Confusion Over MPG Ratings for Electric Cars....How would you rate the Volt

[Fledermaus]

What will fuel-economy numbers look like on the window stickers of electric vehicles coming to the US market next year?” asks Paul Weissler, in his recent article for the Society of Automotive Engineers. Trying to find the answer could short-circuit your brain.

The big numbers on the window sticker for BMW’s Mini E—currently in the hands of hundreds of consumers in a test program—are 33 in the city and 36 on the highway. But those are posted in kilowatt hours per 100 miles. The smaller text explains that the equivalent is 102 mpg city and 94 mpg on the highway.

When plug-in cars hit the US market in the next year or two, consumers will need a lot of help deciphering the efficiency figures of vehicles that carry electric fuel by the kilowatt hour rather than liquid fuel by the gallon. Nissan’s upcoming yet-to-be-named electric car, according to some tests, will get 367 miles per gallon. The Tesla Roadster is reported to get 135 miles per gallon. And the Chevy Volt plug-in hybrid…that depends.

If the EPA uses tests designed for electric cars to evaluate the Chevy Volt, the ratings could exceed 100 mpg. But if the government agency classifies the Volt as a hybrid and tests it as such, the EPA rating would drop to about 50 mpg. The difference could mean success or failure in the marketplace. Corporate Average Fuel Efficiency and sticker numbers for plug-in hybrids, which use gasoline and electricity in various degrees and ways depending on the specific vehicles design, have not yet been determined.

http://www.hybridcars.com/fuel-economy-numbers/confusio...

So, how do you rate a car that was designed to be a pure electric car when driven under 40 miles a day? But unlike the Leaf or Tesla, this car was designed to go on a several hundred mile trip without stopping for several hours to recharge.

I think the Volt should have two sets of EPA ratings on the sticker one for pure electric mode the other for hybrid. I think that would be fair and reasonable.

[NNadir]

1. It may have something to do with the fact that electrons come in coulombs and not gallons.

I always thought that was kind of, um, obvious, but one can never be surprised these days by what one hears in the "flaky science" part of the world.

It just gets worsererer and woreserer.

[Fledermaus]

5. kilowatt hours per 100 miles only?

http://www.hybridcars.com/fuel-economy-numbers/confusio...

[Buzz Clik]

2. Here's the easy way to figure it out:

An electric car's efficiency is affected by its charging and discharging efficiencies, which ranges from 70 to 85%. The electricity generating system in the U.S. loses 9.5% of the power transmitted between the power station and the socket, and the power stations are 33% efficient in turning the calorific value of fuel at the power station to electrical power.<9> Overall this results in an efficiency of 20% to 25% from fuel into the power station, to power into the motor of the grid-charged EV, comparable or slightly better than the average 20% efficiency of gasoline-powered vehicles in urban driving, though worse than the about 45 % of modern Diesel engines running under optimal conditions (e.g. on motorways).

Production and conversion electric cars typically use 10 to 23 kW·h/100 km (0.17 to 0.37 kW·h/mi).<6><10> Approximately 20% of this power consumption is due to inefficiencies in charging the batteries. Tesla Motors indicates that the vehicle efficiency (including charging inefficiencies) of their lithium-ion battery powered vehicle is 12.7 kW·h/100 km (0.21 kW·h/mi) and the well-to-wheels efficiency (assuming the electricity is generated from natural gas) is 24.4 kW·h/100 km (0.39 kW·h/mi).<11> The U.S. fleet average of 10 l/100 km (24 mpg-US) of gasoline is equivalent to 96 kW·h/100 km (1.58 kW·h/mi), and the 3.4 L/100 km (convert70 mpgUS) Honda Insight uses 32 kW·h/100 km (0.52 kW·h/mi) (assuming 9.6 kW·h per liter of gasoline). While hybrid electric vehicles are relatively energy efficient, battery electric vehicles are much more energy efficient.

The greater efficiency of electric vehicles is primarily because most energy in a gasoline-powered vehicle is released as waste heat. With an engine getting only 20% thermal efficiency, a gasoline-powered vehicle using 96 kW·h/100 km of energy is only using 19.2 kW·h/100 km for motion.

http://en.wikipedia.org/wiki/Electric_car#Comparison_wi...

For the Tesla Roadster:

[IDemo]

3. For a hybrid, two figures

One, the MPGe for the energy usage in the first 40 mile "pure" electric mode. Two, the mpg while in the "charge-sustaining" mode, about 50 mpg for the Volt more or less.

The 230 figure is not the best number, IMHO. While it can be argued that *the majority* of drivers, traveling the 50 miles driven per day or less that will yield the average of 230 mpg over time, if you drive a Volt 230 miles continuously (without recharging the battery from a wall socket), you're going to burn closer to four gallons of gas than one. Still not at all bad, and I think the vehicle has real merit. But the marketeers are having a little too much fun with the numbers.

[kristopher]

7. That could be presented in tabular form

To actually help consumers, the information has to provide a quick reference about the way their driving habits affect the total. For the all-electric or all-gasoline it is, of course, easy. The two numbers you suggest for hybrids would need to be weighted by driving habit to produce a final number. For example, if I drive a Volt (40 mile on electric series hybrid) 50 miles per day 6 days per week, I will use much less gasoline (and total energy) than if I drive 100 miles per day 3 days per week.


In the first case I drive a total of 240 miles in electric mode, and 60 miles supported by the gasoline engine.
In the second case I drive 120 miles in electric mode and need the gasoline engine for 180 miles.

There is no easy way to do this.

I like what GM did because it really helps educate the public to the potential of short range battery packs to move us away from petroleum; and right now, whether they are motivated by climate change, energy security or gasoline prices, that is at least one goal that virtually all consumers share.

[wtmusic]

4. Most only care about what it costs: $/mile

National average of electricity combined with mpg, using two scenarios: short (commuter) and long (overnight) travel.

[OKIsItJustMe]

9. People are used to MPG

And they've gotten used to the city/highway/combined figures. (Where "combined" is largest.)

I think the "Petroleum Equivalent Fuel Economy" method works great, to put things into perspective, except for the fact that (in the case of of the "Volt") there is such a radical shift in efficiency when the mode of operation changes (from "pure EV" to "Extended Range EV.")

The EPA method GM used makes sense. I am disappointed that they (apparently) chose not to include the PEFE calculations for the electricity.


Here's a method which just occurred to me. All cars made in the last 10 years have had OBD-II interfaces. In New York State, annual automobile inspections use the OBD-II data (if the car is new enough) for emissions testing. The data actually goes "off to Albany" as part of the inspection process. Every year, data is extracted from every "new" automobile, and most of the "old" ones as well.

It should be possible at that time to determine the "EPA Rating" for each individual car, the data could be compiled by make and model. This would allow a "real world" figure for efficiency. (On average, drivers of this model got this sort of efficiency.)

Unfortunately, figures would have a 1 year lag, meaning that for the first year of sales figures for a radical new design would be approximate (your mileage may vary) but after that first year, the EPA (and customers) should have a very good "real world" figure for the "average driver."

[NNadir]

6. Well, a kWh/km unit would be less illiterate, but we would still face that

little problem in which people assume that electricity is made by plugging into a wall socket.

I am personally disgusted by this kill-to-have-a-car shell game.

There is not now, and never will be, electric, gasoline, hybrid, compressed natural gas, compressed air, hydrogen, flywheel, ammonia or even DME powered "green" car.

The reason that is true that it obviously requires considerable energy to propel ton quantities of metals (or toxic plastics or composits) around to transport people who weigh between 50 and 100 kg.

The car is the distributed energy cowboy individualist Ayn Rand libertarian bullshit "I, me, mine" form of deliberate moral indifference writ large.

It is disgusting how much energy goes into just thinking about this subject. As the planet approaches 7 billion people and somewhere between 700 million and one billion cars, what I see is a bunch of strung out high junkies ripping the cushions open with knives looking for that last quarter.

It's appalling.

[excess_3]

8. gallons per mile ... zero ....n/t