Sunday, November 21, 2010

Measuring the Volt's mileage

As reported by CNN, the Chevy Volt faces one last government hurdle before it goes on sale to the public.  New cars must have an EPA mileage sticker in order to be sold in the US.  The EPA hasn't figured out how to measure the mileage of a plug-in electric hybrid.  No measurement standard, no sticker, no sales.  This isn't a new issue; GM has been in discussions with the EPA since before September 2008, and the issue remains unresolved.  If you haven't looked recently, the current EPA window sticker for a gas-powered vehicle looks like this:

The case of an all-electric car has also been settled.  The relevant portion of the Tesla Roadster is shown below.  A picture of a stylized battery instead of a stylized gas pump, and the kiloWatt-hours for 100 miles in city and highway conditions instead of the MPG figures.  Of course, there's always the caveat that "actual consumption and range may vary."  Nissan has agreed that the range on a full charge for its new Leaf will vary wildly depending on the conditions: from as high as 138 miles, cruising at 38 MPH and an outside temperature of 68 degrees, to as low as 47 miles in stop-and-go traffic averaging 6 MPH, an outside temperature of 86 degrees, and the air conditioning running.

A plug-in hybrid can be rated in several different ways.  You can measure the gasoline mileage when the vehicle is running on gas, as would be the case for most of an extended highway trip.  The Volt will probably be somewhere in the upper 30s as a straight gasoline vehicle.  You can measure the efficiency as an electric vehicle, in kWh per 100 miles.  The Volt will probably be in the same 30-33 range that the Tesla Roadster and the BMW Mini E get, given similar weights, battery packs, and electric motor efficiency.  But what about the cases where you use both modes?

Assume that the Volt delivers on the 40-mile range from a fully charged battery, and gets 37 MPG on gasoline once the battery is depleted.  A 30-mile trip starting with a fully-charge battery gets infinite gasoline mileage because no gasoline is used.  A 30-mile trip starting with a half-charged battery gets 74 MPG (15 miles on gasoline uses 0.405 gallons, divided into 30 miles).  A 120-mile trip (round trip to my repair something at my daughter's house, say) starting with a fully charged battery get 55 MPG).  One proposed EPA methodology, since discarded, would have given the Volt a rating of 235 MPG for city use.  Working backwards, and using the assumptions above, yields about 47.5 miles driving between charges.

I hold out little hope that the EPA will be able to boil things down to a single tidy window sticker that is useful to consumers.  They can provide the basic information: likely battery-pack range, kWh per 100 miles, and gasoline mileage.  But it's going to be up to the individual consumer to know their driving habits in order to determine how much of the time they can operate in electric mode and how much in gasoline.  If Chevy sales staff are going to be helpful, they're going to need some training so that they can sit down with potential customers and help them work through the details.

To use myself as an example:  I currently drive about 7,000 miles per year.  1,000 of that is a single round-trip to my mother's each summer, another 1,000 is probably on days with between 40 and 120 miles driven, and 5,000 is on days less than 40.  On the other hand, Colorado has recently adopted new electric tariffs so that the incremental electricity used to charge a plug-in vehicle would likely be billed at a higher rate in the summer but not in the winter.  Do I drive more in the summer or winter?  To be honest, excluding the trip to my mother's (which would be almost all gasoline anyway), I have no idea.  And I'd be willing to bet most other people don't either.

In the long run, electric cars make sense because that's how you can burn coal, nuclear, hydro, wind, or solar power in your personal transportation system.

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