000?!?! 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.

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The carbon equivalent you use is based on the carbon produced by burning gas, but uses the carbon resulting from the production of the electricity. The carbon created in the production and transportation of 1 Gg is much higher than the carbon created in the production of 1 Ge, isn't it?

If I understand your question correctly, you are talking about the carbon that is released as part of the process of making gasoline (drilling, transportation, processing, etc). Lots of very smart people have been working on the question of "intrinsic" carbon due to transportation, manufacturing, material processing, etc, but I'm not one of them. So while it seems right that gasoline which is produced from oil in the middle east has more intrinsic carbon than coal mined in west virginia, I have no data on that so I ignored it.

“What most people need to learn in life is how to love people and use things instead of using people and loving things.”

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Polls are fun, but can be dangerous. I prefered it when Boston Globe predicted 15 points ahead Coakley and Rassmusson 46 Brown 48 Coakley. We would like Coakley voters to relax and stay home, while Brown supports make sure they get to the polls and bring a friend or two to vote. Polls can bread overconfidence. We have 5 days to keep working, phoning, blogging, tweeting, and texting. Lets not crack the champaign yet gents and ladies. Lets Roll and make sure we have enough votes to pass the trunkload of ballots that will likely appear, the busloads of voters that could be brought in, and observers w/cameras at every polling place.

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Whay does marriage have to be certified by the state? The answer is because it comes with legal rights and privilages (i.e. tax breaks, insurance, etc.) I think the real answer is to eliminate the state from the equation all together. Let each church decide the question of marriage and divorce, but keep the legal drama out of it. A legally binding divorce decree would either be decided by the couple, or in a divorce court that accepts all forms of marriage. The bottom line is, that we should get the govt out of the marriage business altogether.

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99% accurate but you've got a bit of, well, to put it bluntly, garbage there.

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I agree with you 100%. I loved the book Marathoning for Mortals and often refer back mentally to the respect the distance section before every long run. As far as I am concerned, if a runner trains to the best of her ability and tries her best during the marathon and finishes in 6 hours, she deserves that medal just as much as someone who trains hard and runs hard and finishes in 3 hours. Everyone is different in their abilities and thats what I love about the sport of running. It doesnt discriminate. Anyone can do it. Thanks for inspiring me to be a better runner.

Not sure about this news. It seems to be all right, but something is not right

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Your comments regarding the Radiothon and FOJS are duly noted and fair observations. I have had some of the same questions myself. However, I will not be so quick to judge the Radiothon or sell FOJS short.

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39. Commuter:Anecdotal and for what it's worth: Born and bred in Mass and most family is still in state.

Whether one believes in a religion or not, and whether one believes in rebirth or not, there isn't anyone who doesn't appreciate kindness and compassion.

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Im from Massachusetts, currently a registered Democrat. I am voting for Scott Brown, and I am planning to change my party affiliation. This is NOT the Democratic party that my parents and grandparents were so dedicated to.

Yes. I get it. Its sad. So many people are afraid of the antisemite word. What in hell is a semite anyway? I think it means Arab as well???

congrats and good job on the 5k!

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Hi Subhash,In the past, Fletcher hasnt fit any of the US News categories. I just looked through the list. Public affairs would be closest but, as you noted, Fletcher isnt there and neither are our peers. The international politics category refers to traditional academic departments. Multidisciplinary programs such as Fletchers just dont neatly fit anywhere!

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For me, my Garmin is my "official record" of my run. It keeps the data for the log (even if I am not really keeping track). Garmy is always there and always records so later I can go back and see. So I understand. I can be so bummed out when I am ready to work out and Garmys battery is dead! It is almost like, if I workout without a Garmin, it never happened!.

One of the things that this event need not turn into is a bashing of wine ratings. There was nothing here to rate. It is utter ridiculous to ever expect wines scoring at those levels to be precise nailable.

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But, as the saying goes, the last time I confused Burgundy with Bordeaux was at lunch.

I dont see this monster going down w/out a dirty fight.

If you have a place already well done!Otherwise qualifying times are related to age.This means different age groups have a qualifying timeage group 40-45 men is probably around 2 hrs 50 or faster.You have to be able to prove from past race results you can run a marathon at that pace.

Always nice to hear input from within Capcom, even if its not definitive. So yes, please, continue to post this stuff.

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