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I have the greatest respect for the work of the Rocky Mountain Institute, but this recent blog post triggered my general scepticism towards electric cars, or more specifically, their fuel efficiency. This is how the RMI calculates the payback for a Nissan LEAF:

RMI comparison e-cars

I'm assuming their numbers for mpg and kWh/mi as well as the respective fuel prices are sound. Then, an almost 75% cost saving on the fuel indicates a vastly superior fuel efficiency of the electric car. This seems very counterintuitive to me, as I would think producing the electricity, transporting and storing it and then using it to drive the wheels loses a lot of energy along the way, compared to going from thermal directly to kinetic energy.

Now, if we used the same kind of fuel to drive the shaft at the power plant and the one in the car's motor, I guess the large motor at the power plant can be built more efficiently, but I have a hard time imagining that this outweighs all the losses of distributing the power plant's energy via wires and batteries.

Conversely, if we took the electric car and replaced only its motor with the most efficient combustion engine we have, would that show the same superiority over conventional cars, meaning the electrical car's advantage is not the motor technology but that it's generally more efficient?

Intuitively, I would think that distributing the fuel and generating the power in the car makes more thermodynamic sense.

Note that I'm well aware of the fact that there might be other reasons for electric cars (less pollution locally, ability to use things like wind turbines as the energy source etc.). But just focussing on the fuel-to-motion analysis, aren't electric cars much less efficient?

Since the comparison is a financial one, what might be reasons for the great price difference, if the electric really is less efficient thermodynamically?

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It really sounds more like there's a question for Physics.se in here than Skeptics. –  Tacroy Feb 14 '13 at 22:57
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"...if we used the same kind of fuel to drive the shaft at the power plant and the one in the car's motor..." I think this is the crux of the issue. The cost to generate a given amount of energy from a (generally) coal electric plant is very much lower than the cost of energy from gasoline that's been refined, transported to a station, and sold to the consumer. –  Larry OBrien Feb 14 '13 at 23:52
    
The fuel is rarely the same, and internal combustion doesn't use heat to drive its machinations. It uses pressure differentials driven by explosions. If the ICE were significantly more efficient than it is now, you'd be correct. As-is, it loses a lot of energy converting gasoline to kinetic energy. –  MCM Feb 15 '13 at 1:35
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At least here in Germany, about 55% of the price of gasoline is taxes, compared to about 25% on electricity. That would be quite a large factor in this comparison, and totally unrelated to "real" efficiency. –  Jens Feb 15 '13 at 8:44
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Nissan Leaf fuel economy is claimed to be equivalent to 99 MPG - fueleconomy.gov/feg/noframes/32154.shtml –  Tom77 Feb 15 '13 at 10:55

2 Answers 2

up vote 19 down vote accepted

The short answer is yes, electric cars are about 4x as efficient as fossil-fuelled combustion engines, tank-to-wheel: ICE engine efficiency is around 20%. Electric engines tend to be around 80-90%. And the fuel prices in the article are tank prices, so tank-to-wheel is the right measure in this case. If you were interested in the whole-cycle energy efficiency (which is not what your linked claim refers to), then well-to-wheel would be the efficiency you were after, and then it would be very sensitive to how your electricity would be generated.

For the nitty-gritty of the energy consumption of electric, fossil and hydrogen cars, see this paper from George Wallis of the Claverton Energy Group (pdf, 317kb).

Note that pretty much all electric cars benefit from regenerative braking, and very few fossil-cars do.

Efficiencies do depend on the drive cycle: and whereas ICEs tend to be optimised for speeds around 85km/h, the efficiency of electric cars decreases with increasing speed, just as the core physics would lead you to expect:

enter image description here

(source)

For more information on efficiencies power-station to wheel of electric vehicles, which you ask about, but which is not what the claim you've cited refers to, see the US Gov Fuel Economy site.

And please do come over to the new Sustainability Stack Exchange where we take this sort of question too.

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Useful answer - and thanks for pointing me at the sustainability SE! –  Rory Alsop Feb 15 '13 at 9:13
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Thanks, the efficiency of the two motor types was the missing link in my picture. Also, yes the sustainability SE will be useful to me. –  Hanno Fietz Feb 18 '13 at 12:37
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Wouldn't the short answer be "no", not "yes", given your information? The question is whether electric cars are at an efficiency disadvantage. –  Highly Irregular Mar 8 '13 at 7:46
    
It's also worth noting that gasoline production also consumes energy in the form of electricity and natural gas. In states like California that use natural gas to produce electricity, it's almost true that the savings in producing gasoline could power their electric cars. –  Ernie Jul 5 '13 at 21:34

Efficiency is a matter of picking your input and output points. For fuel efficiency, we have chosen the input as the point at which we have paid for it. Thus the electricity is an amount you have paid for after it reaches your home. Any measurement of efficiency from here on out is measured in miles driven per kWh of energy paid for. Thus the transportation/generation losses that you mention are irrelevant, as they happen before you have paid for the energy.

Compare this to gasoline. If we were to assess the energy required to gather/process fossil fuels into their respective types (diesel, gasoline, coal, etc), we could get a measurement of energy from ground to road. We do not do this, we instead choose to measure the efficiency of a vehicle by its use of gasoline after we have already paid for it at the pump.

Overall, remember that articles like this are about money. Thermodynamic efficiency isn't really the game here (at least not from ground to your motor). Cost efficiency is the name of the game. If I drive a certain number of miles to work everyday, then it follows that I probably use a certain amount of fuel every day on that drive as well. How much fuel did I use, and, more importantly, how much did it cost? In most cases, the amount you pay will be less for the electric car's fuel than for a combustion engine's.

Finally, let's go ahead and address the thermodynamics issue. Internal combustion engines in vehicles rarely get above 20% efficient, while a coal-fired plant (the least efficient of the fossil-fuel driven electrical processes, while providing most of the electricity that enters your home) reaches around 30% efficiency. The losses on transmission lines and battery chargers are fairly small, leading me to believe that electric vehicles enjoy a thermodynamic efficiency that beats most internal combustion engines.

Sources: wikipedia http://en.wikipedia.org/wiki/Internal_combustion_engine#Energy_efficiency http://en.wikipedia.org/wiki/Coal_plant#Heat_into_mechanical_energy

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Welcome to Skeptics! Please provide some references to support your claims. And wikipedia is not a primary source. –  Larian LeQuella Feb 15 '13 at 3:46
    
The mods here are so pleasant and nice... –  Chad Feb 15 '13 at 20:04
    
Nice analysis... agree about it needing references before I can upvote though –  Highly Irregular Mar 8 '13 at 7:49

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