Friday, 14 August 2009

The 230 MPG Car

According to the news reports, General Motors has done the impossible. The NYT and hundreds of other press sites have covered the auto maker's announcement that the forthcoming Chevy Volt hybrid car will run 230 miles per (US) gallon of gasoline. Given our interest in low-carbon solutions, that was enough to make the team at Carbon Clear sit up and take notice.

The announcement was a shot across the bow of Toyota, which sells the best-selling Prius hybrid car, and generate more buzz around the long-anticipated Volt. According to the US Environmental Protection Agency's fuel economy website, the Prius gets 48 mpg in city driving, better than anyone else, but pathetic compared to the Volt's 230 mpg.

General Motors based their claim on the fact that the Volt is a plug-in hybrid that can run for 40 miles in electric-only mode, and the battery can be recharged overnight from a household electrical outlet. The gasoline (petrol) motor only kicks in to recharge the electric batteries when the car is driven more than 40 miles in typical conditions. Since the typical American car only travels 33 miles per day and the battery gets recharged overnight, argues GM, the gasoline engine will rarely get called into service. The car might travel on average for 230 miles before an entire gallon of gasoline is consumed.

Voila, 230 mpg.

I can see how this number might be technically accurate. But does it tell customers what they really need to know?

The Volt is expected to cost between $30,000 and $40,000 - considerably more than a conventional car of the same size. I can think of four reasons why people would spend the extra money:
  1. They have money to burn and are caught up in the hype,
  2. They want to reduce consumption of imported fossil fuels,
  3. They want to reduce CO2 emissions from driving,
  4. They want to spend less on fuel.
Let's set aside the first rationale, as it falls outside the normal scope of this blog. How does the Volt rate on the other three?

Rationale #2: Most of the electricity in the United States comes from coal, natural gas, nuclear, and hydropower. The coal, gas, and water, and (much of) the uranium are sourced domestically. So a car that gets mosts of its power from grid electricity rather than gasoline wins on this count. If the 230 mpg figure is accurate, then the Prius uses nearly five times (okay 4.79 times) as much gasoline compare to the Volt. Winner: Volt.

Rationale #3: How does the Volt compare on greenhouse gas emissions? According to the GM press announcement, the Volt can typically travel 40 miles on electricity alone, and its built-in battery has a useable capacity of 8.8 kilowatt-hours. So its daily energy consumption is 8.8 kWh for 40 miles of travel. That's 0.22 kWh/mile.

Greenhouse gas emissions from electricity vary widely depending on the fuel source, but on average the emissions from electricity consumption in the U.S. average 599.9 grams CO2 per kWh. Multiply that number by the Volt's 8.8 kWh daily electricity consumption and we get 5,279 grams of CO2 emissions per day. That's about 132 grams CO2 per mile, or 82 grams CO2 per kilometre. (These are what the carbon reporting standards call "Scope 2" or "energy indirect" emissions: you're using the energy, but the CO2 is coming out of someone else's pipe.)

According to the UK's Car Fuel Data website, the Prius emits 89 grams CO2 per kilometre. Winner: Volt, but not by nearly as wide a margin.

We can look at the CO2 data another way. Let's see how much petrol would have to be consumed to release the Volt's 82 grams CO2 per mile. The US Energy Information Administration says that a gallon of gasoline emits 19.567 pounds CO2/gallon. Converting to metric makes the math easier and gives us 2,346 grams CO2 per litre of gasoline.

Now we have Chevy Volt grams of CO2 per mile, and gasoline grams of CO2 per litre. Manipulating the numbers gives us an equivalent fuel economy of 67.5 miles per (U.S.) gallon. The US EPA says the Prius gets 48 miles per (US) gallon. The UK says the Prius gets 72 miles per (UK) gallon - equivalent to 60 miles per (US gallon). Winner: Volt.

Rationale #4: How much does it cost to drive the Volt? According to the US Energy Information Administration, the average cost of electricity in the year to April 2009 was 9.09 cents per kWh. From our earlier calculations, we learned that the Volt uses 0.22 kWh per mile. So the Chevy Volt's energy costs 1.9998 (let's call it two) cents per mile.

How efficient would a gasoline powered car need to be to achieve the same per-mile fuel costs? The handy US Energy Information Agency website tells us that the average US gasoline price in the week ending August 10th, 2009 was $2.65 per gallon. With two cents (the Volt's per-mile energy cost), we would be able to buy a whopping 0.0135 gallons of gasoline for our car. And if our car were to travel a mile on that amount of fuel, it would need a fuel economy of 75 miles per (US) gallon. Using the larger UK gallons, we would need a fuel economy of 90 mpg to match the driving cost of the Volt.

By comparison, the Prius gets a US EPA rating of 48 mpg, and a UK VCA rating of 72 mpg. Winner: Volt.

Summary: If GM's driving distance and battery capacity numbers hold up in the real world, the company appears poised to take the green consumer car title away from Toyota (and push Honda from second place down to a lowly third). My calculations show a significant greenhouse gas and fuel cost saving compared to the latest model Toyota Prius. The Volt gets the equivalent of between 67 and 75 mpg, depending on whether you're looking at CO2 emissions or dollars per mile. The advantage over the Prius is nowhere near the five-fold difference being trumpted in GM's press releases, but it is real.

  • CO2 emissions per mile: 40% lower using EPA figures for Prius (8% lower using UK figures)
  • Energy cost per mile: 57% lower using EPA figures for Prius

GM has thrown down the gauntlet. I'm eager to see if Toyota and other car producers will rise to the challenge and produce even more efficient vehicles.