ChevyVoltology 101

What's the deal with the Chevy Volt? Well, obviously, it is a cool car. A plug-in hybrid. The problem is in how to quantify its efficiency. Normal hybrids (the non-plug in type) have only one type of energy input, gasoline. The Volt can take gasoline or electricity input. This makes it difficult to compare the efficiency of other cars. What is efficiency? There are several things you could calculate.

Actual MPG

This is the distance the car travels (the miles part) divided by how much gasoline it used (the gallons part). Pretty straight forward? Ah ha! Not so straight forward for the Chevy Volt. The Volt can go 40 miles just on it's batteries, so what if someone travels 35 miles a day? How much gasoline would they use? None.

Equivalent MPG

Ok, so the Volt could get infinite Actual MPG, what about an equivalent MPG? This would be like how much gasoline it would use if it were just a gas input car. To calculate this, I would need to convert the electrical energy input into an equivalent gasoline energy input.

Miles per Dollar (MPD)

What if you don't really care about gasoline or its equivalent? Maybe you just care about the money. Then this would be the cost to operate the car. I will use the cost of electricity and the cost of gasoline. This will also depend on how far you drive each day like MPG.

CO2 per mile (or Mile per CO2)

Maybe you don't care about money, but you care about global warming. If that is the case, then you are concerned with the amount of CO2 produced. This will depend on where you get your electricity from as well as how much you drive a day.

So, what do I need to calculate all this stuff? First, some info about the Chevy Volt.(info from Greendaily)

  • Can travel 40 miles on just the battery.
  • Battery stores 16 kWhr of enegy (5.76 x 107 Joules).
  • Gets a claimed 230 MPG
  • I will assume the whole process of charging and using the battery is 75C% efficient.

Other important info:

  • Energy density of gasoline = 1.21 x 108 Joules per gallon
  • A gasoline powered car is 15%-20% efficient (at converting gasoline energy to mechanical energy)
  • Price of gasoline is $2.50 per gallon (but you can change this)
  • Cost of 1 kiloWatt hour = $0.1159 (national average from Energy Information Administration)
  • 1 gallon of gas produces 10 kg of CO2 - according the this US EPA greenhouse gas calculator
  • According to the same site, 100 kiloWatt Hours of electricity produces 0.07 Metric tons of CO2 (70 kg), or 0.7 kg of CO2 per kiloWatt hour. (I guess that is like an average or something since obviously some electricity production doesn't output any CO2 - nuclear, wind, solar, hydroelectric)

Ok, now on to the calculations. Of course done in zoho spreadsheet (I can't remember why I like zoho better than google docs).

Here you can see the numbers I used and you are welcome to play around with these. One thing I just realized is that I assumed that when it was running on gasoline only, it was only driving the car (and not recharging the batteries). Also, I calculated the fuel efficiency of the car by looking at how much energy it took to go 40 miles (one full charge) and finding the equivalent energy in gasoline. Maybe that is not completely valid, but oh well.

Finally, I would like to make a graph. This is a graph of the actual miles per gallon as a function of distance driven.


Oh. It looks like King of the Road posted about the Chevy Volt also.

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1) 20 mpg driven by my better half, city.
2) 24 mpg driven by Uncle Al, city.
3) 27 mpg mixed with freeway.
4) 31 mpg for 30 miles of 90 mph going 40 miles to the Renaissance Faire, then back, freeway.

Synchronize traffic lights, remove speed limits, keep women out of the driver's seat.

I've been reluctant to calculate my CO2/mile for my future plug-in electric vehicle, but now I guess I will have to. I pay extra for "wind energy" from my electricity supplier, but I don't think that means I actually get electrons from the windmills directly. We have both coal and nuclear plants here.

Neat calculations, but GM has released information that only about half of the battery capacity (8Kwh) will be used to go ~ 40 miles before the ICE goes into charge "sustain" mode. The purpose is to maximize the life of the batteries and get them to last 10 years/150,000 miles. As the battery degrades, the ~40 mile range will stay the same as it goes deeper and deeper into % discharge to compensate for loss of overall capacity.

Interesting calculation but you might want to look at Assuming the 230 MPG equivalent is just using the battery/electric drive, then the referenced website would would apply the "lumped average efficiency of electrical production and transmission in the United States is 0.303". This is the "well-to-tank" efficiency, or, the efficiency of getting the electricity to the battery charger. The "tank-to-wheel efficiency is already included in the 230 MPG figure. Then to get the MPGGE (mile-per-gal-gas-equivalent) well-to-wheel efficiency you just multiply 0.303 X 230 and the answer is 70 miles per gallon. Sounds reasonable. This website has a lot of references which are interesting in themselves.

By Jack Hilift (not verified) on 24 Aug 2009 #permalink